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we present the capabilities of our new code for obtaining doppler maps implementing the maximum likelihood approach. as test data, we used observations of the dwarf nova v2051 ophiuchi. the system was observed in quiescence at least 16 d before the onset of the next outburst. using doppler maps obtained for ten emission lines covering three orbital cycles, we detected spiral structures in the accretion disc of v2051 oph. however, these structures could be biased as our data sampled the orbital period of the binary at only eight different orbital phases. our doppler maps show evolution from a one-arm wave structure in hα to two-armed waves in the other lines. the location of the two-arm structures agrees with simulations showing tidally driven spiral waves in the accretion disc. during consecutive cycles, the qualitative characteristics of the detected structures remained similar but the central absorption increased. for the first time, using the doppler tomography method, we obtained temperature maps of the accretion disc. however, taking into account all the assumptions involved when using our method to retrieve them, the result should be treated with caution. our maps present a relatively flat distribution of the temperature over the disc, showing no temperature increase at the location of the spiral arms. using `ring masking', we have revealed an ionized region located close to the expected location of stream-disc interactions. we found the average temperature of the accretion disc to be 5600 k, which is below the critical limit deduced from the disc instability model. | spiral structures and temperature distribution in the quiescent accretion disc of the cataclysmic binary v2051 ophiuchi |
magnetorotational instability (mri) has the potential to generatevigorous turbulence in protoplanetary disks, although its turbulence strength and accretion stress remain debatable because of the uncertainty of mri with a low ionization fraction. we focus on the heating of electrons by strong electric fields, which amplifies nonideal magnetohydrodynamic effects. the heated electrons frequently collide with and stick to dust grains, which in turn decreases the ionization fraction and is expected to weaken the turbulent motion driven by mri. in order to quantitatively investigate the nonlinear evolution of mri, including the electron heating, we perform magnetohydrodynamical simulation with the unstratified shearing box. we introduce a simple analytic resistivity model depending on the current density by mimicking the resistivity given by the calculation of ionization. our simulation confirms that the electron heating suppresses magnetic turbulence when the electron heating occurs with low current density. we find a clear correlation between magnetic stress and current density, which means that the magnetic stress is proportional to the squared current density. when the turbulent motion is completely suppressed, laminar accretion flow is caused by an ordered magnetic field. we give an analytical description of the laminar stateusing a solution of linear perturbation equations with resistivity. we also propose a formula that successfully predicts the accretion stress in the presence of the electron heating. | electron heating and saturation of self-regulating magnetorotational instability in protoplanetary disks |
the magnetohydrodynamics (mhd) of protoplanetary disks are strongly subject to the nonideal mhd effects arising from the low ionization fraction of the disk gas. a strong electric field induced by gas motions can heat ionized gas particles and can thereby affect the ionization balance in the disks. our previous studies revealed that in dusty protoplanetary disks, the ohmic conductivity decreases with increasing electric field strength until the electrical breakdown of the disk gas occurs. in this study, we extend our previous work to more general cases where both electric and magnetic fields affect the motion of plasma particles, allowing us to study the impacts of plasma heating on all nonideal mhd effects: ohmic, hall, and ambipolar diffusion. we find that the upper limit on the electric current we previously derived applies even in the presence of magnetic fields. although the hall and ambipolar resistivities can either increase or decrease with electric field strength depending on the abundance of charged dust grains, the ohmic resistivity always increases with electric field strength. an order-of-magnitude estimate suggests that a large-scale electric current generated by gas motions in the inner part of protoplanetary disks could exceed the upper limit. this implies that mhd motions of the inner disk, such as the motion driven by the hall-shear instability, could either get suppressed or trigger electrical breakdown (lightning discharge). this may have important implications for gas accretion and chondrule formation in the inner part of protoplanetary disks. | the generalized nonlinear ohm's law: how a strong electric field influences nonideal mhd effects in dusty protoplanetary disks |
we study the development of coherent structures in local simulations of the magnetorotational instability in accretion discs in regimes of on-off intermittency. in a previous paper, we have shown that the laminar and bursty states due to the on-off spatiotemporal intermittency in a one-dimensional model of non-linear waves correspond, respectively, to non-attracting coherent structures with higher and lower degrees of amplitude-phase synchronization. in this paper, we extend these results to a three-dimensional model of magnetized keplerian shear flows. keeping the kinetic reynolds number and the magnetic prandtl number fixed, we investigate two different intermittent regimes by varying the plasma beta parameter. the first regime is characterized by turbulent patterns interrupted by the recurrent emergence of a large-scale coherent structure known as two-channel flow, where the state of the system can be described by a single fourier mode. the second regime is dominated by the turbulence with sporadic emergence of coherent structures with shapes that are reminiscent of a perturbed channel flow. by computing the fourier power and phase spectral entropies in three dimensions, we show that the large-scale coherent structures are characterized by a high degree of amplitude-phase synchronization. | on-off intermittency and amplitude-phase synchronization in keplerian shear flows |
there are several peculiar long-period dwarf-nova-like objects that show rare, low-amplitude outbursts with highly ionized emission lines; 1swasp j162117+441254, bd pav, and v364 lib are among them. some researchers even doubt whether 1swasp j1621 and v364 lib have the same nature as normal dwarf novae. we studied the peculiar outbursts in these three objects via our optical photometry and spectroscopy, and performed numerical modeling of their orbital variations to investigate their properties. we found that their outbursts lasted for a long interval (a few tens of days), and that slow rises in brightness were commonly observed during the early stage of their outbursts. our analyses and numerical modeling suggest that 1swasp j1621 has a very high inclination, close to 90°, plus a faint hot spot. although bd pav seems to have a slightly lower inclination (∼75°), the other properties are similar to those in 1swasp j1621. on the other hand, v364 lib appears to have a massive white dwarf, a hot companion star, and a low inclination (∼35°). in addition, these three objects possibly have a low transfer rate and/or large disks originating from the long orbital periods. we found that these properties of the three objects can explain their infrequent and low-amplitude outbursts within the context of the disk instability model in normal dwarf novae without a strong magnetic field. in addition, we suggest that the highly ionized emission lines in outburst are observed due to a high inclination and/or a massive white dwarf. more instances of this class of object may be unrecognized, since their unremarkable outbursts can be easily overlooked. | on the nature of long-period dwarf novae with rare and low-amplitude outbursts |
context. long gamma ray bursts (grbs) originate from the collapse of massive, rotating stars. some of the grbs exhibit much stronger variability patterns in the prompt grb emission than the usual stochastic variations. we discuss the mechanisms able to account for this effect. aims we aim to model the process of stellar collapse in the scenario of a self-gravitating collapsing star. we account for the changes in kerr metric induced by the growth of the black hole; accretion of angular momentum; and the self-gravity effect due to a large mass of the collapsing stellar core falling onto black hole in a very short time. we also investigate the existence of accretion shocks in the collapsar, and the role of magnetic field in their propagation.methods: we compute the time-dependent axially symmetric general relativistic magnetohydrodynamic model of a collapsing stellar core in the dynamical kerr metric. we explore the influence of self-gravity in such a star, where the newly formed black hole is increasing the mass and changing its spin. the kerr metric evolves according to the mass and angular momentum changes during the collapse. we parameterize the rotation inside the star, and account for the presence of large-scale poloidal magnetic field. for the set of the global parameters, such as the initial black hole spin and the initial content of specific angular momentum in the stellar envelope, we determine the evolution of black hole parameters (mass and spin) and quantify the strength of the gravitational instability. we then estimate the variability timescales and amplitudes.results: we find that the role of the gravitational instability measured by the value of the toomre parameter is relatively important in the innermost regions of the collapsing star. the character of accretion rate variability strongly depends on the assumption of self-gravity in the model, and is also affected by the magnetic field. additional factors are initial spin and rotation of the stellar core. we find that for subcritical rotation of the precollapsed star, a centrifugally supported mini-disc is present at the equatorial plane, and it may be subject to fragmentation due to self-gravitating instability. we also find that self-gravity may play a role in the angular momentum transport and that it generally lowers the final mass and spin of the black hole, while the accretion-rate variability amplitude is much larger in self-gravitating objects. the effect of magnetic field is rather weak, while it seems to decrease the strength of accretion shocks. the magnetisation affects the global properties of the flow in a non-linear way, and is manifested mostly in models with moderate initial black hole spins but supercritical rotation of the collapsing star.conclusions: our computations confirm that gravitational instability can account for flaring activity in grbs and the variations in their prompt emission. rapid variability detected in the brightest grbs (most likely powered by rapidly spinning black holes) is consistent with the self-gravitating collapsar model, where the transonic shocks are formed. the effect should be weakened by magnetic field. | self-gravitating collapsing star and black hole spin-up in long gamma ray bursts |
context. the thermodynamical evolution of gas during the collapse of the primordial star-forming cloud depends significantly on the initial degree of rotation.aims: however, there is no clear understanding of how the initial rotation can affect the heating and cooling process and hence the temperature that leads to the fragmentation of the gas during population iii star formation.methods: we report the results from three-dimensional, smoothed-particle hydrodynamics (sph) simulations of a rotating self-gravitating primordial gas cloud with a modified version of the gadget-2 code, in which the initial ratio of the rotational to the gravitational energy (β0) is varied over two orders of magnitude.results: we find that despite the lack of any initial turbulence and magnetic fields in the clouds, the angular momentum distribution leads to the formation and build-up of a disk that fragments into several clumps. we further examine the behavior of the protostars that form in both idealized as well as more realistic minihalos from the cosmological simulations. the thermodynamical evolution and the fragmentation behavior of the cosmological minihalos are similar to that of the artificial cases, especially in those with a similar β0-parameter. protostars with a higher rotation support exhibit spiral-arm-like structures on several scales, and have lower accretion rates. these type of clouds tend to fragment more, while some of the protostars escape from the cluster with the possibility of surviving until the present day. they also take much longer to form compared to their slowly rotating counterparts.conclusions: we conclude that the use of appropriate initial conditions of the gas in minihalos is a pivotal and decisive quantity to study the evolution and final fate of the primordial stars. | on the effects of rotation in primordial star-forming clouds |
we present our spectroscopic observations of v455 andromedae during the 2007 superoutburst. our observations cover this superoutburst from around the optical peak of the outburst to the post-superoutburst stage. during the early superhump phase, the emission lines of the balmer series, he i, he ii, bowen blend, and c iv/n iv blend were detected. the he ii 4686 line exhibited a double-peaked emission profile, where balmer emission lines were single-peaked, which is unexpected from its high inclination. in the ordinary superhump phase, the balmer series transitioned to double-peaked emission profiles, and high-ionization lines were significantly weakened. these transitions of the line profiles should be related to the structural transformation of the accretion disk, as expected between the early and ordinary superhump transition in the thermal-tidal instability model. the doppler map of hα during the early superhump phase exhibits a compact blob centered at the primary white dwarf. in analogy to sw sex-type cataclysmic variables, this feature could emerge from the disk wind and/or the mass accretion column on to the magnetized white dwarf. the doppler map of he ii 4686 å is dominated by the ring-like structure and imposed two flaring regions with the velocity of ~300 km s-1, which is too slow for a keplerian accretion disk. the phase of the flaring regions was coincident with the inner spiral arm structure identified during the early superhump phase. our disk wind model with the enhanced emission from the wind component launched from the inner arm structure successfully reproduced the observed properties of he ii 4686 å. therefore, v455 and is the first case in dwarf nova outbursts where the presence of the disk wind is inferred from an optical spectrum. | spectroscopic observations of v455 andromedae superoutburst in 2007: the most exotic spectral features in dwarf nova outbursts |
x-ray variabilities with spectral state transitions in bright low-mass x-ray binaries containing a neutron star are investigated by using the one-day bin light curves of maxi/gsc (gas slit camera) and swift/bat (burst alert telescope). four sources (4u 1636-536, 4u 1705-44, 4u 1608-52, and gs 1826-238) exhibited small-amplitude x-ray variabilities with spectral state transitions. such "mini-outbursts" were characterized by smaller amplitudes (several times) and shorter duration (less than several tens of days) than those of "normal outbursts." a theoretical model of disk instability by mineshige and osaki (pasj, 37, 1, 1985) predicts both large-amplitude outbursts and small-amplitude variabilities. we interpret the normal outbursts as the former prediction of this model, and the mini-outbursts as the latter. here, we can also call the mini-outburst a "purr-type outburst" referring to the theoretical work. we suggest that similar variabilities lasting for several tens of days without spectral state transitions, which are often observed in the hard state, may be repeats of mini-outbursts. | x-ray variability with spectral state transitions in ns-lmxbs observed with maxi/gsc and swift/bat |
we report long-term simulations of black hole-neutron star binary mergers where the neutron star possesses an asymmetric magnetic field dipole. focusing on the scenario where the neutron star is tidally disrupted by the black hole, we track the evolution of the binary up to ≈100 ms after the merger. we uncover more than one episode of thermally driven winds being launched along a funnel wall in all these cases beginning from ≈25 ms after the merger. on the other hand, we are unable to conclude presently whether the amount of ejected mass increases with the degree of asymmetry. a large-scale magnetic field configuration in the poloidal direction is formed along the funnel wall accompanied by the generation of a large poynting flux. the magnetic field in the accretion disk around the black hole remnant is amplified by both magnetic winding and the nonaxisymmetric magnetorotational instability (mri). the mri growth is estimated to be in the ideal magnetohydrodynamics (mhd) regime and thus would be free from significant effects induced by potential neutrino radiation. however, the asymmetry in the magnetic field leads to increased turbulence, which causes the vertical magnetic field in the accretion disk to grow largely in a nonlinear manner. | effects of magnetic field topology in black hole-neutron star mergers: long-term simulations |
i have run hydrodynamic simulations that follow the colliding wind structure of the massive binary system hd 166734 along its binary orbit. i show that close to periastron passage the secondary wind is suppressed and the secondary accretes mass from the primary wind. the system consists of two blue supergiants with masses of m_1 ≈ 39.5 ~m_{⊙} and m_2 ≈ 30.5 ~m_{⊙}, on an orbit of p ≃ 34.538 d with an eccentricity of e ≈ 0.618. this close o-o binary with high eccentricity is observed through its orbit in x-rays, where it shows an unusually long minimum close to periastron passage. i use advanced simulations with wind acceleration and a prescription treatment of accretion and i simulate the entire orbit at high resolution, which captures the instabilities in the winds. i find that the colliding wind structure is unstable even at apastron. as the stars approach periastron passage, the secondary wind is quenched by the primary wind and the accretion on to the secondary begins. the accretion phase lasts for ≃12 d, and the amount of accreted mass obtained per cycle is m_{acc} ≃ 1.3 × 10^{-8} m_{⊙ }. the accretion phase can account for the observed decline in x-ray emission from the system. | wind collision and accretion simulations of the massive binary system hd 166734 |
white dwarfs are physically simple and numerous. their properties provide insight into stellar evolution and have applications to many astrophysical questions. in this dissertation, we present new measurements of white dwarf properties in two environments that help further our knowledge of the structure and evolution of white dwarfs. we have undertaken a series of observations that enable the measurement of fundamental parameters of the white dwarf in two magnetic cataclysmic variables. we have chosen these particular systems because the lack of accretion disk and fortunate geometry leading to eclipses makes it possible to observe and characterize the white dwarf. in one system, we confirm that lsq1725-64 is a magnetic cataclysmic variable by estimating the magnetic field strength of the white dwarf from zeeman splitting. we measure the effective temperature of the white dwarf and the spectral type of the secondary star from spectroscopy during a state of low accretion. our precise eclipse measurements allow us to estimate the white dwarf mass and other binary parameters of lsq1725-64. the spectral type and color of the secondary, as well as the eclipse length, are consistent with other secondaries that have not yet evolved through the period minimum expected for cataclysmic variables. in ctcv1928-50, we detect h alpha emission from the heated face of the secondary that we use to measure the radial velocity amplitude of the secondary star. we combine this with previous measurements to estimate a white dwarf mass and other binary parameters. our measurements in these two systems add to a limited number of measured white dwarf parameters in magnetic cataclysmic variables. we have also completed a spectroscopic survey of pulsating, hydrogen-dominated atmosphere white dwarfs. these pulsations have long offered the promise to conduct seismology of white dwarfs to understand their internal structure and composition. we have spectroscopically observed 122 da white dwarfs that are either pulsating or close to the da instability strip. we estimate t eff and log(g) for each white dwarf from the shape of the balmer line profiles. these parameters provide important initial constraints for both absolute and relative seismology. we have completed a careful study of ten systematics involved in the determination of t eff and log(g). understanding and limiting these systematics has permitted us to obtain the most systematically consistent set of atmospheric parameters ever determined. this work will feed efforts to study and understand pulsating white dwarfs as a whole class instead of individually. | fundamental properties of white dwarfs alone and in binaries |
young protostellar discs provide the initial conditions for planet formation. the properties of these discs may be different from those of late-phase (t tauri) discs due to continuing infall from the envelope and protostellar variability resulting from irregular gas accretion. we use a set of hydrodynamic simulations to determine the structure of discs forming in collapsing molecular clouds. we examine how radiative feedback from the host protostar affects the disc properties by examining three regimes: without radiative feedback, with continuous radiative feedback and with episodic feedback, similar to fu ori-type outbursts. we find that the radial surface density and temperature profiles vary significantly as the disc accretes gas from the infalling envelope. these profiles are sensitive to the presence of spiral structure, induced by gravitational instabilities, and the radiative feedback provided by the protostar, especially in the case when the feedback is episodic. we also investigate whether mass estimates from position-velocity (pv) diagrams are accurate for early-phase discs. we find that the protostellar system mass (i.e. the mass of the protostar and its disc) is underestimated by up to 20 per cent, due to the impact of an enhanced radial pressure gradient on the gas. the mass of early-phase discs is a significant fraction of the mass of the protostar, so pv diagrams cannot accurately provide the mass of the protostar alone. the enhanced radial pressure gradient expected in young discs may lead to an increased rate of dust depletion due to gas drag, and therefore to a reduced dust-to-gas ratio. | the structure of young embedded protostellar discs |
we derive expressions for the local ideal magnetohydrodynamic (mhd) equations for a warped astrophysical disc using a warped shearing box formalism. a perturbation expansion of these equations to first order in the warping amplitude leads to a linear theory for the internal local structure of magnetized warped discs in the absence of magnetorotational instability (mri) turbulence. in the special case of an external magnetic field oriented normal to the disc surface, these equations are solved semi-analytically via a spectral method. the relatively rapid warp propagation of low-viscosity keplerian hydrodynamic warped discs is diminished by the presence of a magnetic field. the magnetic tension adds a stiffness to the epicyclic oscillations, detuning the natural frequency from the orbital frequency and thereby removing the resonant forcing of epicyclic modes characteristic of hydrodynamic warped discs. in contrast to a single hydrodynamic resonance, we find a series of alfvénic-epicyclic modes which may be resonantly forced by the warped geometry at critical values of the orbital shear rate q and magnetic field strength. at these critical points large internal torques are generated and anomalously rapid warp propagation occurs. as our treatment omits mri turbulence, these results are of greatest applicability to strongly magnetized discs. | a local model of warped magnetized accretion discs |
we study the stellar populations of bulges of milky way-like (mw-like) galaxies with the aim of identifying the physical processes involved in the formation of the bulge of our galaxy. we use the semi-analytic model of galaxy formation and evolution sag adapted to this aim; these kind of models can trace the properties of galaxies and their components like stellar discs, bulges and haloes, but resolution limits prevent them from reaching the scale of stellar populations (sps). properties of groups of stars formed during single star formation events are stored and tracked in the model and results are compared with observations of stars in the galactic bulge. mw-like galaxies are selected using two different criteria. one of them considers intrinsic photo-metric properties and the second is focused on the cosmological context of the local group (lg) of galaxies. we compare our model results with spectroscopic and photometric stellar metallicity distributions. we find that 87 per cent of stars in bulges of mw-type galaxies in our model are accreted and formed in starbursts during disc instability events. mergers contribute to 13 per cent of the mass budget of the bulge and are responsible for the low metallicity tail of the distribution. abundance ratios of α elements with respect to iron, [α/fe], are measured in sps of model galaxies. the patterns found in the model for sps with different origins help to explain the lack of a gradient of [α/fe] ratios in observed stars along the minor axis of the bulge. | stellar populations in a semi-analytic model i: bulges of milky way-like galaxies |
we conduct a linear stability calculation of an ideal keplerian flow on which a sinusoidal zonal flow is imposed. the analysis uses the shearing sheet model and is carried out both in isothermal and adiabatic conditions, with and without self-gravity (sg). in the non-sg regime, a structure in the potential vorticity (pv) leads to a non-axisymmetric kelvin-helmholtz (kh) instability; in the short-wavelength limit its growth rate agrees with the incompressible calculation by lithwick, which only considers perturbations elongated in the streamwise direction. the instability's strength is analysed as a function of the structure's properties, and zonal flows are found to be stable if their wavelength is ≳8 h, where h is the disc's scaleheight, regardless of the value of the adiabatic index γ. the non-axisymmetric kh instability can operate in rayleigh-stable conditions, and it therefore represents the limiting factor to the structure's properties. introducing sg triggers a second non-axisymmetric instability, which is found to be located around a pv maximum, while the kh instability is linked to a pv minimum, as expected. in the adiabatic regime, the same gravitational instability is detected even when the structure is present only in the entropy (not in the pv) and the instability spreads to weaker sg conditions as the entropy structure's amplitude is increased. this eventually yields a non-axisymmetric instability in the non-sg regime, albeit of weak strength, localized around an entropy maximum. | non-axisymmetric instabilities in discs with imposed zonal flows |
rapidly rotating neutron stars in low mass x-ray binaries have been proposed as an interesting source of gravitational waves. in this chapter we present estimates of the gravitational wave emission for various scenarios, given the (electromagnetically) observed characteristics of these systems. first of all we focus on the r-mode instability and show that a "minimal" neutron star model (which does not incorporate exotica in the core, dynamically important magnetic fields or superfluid degrees of freedom), is not consistent with observations. we then present estimates of both thermally induced and magnetically sustained mountains in the crust. in general magnetic mountains are likely to be detectable only if the buried magnetic field of the star is of the order of b ≈ 10^{12} g. in the thermal mountain case we find that gravitational wave emission from persistent systems may be detected by ground based interferometers. finally we re-asses the idea that gravitational wave emission may be balancing the accretion torque in these systems, and show that in most cases the disc/magnetosphere interaction can account for the observed spin periods. | gravitational waves from rapidly rotating neutron stars |
we observed the 2014 superoutburst of the su uma-type intermediate polar cc scl. we detected superhumps with a mean period of 0.05998(2) d during the superoutburst plateau and during three nights after the fading. during the post-superoutburst stage after three nights, a stable superhump period of 0.059523(6) d was detected. we found that this object is an eclipsing system with an orbital period of 0.058567233(8) d. by assuming that the disk radius in the post-superoutburst phase is similar to those in other su uma-type dwarf novae, we obtained a mass ratio of q = 0.072(3) from the dynamical precession rate of the accretion disk. the eclipse profile during outbursts can be modeled by an inclination of 80.6°6 ± 0.5°. the 2014 superoutburst was preceded by a precursor outburst and the overall appearance of the outburst was similar to superoutbursts in ordinary su uma-type dwarf novae. we showed that the standard thermal-tidal instability model can explain the outburst behavior in this system and suggest that inner truncation of the disk by magnetism of the white dwarf does not strongly affect the behavior in the outer part of the disk. | cc sculptoris: eclipsing su uma-type intermediate polar |
the central molecular zone (cmz) of the milky way shows several peculiar properties: a large star formation rate (sfr), some of the most massive young star clusters and molecular clouds in the galaxy, and a twisted ring morphology in molecular gas. in this paper, i use smoothed particle hydrodynamics simulations to show that most of these properties can be explained as due to a recent outburst of active galactic nucleus (agn) activity in sgr a*, the central supermassive black hole of the milky way. in particular, the narrow ring of dense gas, massive gas clouds, young star clusters and an elevated sfr can all be caused by the passage of an agn outflow through the system, which compresses the gas and triggers fragmentation. furthermore, i show that the asymmetric distribution of gas, as observed in the cmz, can be produced by outflow-induced instabilities from an initially axisymmetric gas disc. angular momentum mixing in the disc produces some low angular momentum material, which can subsequently feed sgr a*. these processes can occur in any galaxy that experiences an agn episode, leading to bursts of nuclear star formation much stronger than pure bar-driven mass inflows would predict. | agn activity and nuclear starbursts: sgr a* activity shapes the central molecular zone |
we present an instability for exciting incompressible modes (e.g., gravity or rossby modes) at the surface of a star accreting through a boundary layer. the instability excites a stellar mode by sourcing an acoustic wave in the disk at the boundary layer, which carries a flux of energy and angular momentum with the opposite sign as the energy and angular momentum density of the stellar mode. we call this instability the acoustic chandrasekhar-friedman-schutz (cfs) instability, because of the direct analogy to the cfs instability for exciting modes on a rotating star by emission of energy in the form of gravitational waves. however, the acoustic cfs instability differs from its gravitational wave counterpart in that the fluid medium in which the acoustic wave propagates (i.e., the accretion disk) typically rotates faster than the star in which the incompressible mode is sourced. for this reason, the instability can operate even for a non-rotating star in the presence of an accretion disk. we discuss applications of our results to high-frequency quasi-periodic oscillations in accreting black hole and neutron star systems and dwarf nova oscillations in cataclysmic variables. | incompressible modes excited by supersonic shear in boundary layers: acoustic cfs instability |
by using an n-body simulation of a bulge that was formed via a bar instability mechanism, we analyse the imprints of the initial (i.e. before bar formation) location of stars on the bulge kinematics, in particular on the heliocentric radial velocity distribution of bulge stars. four different latitudes were considered: b = -4°, -6°, -8°, and -10°, along the bulge minor axis as well as outside it, at l = ± 5° and l = ± 10°. the bulge x-shaped structure comprises stars that formed in the disk at different locations. stars formed in the outer disk, beyond the end of the bar, which are part of the boxy peanut-bulge structure may show peaks in the velocity distributions at positive and negative heliocentric radial velocities with high absolute values that can be larger than 100 km s-1, depending on the observed direction. in some cases the structure of the velocity field is more complex and several peaks are observed. stars formed in the inner disk, the most numerous, contribute predominantly to the x-shaped structure and present different kinematic characteristics. they display a rather symmetric velocity distribution and a smaller fraction of high-velocity stars. the stellar stream motion, which is induced by the bar changes with the star initial position, can reach more than 40 km s-1 for stars that originated in the external disk, depending on the observed direction. otherwise it is smaller than approximately 20 km s-1. in all cases, it decreases from b = -4° to -10°. our results may enable us to interpret the cold high-velocity peak observed in the apogee commissioning data, as well as the excess of high-velocity stars in the near and far arms of the x-shaped structure at l = 0° and b = -6°. when compared with real data, the kinematic picture becomes more complex due to the possible presence in the observed samples of classical bulge and/or thick disk stars. overall, our results point to the existence of complex patterns and structures in the bulge velocity fields, which are generated by the bar. this suggests that caution should be used when interpreting the bulge kinematics: the presence of substructures, peaks and clumps in the velocity fields is not necessarily a sign of past accretion events. | a new look at the kinematics of the bulge from an n-body model |
in this paper we investigate effects of the r-mode instability on a newborn rapidly-rotating magnetar with fall-back accretion. such a magnetar could usually occur in core-collapse supernovae and gamma-ray bursts. we find that the magnetar's spin and r-mode evolution are influenced by accretion. if the magnetar is sufficiently spun up to a few milliseconds, gravitational radiation leads to the growth of the r-mode amplitude significantly. the maximum r-mode amplitude reaches an order of 0.001 when the damping due to the growth of a toroidal magnetic field balances the growth of the r-mode amplitude. if such a sufficiently spun-up magnetar was located at a distance less than 1 mpc, then gravitational waves would be detectable by the einstein telescope but would have an extremely low event rate. however, if the spin-up is insufficient, the growth of the r-mode amplitude is mainly due to the accretion torque. in this case, the maximum r-mode amplitude is of the order of 10-6-10-5. | evolution of newborn rapidly rotating magnetars: effects of r-mode and fall-back accretion |
criterion of the jeans modes for self-gravitational systems in eddington-inspired born-infeld (eibi) gravity is revisited based on the framework of nonextensive statistics and kinetic theory. the results show the nonextensive parameter q has significant effects on the growth rate and critical wave-number of jeans instability for the eibi-gravitational systems. also, we prove that the standard dispersion relationship proposed by binney can be recovered if q → 1 and τ → 0 . concept of jeans critical value (that is, the jeans thresholds of eibi gravitational systems) obtains a more tight emendation and the results of present work may provide theoretical references for the evolution of stars such as neutron stars, accretion disks, and relevant plasma physics, etc. | jeans instability of eddington-inspired born-infield gravitational systems in the context of nonextensive statistics |
we study the local thermal stability of thin accretion disks. we present a full stability analysis in the presence of a magnetic field and, more importantly, wind. we use a general model suitable for adequately describing several kinds of winds. first, we explicitly show that the magnetic field, irrespective of the type of wind, has a stabilizing effect. this is also true when there is no wind. in this case, we confirm other works presented in the literature. however, our main objective is to investigate the local thermal stability of the disk in the presence of wind. in this case, interestingly, the response of the disk is directly related to the type of wind. in other words, in some cases, the wind can stabilize the disk. on the other hand, in some cases it can destabilize the disk. we find that in some thin disk models where the magnetic pressure cannot explain the stability of the disk by including a typical contribution for magnetic pressure, wind can provide a viable explanation for the thermal stability. | thermal instability of thin accretion disks in the presence of wind and a toroidal magnetic field |
we investigate the stability of a neutrino-dominated accretion flow (ndaf), which is expected to be formed in the gravitational collapse of a massive star or the merger of a neutron star binary, based on the variable-α prescription. recent magnetohydrodynamic simulations shows that the viscosity parameter α is proportional to the power of the magnetic prandtl number pm = ν/η, where ν and η are the kinematic viscosity and electric resistivity of the fluid, respectively. in the inner region of a hyperaccretion flow, the viscosity and resistivity are carried by mildly, relativistically degenerated electrons. we fit the dependence of the magnetic prandtl number on density and temperature by a simple analytic form, and derive the condition for an ndaf to be dynamically unstable. as demonstrations we perform simple one-dimensional simulations of ndafs with variable-α and show that the mass accretion becomes highly time-variable in the unstable branch. this mechanism may account for the rapid variability observed in the prompt emission of gamma-ray bursts. the mass ejection from a hyperaccretion flow due to viscous heating, which makes a kilonova/macronova emission in the merger of a neutron star binary, is also briefly discussed. | neutrino-dominated accretion flows with magnetic prandtl number-dependent mri-driven turbulence |
we develop a framework for magnetohydrodynamical (mhd) simulations in a local cylindrical shearing box by extending the formulation of the cartesian shearing box. we construct shearing-periodic conditions at the radial boundaries of a simulation box from the conservation relations of the basic mhd equations, taking into account the explicit radial dependence of physical quantities. we demonstrate quasi-steady mass accretion, which cannot be handled by the standard cartesian shearing box model, with an ideal mhd simulation in a vertically unstratified cylindrical shearing box for up to 200 rotations. in this demonstrative run we set up (i) net vertical magnetic flux, (ii) a locally isothermal equation of state, and (iii) a sub-keplerian equilibrium rotation, whereas the sound velocity and the initial alfvén velocity have the same radial dependence as that of the keplerian velocity. inward mass accretion is induced to balance the outward angular momentum flux of the mhd turbulence triggered by the magnetorotational instability in a self-consistent manner. we discuss detailed physical properties of the saturated magnetic field, in comparison to the results of a cartesian shearing box simulation. | magnetohydrodynamics in a cylindrical shearing box |
a turbulent state of spectrally stable shear flows can be developed and sustained according to the bypass scenario of transition. if it works in non-magnetized boundless and homogeneous quasi-keplerian flow, then transiently growing shearing vortices should supply turbulence with energy. employing the large shearing box approximation, as well as a set of global disc models, we study the optimal growth of the shearing vortices in such a flow in the whole range of azimuthal length-scales, λy, compared to the flow scaleheight, h. it is shown that with the account of the viscosity the highest possible amplification of shearing vortices, gmax , attains maximum at λy ≲ h and declines towards both the large scales λy ≫ h and the small scales λy ≪ h in good agreement with analytical estimations based on balanced solutions. our main attention is directed to the large-scale vortices λy ≫ h, which produce gmax∝ (ω/κ)4, where ω and κ denote local rotational and epicyclic frequencies, respectively. it is demonstrated that the large-scale vortices acquire high-density perturbation as they approach the instant of swing. at the same time, their growth is not affected by bulk viscosity. we check that gmaxobtained globally is comparable to its local counterpart, and the shape and localization of global optimal vortices can be explained in terms of the local approach. the obtained results allow us to suggest that the critical reynolds number of subcritical transition to turbulence in quasi-keplerian flow, as well as the corresponding turbulent effective azimuthal stress should substantially depend on shear rate. | transient growth of perturbations on scales beyond the accretion disc thickness |
dwarf novæ (dne) and low-mass x-ray binaries (lmxbs) are compact binaries showing variability on time-scales from years to less than seconds. here, we focus on explaining part of the rapid fluctuations in dne, following the framework of recent studies on the monthly eruptions of dne that use a hybrid disc composed of an outer standard disc and an inner magnetized disc. we show that the ionization instability, which is responsible for the monthly eruptions of dne, is also able to operate in the inner magnetized disc. given the low density and the fast accretion time-scale of the inner magnetized disc, the ionization instability generates small, rapid heating and cooling fronts propagating back and forth in the inner disc. this leads to quasi-periodic oscillations (qpos) with a period of the order of 1000 s. a strong prediction of our model is that these qpos can only develop in quiescence or at the beginning/end of an outburst. we propose that these rapid fluctuations might explain a subclass of already observed qpos in dne as well as a, still to observe, subclass of qpos in lmxbs. we also extrapolate to the possibility that the radiation pressure instability might be related to type b qpos in lmxbs. | qpos in compact binaries from small-scale eruptions in an inner magnetized disc |
accretion disks in binary systems can experience hydrodynamical influences at both their inner and outer edges. the former is typical for protoplanetary disks around young t tauri stars, while the latter is typical for circumstellar disks in close binaries. this influence excites perturbations with various scales and amplitudes in the disk. the nonlinear evolution of perturbations with a finite, but small amplitude against the background of a sub-keplerian flow is investigated. nonlinear effects at the fronts of perturbation waves lead to the formation of discontinuities in the density and radial velocity; i.e., to formation of shocks. the tangential flow in the neighborhood of the shock becomes equivalent to a flow in a boundary layer. due to an instability of the tangential flow, the disk becomes turbulent. the characteristics of the turbulence depend on the parameters of the perturbations, but the shakura-syunyaev α parameter does not exceed 0.1. | excitation of turbulence in accretion disks of binary stars by non-linear perturbations |
while collisional accumulation is nearly universally accepted as the formation mechanism of rock and ice worlds, the situation regarding gas giant planet formation is more nuanced. gas accretion by solid cores formed by collisional accumulation is the generally favored mechanism, but observations increasingly suggest that gas disk gravitational instability might explain the formation of at least the massive or wide-orbit gas giant exoplanets. this paper continues a series aimed at refining three-dimensional (3d) hydrodynamical models of disk instabilities, where the handling of the gas thermodynamics is a crucial factor. boss (2017, 2021) used the β cooling approximation to calculate 3d models of disks with initial masses of 0.091 m ⊙ extending from 4 to 20 au around 1 m ⊙ protostars. here we employ 3d flux-limited diffusion (fld) approximation models of the same disks, in order to provide a superior treatment of disk gas thermodynamics. the new models have quadrupled spatial resolution compared to previous 3d fld models, in both the radial and azimuthal spherical coordinates, resulting in the highest spatial resolution 3d fld models to date. the new models continue to support the hypothesis that such disks can form self-gravitating, dense clumps capable of contracting to form gas giant protoplanets, and suggest that the fld models yield similar numbers of clumps as β cooling models with β ~ 1 to ~10, including the critical value of β = 3 for fragmentation proposed by gammie. | flux-limited diffusion approximation models of giant planet formation by disk instability. ii. quadrupled spatial resolution |
we carry out a series of local, shearing-box simulations of the outer regions of protoplanetary disks, where ambipolar diffusion is important due to low ionization levels, to better characterize the nature of turbulence and angular momentum transport in these disks. these simulations are divided into two groups, one with far-ultraviolet (fuv) ionization included, and one without fuv. in both cases, we explore a large range in diffusivity values. we find that in the simulations without fuv, the properties of the turbulence are similar to the unstratified simulations of bai & stone; for a given diffusivity, the magnetorotational instability (mri) can still be present so long as the magnetic field is sufficiently weak. furthermore, the dynamics of the midplane in these simulations are primarily controlled by the mri. in the fuv simulations on the other hand, the mri-active fuv layers transport strong toroidal magnetic flux to the midplane, which shuts off the mri. instead, angular momentum transport at the midplane is dominated by laminar magnetic fields, resulting in lower levels of turbulent maxwell stress compared to the no-fuv simulations. finally, we perform a temporal correlation analysis on the fuv simulations, confirming our result that the dynamics in the midplane region is strongly controlled by the fuv-ionized layers. | the nature of turbulence in the outer regions of protoplanetary disks |
context. the rather elusive high-frequency quasi-periodic oscillations (hfqpos) observed in the x-ray light curve of black holes have been seen in a wide range of frequencies, even within one source. also notable is the detection of "pairs" of hfqpos with a close-to-integer ratio between the frequencies.aims: the aim of this paper is to investigate some of the possible observables that we could obtain from the rossby wave instability (rwi) active in the accretion disc surrounding the compact object.methods: using the newly developed gr-amrvac code able to follow the evolution of the rwi in a full general relativistic framework, we explore how rwi can reproduce observed hfqpo frequency ratios and whether or not it is compatible with observations. in order to model the emission coming from the disc we have linked our general relativistic simulations to the general relativistic ray-tracing gyoto code and delivered synthetic observables that can be confronted with actual data from binary systems hosting hfqpos.results: we demonstrate that some changes in the physical conditions prevailing in the part of the disc where rwi can be triggered lead to various dominant rwi modes whose ratio recovers frequency ratios observed in various x-ray binary systems. in addition we also show that when rwi is triggered near to the last stable orbit of a spinning black hole, the amplitude of the x-ray modulation increases with the spin of the black hole. revisiting published data on x-ray binary systems, we show that this type of relationship actually exists in five systems where an indirect measurement of the spin of the black hole is available. | rossby wave instability and high-frequency quasi-periodic oscillations in accretion discs orbiting around black holes |
x-ray pulsars shine thanks to the conversion of the gravitational energy of accreted material to x-ray radiation. the accretion rate is modulated by geometrical and hydrodynamical effects in the stellar wind of the pulsar companions and/or by instabilities in accretion disks. wind-driven flows are highly unstable close to neutron stars and responsible for x-ray variability by factors ≳ 103 on time scale of hours. disk-driven flows feature slower state transitions and quasiperiodic oscillations related to orbital motion and precession or resonance. on shorter time scales, and closer to the surface of the neutron star, x-ray variability is dominated by the interactions of the accreting flow with the spinning magnetosphere. when the pulsar magnetic field is large, the flow is confined in a relatively narrow accretion column, whose geometrical properties drive the observed x-ray emission. in low magnetized systems, an increasing accretion rate allows the ignition of powerful explosive thermonuclear burning at the neutron star surface. transitions from rotation- to accretion-powered activity has been observed in rare cases and proved the link between these classes of pulsars. | x-ray pulsars |
a set of strongly magnetized accreting white dwarfs (polars) shows quasi-periodic oscillations (qpos) with frequency about a hz in their optical luminosity. these hz-frequency qpos are thought to be generated by intensity variations of the emitted radiation originating at the post-shock accretion column. thermal instability in the post-shock region, triggered by efficient cooling process at the base, is believed to be the primary reason behind the temporal variability. here, we study the structure and the dynamical properties of the post-shock accretion column including the effects of bremsstrahlung and cyclotron radiation. we find that the presence of significant cyclotron emission in optical band reduces the overall variability of the post-shock region. in the case of a larger post-shock region above the stellar surface, the effects of stratification due to stellar gravity become important. an accretion column, influenced by the strong gravity, has a smaller variability as the strength of the thermal instability at the base of the column is reduced. on the other hand, the cool, dense plasma, accumulated just above the stellar surface, may enhance the post-shock variability due to the propagation of magnetic perturbations. these characteristics of the post-shock region are consistent with the observed properties of v834 cen and in general with cataclysmic variable sources that exhibit qpo frequency of about a hz. | quasi-periodic oscillations from post-shock accretion column of polars |
stunted outbursts are ∼0.ͫ6 eruptions, typically lasting 5-10 days, which are found in some novalike cataclysmic variables, including uu aqr. the mechanism responsible for stunted outbursts is uncertain but is likely related to an accretion disk instability or to variations in the mass transfer rate. a campaign to monitor the eclipse light curves in uu aqr has been conducted in order to detect any light curve distortions due to the appearance of a hot spot on the disk at the location of the impact point of the accretion stream. if stunted outbursts are due to a temporary mass transfer enhancement, then predictable deformations of the orbital light curve are expected to occur during such outbursts. this study used 156 eclipses on 135 nights during the years 2000-2012. during this interval, random samples found the system to be in stunted outbursts 4%-5% of the time, yielding ∼7 eclipses obtained during some stage of stunted outburst. about half of the eclipses obtained during stunted outbursts showed clear evidence for hot spot enhancement, providing strong evidence that the stunted outbursts in uu aqr are associated with mass transfer variations. the other half of the eclipses during stunted outburst showed little or no evidence for hot spot enhancement. furthermore, there were no systematic changes in the hot spot signature as stunted outbursts progressed. therefore, we have tentatively attributed the changes in hot spot visibility during stunted outburst to random blobby accretion, which likely further modulates the strength of the accretion stream on orbital timescales. | orbital light curves of uu aquarii in stunted outburst |
the prototype dwarf nova ss cyg unexpectedly exhibited an anomalous event in its light curve in the early few months of 2021 in which regular dwarf nova-type outbursts stopped, and only small-amplitude fluctuations occurred. inspired by this event, we have performed numerical simulations of light curves of ss cyg by varying mass transfer rates and varying viscosity parameters in the cool disk. we have also studied the effect of gas-stream overflows beyond the outer disk edge in the light curve simulations. we have confirmed that the enhanced mass transfer is not likely to be responsible for the 2021 anomalous event or its forerunner. we have found that the enhancement of the viscosity in the disk may reproduce the forerunner of that event but may not be sufficient to explain the 2021 anomalous event, although the latter result might be particular to the thermal equilibrium curve we used. within our simulations, a model of the gas-stream overflow with a slightly higher mass transfer rate than that of our standard model reproduces light curves similar to the 2021 anomalous event. we suggest that the gas-stream overflow is necessary to reproduce that event. the gas-stream overflow may also be responsible for the abnormally high x-ray flux during the normal quiescent state in ss cyg. | the light curve simulations of the 2021 anomalous event in ss cygni |
we analytically study the dynamical and thermal properties of the optically thin gases at the parsec-scale when they are spherically accreted on to low luminous active galactic nuclei. the falling gases are irradiated by the central x-ray radiation with the compton temperature of 5-15 × 107 k. the radiative heating/cooling and the bulge stellar potential in galaxies are taken into account. we analyse the effect of accretion rate, luminosity, gas temperature, and compton temperature on steady solutions of dynamical and thermal properties. the steady solutions are obviously different from bondi solution. compared to our models, the bondi model underestimates the accretion rate. we give the boundary between thermal stability and instability. the boundary is significantly affected by compton temperature. when compton temperature is higher, the falling gases tend to become thermally unstable. when thermal instability takes place in the irradiated gases, the gases become two phases (i.e. hot gases and cool gases) and the hot gases may become outflows. this effect may reduce the accretion rates. | dynamical and thermal properties of the parsec-scale gases spherically accreted on to low luminous active galactic nuclei |
intermediate polars (ips) are cataclysmic variables with mildly magnetized white dwarfs (wds). this analysis of the long-term optical activity of five examples of ips with accretion discs used data from the catalina real-time transient survey, digital access to a sky century @ harvard (dasch) and the american association of variable star observers (aavso). it is shown that each of these ips had their most preferred value of absolute magnitude mopt, even if it significantly varied on the superorbital time-scale. the values of mopt of these ips were in the zone of thermal-viscous instability (tvi) of the disc most of the time. the properties of a series of outbursts of v426 oph can be explained by an intermittently operating tvi. the activity of tv col and dw cnc is interpreted as caused by a gradually variable mass inflow rate from the secondary to a cool disc. the mass transfer rate from the secondary varied on a well-determined time-scale. it is shown that mopt of ei uma, close to the peaks of outbursts of non-magnetic dwarf novae, fluctuated on the time-scale of days; it also produced short flares, ascribed to the bursts of matter from the donor. hy leo, with a presumably cool disc, fluctuated between its high and low states. a temporary brightening from an extended low state is ascribed to a short, intense burst of matter from the donor to the remaining cool disc or torus. | a study of the long-term activity of five intermediate polars with accretion discs |
the accreting millisecond x-ray pulsar swift j1756.9-2508 launched into an outburst in april 2018 and june 2019 - 8.7 years after the previous period of activity. we investigated the temporal, timing, and spectral properties of these two outbursts using data from nicer, xmm-newton, nustar, integral, swift, and insight-hxmt. the two outbursts exhibited similar broadband spectra and x-ray pulse profiles. for the first time, we report the detection of the pulsed emission up to ∼100 kev that was observed by insight-hxmt during the 2018 outburst. we also found the pulsation up to ∼60 kev that was observed by nicer and nustar during the 2019 outburst. we performed a coherent timing analysis combining the data from the two outbursts. the binary system is well described by a constant orbital period over a time span of ∼12 years. the time-averaged broadband spectra are well fitted by the absorbed thermal comptonization model compps in a slab geometry with an electron temperature, kte = 40-50 kev, thomson optical depth τ ∼ 1.3, blackbody seed photon temperature ktbb, seed ∼ 0.7-0.8 kev, and hydrogen column density of nh ∼ 4.2 × 1022 cm−2. we searched the available data for type-i (thermonuclear) x-ray bursts, but found none, which is unsurprising given the estimated low peak accretion rate (≈0.05 of the eddington rate) and generally low expected burst rates for hydrogen-poor fuel. based on the history of four outbursts to date, we estimate the long-term average accretion rate at roughly 5 × 10−12 m⊙ yr−1 for an assumed distance of 8 kpc. the expected mass transfer rate driven by gravitational radiation in the binary implies the source may be no closer than 4 kpc. swift j1756.9-2508 is the third low mass x-ray binary exhibiting "double" outbursts, which are separated by much shorter intervals than what we typically see and are likely to result from interruption of the accretion flow from the disk onto the neutron star. such behavior may have important implications for the disk instability model. | broadband x-ray spectra and timing of the accreting millisecond pulsar swift j1756.9-2508 during its 2018 and 2019 outbursts |
the presence of a surface at the inner boundary, such as in a neutron star or a white dwarf, allows the existence of a standing shock in steady spherical accretion. the standing shock can become unstable in 2d or 3d; this is called the standing accretion shock instability (sasi). two mechanisms - advective-acoustic and purely acoustic - have been proposed to explain sasi. using axisymmetric hydrodynamic and magnetohydrodynamic simulations, we find that the advective-acoustic mechanism better matches the observed oscillation time-scales in our simulations. the global shock oscillations present in the accretion flow can explain many observed high frequency (≳100 hz) quasi-periodic oscillations (qpos) in x-ray binaries. the presence of a moderately strong magnetic field adds more features to the shock oscillation pattern, giving rise to low frequency modulation in the computed light curve. this low frequency modulation can be responsible for ∼100 hz qpos (known as hhz qpos). we propose that the appearance of hhz qpo determines the separation of twin peak qpos of higher frequencies. | magnetized sasi: its mechanism and possible connection to some qpos in xrbs |
fourteen years after its eruption as a classical nova (cn), v1047 cen (nova cen 2005) began an unusual re-brightening in 2019 april. the amplitude of the brightening suggests that this is a dwarf nova (dn) eruption in a cn system. very few cne have had dn eruptions within decades of the main cn outburst. the 14 yr separating the cn and dn eruptions of v1047 cen is the shortest of all instances recorded thus far. explaining this rapid succession of cn and dn outbursts in v1047 cen may be challenging within the framework of standard theories for dn outbursts. following a cn eruption, the mass accretion rate is believed to remain high (\dot{m}∼ {10}-8 {m}⊙{yr}}-1) for a few centuries, due to the irradiation of the secondary star by the still-hot surface of the white dwarf. thus a dn eruption is not expected to occur during this high mass accretion phase as dn outbursts, which result from thermal instabilities in the accretion disk, and arise during a regime of low mass accretion rate (\dot{m}∼ {10}-10 {m}⊙{yr}}-1). here we present near-infrared spectroscopy to show that the present outburst is most likely a dn eruption, and discuss the possible reasons for its early occurrence. even if the present re-brightening is later shown to be due to a cause other than a dn outburst, the present study provides invaluable documentation of this unusual event. | infrared spectroscopy of the recent outburst in v1047 cen (nova centauri 2005) |
circumstellar discs are thought to be self-gravitating at very early times. if the disc is relatively cool, extended and accreting sufficiently rapidly, it can fragment into bound objects of order a few jupiter masses and upwards. given that the fragment's initial angular momentum is non-zero, and it will continue to accrete angular momentum from the surrounding circumstellar disc, we should expect that the fragment will also possess a relatively massive disc at early times. therefore, we can ask: is disc fragmentation a hierarchical process? or, can a disc fragment go on to produce its own self-gravitating circumfragmentary disc that produces daughter fragments? we investigate this using a set of nested 1d self-gravitating disc models. we calculate the radial structure of a marginally stable, self-gravitating circumstellar disc, and compute its propensity to fragmentation. we use this data to construct the local fragment properties at this radius. for each circumstellar disc model that results in fragmentation, we then compute a marginally stable self-gravitating circumfragmentary disc model. in general, the circumfragmentary discs are geometrically thick and truncated inside the hill radius, and are hence stable against daughter fragmentation. the typical steady-state accretion rate is between 0.1 and 10 percent of the local circumstellar disc accretion rate. the lifetime of the circumfragmentary discs' self-gravitating phase is somewhat less than 0.1 myr, quite comparable with that of the circumstellar disc. we should therefore expect that disc fragments will not produce satellites via gravitational instability, but equally the self-gravitating phase of circumfragmentary discs is likely to affect the properties of satellites subsequently formed via core accretion. | daughter fragmentation is unlikely to occur in self-gravitating circumstellar discs |
one of the currently favored scenarios for the formation of globular clusters (gcs) with multiple stellar populations is that an initial massive stellar system forms (`first generation', fg), subsequently giving rise to gaseous ejecta which is converted into a second generation (sg) of stars to form a gc. we investigate, for the first time, the sequential formation processes of both fg and sg stars from star-forming massive gas clumps in gas-rich dwarf disk galaxies. we adopt a novel approach to resolve the two-stage formation of gcs in hydrodynamical simulations of dwarf galaxies.in the new simulations, new gas particles that are much less massive than their parent star particle are generated around each new star particle when the new star enters into the asymptotic giant branch (agb) phase. furthermore, much finer maximum time step width (<10^5 yr) and smaller softening length (<2 pc) are adopted for such agb gas particles to properly resolve the ejection of gas from agb stars and agb feedback effects. therefore, secondary star formation from agb ejecta can be properly investigated in galaxy-scale simulations. an fg stellar system can first form from a massive gas clump developing due to gravitational instability within its host gas-rich dwarf galaxy. initially the fg stellar system is not a single massive cluster, but instead is composed of several irregular stellar clumps (or filaments) with a total mass larger than 10^6 msun. while the fg system is dynamically relaxing, gaseous ejecta from agb stars can be gravitationally trapped by the fg system and subsequently converted into new stars to form a compact sg stellar system within the fg system. interestingly, about 40% of agb ejecta is from stars that do not belong to the fg system (`external gas accretion'). the mass-density relation for sg stellar systems can be approximated as rho_sg ~ m_sg^1.5. | formation of globular clusters with multiple stellar populations from massive gas clumps in high-z gas-rich dwarf galaxies |
numerical models of gas inflow towards a supermassive black hole (smbh) show that star formation may occur in such an environment through the growth of a gravitationally unstable gas disc. we consider the effect of nuclear activity on such a scenario. we present the first three-dimensional grid-based radiative hydrodynamic simulations of direct collisions between infalling gas streams and a 4 × 106 m⊙ smbh, using ray-tracing to incorporate radiation consistent with an active galactic nucleus (agn). we assume inflow masses of ≈105 m⊙ and explore radiation fields of 10 per cent and 100 per cent of the eddington luminosity (ledd). we follow our models to the point of central gas disc formation preceding star formation and use the toomre q parameter (qt) to test for gravitational instability. we find that radiation pressure from uv photons inhibits inflow. yet, for weak radiation fields, a central disc forms on time-scales similar to that of models without feedback. average densities of >108 cm-3 limit photoheating to the disc surface allowing for qt ≈ 1. for strong radiation fields, the disc forms more gradually resulting in lower surface densities and larger qt values. mass accretion rates in our models are consistent with 1-60 per cent of the eddington limit, thus we conclude that it is unlikely that radiative feedback from agn activity would inhibit circumnuclear star formation arising from a massive inflow event. | gas inflow and star formation near supermassive black holes: the role of nuclear activity |
we analyse the time evolution of the luminosity of a cluster of population iii protostars formed in the early universe. we argue from the jeans criterion that primordial gas can collapse to form a cluster of first stars that evolve relatively independently of one another (i.e. with negligible gravitational interaction). we model the collapse of individual protostellar clumps using non-axisymmetric numerical hydrodynamics simulations. each collapse produces a protostar surrounded by a massive disc (i.e. mdisc /m* ≳ 0.1), whose evolution we follow for a further 30-40 kyr. gravitational instabilities result in the fragmentation and the formation of gravitationally bound clumps within the disc. the accretion of these fragments by the host protostar produces accretion and luminosity bursts on the order of 106 l⊙. within the cluster, we show that a simultaneity of such events across several protostellar cluster members can elevate the cluster luminosity to 5-10 times greater than expected, and that the cluster spends ∼15 per cent of its star-forming history at these levels. this enhanced luminosity effect is particularly enabled in clusters of modest size with ≃10-20 members. in one such instance, we identify a confluence of burst events that raise the luminosity to nearly 1000 times greater than the cluster mean luminosity, resulting in l > 108 l⊙. this phenomenon arises solely through the gravitational-instability-driven episodic fragmentation and accretion that characterizes this early stage of protostellar evolution. | the luminosity of population iii star clusters |
while it has been observed that the parameters intrinsic to the type c low-frequency quasi-periodic oscillations are related in a nonlinear manner among themselves, there has been, up to now, no model to explain or reproduce how the frequency, the fwhm, and the rms amplitude of the type c low-frequency quasi-periodic oscillations behave with respect to one another. here we are using a simple toy model representing the emission from a standard disk and a spiral such as that caused by the accretion-ejection instability to reproduce the overall observed behavior and shed some light on its origin. this allows us to prove the ability of such a spiral structure to be at the origin of flux modulation over more than an order of magnitude in frequency. | reproducing the correlations of type c low-frequency quasi-periodic oscillation parameters in xte j1550-564 with a spiral structure |
we have analysed a series of suzaku data and one data set of xmm-newton of the su uma type dwarf nova vw hyi in optical quiescence. the observed spectra in the 0.2-10 kev band are moderately well represented by multitemperature thermal plasma emission models with a maximum temperature of 5-9 kev and bolometric luminosity of (2.4-5.2) × 1030 erg s-1. the mass accretion rate derived from the hard x-ray spectra does not show any clear trend as a function of time since the last superoutburst, in contradiction to theoretical predictions of the disc behaviour of an su uma type dwarf nova. the mass accretion rate, on the other hand, shows a clear declining trend with time since the last outburst (including the superoutburst). the rate of decline is of the same order as that evaluated from the hard x-ray light curves of the other two dwarf novae ss cyg and su uma. the standard disc instability model, on the other hand, predicts that the mass accretion rate should increase throughout the optically quiescent phase. we need further observation and theoretical consideration to resolve this discrepancy. | variation of mass accretion rate on to the white dwarf in the dwarf nova vw hyi in quiescence |
context. the most successful scenario for the origin of astrophysical jets requires a large-scale magnetic field anchored in a rotating object (black hole or star) and/or its surrounding accretion disk. platform jet simulations, where the mass load onto the magnetic field is not computed by solving the vertical equilibrium of the disk but is imposed as a boundary condition, are very useful for probing the jet acceleration and collimation mechanisms. the drawback of such simulations is the very large parameter space: despite many previous attempts, it is very difficult to determine the generic results that can be derived from them.aims: we wish to establish a firm link between jet simulations and analytical studies of magnetically driven steady-state jets from keplerian accretion disks. in particular, the latter have predicted the existence of recollimation shocks - due to the dominant hoop stress -, which have so far never been observed in platform simulations.methods: we performed a set of axisymmetric magnetohydrodynamics (mhd) simulations of nonrelativistic jets using the pluto code. the simulations are designed to reproduce the boundary conditions generally expected in analytical studies. we vary two parameters: the magnetic flux radial exponent α and the jet mass load κ. in order to reach the huge unprecedented spatial scales implied by the analytical solutions, we used a new method allowing us to boost the temporal evolution.results: we confirm the existence of standing recollimation shocks at large distances. as in self-similar studies, their altitude evolves with the mass load κ. the shocks are weak and correspond to oblique shocks in a moderately high, fast magnetosonic flow. the jet emitted from the disk is focused toward the inner axial spine, which is the outflow connected to the central object. the presence of this spine is shown to have a strong influence on jet asymptotics. we also argue that steady-state solutions with α ≥ 1 are numerically out of range.conclusions: internal recollimation shocks may produce observable features such as standing knots of enhanced emission and a decrease in the flow rotation rate. however, more realistic simulations (e.g. fully three-dimensional) must be carried out in order to investigate nonaxisymmetric instabilities and with ejection only from a finite zone in the disk, so as to to verify whether these mhd recollimation shocks and their properties are maintained. | numerical simulations of mhd jets from keplerian accretion disks. i. recollimation shocks |
shear flows have a significant impact on the dynamics in an assortment of different astrophysical objects, including accretion discs and stellar interiors. investigating shear flow instabilities in a polytropic atmosphere provides a fundamental understanding of the motion in stellar interiors where turbulent motions, mixing processes, and magnetic field generation take place. here, a linear stability analysis for a fully compressible fluid in a two-dimensional cartesian geometry is carried out. our study focuses on determining the critical richardson number for different mach numbers and the destabilising effects of high thermal diffusion. we find that there is a deviation in the predicted stability threshold for moderate mach number flows, along with a significant effect on the growth rate of the linear instability for small péclet numbers. we show that in addition to a kelvin-helmholtz instability, a holmboe instability can appear, and we discuss the implication of this in stellar interiors. | shear instabilities in a fully compressible polytropic atmosphere |
this work presents a linear analytical calculation on the stability and evolution of a compressible, viscous self-gravitating (sg) keplerian disc with both horizontal thermal diffusion and a constant cooling time-scale when an axisymmetric structure is present and freely evolving. the calculation makes use of the shearing sheet model and is carried out for a range of cooling times. although the solutions to the inviscid problem with no cooling or diffusion are well known, it is non-trivial to predict the effect caused by the introduction of cooling and of small diffusivities; this work focuses on perturbations of intermediate wavelengths, therefore representing an extension to the classical stability analysis on thermal and viscous instabilities. for density wave modes, the analysis can be simplified by means of a regular perturbation analysis; considering both shear and thermal diffusivities, the system is found to be overstable for intermediate and long wavelengths for values of the toomre parameter q ≲ 2; a non-sg instability is also detected for wavelengths ≳18h, where h is the disc scale-height, as long as γ ≲ 1.305. the regular perturbation analysis does not, however, hold for the entropy and potential vorticity slow modes as their ideal growth rates are degenerate. to understand their evolution, equations for the axisymmetric structure's amplitudes in these two quantities are analytically derived and their instability regions obtained. the instability appears boosted by increasing the value of the adiabatic index and of the prandtl number, while it is quenched by efficient cooling. | zonal flow evolution and overstability in accretion discs |
x-ray emission associated to accretion onto compact objects displays important levels of photometric and spectroscopic time-variability. when the accretor orbits a supergiant star, it captures a fraction of the supersonic radiatively-driven wind which forms shocks in its vicinity. the amplitude and stability of this gravitational beaming of the flow conditions the mass accretion rate responsible, in fine, for the x-ray luminosity of those supergiant x-ray binaries. the capacity of this low angular momentum inflow to form a disc-like structure susceptible to be the stage of well-known instabilities remains at stake. using state-of-the-art numerical setups, we characterized the structure of a bondi-hoyle-lyttleton flow onto a compact object, from the shock down to the vicinity of the accretor, typically five orders of magnitude smaller. the evolution of the mass accretion rate and of the bow shock which forms around the accretor (transverse structure, opening angle, stability, temperature profile) with the mach number of the incoming flow is described in detail. the robustness of those simulations based on the high performance computing mpi-amrvac code is supported by the topology of the inner sonic surface, in agreement with theoretical expectations. we developed a synthetic model of mass transfer in supergiant x-ray binaries which couples the launching of the wind accordingly to the stellar parameters, the orbital evolution of the streamlines in a modified roche potential and the accretion process. we show that the shape of the permanent flow is entirely determined by the mass ratio, the filling factor, the eddington factor and the alpha-force multiplier. provided scales such as the orbital period are known, we can trace back the observables to evaluate the mass accretion rates, the accretion mechanism (stream or wind-dominated) and the shearing of the inflow. | wind accretion onto compact objects |
the dynamical evolution of protoplanetary disks is of key interest for building a comprehensive theory of planet formation and to explain the observational properties of these objects. using the magnetohydrodynamics code athena++, with an isothermal shearing box setup, we study the boundary between the active and dead zone, where the accretion rate changes and mass can accumulate. we quantify how the turbulence level is affected by the presence of a non-uniform ohmic resistivity in the radial x direction that leads to a region of inhibited turbulence (or dead zone). comparing the turbulent activity to that of ideal simulations, the turbulence-inhibited area shows density fluctuations and magnetic activity at its boundaries, driven by energy injection from the active (ideal) zone boundaries. we find magnetic dissipation to be significantly stronger in the ideal regions, and the turbulence penetration through the boundary of the dead zone is determined by the value of the resistivity itself, through the ohmic dissipation process, though the thickness of the transition does not play a significant role in changing the dissipation. we investigate the 1d spectra along the shearing direction: magnetic spectra appear flat at large scales both in ideal as well as resistive simulations, though a kolmogorov scaling over more than one decade persists in the dead zone, suggesting the turbulent cascade is determined by the hydrodynamics of the system: magnetorotational instability dynamo action is inhibited where sufficiently high resistivity is present. | transition region from turbulent to dead zone in protoplanetary disks: local shearing box simulations |
close binaries containing a white dwarf accreting from a disk that receives material from a late stellar companion show an interesting variety of behaviors that depend on their accretion rates and the changes in this rate. the highest accretion rates are evident during the bright states of novalikes, as well as the superoutbursts of the shortest period dwarf novae followed by normal outbursts and the z cam systems. while the normal outbursts and superoutbursts of dwarf novae can be understood from the standpoint of disk and tidal instabilities, the changes in rates in novalikes when they enter low brightness states and the cause of extremely high rates for the systems with orbital periods between 3 and 4 h remain elusive. this paper highlights some recent insights and continuing problems found from x-ray, uv and optical observations of these high and low states, as well as the prospects for increased understanding from the anticipated future ground and space missions. | insights from multi-wavelength observations during high and low states of non-magnetic cvs |
the persistent low-luminosity neutron star x-ray binary 4u 1812-12 is a potential member of the scarce family of ultra-compact systems. we performed deep photometric and spectroscopic optical observations with the 10.4 m gran telescopio canarias in order to investigate the chemical composition of the accreted plasma, which is a proxy for the donor star class. we detect a faint optical counterpart (g ∼ 25, r ∼ 23) that is located in the background of the outskirts of the sharpless 54 h ii region, whose characteristic nebular lines superimpose on the x-ray binary spectrum. once this is corrected for, the actual source spectrum lacks hydrogen spectral features. in particular, the hα emission line is not detected, with an upper limit (3σ) on the equivalent width of < 1.3 å. helium (he i) lines are also not observed, even though our constraints are not restrictive enough to properly test the presence of this element. we also provide stringent upper limits on the presence of emission lines from other elements, such as c and o, which are typically found in ultra-compact systems with c-o white dwarfs donors. the absence of hydrogen features, the persistent nature of the source at low luminosity, and the low optical-to-x-ray flux ratio confirm 4u 1812-12 as a compelling ultra-compact x-ray binary candidate, for which we tentatively propose a he-rich donor based on the optical spectrum and the detection of short thermonuclear x-ray bursts. in this framework, we discuss the possible orbital period of the system according to disc instability and evolutionary models. | optical spectroscopy of 4u 1812-12. an ultra-compact x-ray binary seen through an h ii region |
hete j1900.1-2455 is an ultra-compact low-mass x-ray binary that underwent a long-lasting (about 10 yr) active state. the analysis presented here of its activity uses the observations of rxte/asm, swift/bat, and iss/maxi for investigating this active state and the relation of time evolution of fluxes in the hard and medium x-ray bands. we show that the variations of the flux of hete j1900.1-2455 on the time-scales of days and weeks have the form both of the outbursts and occasional high-state episodes. these outbursts are accompanied by the large changes of the hardness of the spectrum in the surroundings of the peaks of their soft x-ray flux. the very strong peaks of these outbursts occur in the soft x-ray band (2-4 kev) and are accompanied by a large depression in the 15-50 kev band flux. we interpret these events as an occasional occurrence of a thermal-viscous instability of the accretion disc that gives rise to the outbursts similar to those in the soft x-ray transients. on the other hand, the 2-4 and the 15-50 kev band fluxes are mutually correlated in the high-state episodes, much longer than the outbursts. in the interpretation, the episodes of the x-ray high states of hete j1900.1-2455 during the active state bear some analogy with the standstills in the z cam type of cataclysmic variables. | x-ray outbursts and high-state episodes of hete j1900.1-2455 |
gro j1744-28 (the bursting pulsar) is a neutron star low-mass x-ray binary which shows highly structured x-ray variability near the end of its x-ray outbursts. in this letter we show that this variability is analogous to that seen in transitional millisecond pulsars such as psr j1023+0038: `missing link' systems consisting of a pulsar nearing the end of its recycling phase. as such, we show that the bursting pulsar may also be associated with this class of objects. we discuss the implications of this scenario; in particular, we discuss the fact that the bursting pulsar has a significantly higher spin period and magnetic field than any other known transitional pulsar. if the bursting pulsar is indeed transitional, then this source opens a new window of opportunity to test our understanding of these systems in an entirely unexplored physical regime. | the bursting pulsar gro j1744-28: the slowest transitional pulsar? |
the structured jets are postulated to account for emission properties of high energy sources across the mass scale, from stellar mass black holes in grbs to supermassive black holes in agn. their active cores contain magnetized accretion disks and rotation of the kerr black hole provides mechanism for jet launching. this process works most effectively if the mode of accretion turns out to be magnetically arrested. in this mode, the modulation of jets launched from the engine is related to internal instabilities in the accretion flow, that work on smallest time and spatial scales. we find that mad scenario can explain the variability of both stellar and supermassive engines, but in some cases magnetic fields may lead to jet quenching. the effect is important mainly for gamma ray burst jets, where the jet can be chocked inside the magnetically driven winds. | can mad accretion disks launching structured jets explain both grb and agn engines? |
the classical alpha-disc model assumes that the turbulent stress scales linearly with - and responds instantaneously to - the pressure. it is likely, however, that the stress possesses a non-negligible relaxation time and will lag behind the pressure on some time-scale. to measure the size of this lag we carry out unstratified 3d magnetohydrodynamic shearing box simulations with zero-net-magnetic-flux using the finite-volume code pluto. we impose thermal oscillations of varying periods via a cooling term, which in turn drives oscillations in the turbulent stress. our simulations reveal that the stress oscillations lag behind the pressure by ~5 orbits in cases where the oscillation period is several tens of orbits or more. we discuss the implication of our results for thermal and viscous overstability in discs around compact objects. | the stress-pressure lag in mri turbulence and its implications for thermal instability in accretion discs |
this ph.d. thesis explores the modeling of dynamics in magnetospheres around compact objects (black holes and neutron stars), and their implications in the formation of high energy phenomena such as magnetar flares and the highly variable teraelectron volt (tev) emission of some active galactic nuclei, by means of numerical simulations. the amazing images of black hole (bh) shadows from the galactic center and the m87 galaxy provide a first direct glimpse into the physics of accretion flows in the most extreme environments of the universe. the efficient extraction of energy in the form of collimated outflows or jets from a rotating bh is directly linked to the topology of the surrounding magnetic field. general relativistic force-free electrodynamics (grffe) is one possible plasma limit employed to analyze energetic outflows in which strong magnetic fields are dominant over all inertial phenomena. it is capable to model energy flows in astrophysical scenarios in global simulations and we have exploited this for several applications. in this work, we present numerical strategies capable of modeling both, stationary, and fully dynamic force-free magnetospheres of compact objects. while we solve for equilibrium bh magnetospheres with a newly developed solver of the so-called grad-shafranov equation, the dynamical evolution is provided by an implementation of grffe on the infrastructure of the einstein toolkit. this ph.d. thesis reviews the methodology behind this newly developed code package and its application to magnetars and rapidly spinning bhs in detail. it combines a series of numerical and astrophysical tests to substantiate the astrophysical findings obtained by large scale numerical simulations. we put special emphasis on the correct modeling of plasma waves and identify the limitations of the employed method with great care. we give a detailed account of the techniques employed to conservation the force-free character of the simulated plasma. scientific results of this project are presented by a series of publications. we improved the numerical techniques used to solve for equilibrium magnetospheres of kerr bhs across their singular surfaces by biased discretization stencils. as a direct application, we reproduced an array of bh magnetoshperes found throughout the literature and provided a first detailed review of convergence properties. furthermore, we identified instabilities in the high energy branches of twisted magnetar magnetospheres. after their onset, a substantial fraction of the magnetospheric energy is released and may act as the triggering mechanism of the most powerful soft-gamma repeaters (sgrs). we provide a consistent argumentation for the connection of these simulations to the bolometric fingerprint expected from the observation of giant magnetar flares. finally, we confirmed the possibility of energy extraction by the blandford/znajek mechanism from rapidly spinning bhs in 3d dynamical magnetospheres induced by the accretion of small scale magnetic structures. we presented an extensive parameter study in which we analyzed the influence of magnetic loop geometry on the efficiency of the striped jet like outflow. we directly associated efficient episodes of energy extraction with the establishment of conditions which are favorable to the blandford/znajek process, arising quite naturally in the course of our simulations. | dynamics in the magnetospheres of compact objects |
accretion disks around young binary stars are subject to strong forces exerted by the system components. gas-dynamical interactions excite strong non-linear perturbations in the disk, which can give rise to turbulence. this study considers a mechanism for the excitation of turbulence based on the instability of non-linear waves in a rotating flow. it is shown that the spectrum of non-linear perturbations excited in the inner part of the disk leads to turbularization of the flow. estimates of the shakura-syunyaev index, α ∼ 0.01-0.05, are obtained for use in numerical models of accretion disks. | excitation of turbulence in protoplanetary disks around binary stars |
the spectra of disc-dominated cataclysmic variables (cvs) often deviate from the spectra of accretion disc models; in particular, the balmer jump and absorption lines are found to be shallower in the observations than in the models. we carried out a combined ultraviolet-optical spectral analysis of two dwarf novae (dne): uz ser in outburst, decline, and quiescence, and cy lyr on the rise to outburst and in outburst. we fit the balmer jump and absorption lines, the continuum flux level and slope by adjusting the accretion rate, inclination, and disc outer radius. for both systems, we find an accretion rate $\dot{m} \approx 8 \times 10^{-9}\,\mathrm{ m}_\odot\,\mathrm{ yr}^{-1}$ in outburst, and $\dot{m} \approx 2-3 \times 10^{-9}\,\mathrm{ m}_\odot\,\mathrm{ yr}^{-1}$ for the rise and decline phases. the outer disc radius we derive is smaller than expected (rdisc ≈ 0.2a, where a is the binary separation), except during late rise (for cy lyr) where rdisc = 0.3a. uz ser also reveals a 60 000 k white dwarf. these results show that during a dn cycle the radius of the disc is the largest just before the peak of the outburst, in qualitative agreement with the disc instability model for dn outbursts. we suspect that an additional emitting component (e.g. disc wind) is also at work to reduce the slope of the continuum and size of the balmer jump and absorption lines. we stress that both the outer disc radius and disc wind need to be taken into account for more realistic disc modelling of cvs. | the hydrogen balmer lines and jump in absorption in accretion disc modelling - an ultraviolet-optical spectral analysis of the dwarf novae uz serpentis and cy lyrae |
context. self-gravitational rotating bodies do not have spherically symmetric geometries. the study of physical events appearing in fast-spinning compact stars and accretion disks, for example those due to localized thermonuclear ignitions in white dwarfs or to the role played by hydrodynamic instabilities in stars and disks, often requires 3d simulations. when the numerical simulations are carried out with the smoothed particle hydrodynamics (sph) technique a critical point arises as to how to build a stable initial model with rotation because there is no well-established method for that purpose.aims: we want to provide a portable, easy-to-implement methodology for sph simulations to procedurally generate physically sound, stable initial conditions for rotating bodies.methods: we explain and validate an easy and versatile novel relaxation method to obtain 3d equilibrium configurations of rotating bodies with sph. as detailed below, this method is able to relax barotropic, p(ρ), structures either in rigid or differential rotation. the relaxation procedure strongly relies on the excellent conservation of angular momentum that characterizes the sph technique.results: we applied our proposal to obtain stable rotating structures of single white dwarfs, compact binaries harboring two white dwarfs, high-density stars approached as polytropes and accretion disks either in rigid or differential rotation.conclusions: we present a novel relaxation method to build 3d rotating structures of barotropic bodies using the sph technique. the method has been successfully applied to a variety of zero-temperature white dwarfs and polytropic self-gravitating structures. our sph results have been validated by comparing the main features (energies, central densities, and the polar-to-equatorial radius ratio) to those obtained with independent grid-based methods, for example, the self-consistent field method, showing that both methods agree within a few percent. | self-gravitating barotropic equilibrium configurations of rotating bodies with smoothed particle hydrodynamics |
linearized and flattened light curves (29 in b band and 29 in v band) of the recurent nova rs oph, taken in 2008--2017 by 5 telescopes, are analyzed. the purposes are (i) characterizing of the flickering phenomenon by various ways and (ii) justification of the "resonance" distribution of the modes of the flickering quasi-periods, found in paper i. the typical observing circumstances are: monitoring duration 60--150 min, number of ccd frames 50--200 and time step 0.5-1.5 min. part 1 presents photometric diagrams, standard and range deviations of the light curves in dependence on the average flux, as well as the distributions of the skewness and kurtosis of the histograms of the light curves. the intra-night flickering contains local disk obscures/shots and global disk instabilities. it seems both processes alternate their superiority. the average skewness, 0.12±0.30, gives weak privilege to the local shots. the average kurtosis, --0.17±0.62, gives some privilege to the global instabilities. in contrast to the random process with bell-shaped distribution of the deviations, deficit of small and large deviations is obvious in about 1/2 of the cases. presence of light variations, close to oscillations, give some priority to the global disk instabilities. in part 2 every light curve is scanned digitally by a system of data windows with sizes θ. six fractal parameters are derived as average values for each θ and regarded as functions on θ in log-log coordinates. the average structure gradient, 0.48±0.16, also does not point out the prior process of the light variations. the mean value of the hurst gradient, 0.22 ± 0.06, giving fractal dimension 1.78 ± 0.06, corresponds to auto-correlation at short time scale. the mean value of the fractal dimension, 1.48 ± 0.06, too. the asymmetry function, introduced in this work, shows that the mean duration of the elementary shots is 3.6±0.8 min. it seems the relatively weak and short local shots dominate at time scales of a few minutes. part 3 presents 97 quasi-periods and their distributions. the quasi-periods are detected by the local minima of the structure functions as 58 primary, 20 secondary, 10 tertiary and 13 very short. all quasi-periods are confirmed by the local maxima of the relevant auto-correlation function. the distribution of the quasi-periods shows 6 modes, at 8, 13, 21, 30, 48 and 73 min. the modes of the quasi-period p follow exponential (or power) function on the number of the mode m with standard deviation of 4.7%. the power dependence is p_m=3.48× 1.55^m. this function predicts modes also at 3.5, 5.3 and 115 min, which are not detected in this work. however, the characteristic duration of the elementary shot, 3.6±0.8 min, corresponds well to the predicted mode at 3.5 min. because 1.55 ≈ 3/2, the modes of the quasi-periods like obey 3/2 resonance with unknown reason. the relative energy of the quasi-period (i.e. of the quasi-periodic structure, including the energy of its substructures) correlates with the logarithm of the quasi-period under a slope coefficients 0.02. generally, the flickering light curves are too complicated and various, without a possible classification (at least in the present work). unique systematic is only the resonance-like sequence of the quasi-period modes. | intra-night flickering of rs ophiuchi: ii. exponentially distributed quasi-period modes |
in this study, we addressed the question of how the growth rate of the magnetorotational instability is modified when the radial component of the stellar dipole magnetic field is taken into account in addition to the vertical component. considering a fiducial radius in the disc where diamagnetic currents are pronounced, we carried out a linear stability analysis to obtain the growth rates of the magnetorotational instability for various parameters such as the ratio of the radial-to-vertical component and the gradient of the magnetic field, the alfvenic mach number, and the diamagnetization parameter. our results show that the interaction between the diamagnetic current and the radial component of the magnetic field increases the growth rate of the magnetorotational instability and generates a force perpendicular to the disc plane that may induce a torque. it is also shown that considering the radial component of the magnetic field and taking into account a radial gradient in the vertical component of the magnetic field causes an increase in the magnitudes of the growth rates of both the axisymmetric (m = 0) and the non-axisymmetric (m = 1) modes. | magnetorotational instability in diamagnetic, misaligned protostellar discs |
a simple look at the steady high-energy universe reveals a clear correlation with outflows generated around compact objects (winds and jets). in the case of relativistic jets, they are thought to be produced as a consequence of the extraction of rotational energy from a kerr black hole (blandford-znajek), or from the disc (blandford-payne). a fraction of the large energy budget provided by accretion and/or black hole rotational energy is invested into jet formation. after formation, the acceleration and collimation of these outflows allow them to propagate to large distances away from the compact object. the synchrotron cooling times demand that re-acceleration of particles takes place along the jets to explain high-energy and very-high-energy emission from kiloparsec scales. at these scales, jets in radio galaxies are divided in two main morphological/luminosity types, namely, fanaroff-riley type i and ii (fri, frii), the latter being more luminous, collimated and edge-brightened than the former, which show clear hints of decollimation and deceleration. in this contribution, i summarise a set of mechanisms that may contribute to dissipate magnetic and kinetic energy: magnetohydrodynamic instabilities or jet-obstacle interactions trigger shocks, shearing and mixing, which are plausible scenarios for particle acceleration. i also derive an expression for the expected distance in which the entrainment by stellar winds starts to be relevant, which is applicable to fri jets. finally, i discuss the differences in the evolutionary scenarios and the main dissipative mechanisms that take place in extragalactic and microquasar jets. | propagation and stability of relativistic jets |
accreting millisecond x-ray pulsars (amxps) are an important subclass of low-mass x-ray binaries (lmxbs), in which coherent millisecond x-ray pulsations can be observed during outburst states. they have dual characteristics of lmxbs and millisecond pulsars, providing a direct confirmation for the recycling scenario. however, their formation is not well understood. in this work, we simulate the evolution of lmxbs with the mesa code to explore the formation and evolution of amxps. based on the binary evolutionary model of lmxbs and the model of accretion disk instability, we find that most of the observed amxps can be produced from lmxbs with orbital periods at the onset of roche lobe overflow close to the bifurcation period and their observed properties can be explained by our models. the amxps with main sequence (ms) donors ultimately evolve into amxps with extremely low-mass he white dwarf donors. moreover, our results indicate that these amxps with ms donors are likely to have donor stars near the terminal-age main sequence. | formation of accreting millisecond x-ray pulsars |
context. enhanced angular momentum transfer through the boundary layer near the surface of weakly magnetised accreting star is required in order to explain the observed accretion timescales in low-mass x-ray binaries, cataclysmic variables, or young stars with massive protoplanetary discs. the accretion disc boundary layer is locally represented by incompressible homogeneous and boundless flow of the cyclonic type, which is linearly stable. its non-linear instability at the shear rates of the order of the rotational frequency remains an issue.aims: we put forward a conjecture that hydrodynamical subcritical turbulence in such a flow is sustained by the non-linear feedback from essentially three-dimensional vortices, which are generated by quasi-two-dimensional trailing shearing spirals grown to high amplitude via the swing amplification. we refer to those three-dimensional vortices as cross-rolls, since they are aligned in the shearwise direction in contrast to streamwise rolls generated by the anti-lift-up mechanism in rotating shear flow on the rayleigh line.methods: transient growth of cross-rolls is studied analytically and further confronted with direct numerical simulations (dns) of the dynamics of non-linear perturbations in the shearing box approximation.results: a substantial decrease of transition reynolds number rt is revealed as one changes a cubic box to a tall box. dns performed in a tall box show that rt as a function of shear rate accords with the line of constant maximum transient growth of cross-rolls. the transition in the tall box has been observed until the shear rate is three times higher than the rotational frequency, when rt ∼ 50 000.conclusions: assuming that the cross-rolls are also responsible for turbulence in the keplerian flow, we estimate r t ≲ 108 in this case. our results imply that non-linear stability of keplerian flow should be verified by extending turbulent solutions found in the cyclonic regime across the solid-body line rather than entering a quasi-keplerian regime from the side of the rayleigh line. the most favourable shear rate to test the existence of turbulence in the quasi-keplerian regime may be sub-keplerian and equal approximately to 1/2. | subcritical transition to turbulence in accretion disc boundary layer |
a number of white dwarf stars show absoption lines of heavy elements in their spectra. many of them also exhibit infra-red excess in their spectral energy distribution. these observations prove that these white dwarfs are surrounded by an orbiting debris disk resulting from the disruption of rocky planetesimals, remnants of the primordial planetary system. part of the material from the debris disk is accreted onto the white dwarfs, explaining the presence of heavy elements in their outer layers. previous attempts to estimate the accretion rates have overlooked the importance of the fingering convection. the fingering convection is an instability triggered by the accumulation in the white dwarf outer layers of material heavier than the underlying h-rich (for the da) or the he-rich (for the db) composition. the fingering convection induces a deep mixing of the accreted material. our preliminary simulations of the fingering convection show that the effect may be important in da white dwarfs. the accretion rates needed in order to reproduce the observed heavy element abundances exceed by order of magnitudes the accretion rates estimated when this extra-mixing is ignored. by contrast, in the cases of the db white dwarfs that we have considered in our simulations the fingering convection either does not occur or has very little effects on the derived accretion rates. we have undertaken a systematic exploration of the consequences of the fingering convection in accreting white dwarfs. | fingering convection and its consequences for accreting white dwarfs |
3xmm j031820.8-663034, first detected by rosat in ngc 1313, is one of a few known transient ultraluminous x-ray sources (ulxs). in this paper, we present decades of x-ray data of this source from rosat, xmm-newton, chandra, and the neil gehrels swift observatory. we find that its x-ray emission experienced four outbursts since 1992, with a typical recurrent time ∼1800 days, an outburst duration ∼240-300 days, and a nearly constant peak x-ray luminosity ∼1.5 × 1039 erg s-1. the upper limit of x-ray luminosity at the quiescent state is ∼5.6 × 1036 erg s-1, and the total energy radiated during one outburst is ∼1046 erg. the spectra at the high luminosity states can be described with an absorbed disk blackbody, and the disk temperature increases with the x-ray luminosity. we compare its outburst properties with other known transient ulxs including eso 243-49 hlx-1. as its peak luminosity only marginally puts it in the category of ulxs, we also compare it with normal transient black hole binaries. our results suggest that the source is powered by an accreting massive stellar-mass black hole, and the outbursts are triggered by the thermal-viscous instability. | recurrent outbursts revealed in 3xmm j031820.8-663034 |
recent numerical simulations of magnetized accretion discs show that the radial-azimuthal component of the stress tensor due to the magnetorotational instability is well represented by a power-law function of the gas pressure rather than a linear relation that has been used in most of the accretion disc studies. the exponent of this power-law function that depends on the net flux of the imposed magnetic field is reported in the range between zero and unity. however, the physical consequences of this power-law stress-pressure relation within the framework of the standard disc model have not been explored so far. in this study, the structure of an accretion disc with a power-law stress-pressure relation is studied using analytical solutions in the steady state and time-dependent cases. the derived solutions are applicable to different accreting systems, and as an illustrative example, we explore structure of protoplanetary discs using these solutions. we show that the slopes of the radial surface density and temperature distributions become steeper with decreasing the stress exponent. however, if the disc opacity is dominated by icy grains and value of the stress exponent is less than about 0.5, the surface density and temperature profiles become so steep that make them unreliable. we also obtain analytical solutions for the protoplanetary discs that are irradiated by the host star. using these solutions, we find that the effect of the irradiation becomes more significant with decreasing the stress exponent. | properties of an accretion disc with a power-law stress-pressure relationship |
we observed the deeply eclipsing su uma-type dwarf nova v4140 sgr and established the very short supercycle of 69.7(3) d. there were several short outbursts between superoutbursts. these values, together with the short orbital period (0.06143 d), were similar to, but not as extreme as, those of er uma-type dwarf novae. the object is thus the first, long sought, eclipsing er uma-like object. this er uma-like nature can naturally explain the high (apparent) quiescent viscosity and unusual temperature profile in quiescence, which were claimed observational features against the thermal-tidal instability model. the apparently unusual outburst behavior can be reasonably explained by a combination of this er uma-like nature and the high orbital inclination, and there is no need to introduce mass transfer bursts from its donor star. | detection of the supercycle in v4140 sagittarii: first eclipsing er ursae majoris-like object |
we use an n-body simulation, constructed using gadget-2, to investigate an accretion flow onto an astrophysical disk that is in the opposite sense to the disk's rotation. in order to separate dynamics intrinsic to the counter-rotating flow from the impact of the flow onto the disk, we consider an initial condition in which the counter-rotating flow is in an annular region immediately exterior the main portion of the astrophysical disk. such counter-rotating flows are seen in systems such as ngc 4826 (known as the "evil eye galaxy"). interaction between the rotating and counter-rotating components is due to two-stream instability in the boundary region. a multi-armed spiral density wave is excited in the astrophysical disk and a density distribution with high azimuthal mode number is excited in the counter-rotating flow. density fluctuations in the counter-rotating flow aggregate into larger clumps and some of the material in the counter-rotating flow is scattered to large radii. accretion flow processes such as this are increasingly seen to be of importance in the evolution of multi-component galactic disks. | instability of counter-rotating stellar disks |
> in this paper, we analyse the linear stability of a stellar accretion disk having a stratified morphology. the study is performed in the framework of ideal magneto-hydrodynamics and therefore results in a characterization of the linear unstable magneto-rotational modes. the peculiarity of the present scenario consists of adopting the magnetic flux function as the basic dynamical variable. such a representation of the dynamics allows us to take account of the co-rotation theorem as a fundamental feature of the ideal plasma equilibrium and to evaluate its impact on the perturbation evolution. according to the alfvenic nature of the magneto-rotational instability, we consider an incompressible plasma profile and perturbations propagating along the background magnetic field. furthermore, we develop a local perturbation analysis around fiducial coordinates of the background configuration and deal with very small-scale linear dynamics in comparison to the background inhomogeneity size. the main issue of the present study is that the condition for the emergence of unstable modes is the same in the stratified plasma disk as in the case of a thin configuration. such a feature is the result of the cancellation of the vertical derivative of the disk angular frequency from the dispersion relation, which implies that only the radial profile of the differential rotation is responsible for the trigger of the growing modes. | morphology of two-dimensional mri in axial symmetry |
we report the analysis of time series of infrared jhks photometry of the dwarf nova v2051 oph in quiescence. we modelled the ellipsoidal variations caused by the distorted mass-donor star to infer its jhks fluxes. from its infrared colours, we estimate a spectral type of m(8.0 ± 1.5) and an equivalent blackbody temperature of tbb = (2700 ± 270) k. we used the barnes & evans relation to infer a photometric parallax distance of dbe = (102 ± 16) pc to the binary. at this short distance, the corresponding accretion disc temperatures in outburst are too low to be explained by the disc-instability model for dwarf nova outbursts, underscoring a previous suggestion that the outbursts of this binary are powered by mass-transfer bursts. | infrared photometry of the dwarf nova v2051 ophiuchi - i. the mass-donor star and the distance |
supermassive black holes (smbhs) with m• ∼ 109 m⊙ at z > 6 likely originate from massive seed black holes (bhs). we investigate the consequences of seeding smbhs with direct collapse bhs (dcbhs) (m_{\bullet }=10^{4-6} m_⊙) on proto-galactic disc growth. we show that even in the absence of direct feedback effects, the growth of seed bhs reduces the development of gravitational instabilities in host galaxy discs, suppressing star formation and confining stars to a narrow ring in the disc and leading to galaxies at z ∼ 6 which lie above the local bh-stellar mass relation. the relative magnitude of cosmic and bh accretion rates governs the evolution of the bh-stellar mass relation. for typical dcbh formation epochs, zi ∼ 10, we find that star formation is inhibited in haloes growing at the average rate predicted by λcold dark matter that host bhs capable of reaching m_{\bullet }∼ 10^9 m_{⊙} by z ≳ 6. slower growing bhs cause a delay in the onset of star formation; an m• ∼ 106 m⊙ seed growing at 0.25 times the eddington limit will delay star formation by ∼100 myr. this delay is reduced by a factor of ∼10 if the halo growth rate is increased by {∼ } 0.6 σ. our results suggest that smbhs seeded by dcbhs and their host galaxies form in separate progenitor haloes. in the absence of subsequent mergers, higher than average cosmic accretion or earlier seed formation (zi ∼ 20) are required to place the evolving bh on the local bh-stellar mass relation by z = 6. | how black holes stop their host galaxy from growing without agn feedback |
we follow the development of nonaxisymmetric instabilities of self-gravitating disks from the linear regime to the nonlinear regime. particular attention is paid to comparison of nonlinear simulation results with previous linear and quasi-linear modeling results to study the mass and angular momentum transport driven by nonaxisymmetric disk instabilities. systems with star-to-disk mass ratios of and 5 and inner-to-outer disk radius ratios of to 0.66 are investigated. in disks where self-gravity is important, systems with small and large , jeans-like j modes are dominant and the gravitational stress drives angular momentum transport. in disks where self-gravity is weak, systems with large and large , shear-driven p modes dominate and the reynolds stress drives angular momentum transport. in disks where self-gravity is intermediate in strength between disks where p modes dominate and disks where j modes dominate, i modes control the evolution of the system and the reynolds and gravitational stresses both play important roles in the angular momentum transport. in all cases, redistribution of angular momentum takes place on the characteristic disk timescale defined as the orbital period at the location of maximum density in the disk midplane. the disk susceptible to one-armed modes behaves differently than disks dominated by multi-armed spirals. coupling between the star and the disk driven by one-armed modes leads to angular momentum transfer between the star and disk even when instability is in the linear regime. all modes drive spreading of the disk material and eventually accretion onto the star. the disks dominated by an i mode and one-armed mode do not lead to prompt fission or fragmentation. the j mode dominated disk fragments after instability develops. | nonaxisymmetric instabilities in self-gravitating disks iii. angular momentum transport |
the nature of accretion in symbiotic binaries, in which the red giant transfers material to a white dwarf (wd) companion, has been difficult to uncover. the accretion flows in a symbiotic binary are most clearly observable, however, when there is no quasi-steady shell burning on the wd to hide them. through observations in the high energy regime, which provide a view of the innermost accretion structures, we have studied two unprecedented events in two systems, rt cru and t crb, which host similar massive white dwarf but have very different orbital periods with correspondingly different accretion mechanism. in the past 20 years, rt cru has experienced two similar optical brightening events, separated by 4000 days and with amplitudes of v1.5 mag, reminiscent of dwarf-novae-type outbursts, but the hard x-ray behavior does not correspond to that observed in well-known dwarf nova. an alternative explanation for the brightening events could be that they are due to an enhancement of the accretion rate as the wd travels through the red giant wind in a wide orbit, with a period of about 4000 days. we have witnessed a change in the accretion rate for the first time in the symbiotic recurrent nova t crb. optical, uv and high energy data indicate that during an optical brightening event that started in early 2014 (v1.5), the hard x-ray emission has almost vanished and the x-ray spectrum became much softer and a bright, new, blackbody-like component appeared. we suggest that the optical brightening event, that could be a similar event to that observed about 8 years before the most recent thermonuclear outburst in 1946 is due to a disk instability. | accretion disks in symbiotic stars |
magnetorotational instability (mri), also known as the balbus-hawley instability, is thought to have an important role on the initiation of turbulence and angular momentum transport in accretion disks. in this work, we investigate the effect of the magnetic field gradient in the azimuthal direction on mri. we solve the magnetohydrodynamic equations by including the azimuthal component of the field gradient. we find the dispersion relation and calculate the growth rates of the instability numerically. the inclusion of the azimuthal magnetic field gradient produces a new unstable region in the wavenumber space. it also modifies the growth rate and the wavelength range of the unstable mode: the higher the magnitude of the field gradient, the greater the growth rate and the wider the unstable wavenumber range. such a gradient in the magnetic field may be important in t tauri disks, where the stellar magnetic field has an axis that is misaligned with respect to the rotation axis of the disk. | magnetic field gradient effects on the magnetorotational instability |
active states in white dwarfs are usually associated with light curve's effects that concern to the bursts, flickering or flare-up occurrences. it is common that a gas-dynamics source exists for each of these processes there. we consider the white dwarf binary stars with accretion disc around the primary. we suggest a flow transformation modeling of the mechanisms that are responsible for ability to cause some flow instability and bring the white dwarfs system to the outburst's development. the processes that cause the accretion rate to sufficiently increase are discussed. then the transition from a quiescent to an active state is realized. we analyze a quasi-periodic variability in the luminosity of white dwarf binary stars systems. the results are supported with an observational data. | active states and structure transformations in accreting white dwarfs |
3d numerical simulations demonstrate the formation of precessional spiral density waves in accretion disks of close binary stars. the precesional wave occurs in the keplerian disk as a result of gravitational action of the donor-star. the wave causes the appearance of strong density and velocity gradients in the disk. linear stability analysis shows that the presence of a radial velocity gradient leads to the instability of radial modes. the perturbation becomes unstable if the radial velocity variations are of the same order or greater than the sound speed on the characteristic wave scale of the perturbations. the unstable perturbations rapidly grow with time and give rise to the emergence and growth of turbulence in the accretion disk. the obtained viscosity (0.01 in terms of shakura-sunyaev parameter) is in agreement with observations. | precessional density wave as a reason of turbulence in accretion disks of non-magnetic close binary stars |
the outburst activity of the symbiotic system ag dra has been studied using extensive spectroscopic observational material. high luminosity and temperature of the hot component of ag dra indicate that quasi-steady thermonuclear shell burning takes place on the surface of the white dwarf. the major (cool) outbursts at the beginning of active phases might occur due to enhanced thermonuclear burning triggered by disk instability. smaller scale hot outbursts might be explained by the accretion disc instability model like in dwarf novae. we discovered significant similarities in photometric and spectroscopic behaviour of ag dra and prototypical symbiotic star z and. | outburst activity of the symbiiotic binary ag dra |
high-energy radiation from the central t tauri and protostars plays an important role in shaping protoplanetary disks and influences their evolution. such radiation, in particular x-rays and extreme-ultraviolet (euv) radiation, is predominantly generated in unstable stellar magnetic fields (e.g., the stellar corona), but also in accretion hot spots. even jets may produce x-ray emission. cosmic rays, i.e., high-energy particles either from the interstellar space or from the star itself, are of crucial importance. both highenergy photons and particles ionize disk gas and lead to heating. ionization and heating subsequently drive chemical networks, and the products of these processes are accessible through observations of molecular line emission. furthermore, ionization supports the magnetorotational instability and therefore drives disk accretion, while heating of the disk surface layers induces photoevaporative flows. both processes are crucial for the dispersal of protoplanetary disks and therefore critical for the time scales of planet formation. this chapter introduces the basic physics of ionization and heating starting from a quantum mechanical viewpoint, then discusses relevant processes in astrophysical gases and their applications to protoplanetary disks, and finally summarizes some properties of the most important high-energy sources for protoplanetary disks. 14th lecture from summer school "protoplanetary disks: theory and modelling meet observations" | ionization and heating by x-rays and cosmic rays |
by performing multi-dimensional radiation-mhd simulations, we reveal the inflow-outflow structure around the black holes (bhs) and the neutron stars (nss). in both cases, the quasi-steady, super-eddington disk forms around the central objects. the disk is supported by the strong radiation pressure and the radiatively-driven outflows are launched from the disk surface. the hot outflow produces the high-energy x-ray photons via comptonization. in addition, powerful outflows fragments into many gas clouds by the radiation rt instability. such gas clouds would lead to the x-ray time variation with several seconds for the stellar mass black holes. our results basically consistent with the recent observations of ultraluminous x-ray sources. | numerical simulations of sueper-eddington accretion flows and outflows around black holes and neutron stars |
population iii accretion disks are prone to gravitational instabilities that can cause them to fragment. studies of present-day star formation find that magnetic fields can help to stabilize protostellar accretion disks and suppress fragmentation. however, the role of magnetic fields during pop iii star formation is uncertain. the small-scale dynamo (ssd) can amplify weak seed fields to dynamically significant values, but it is unclear whether the resulting field is strong enough to suppress fragmentation. here, we use an extended toomre criterion to estimate the critical field strength b_crit needed for disk stabilization. we compare this with estimates of the field strength produced by the ssd, b_sat. we show that in general, b_crit > b_sat, implying that the magnetic field produced by the small-scale dynamo is too weak to fully stabilize the disk. | stabilizing population iii accretion disks with magnetic fields |
we propose a 100ks observation of the unique outbursting seyfert galaxyic 3599, to be triggered if it flares again in 2019. two giant-amplitudex-ray outbursts (factor>100) in 1990 and 2010 were accompanied by dramaticemission-line response never seen before in any agn, but high-resolutionx-ray spectra where missed. among the two outburst models - a repeatstripping of the same star in orbit of a tidal disruption event oran accretion-disk instability - the former scenario predicts a giantoutburst in 2019, which will be detected in swift monitoring, if ithappens. we propose to obtain the first high-resolution x-ray spectrumof ic 3599 in one of these rare outburst states in order to understandaccretion and wind physics under extreme conditions. | the nature of the x-ray outbursts in ic 3599 |
we develop a semi-analytic formalism for the determination of the evolution of the stellar mass accretion rate for specified density and velocity profiles that emerge from the runaway collapse of a prestellar cloud core. in the early phase, when the infall of matter from the surrounding envelope is substantial, mass is transported inward because of envelope-induced gravitational instability in a protostellar disk. in this phase, we model the mass accretion rate assuming isothermal free-fall collapse of a molecular cloud core that feeds the disk, and episodic gravitational instability and mass accretion bursts according to the toomre q criterion. in the late phase, when the gas reservoir of the envelope is depleted, mass is being accreted onto the star due to gravitational torques within the disk, in a manner that analytic theory suggests has the form ∝ t-6/5. our model provides a self-consistent evolution of the mass accretion rate by joining the spherical envelope accretion (dominant at the earlier stage) with the disk accretion (important at the later stage), and accounts for the presence of episodic accretion bursts at suitable times. we compare our semi-analytic results with that of a numerical model for mass accretion described in vorobyov & basu (2007). the burst modes may hold the key to explaining the long-standing 'luminosity problem'. we investigate whether the bursts are needed to provide a good match to the observed distribution of bolometric luminosities of ysos, or alternately whether the overall declining profile of the mass accretion rate is enough to give a good fit to the luminosity distribution. | a semi-analytic model for the temporal evolution of the episodic disk-to-star accretion rate during star formation |
kepler revealed the common existence of tightly-packed super-earth systems around solar-type stars, existing entirely inside the orbit of our venus. those systems must be stable for the ages of their host stars (~10^9 years); their formation mechanism must provide inter-planet spacings that permit long-term stability. if one postulates that most planetary systems form with tightly-packed inner planets, their current absence in some systems could be explained by the collisional destruction of the inner system after a period of meta-stability.we posit that our solar system also originally had a system of multiple planets interior to the orbit of venus. this would resolve a known issue that the energy/angular momentum of our inner-planet system is best explained by accreting the current terrestrial planets from a disk limited to 0.7-1.1 au; in our picture the disk material closer to the sun also formed planets, but they have since been destroyed. by studying the orbital stability of systems like the known kepler systems, we demonstrate that orbital excitation and collisional destruction could be confined to just the inner parts of the system. in this scenario, mercury is the final remnant of the inner system's destruction via a violent multi-collision (and/or hit-and-run disruption) process. this would provide a natural explanation for mercury's unusually high eccentricity and orbital inclination; it also fits into the general picture of long-timescale secular orbital instability, with mercury’s current orbit being unstable on 5 gyr time scales. the common decade spacing of instability time scales raises the intriguing possibility that this destruction occurred roughly 0.6 gyr after the formation of our solar system and that the lunar cataclysm is a preserved record of this apocalyptic event that began when slow secular chaos generated orbital instability in our former super-earth system. | consolidating and crushing exoplanets: did it happen here? |
we will attempt to confirm the detection of an accreting protoplanet detected after the cycle 30 deadline with the magao-x instrument in a narrow-band h-alpha filter. the observations were taken under exceptional conditions, and it is highly unlikely that similar conditions will be met during a second epoch observation from the ground. the protoplanet, detected with >4 sigma fidelity, is located at ~1 arcsec from the central star, and it is accreting at a relatively low rate (macc = 8.5 x 10^-9 mj/yr assuming zero extinction). the analysis will allow us to (1) confirm the existence of the accreting protoplanet, (2) investigate its accretion history, (3) search for indications of accretion variability, (4) provide valuable information to numerical simulations of planet formation and planet-disk interactions. given the young age of the star (<1-2 myr), the protoplanet candidate as209c has the potential of revolutionize the field of planet formation, demonstrating that planets can form quickly at large separation from their star via disk instability. indeed, core accretion requires much longer timescales for the protoplanet candidate to form. we will use the narrow-band filter f656n of the hst/wfc3 instrument for 3 orbit in order to achieve the sensitivity necessary to confirm the companion detection. if confirmed, this candidate would be hst's first bona fide discovery of a protoplanet. | confirming an accreting planet in a very young protoplanetary disk |
the origin of the soft x-ray excess in luminous active galactic nuclei (agn) such as type 1 seyfert galaxies is thought to be the warm compton region with a temperature of 106-107 k. this temperature is higher than that of the optically thick standard disk around a supermassive black hole and lower than the optically thin and radiatively inefficient accretion flow (riaf). recently, transitions between a hard x-ray dominant state and a soft x-ray dominant state have been observed in changing look agn, which changes between type 1 with broad emission lines and type 2 without broad emission lines. this hard-to-soft state transition is similar to the state transitions observed in stellar-mass black hole candidates. the origin of the state transition may be the cooling instability which grows when the accretion rate exceeds the upper limit of riaf. igarashi et al. (2020) reported the results of global three-dimensional radiation magnetohydrodynamic simulations of the accretion flow into a supermassive black hole carried out by using a radiation magnetohydrodynamic code cans+r. they found that when the accretion rate exceeds 10% of the eddington accretion rate, a warm compton region with a temperature of 107-108 k is formed outside the riaf near the black hole and that radial oscillation is excited in this region when the radiation pressure becomes dominant. the critical luminosity for this transition is 0.5% of the eddington luminosity, which is consistent with observations of changing look agn. however, the temperature of the warm region is higher than that of the soft x-ray emitting region of agn obtained by using the observed radiation spectra. here we report the results of radiation magnetohydrodynamic simulations carried out by implementing the effects of compton cooling to the cans+r code. as a result, the temperature of the thomson thick, warm compton region decreased to 106-107 k. we discuss the structure and time variation of the warm compton region. | radiation magnetohydrodynamic simulations of sub-eddington accretion flows in agn |
short-period super-earths and mini-neptunes have been shown to be common, yet it is still not understood how and where inside protoplanetary discs they could have formed. to form these planets at the short periods at which they are detected, the inner regions of protoplanetary discs must be enriched in dust. dust could accumulate in the inner disc if the innermost regions accrete via the magneto-rotational instability (mri). we developed a model of the inner disc which includes mri-driven accretion, disc heating by both accretion and stellar irradiation, vertical energy transport, dust opacities, dust effects on disc ionization, thermal and non-thermal sources of ionization. the inner disc is assumed to be in steady state, and the dust is assumed to be well-mixed with the gas. using this model, we explore how various disc and stellar parameters affect the structure of the inner disc and the possibility of dust accumulation. we show that properties of dust strongly affect the size of the mri-accreting region and whether this region exists at all. increasing the dust-to-gas ratio increases the size of this region, suggesting that dust may accumulate in the inner disc without suppressing the mri. overall, conditions in the inner disc may be more favourable to planet formation earlier in the disc lifetime, while the disc accretion rate is higher. | mri-accreting inner regions of protoplanetary discs |
in the early universe, we observe supermassive black holes with masses of up to a billion times the mass of the sun, accreting at or even above the eddington limit. these high-redshift quasars are some of the most luminous objects in the universe, and raise many questions about the formation and growth of the first black holes. investigating their host galaxies provides a useful probe for understanding these high-redshift quasars. in the local universe, there are clear correlations between the mass of a supermassive black hole and the properties of its host galaxy, indicating a black hole--galaxy co-evolution. exploring how these black hole--host relations evolve with redshift can give valuable insights into why these relations exist. studying the host galaxies of high-redshift quasars thus provides vital insights into the early growth of supermassive black holes and the black hole--galaxy connection. in this thesis i use three techniques to study the host galaxies of high-redshift quasars: the meraxes semi-analytic model, the bluetides hydrodynamical simulation, and observations with the hubble space telescope. meraxes is a semi-analytic model designed to study galaxy formation and evolution at high redshift. using this model, i study the sizes, angular momenta and morphologies of high-redshift galaxies. i also use meraxes to study the evolution of black holes and their host galaxies from high redshift to the present day. the model predicts no significant evolution in the black hole--host mass relations out to high redshift, with the growth of galaxies and black holes tightly related even in the early universe. i also examine the growth mechanisms of black holes in meraxes, finding that the majority of black hole growth is caused by internal disc instabilities, and not by galaxy mergers. i then use the bluetides cosmological hydrodynamical simulation to investigate the detailed properties of quasar host galaxies at z=7. i find that the hosts of quasars are generally highly star-forming and bulge dominated, and are significantly more compact than the typical high-redshift galaxy. using bluetides i make predictions for observations of quasars with the james webb space telescope, finding that detecting quasar hosts at these redshifts may be possible, but will still be challenging with this groundbreaking instrument. finally, i use observations from the hubble space telescope to obtain deep upper limits on the rest-frame ultraviolet luminosities of six z~6 quasars. i also detect up to 9 potential companion galaxies surrounding these quasars, which may be interacting with their host galaxies. observations with the upcoming james webb space telescope are needed to detect quasar host galaxies in the rest-frame ultraviolet and optical for the first time. | the host galaxies of high-redshift quasars |
all giant planets in the solar system have two types of moons as defined by their orbits and mode of origin. the first type, referred to as the regular moons, has tight circular orbits close to the equatorial plane of the host, implying primordial accretion in the circum-planetary disc. the second type, called the irregular moons, in contrast, is characterised by wide, highly-eccentric and -inclined orbits and are believed to be captured by their host from heliocentric orbits through some form of dissipation. however, the neptunian moon triton, 3000 km across, does not neatly fit in any of the two categories — it is orbiting the host rather close-in but in a direction opposite to the spin of neptune. the obvious incompatibility between its retrograde orbit and an in-situ accretion origin suggests that it was captured by neptune, for example, as a component of a binary asteroid pair. another moon in the system, nereid, is a distant irregular satellite. it is the largest of its kind and at the same time features the tightest and the most eccentric orbit for an irregular moon. here we explore an in-situ formation formation for these two moons. we assume that both initially formed as regular satellites at neptune. then a planetary encounter triggers an evolutionary sequence of events for these two moons towards their observed orbits. such an encounter cannot happen in the present solar system. but rather in the early solar system, there is an instability period as envisioned by the nice scenario. specifically in a later version of the nice scenario where three gas giants (ig) are initially orbiting the sun; during the instability period the additional ig gains a significant orbital eccentricity, allowing it to encounter other planets until finally ejected. here, we model such an encounter between a moon-bearing neptune and an ig. we find that during the encounter, about half of the pre-existing neptunian moons are ejected and the surviving moons are highly excited. among the survivors, a few per cent gain retrograde orbits (triton analogues, tas) while a similar fraction acquire wide, eccentric orbit (nereid analogues, nas). while the nas orbits match that of nereid quite well, those of the tas are highly eccentric. often the orbit of the ta intersect that of the na; then the latter will be removed due to scattering or collision within a myr. how can the na survive then? a further issue is after the neptune-ig encounter, some of the other moons may also survive. why are these additional moons not observed today? we find that if these moons are small, collisions between them and the ta would eliminate the former without endangering the latter. collisions also shrink the orbit of the ta, decouple from that of na and hence na is protected. finally, tides takes control and circularise ta"s orbit on gyr timescale.an illustration of our model is shown in figure 1 and an example from the numerical simulations in figure 2. depending on how stringently we define a ta and na, our model has an efficiency of 10^-5 - 10^-3.in this in-situ formation model, triton and nereid accrete in the circum-planetary disk (see also, harrington & van flandern, 1979, icarus, 39, 131; li et al. 2020, a&a, in press, doi: 10.1051/0004-6361/201936672) whereas the conventional capture model (e.g., agnor & hamilton, 2006, nature, 441, 192; nesvorny et al., 2007, aj, 133, 1962) predicts that the two form in the circum-stellar disk. the environment, e.g., the temperature, in the two disks could be rather different, potentially leading to different compositional properties for example the fraction of volatiles. hence, further observations as well as space missions would be helpful to constrain the formation path of the two moons. full details can be found in li & christou (2020, aj, 159, 184). the authors thank dr. craig b. agnor for direct contributions to this work. dl acknowledges financial support from knut and alice wallenberg foundation (2014.0017 and 2012.0150) and from vetenskapsrådet (2017-04945). the authors also thank the royal physiographic society of lund. astronomical research at the armagh observatory and planetarium is funded by the northern ireland department for communities (dfc). | an in-situ formation for triton and nereid |
v630 cas is one of several long-period cataclysmic variables withrare, long-duration, dwarf nova outbursts. the characteristics of theseoutbursts are likely shaped by the large physical size of the accretiondisks. moreover, they often are luminous (> 10e33 ergs/s) x-ray sourcesin quiescence, severely challenging the disk instability model. here wepropose an xmm-newton observation of v630 cas in quiescence to constrainthe white dwarf mass and the accretion rate, possibly adding it to thelist of troubling systems with a high accretion rate white dwarf witha large, quiescent accretion disk. we also propose to obtain fast uvphotometry with the om, since uv flickering is a key diagnostic ofaccretion for the related class of symbiotic stars. | probing accretion disks on intermediate size scales: the case of v630 cas |
the work is devoted to synthetic light curves modeling for the observed binary stellar systems and an interpretation of their characteristic features including pre-eclipse humps, asymmetry in the vicinity of the eclipse and variability of light curves from revolution to revolution. the mathematical model of the accretion disk including the radiation cooling of the plasma, gravitational forces and incomplete plasma ionization has been studied. the results of the plasma flows calculation allow one to use the radiated energy for synthetic light curves construction. in a number of simulations the formation of an inclined accretion disk is noted. the inclination may be caused by the instability of the circumstellar plasma flowing around the disk at the initial stages of its formation. the disk obtained in the calculations retrogradely precesses with a period of about 40 orbital periods of the system. due to this precession in different orbital periods the jet enters the disk in different places. this may explain the significant variability of the light curves of the binary star system as well as the presence of brightness humps at the eclipse. | mathematical modeling of inclined accretion disks in cataclysmic variables |
we report an extensive campaign of v-band time-series photometry of the new x-ray transient maxi j1820+070 = asassn-18ey. about 90 days into its outburst, the star developed large-amplitude photometric waves with a period of 0.690(2) days. these are likely to be superhumps characteristic of an eccentric instability in the accretion disk of compact binaries with a low mass ratio. such waves are common in the high state of cataclysmic variables. their presence here along with the luminosity, spectrum, and fast variability of the x-rays probably establishes the accreting star as a black hole of >4 solar masses. | asassn-18ey (maxi j1820+070): king of the black-hole superhumps |
close binaries containing a white dwarf accreting from a disk that receives material from a late stellar companion show an interesting variety of behavior that depends on their accretion rate and the changes in this rate. the highest accretion rates are evident during the bright states of novalikes, as well as the superoutbursts of the shortest period dwarf novae, followed by normal outbursts and the standstills of z cam systems. while the normal outbursts and superoutbursts of dwarf novae can be understood from the standpoint of disk and tidal instabilities, the changes in rates in novalikes when they enter low brightness states and the cause of extremely high rates for the systems between 3-4 hrs (sw sex) remain elusive. the differences found from x-ray, uv and optical observations between these high and low states will be summarized as well as the prospects for increased understanding from the anticipated long term extensive coverage that will be provided by lsst and its resulting followup. | insights from multi-wavelength observations during high and low accretion states of non-magnetic cvs |
active galactic nuclei (agns) and x-ray binaries (xrbs) contain at their cores supermassive black holes (smbh), and stellar mass black holes or neutron stars, respectively. these objects bind matter gravitationally, leading to the formation of accretion disks. the accretion of matter leads to efficient conversion of gravitational potential energy into radiation. when the force and/or energy imparted by this radiation on matter exceeds the attractive force due to the local gravitational field, it leads to the launching of matter in the form of outflows or winds. these outflows produce blueshifted (and occasionally redshifted) emission or absorption components in the spectra of these sources. for instance, seyfert galaxies generally display absorption lines with relatively small blueshifts (~ 100 -1000 km/s) in their uv and x-ray spectra, indicating the presence of mass outflows. in some such galaxies, the x-ray absorbers (the so-called warm absorbers, was) and uv absorbers have very similar velocities which suggests that these absorbers are nearly cospatial. this, in turn, suggests that regions of very different temperatures coexist and that the absorption occurs in a multi-phase outflow. in our work, we study two distinct effects of disk winds/outflows: (a) self-regulating effects on accretion disks, and (b) effects on the background continuum radiation due to absorption. the self-regulating disk-wind feedback process is an example of how disk winds can regulate the time evolution of the entire accretion disk. we develop a one-dimensional radial hydrodynamic code to study the evolution of such a disk. by relaxing the condition of a fixed wind launching radius, we find that an accretion disk is destabilized for much stronger winds than previously predicted for a fixed launching radius model. we then compute thermally-driven outflow models irradiated by an agn continuum, using the magnetohydrodynamics code athena++. an isobaric perturbation can trigger thermal instability leading to the formation of multi-phase outflows that can have distinguishing effects on the absorption line profiles. to study such effects, we develop and test a pipeline written in python, which combines results from athena++ and the photoionization calculation code xstar. we use this pineline to generate the absorption line profiles based on our outflow solutions. we discuss how absorption line profiles produced by different species could help to distinguish smooth outflow from multi-phase ones. | the effects of winds on accretion disks and spectra of x-ray binaries and active galactic nuclei |
we illustrate the peculiar x-ray variability displayed by the accreting millisecond x-ray pulsar igr j00291+5934 in a 80 ks-long joint nustar and xmm-newton observation performed during the source outburst in 2015. the lightcurve of the source is characterized by a flaring behavior, with typical rise and decay timescales of ∼120 s. the flares are accompanied by a remarkable spectral variability, with the x- ray emission being generally softer at the peak of the flares. a strong qpo is detected at ∼8 mhz in the power spectrum of the source and clearly associated to its flaring-like behaviour. this feature has the strongest power at soft x-rays (<3 kev). we carried out a dedicated hardness-ratio resolved spectral analysis and a qpo-phase resolved spectral analysis together with an in-depth study of the source timing properties to investigate the origin of this behaviour. we discuss that it could be due either a disk-instability like the hearth-beat in the black-hole binary grs 1915+105, or, less likely, to unstable nuclear burning on the neutron star surface, as observed in the burster 4u 1636-536. this phenomenology could be ideally studied with the large throughput and wide energy coverage of present and future instruments. | a soft mhz quasi periodic oscillation in the fastest accreting millisecond pulsar. |
protoplanetary disks are observed to undergo episodes of powerful eruptions known as fuor- and exor-type outbursts. thermal instability has been proposed as one of the underlying mechanisms behind this outburst activity, which can produce self-regulating accretion events. this instability sets in at high temperatures, above 5000 k, because of steep opacity dependence on temperature caused by ionization of hydrogen. in this presentation i will talk about our latest findings related to new aspects of the thermal instability. with the help of global protoplanetary disk simulations, we show that "generalized" thermal instabilities can set in at temperatures as low as 1500 k. the molecular line absorption, primarily from water vapor, results in regions where the opacity is strongly increasing with temperature. this mechanism can cause formation of locally unstable regions, and as these lower temperatures are readily achieved in the inner parts of the disk, encounters of such unstable regions with magnetospheric boundary may be responsible for some of the short duration (exor) outbursts. the water content in the protoplanetary disks may thus significantly influence the eruptive phase of the young stars. | links between water and episodic accretion in young stars |
gravitational instability has been extensively studied in contexts of protoplanetary disks as well as agn accretion disks around supermassive blackholes (smbhs). when a gravitationally stable standard shakura-sunyaev accretion disk, with its accretion parameter alpha driven primarily by magnetorotational instability (mri), extends outwards from the inner region of a quasar disk, at some critical radius the toomre parameter q will drop below order unity. beyond this radius, the disk becomes gravitationally unstable, but is thought to be able to eventually self-regulate at a state where q ~ 1, with the effective sound speed/pressure given by the combination of gas and radiation pressure. analytical models have been applied to described this region as a constant-q disk where steady state alpha is an explicit function of distance to the smbh r, parametrized by the constants q and accretion rate ṁ (e.g. goodman 2003). to gain energy balance this radius-dependent effective viscosity alpha needs to be ~ tcool /omega in steady state, here tcool is the local characteristic energy cooling time. however, extensive gas-pressure-only simulations show that typically when tcool is smaller than some threshold value tc ~ 3/omega, the disk will fragment into a number of clumps, although the accurate value for the boundary threshold tc is uncertain (e.g. gammie 2001, johnson & gammie 2003, rice et al. 2005). for a disk with a constant accretion rate ṁ, tc ≪ 1/omega far enough from the smbh so fragmentation/star formation always occurs at large distances, but towards the central smbh the radiation pressure fraction increases and inevitably dominates the total pressure at smaller radii. a radiation-supported disk has an effective adiabatic index of 4/3 and is much more prone to fragmentation than disk supported by gas pressure (jiang & goodman 2011). in this study we perform 3d local shearing box simulations with radiation transfer, using state-of-the-art implicit radiation module of athena++ to determine how tc depends on radiation-over-gas pressure ratio pr/pg along the fragmentation/steady-turbulence boundary, in particular how non-negligible fractions of radiation pressure extends tc to be much larger than the classical order-unity threshold. our simulations imply that if the increase of local tcool cannot overcome the destabilizing effects of growing pr/pg as we follow a constant ṁ accretion disk inwards towards the smbh, a steady state may never be reached at q~1 and fragmentation is inevitable for gravitationally unstable regions. | radiation hydrodynamic simulations of gravitational instability in agn accretion disks: the radiation-modified fragmentation boundary |
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