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The Celestial Sign in the Anglo-Saxon Chronicle in the 770s: Insights on
Contemporary Solar Activity: The anomalous concentration of radiocarbon in 774/775 attracted intense
discussion on its origin, including the possible extreme solar event(s)
exceeding any events in observational history. Anticipating such extreme solar
events, auroral records were also surveyed in historical documents and those
including the red celestial sign after sunset in the Anglo-Saxon Chronicle
(ASC) were subjected to consideration. Usoskin et al. (2013: U13) interpreted
this record as an aurora and suggested enhanced solar activity around 774/775.
Conversely, Neuhauser and Neuhauser (2015a, 2015b: N15a and N15b) interpreted
"after sunset" as during sunset or twilight; they considered this sign as a
halo display and suggested a solar minimum around 774. However, so far these
records have not been discussed in comparison with eyewitness auroral records
during the known extreme space-weather events, although they were discussed in
relationship with potential extreme events in 774/775. Therefore, we
reconstruct the observational details based on the original records in the ASC
and philological references, compare them with eyewitness auroral observations
during known extreme space-weather events, and consider contemporary solar
activity. We clarify the observation was indeed "after sunset", reject the
solar halo hypothesis, define the observational time span between 25 Mar. 775
and 25 Dec. 777, and note the parallel halo drawing in 806 in the ASC shown in
N15b was not based on the original observation in England. We show examples of
eyewitness auroral observations during twilight in known space-weather events,
and this celestial sign does not contradict the observational evidence.
Accordingly, we consider this event happened after the onset of the event in
774/775, but shows relatively enhanced solar activity, with other historical
auroral records in the mid-770s, as also confirmed by the Be data from ice
cores. | astro-ph_SR |
Stellar mid-life crisis: subcritical magnetic dynamos of solar-like
stars and the breakdown of gyrochronology: Recent observations reveal the surprising breakdown of stellar gyrochronology
relations at about the Sun's age hinting that middle-aged, solar-like stars
transition to a magnetically inactive future. We provide a theoretical basis
for these intriguing observations inspired by simulations with a mathematical
dynamo model that can explore long-term solar cycle fluctuations. We reproduce
the observed bimodal distribution of sunspot numbers, but only for subcritical
dynamos. Based on a bifurcation analysis, we argue that ageing of solar-like
stars makes the magnetically-weak dynamo regime readily accessible. Weak
magnetic field production in this regime compromises wind-driven angular
momentum losses thus disrupting the hegemony of magnetic braking on stellar
rotational spin-down. This hypothesis of {\emph{subcritical magnetic dynamos}}
of solar-like stars provides a self-consistent, unifying physical basis for a
diversity of solar-stellar phenomena such as why stars beyond their mid-life do
not spin-down as fast as in their youth, the break-down of stellar
gyrochronology relations, the observed bimodal distribution of long-term
sunspot observations and recent findings suggesting that the Sun may be
transitioning to a magnetically inactive future. | astro-ph_SR |
On the energy dissipation rate at the inner edge of circumbinary discs: We study, by means of numerical simulations and analysis, the details of the
accretion process from a disc onto a binary system. We show that energy is
dissipated at the edge of a circumbinary disc and this is associated with the
tidal torque that maintains the cavity: angular momentum is transferred from
the binary to the disc through the action of compressional shocks and viscous
friction. These shocks can be viewed as being produced by fluid elements which
drift into the cavity and, before being accreted, are accelerated onto
trajectories that send them back to impact the disc. The rate of energy
dissipation is approximately equal to the product of potential energy per unit
mass at the disc's inner edge and the accretion rate, estimated from the disc
parameters just beyond the cavity edge, that would occur without the binary.
For very thin discs, the actual accretion rate onto the binary may be
significantly less. We calculate the energy emitted by a circumbinary disc
taking into account energy dissipation at the inner edge and also irradiation
arising there from reprocessing of light from the stars. We find that, for
tight PMS binaries, the SED is dominated by emission from the inner edge at
wavelengths between 1-4 and 10 $\mu$m. This may apply to systems like CoRoT
223992193 and V1481 Ori. | astro-ph_SR |
The First Extensive Exploration of UV-bright Stars in the Globular
Cluster NGC 2808: In this study, we identified and characterized the hot and luminous UV-bright
stars in the globular cluster NGC 2808. We combined data from the Ultra Violet
Imaging Telescope (UVIT) on-board the Indian space satellite, AstroSat, with
the Hubble Space Telescope UV Globular Cluster Survey (HUGS) data for the
central region (within $\sim$ $\ang[angle-symbol-over-decimal]{;2.7;} \times
\ang[angle-symbol-over-decimal]{;2.7;}$) and Gaia and ground-based optical
photometry for the outer parts of the cluster. We constructed the UV and
UV-optical color-magnitude diagrams, compared the horizontal branch (HB)
members with the theoretical zero-age HB and terminal-age HB models and
identified 34 UV-bright stars. The spectral energy distributions of the
UV-bright stars were fitted with theoretical models to estimate their effective
temperatures (12500 K - 100,000 K), radii (0.13 to 2.2 $R_{\odot}$), and
luminosities ($\sim 40$ to $3000$ $L_{\odot}$) for the first time. These stars
were then placed on the H-R diagram, along with theoretical post-HB (pHB)
evolutionary tracks to assess their evolutionary status. The models suggest
that most of these stars are in the AGB-manqu\'e phase and all, except three,
have evolutionary masses $<$ 0.53 $M_{\odot}$. We also calculated the
theoretically expected number of hot post-(early)-AGB (p(e)AGB) stars in this
cluster and found the range to match our observations. Seven UV-bright stars
located in the outer region of the cluster, identified from the AstroSat/UVIT
images, are ideal candidates for detailed follow-up spectroscopic studies. | astro-ph_SR |
Testing Angular Momentum Transport and Wind Loss in Intermediate Mass
Core Helium Burning Stars: Stars between two and three solar masses rotate rapidly on the main sequence,
and their rotation rates in the core helium burning (secondary clump) phase can
therefore be used to test models of angular momentum loss used for
gyrochronology in a new regime. Because both their core and surface rotation
rates can be measured, these stars can also be used to set strong constraints
on angular momentum transport inside stars. We find that they are rotating
slower than angular momentum conservation and rigid rotation would predict. Our
results are insensitive to the degree of core-envelope coupling because of the
small moment of inertia of the radiative core. We discuss two possible
mechanisms for slowing down the surfaces of these stars: (1) substantial
angular momentum loss, and (2) radial differential rotation in the surface
convection zone. Modern angular momentum loss prescriptions used for solar-type
stars predict secondary clump surface rotation rates in much better agreement
with the data than prior variants used in the literature, and we argue that
such enhanced loss is required to understand the combination of core and
surface rotation rates. However, we find that the assumed radial differential
rotation profile in convective regions has a strong impact on the predicted
surface rotation rates, and that a combination of enhanced loss and radial
differential rotation in the surface convection zone is also consistent with
the data. We discuss future tests that can quantify the impact of both
phenomena. Current data tentatively suggests that some combination of the two
processes fits the data better than either one alone. | astro-ph_SR |
Binary stars as the key to understanding planetary nebulae: Planetary nebulae are traditionally considered to represent the final
evolutionary stage of all intermediate-mass stars ($\sim$0.7-8Msol). Recent
evidence seems to contradict this picture. In particular, since the launch of
the Hubble Space Telescope it has become clear that planetary nebulae display a
wide range of striking morphologies which cannot be understood in a single star
scenario, instead pointing towards a binary evolution in a majority of systems.
Here, we summarise our current understanding of the importance of binarity in
the formation and shaping of planetary nebulae, as well as the surprises that
recent observational studies have revealed with respect to our understanding of
binary evolution in general. These advances have critical implications,
including for the understanding of mass transfer processes in binary stars -
particularly the all-important but ever-so poorly understood `common envelope
phase' - as well as the formation of cosmologically important type Ia
supernovae. | astro-ph_SR |
A detailed study of the barium central star of the planetary nebula
Abell 70: We present a detailed study of the barium star at the heart of the planetary
nebula Abell 70. Time-series photometry obtained over a period of more than ten
years demonstrates that the barium-contaminated companion is a rapid rotator
with temporal variability due to spots. The amplitude and phasing of the
photometric variability changes abruptly, however there is no evidence for a
change in the rotation period (P = 2.06~d) over the course of the observations.
The co-addition of 17 high-resolution spectra obtained with VLT-UVES allow us
to measure the physical and chemical properties of the companion, confirming it
to be a chromospherically-active, late G-type sub-giant with more than +1~dex
of barium enhancement. We find no evidence of radial velocity variability in
the spectra, obtained over the course of approximately 130~d with a single
additional point some 8 years later, with the radial velocities of all epochs
approximately $-$10 \kms{} from the previously measured systemic velocity of
the nebula. This is perhaps indicative that the binary has a relatively long
period (P $\gtrsim$ 2~yr) and high eccentricity ($e\gtrsim$ 0.3), and that all
the observations were taken around radial velocity minimum. However, unless the
binary orbital plane is not aligned with the waist of the nebula or the
systemic velocity of the binary is not equal to the literature value for the
nebula, this would imply an unfeasibly large mass for the nebular progenitor. | astro-ph_SR |
GALEX J184559.8-413827: a new extreme helium star identified using SALT: A high-resolution spectrum of the helium-rich `hot subdwarf' GALEX
J184559.8-413827 (J1845-4138) obtained with SALT HRS demonstrates it to be the
first extreme helium (EHe) star to be discovered in nearly 40 years. A
quantitative analysis demonstrates it to have an atmosphere described by Teff =
26 170 +/- 750 K, log g /(cm s^-2) = 4.22 +/- 0.10, and a surface chemistry
characterised by CNO-processed helium, a 1% contamination of hydrogen (by
number), and a metallicity 0.4 dex subsolar. Its distance and position are
consistent with membership of the Galactic bulge. Its sharp absorption lines
place strong constraints on both the rotation and microturbulent velocities.
Spectroscopically, J1845-4138 closely resembles the pulsating EHe star V652
Her, generally considered to be the product of a double helium white dwarf
merger evolving to become a helium-rich sdO star. | astro-ph_SR |
Driving Turbulence and Triggering Star Formation by Ionizing Radiation: We present high resolution simulations on the impact of ionizing radiation of
massive O-stars on the surrounding turbulent interstellar medium (ISM). The
simulations are performed with the newly developed software iVINE which
combines ionization with smoothed particle hydrodynamics (SPH) and
gravitational forces. We show that radiation from hot stars penetrates the ISM,
efficiently heats cold low density gas and amplifies over-densities seeded by
the initial turbulence. The formation of observed pillar-like structures in
star forming regions (e.g. in M16) can be explained by this scenario. At the
tip of the pillars gravitational collapse can be induced, eventually leading to
the formation of low mass stars. Detailed analysis of the evolution of the
turbulent spectra shows that UV-radiation of O-stars indeed provides an
excellent mechanism to sustain and even drive turbulence in the parental
molecular cloud. | astro-ph_SR |
Active red giants: close binaries versus single rapid rotators: The objective of this work is to determine what fraction of red-giant (RG)
stars shows photometric rotational modulation, and understand its origin. One
of the underlying questions is the role of close binarity in this population,
standing upon the fact that RGs in short-period binary systems (<150 days or
so) have been observed to display strong rotational modulation. We select a
sample of about 4500 relatively bright RGs observed by Kepler, and show that
370 of them (8%) display rotational modulation. Almost all have oscillation
amplitudes below the median of the sample, while 30 of them are not oscillating
at all. Of the 85 of these RGs with rotational modulation chosen for follow-up
radial-velocity observation and analysis, 34 show clear evidence of
spectroscopic binarity. Surprisingly, 26 of the 30 non-oscillators are in this
group of binaries. To the contrary, about 85% of the active RGs with detectable
oscillations are not part of close binaries. With the help of stellar masses
and evolutionary states computed from the oscillation properties, it appears
that low-mass red-giant branch stars tend to be magnetically inactive, while
intermediate-mass ones tend to be highly active. The opposite trends are true
for helium-core burning (red clump) stars, whereby the lower-mass clump stars
are comparatively more active and the higher-mass ones less so. In other words,
we find that low-mass red-giant branch stars gain angular momentum as they
evolve to clump stars, while higher-mass ones lose angular momentum. The trend
observed with low-mass stars leads to possible scenarios of planet engulfment
or other merging events during the shell-burning phase. Regarding
intermediate-mass stars, the rotation periods are long with respect to
theoretical expectations reported in the literature, which reinforces the
existence of an unidentified sink of angular momentum after the main sequence. | astro-ph_SR |
Eccentricity and Spin-Orbit Misalignment in Short-Period Stellar
Binaries as a Signpost of Hidden Tertiary Companions: Eclipsing binaries are observed to have a range of eccentricities and
spin-orbit misalignments (stellar obliquities). Whether such properties are
primordial, or arise from post-formation dynamical interactions remains
uncertain. This paper considers the scenario in which the binary is the inner
component of a hierarchical triple stellar system, and derives the requirements
that the tertiary companion must satisfy in order to raise the eccentricity and
obliquity of the inner binary. Through numerical integrations of the secular
octupole-order equations of motion of stellar triples, coupled with the spin
precession of the oblate primary star due to the torque from the secondary, we
obtain a simple, robust condition for producing spin-orbit misalignment in the
inner binary: In order to excite appreciable obliquity, the precession rate of
the stellar spin axis must be smaller than the orbital precession rate due to
the tertiary companion. This yields quantitative requirements on the mass and
orbit of the tertiary. We also present new analytic expressions for the maximum
eccentricity and range of inclinations allowing eccentricity excitation
(Lidov-Kozai window), for stellar triples with arbitrary masses and including
the non-Keplerian potentials introduced by general relativity, stellar tides
and rotational bulges. The results of this paper can be used to place
constraints on unobserved tertiary companions in binaries that exhibit high
eccentricity and/or spin-orbit misalignment, and will be helpful in guiding
efforts to detect external companions around stellar binaries. As an
application, we consider the eclipsing binary DI Herculis, and identify the
requirements that a tertiary companion must satisfy to produce the observed
spin-orbit misalignment. | astro-ph_SR |
The winds of young Solar-type stars in Coma Berenices and Hercules-Lyra: We present wind models of ten young Solar-type stars in the Hercules-Lyra
association and the Coma Berenices cluster aged around 0.26 Gyr and 0.58 Gyr
respectively. Combined with five previously modelled stars in the Hyades
cluster, aged 0.63 Gyr, we obtain a large atlas of fifteen observationally
based wind models. We find varied geometries, multi-armed structures in the
equatorial plane, and a greater spread in quantities such as the angular
momentum loss. In our models we infer variation of a factor of ~6 in wind
angular momentum loss $\dot J$ and a factor of ~2 in wind mass loss $\dot M$
based on magnetic field geometry differences when adjusting for the unsigned
surface magnetic flux. We observe a large variation factor of ~4 in wind
pressure for an Earth-like planet; we attribute this to variations in the
'magnetic inclination' of the magnetic dipole axis with respect to the stellar
axis of rotation. Within our models, we observe a tight correlation between
unsigned open magnetic flux and angular momentum loss. To account for possible
underreporting of the observed magnetic field strength we investigate a second
series of wind models where the magnetic field has been scaled by a factor of
5. This gives $\dot M \propto B^{0.4}$ and $\dot J \propto B^{1.0}$ as a result
of pure magnetic scaling. | astro-ph_SR |
Episodic Accretion on to Strongly Magnetic Stars: Some accreting neutron stars and young stars show unexplained episodic flares
in the form of quasi-periodic oscillations or recurrent outbursts. In a series
of two papers we present new work on an instability that can lead to episodic
outbursts when the accretion disc is truncated by the star's strong magnetic
field close to the corotation radius (where the Keplerian frequency matches the
star's rotational frequency). In this paper we outline the physics of the
instability and use a simple parameterization of the disc-field interaction to
explore the instability numerically, which we show can lead to repeated bursts
of accretion as well as steady-state solutions, as first suggested by Sunyaev
and Shakura. The cycle time of these bursts increases with decreasing accretion
rate. These solutions show that the usually assumed `propeller' state, in which
mass is ejected from the system, does not need to occur even at very low
accretion rates. | astro-ph_SR |
Two-dimensional simulations of internal gravity waves in a 5 $M_{\odot}$
Zero-Age-Main-Sequence model: Main-sequence intermediate-mass stars present a radiative envelope that
supports internal gravity waves (IGWs). Excited at the boundary with the
convective core, IGWs propagate towards the stellar surface and are suspected
to impact physical processes such as rotation and chemical mixing. Using the
fully compressible time-implicit code MUSIC, we study IGWs in two-dimensional
simulations of a zero-age-main-sequence 5 solar mass star model up to 91\% of
the stellar radius with different luminosity and radiative diffusivity
enhancements. Our results show that low frequency waves excited by core
convection are strongly impacted by radiative effects as they propagate. This
impact depends on the radial profile of radiative diffusivity which increases
by almost 5 orders of magnitude between the centre of the star and the top of
the simulation domain. In the upper layers of the simulation domain, we observe
an increase of the temperature. Our study suggests that this is due to heat
added in these layers by IGWs damped by radiative diffusion. We show that
non-linear effects linked to large amplitude IGWs may be relevant just above
the convective core. Both these effects are intensified by the artificial
enhancement of the luminosity and radiative diffusivity, with enhancement
factors up to $10^4$ times the realistic values. Our results also highlight
that direct comparison between numerical simulations with enhanced luminosity
and observations must be made with caution. Finally, our work suggests that
thermal effects linked to the damping of IGWs could have a non-negligible
impact on stellar structure. | astro-ph_SR |
Mode identification in three pulsating hot subdwarfs observed with TESS
satellite: We report on the detection of pulsations of three pulsating subdwarf B stars
observed by the TESS satellite and our results of mode identification in these
stars based on an asymptotic period relation. SB 459 (TIC 067584818), SB 815
(TIC 169285097) and PG 0342+026 (TIC 457168745) have been monitored during
single sectors resulting in 27 days coverage. These datasets allowed for
detecting, in each star, a few tens of frequencies, which we interpreted as
stellar oscillations. We found no multiplets, though we partially constrained
mode geometry by means of period spacing, which recently became a key tool in
analyses of pulsating subdwarf B stars. Standard routine that we have used
allowed us to select candidates for trapped modes that surely bear signatures
of non-uniform chemical profile inside the stars. We have also done statistical
analysis using collected spectroscopic and asteroseismic data of previously
known subdwarf B stars along with our three stars. Making use of high precision
trigonometric parallaxes from the Gaia mission and spectral energy
distributions we converted atmospheric parameters to stellar ones. Radii,
masses and luminosities are close to their canonical values for extreme
horizontal branch stars. In particular, the stellar masses are close to the
canonical one of 0.47 M$_\odot$ for all three stars but uncertainties on the
mass are large. The results of the analyses presented here will provide
important constrains for asteroseismic modelling. | astro-ph_SR |
The Vertical Current Approximation Nonlinear Force-Free Field Code -
Description, Performance Tests, and Measurements of Magnetic Energies
Dissipated in Solar Flares: In this work we provide an updated description of the Vertical Current
Approximation Nonlinear Force-Free Field (VCA-NLFFF) code, which is designed to
measure the evolution of the potential, nonpotential, free energies, and the
dissipated magnetic energies during solar flares. This code provides a
complementary and alternative method to existing traditional NLFFF codes. The
chief advantages of the VCA-NLFFF code over traditional NLFFF codes are the
circumvention of the unrealistic assumption of a force-free photosphere in the
magnetic field extrapolation method, the capability to minimize the
misalignment angles between observed coronal loops (or chromospheric fibril
structures) and theoretical model field lines, as well as computational speed.
In performance tests of the VCA-NLFFF code, by comparing with the NLFFF code of
Wiegelmann (2004), we find agreement in the potential, nonpotential, and free
energy within a factor of about 1.3, but the Wiegelmann code yields in the
average a factor of 2 lower flare energies. The VCA-NLFFF code is found to
detect decreases in flare energies in most X, M, and C-class flares. The
successful detection of energy decreases during a variety of flares with the
VCA-NLFFF code indicates that current-driven twisting and untwisting of the
magnetic field is an adequate model to quantify the storage of magnetic
energies in active regions and their dissipation during flares. - The VCA-NLFFF
code is also publicly available in the Solar SoftWare (SSW). | astro-ph_SR |
Precise mass measurements for the double neutron star system J1829+2456: PSR J1829+2456 is a radio pulsar in a relativistic binary system with another
neutron star. It has a rotational period of 41 ms and a mildly eccentric ($e =
0.14$) 28-hr orbit. We have continued its observations with the Arecibo radio
telescope and have now measured the individual neutron star masses of this
system. The pulsar and companion masses are $1.306\,\pm\,0.007\,M_{\odot}$ and
$1.299\,\pm\,0.007\,M_{\odot}$ (2$\sigma$ - 95% confidence, unless stated
otherwise), respectively. We have also measured the proper motion for this
system and used it to estimate a space velocity of 49$^{+77}_{-30}$ km s$^{-1}$
with respect to the local standard of rest. The relatively low values for
companion mass, space velocity and orbital eccentricity in this system make it
similar to other double neutron star systems in which the second-formed neutron
star is thought to have formed in a low-kick, low mass-loss, symmetric
supernova. | astro-ph_SR |
Imaging Discovery of the Debris Disk Around HIP 79977: We present Subaru/HiCIAO H-band high-contrast images of the debris disk
around HIP 79977, whose pres- ence was recently inferred from an infrared
excess. Our images resolve the disk for the first time, allowing
characterization of its shape, size, and dust grain properties. We use angular
differential imaging (ADI) to reveal the disk geometry in unpolarized light out
to a radius of ~2", as well as polarized differential imaging (PDI) to measure
the degree of scattering polarization out to ~1.5". In order to strike a
favorable balance between suppression of the stellar halo and conservation of
disk flux, we explore the application of principal component analysis (PCA) to
both ADI and reference star subtraction. This allows accurate forward modeling
of the effects of data reduction on simulated disk images, and thus direct
comparison with the imaged disk. The resulting best-fit values and well-fitting
intervals for the model parameters are a surface brightness power-law slope of
S_out = -3.2 [-3.6,-2.9], an inclination of i = 84{\deg} [81{\deg},86{\deg}], a
high Henyey-Greenstein forward-scattering parameter of g = 0.45 [0.35, 0.60],
and a non-significant disk-star offset of u = 3.0 [-1.5, 7.5] AU = 24 [-13, 61]
mas along the line of nodes. Furthermore, the tangential linear polarization
along the disk rises from ~10% at 0.5" to ~45% at 1.5". These measurements
paint a consistent picture of a disk of dust grains produced by collisional
cascades and blown out to larger radii by stellar radiation pressure. | astro-ph_SR |
The Mass-Loss Induced Eccentric Kozai Mechanism: A New Channel for the
Production of Close Compact Object-Stellar Binaries: Over a broad range of initial inclinations and eccentricities an appreciable
fraction of hierarchical triple star systems with similar masses are
essentially unaffected by the Kozai-Lidov mechanism (KM) until the primary in
the central binary evolves into a compact object. Once it does, it may be much
less massive than the other components in the ternary, enabling the "eccentric
Kozai mechanism (EKM):" the mutual inclination between the inner and outer
binary can flip signs driving the inner binary to very high eccentricity,
leading to a close binary or collision. We demonstrate this "Mass-loss Induced
Eccentric Kozai" (MIEK) mechanism by considering an example system and defining
an ad-hoc minimal separation between the inner two members at which tidal
affects become important. For fixed initial masses and semi-major axes, but
uniform distributions of eccentricity and cosine of the mutual inclination,
~10% of systems interact tidally or collide while the primary is on the MS due
to the KM or EKM. Those affected by the EKM are not captured by earlier
quadrupole-order secular calculations. We show that fully ~30% of systems
interact tidally or collide for the first time as the primary swells to AU
scales, mostly as a result of the KM. Finally, ~2% of systems interact tidally
or collide for the first time after the primary sheds most of its mass and
becomes a WD, mostly as a result of the MIEK mechanism. These findings motivate
a more detailed study of mass-loss in triple systems and the formation of close
NS/WD-MS and NS/WD-NS/WD binaries without an initial common envelope phase. | astro-ph_SR |
Using Machine Learning Methods to Forecast If Solar Flares Will Be
Associated with CMEs and SEPs: Among the eruptive activity phenomena observed on the Sun, the most
technology threatening ones are flares with associated coronal mass ejections
(CMEs) and solar energetic particles (SEPs). Flares with associated CMEs and
SEPs are produced by magnetohydrodynamical processes in magnetically active
regions (ARs) on the Sun. However, these ARs do not only produce flares with
associated CMEs and SEPs, they also lead to flares and CMEs, which are not
associated with any other event. In an attempt to distinguish flares with
associated CMEs and SEPs from flares and CMEs, which are unassociated with any
other event, we investigate the performances of two machine learning
algorithms. To achieve this objective, we employ support vector machines (SVMs)
and multilayer perceptrons (MLPs) using data from the Space Weather Database of
Notification, Knowledge, Information (DONKI) of NASA Space Weather Center, {\it
the Geostationary Operational Environmental Satellite} ({\it GOES}), and the
Space-Weather Heliospheric and Magnetic Imager Active Region Patches (SHARPs).
We show that True Skill Statistics (TSS) and Heidke Skill Scores (HSS)
calculated for SVMs are slightly better than those from the MLPs. We also show
that the forecasting time frame of 96 hours provides the best results in
predicting if a flare will be associated with CMEs and SEPs (TSS=0.92$\pm$0.09
and HSS=0.92$\pm$0.08). Additionally, we obtain the maximum TSS and HSS values
of 0.91$\pm$0.06 for predicting that a flare will not be associated with CMEs
and SEPs for the 36 hour forecast window, while the 108 hour forecast window
give the maximum TSS and HSS values for predicting CMEs will not be
accompanying any events (TSS=HSS=0.98$\pm$0.02). | astro-ph_SR |
Magnetospherically driven optical and radio aurorae at the end of the
stellar main sequence: Aurorae are detected from all the magnetized planets in our Solar System,
including Earth. They are powered by magnetospheric current systems that lead
to the precipitation of energetic electrons into the high-latitude regions of
the upper atmosphere. In the case of the gas-giant planets, these aurorae
include highly polarized radio emission at kilohertz and megahertz frequencies
produced by the precipitating electrons, as well as continuum and line emission
in the infrared, optical, ultraviolet and X-ray parts of the spectrum,
associated with the collisional excitation and heating of the
hydrogen-dominated atmosphere. Here we report simultaneous radio and optical
spectroscopic observations of an object at the end of the stellar main
sequence, located right at the boundary between stars and brown dwarfs, from
which we have detected radio and optical auroral emissions both powered by
magnetospheric currents. Whereas the magnetic activity of stars like our Sun is
powered by processes that occur in their lower atmospheres, these aurorae are
powered by processes originating much further out in the magnetosphere of the
dwarf star that couple energy into the lower atmosphere. The dissipated power
is at least four orders of magnitude larger than what is produced in the Jovian
magnetosphere, revealing aurorae to be a potentially ubiquitous signature of
large-scale magnetospheres that can scale to luminosities far greater than
those observed in our Solar System. These magnetospheric current systems may
also play a part in powering some of the weather phenomena reported on brown
dwarfs. | astro-ph_SR |
Detailed computation of hot-plasma atomic spectra: We present recent evolutions of the detailed opacity code SCO-RCG which
combines statistical modelings of levels and lines with fine-structure
calculations. The code now includes the Partially-Resolved-Transition-Array
model, which allows one to replace a complex transition array by a small-scale
detailed calculation preserving energy and variance of the genuine transition
array and yielding improved high-order moments. An approximate method for
studying the impact of strong magnetic field on opacity and emissivity was also
recently implemented. The Zeeman line profile is modeled by fourth-order
Gram-Charlier expansion series, which is a Gaussian multiplied by a linear
combination of Hermite polynomials. Electron collisional line broadening is
often modeled by a Lorentzian function and one has to calculate the convolution
of a Lorentzian with Gram-Charlier distribution for a huge number of spectral
lines. Since the numerical cost of the direct convolution would be prohibitive,
we propose, in order to obtain the resulting profile, a fast and precise
algorithm, relying on a representation of the Gaussian by cubic splines. | astro-ph_SR |
Observations of magnetic reconnection and particle acceleration
locations in solar coronal jets: We present a multi-wavelength analysis of two flare-related jets on November
13, 2014, using data from SDO/AIA, RHESSI, Hinode/XRT, and IRIS. Unlike most
coronal jets where hard X-ray (HXR) emissions are usually observed near the jet
base, in these events HXR emissions are found at several locations, including
in the corona. We carry out the first differential emission measure (DEM)
analysis that combines both AIA (and XRT when available) bandpass filter data
and RHESSI HXR measurements for coronal jets, and obtain self-consistent
results across a wide temperature range and into non-thermal energies. In both
events, hot plasma first appeared at the jet base, but as the base plasma
gradually cooled, hot plasma also appeared near the jet top. Moreover,
non-thermal electrons, while only mildly energetic, are found in multiple HXR
locations and contain a large amount of total energy. Particularly, the
energetic electrons that produced the HXR sources at the jet top were
accelerated near the top location, rather than traveling from a reconnection
site at the jet base. This means that there was more than one particle
acceleration site in each event. Jet velocities are consistent with previous
studies, including upward and downward velocities around ~200 km/s and ~100
km/s respectively, and fast outflows of 400-700 km/s. We also examine the
energy partition in the later event, and find that the non-thermal energy in
accelerated electrons is most significant compared to other energy forms
considered. We discuss the interpretations and provide constraints on
mechanisms for coronal jet formation. | astro-ph_SR |
Hubble Space Telescope Snapshot Survey for Resolved Companions of
Galactic Cepheids: Final Results: Cepheids in multiple systems provide information on the outcome of the
formation of massive stars. They can also lead to exotic end-stage objects.
This study concludes our survey of 70 galactic Cepheids using the {\it Hubble
Space Telescope\} (\HST) Wide Field Camera~3 (WFC3) with images at two
wavelengths to identify companions closer than $5\arcsec$. In the entire WFC3
survey we identify 16 probable companions for 13 Cepheids. The seven Cepheids
having resolved candidate companions within $2"$ all have the surprising
property of themselves being spectroscopic binaries (as compared with a 29\%
incidence of spectroscopic binaries in the general Cepheid population). That is
a strong suggestion that an inner binary is linked to the scenario of a third
companion within a few hundred~AU\null. This characteristic is continued for
more widely separated companions. Under a model where the outer companion is
formed first, it is unlikely that it can anticipate a subsequent inner binary.
Rather it is more likely that a triple system has undergone dynamical
interaction, resulting in one star moving outward to its current location. {\it
Chandra\} and {\it Gaia\} data as well as radial velocities and \HSTSTIS and
{\it IUE\} spectra are used to derive properties of the components of the
Cepheid systems.
The colors of the companion candidates show a change in distribution at
approximately 2000~AU separations, from a range including both hot and cool
colors for closer companions, to only low-mass companions for wider
separations. | astro-ph_SR |
Probing X-ray photoevaporative winds through their interaction with
ionising radiation in cluster environments: the case for X-ray proplyds: We show that if young low mass stars undergo vigorous X-ray driven disc
winds, these may be detected in clusters through their interaction with
ionising radiation from massive stars. We argue that in the ONC (Orion Nebula
Cluster) one should see $\sim$ 10s of `X-ray proplyds' ( objects with optically
imaged offset ionisation fronts) in the range $0.3-0.6$pc from $\theta_1$C Ori
(the dominant O star in the ONC). Such objects lie outside the central `FUV
zone' in the ONC where proplyds are instead well explained by neutral winds
driven by external Far Ultraviolet (FUV) emission from $\theta_1$C. We show
that the predicted numbers and sizes of X-ray proplyds are compatible with
those observed and that this may also explain at least some of the far flung
proplyds seen in the Carina nebula. We compare the sizes of observed proplyds
outside the FUV region of the ONC with model predictions based on the current
observed X-ray luminosities of these sources ( bearing in mind that the current
size is actually set by the X-ray luminosity a few hundred years ago, $\sim$
the flow time to the ionisation front). We discuss whether variability on this
timescale can plausibly explain proplyd size data on a case by case basis. We
also show that the predicted radio free-free emission signature of X-ray
proplyds is readily detectable. Monitoring is however required to distinguish
such emission from non-thermal emission from active coronae. We also predict
that it is only at $> 1$ pc from $\theta_1$C that the free-free emission from
such offset ionised structures would be clearly distinguishable from an
externally driven ionised disc wind. We argue that the fortuitous proximity of
massive stars in the ONC can be used as a beacon to light up internally driven
X-ray winds and that this is a promising avenue for observationally testing
X-ray photoevaporation. | astro-ph_SR |
Spicule Dynamics over Plage Region: We studied spicular jets over a plage area and derived their dynamic
characteristics using Hinode Solar Optical Telescope (SOT) high-resolution
images. The target plage region was near the west limb of the solar disk. This
location permitted us to study the dynamics of spicular jets without the
overlapping effect of spicular structures along the line of sight.
In this work, to increase the ease with which we can identify spicules on the
disk, we applied the image processing method `MadMax' developed by Koutchmy et
al. (1989). It enhances fine, slender structures (like jets), over a diffuse
background. We identified 169 spicules over the target plage. This sample
permits us to derive statistically reliable results regarding spicular
dynamics.
The properties of plage spicules can be summarized as follows: (1) In a plage
area, we clearly identified spicular jet features. (2) They were shorter in
length than the quiet region limb spicules, and followed ballistic motion under
constant deceleration. (3) The majority (80%) of the plage spicules showed the
cycle of rise and retreat, while 10% of them faded out without a complete
retreat phase. (4) The deceleration of the spicule was proportional to the
velocity of ejection (i.e. the initial velocity). | astro-ph_SR |
Resistive magnetohydrodynamic simulations of the ideal tearing mode: We study the linear and nonlinear evolution of the tearing instability on
thin current sheets by means of two-dimensional numerical simulations, within
the framework of compressible, resistive magnetohydrodynamics. In particular we
analyze the behavior of current sheets whose inverse aspect ratio scales with
the Lundquist number $S$ as $S^{-1/3}$. This scaling has been recently
recognized to yield the threshold separating fast, ideal reconnection, with an
evolution and growth which are independent of $S$ provided this is high enough,
as it should be natural having the ideal case as a limit for $S\to\infty$. Our
simulations confirm that the tearing instability growth rate can be as fast as
$\gamma\approx 0.6\,{\tau_A}^{-1}$, where $\tau_A$ is the ideal Alfv\'enic time
set by the macroscopic scales, for our least diffusive case with $S=10^7$. The
expected instability dispersion relation and eigenmodes are also retrieved in
the linear regime, for the values of $S$ explored here. Moreover, in the
nonlinear stage of the simulations we observe secondary events obeying the same
critical scaling with $S$, here calculated on the \emph{local}, much smaller
lengths, leading to increasingly faster reconnection. These findings strongly
support the idea that in a fully dynamic regime, as soon as current sheets
develop, thin and reach this critical threshold in their aspect ratio, the
tearing mode is able to trigger plasmoid formation and reconnection on the
local (ideal) Alfv\'enic timescales, as required to explain the explosive
flaring activity often observed in solar and astrophysical plasmas. | astro-ph_SR |
Signature of mass supply to quiet coronal loops: Aims. The physical implication of large blue shift of Ne viii in the quiet
Sun region is investigated in this paper. Methods. We compare the significant
Ne viii blue shifts, which are visible as large blue patches on the
Doppler-shift map of a middlelatitude quiet-Sun region observed by SUMER, with
the coronal magnetic-field structures as reconstructed from a simultaneous
photospheric magnetogram by means of a force-free-field extrapolation. Results.
We show for the first time that coronal funnels also exist in the quiet Sun.
The region studied contains several small funnels that originate from network
lanes, expand with height and finally merge into a single wide open-field
region. However, the large blue shifts of the Ne viii line are not generally
associated with funnels. A comparison between the projections of coronal loops
onto the solar x-y-plane and the Ne viii dopplergram indicates that there are
some loops that reveal large Ne viii blue shifts in both legs, and some loops
with upflow in one and downflow in the other leg. Conclusions. Our results
suggest that strong plasma outflow, which can be traced by large Ne viii blue
shift, is not necessarily associated with the solar wind originating in coronal
funnels but appears to be a signature of mass supply to coronal loops. Under
the assumption that the measured Doppler shift of the Ne viii line represents
the real outflow velocity of the neon ions being markers of the proton flow, we
estimate the mass supply rate to coronal loops to be about 10\^{34} s\^{-1}. | astro-ph_SR |
The first photometric investigation of the contact binary OQ Cam: The first charge-coupled device photometric light curves in B, V, R${_c}$,
and I${_c}$ bands of the short-period contact binary star OQ Cam are presented.
Through analyzing the light curves with the Wilson-Devinney code, it is found
that OQ Cam is a W-type shallow contact binary star with a contact degree of f
=20.2%. Based on the O-C analyzing, the orbit period displays a long term
increasing with a rate of $dP/dt=4.40\times10{^{-7}} day \cdot year{^{-1}}$.
The increasing in orbit period can be interpreted by mass transfer from the
less massive component to the more massive one. As the orbit period increasing,
OQ Cam would evolve from the present shallow contact configuration to a none
contact stage. So it may be a potential candidate to confirm the thermal
relaxation oscillation theory. | astro-ph_SR |
A Low-Mass Pre-Main-Sequence Eclipsing Binary in Lower Centaurus Crux
Discovered with TESS: We report the discovery of 2M1222-57 as a low-mass, pre-main-sequence (PMS)
eclipsing binary (EB) in the Lower Centaurus Crux (LCC) association for which,
using Gaia parallaxes and proper motions with a neural-net age estimator, we
determine an age of 16.2$\pm$2.2 Myr. The broadband spectral energy
distribution (SED) shows clear excess at ~10 um indicative of a circumbinary
disk, and new speckle-imaging observations reveal a faint, tertiary companion
separated by ~100 AU. H-alpha emission is modulated on the orbital period,
consistent with theoretical models of orbitally pulsed accretion streams
reaching from the inner disk edge to the central stars. From a joint analysis
of spectroscopically determined radial velocities and TESS light curves,
together with additional tight constraints provided by the SED and the Gaia
parallax, we measure masses for the eclipsing stars of 0.74 Msun and 0.67 Msun;
radii of 0.98 Rsun and 0.94 Rsun; and effective temperatures of 3750 K and 3645
K. The masses and radii of both stars are measured to an accuracy of ~1%. The
measured radii are inflated, and the temperatures suppressed, relative to
predictions of standard PMS evolutionary models at the age of LCC; also, the Li
abundances are ~2 dex less depleted than predicted by those models. However,
models that account for the global and internal effects of surface magnetic
fields are able to simultaneously reproduce the measured radii, temperatures,
and Li abundances at an age of 17.0$\pm$0.5 Myr. Altogether, the 2M1222-57
system presents very strong evidence that magnetic activity in young stars
alters both their global properties and the physics of their interiors. | astro-ph_SR |
Lagrangian coherent structures in nonlinear dynamos: Turbulence and chaos play a fundamental role in stellar convective zones
through the transportof particles, energy and momentum, and in fast dynamos,
through the stretching, twisting and folding of magnetic flux tubes. A
particularly revealing way to describe turbulent motions is through the
analysis of Lagrangian coherent structures (LCS), which are material lines or
surfaces that act as transport barriers in the fluid. We report the detection
of Lagrangian coherent structures in helical MHD dynamo simulations with scale
separation. In an ABC--flow, two dynamo regimes, a propagating coherent
mean--field regime and an intermittent regime, are identified as the magnetic
diffusivity is varied. The sharp contrast between the chaotic tangle of
attracting and repelling LCS in both regimes permits a unique analysis of the
impact of the magnetic field on the velocity field. In a second example, LCS
reveal the link between the level of chaotic mixing of the velocity field and
the saturation of a large--scale dynamo when the magnetic field exceeds the
equipartition value. | astro-ph_SR |
Discovery of a new photometric sub-class of faint and fast classical
novae: We present photometric and spectroscopic follow-up of a sample of
extragalactic novae discovered by the Palomar 60-inch telescope during a search
for "Fast Transients In Nearest Galaxies" (P60-FasTING). Designed as a fast
cadence (1-day) and deep (g < 21 mag) survey, P60-FasTING was particularly
sensitive to short-lived and faint optical transients. The P60-FasTING nova
sample includes 10 novae in M31, 6 in M81, 3 in M82, 1 in NGC2403 and 1 in
NGC891. This significantly expands the known sample of extragalactic novae
beyond the Local Group, including the first discoveries in a starburst
environment. Surprisingly, our photometry shows that this sample is quite
inconsistent with the canonical Maximum Magnitude Rate of Decline (MMRD)
relation for classical novae. Furthermore, the spectra of the P60-FasTING
sample are indistinguishable from classical novae. We suggest that we have
uncovered a sub-class of faint and fast classical novae in a new phase space in
luminosity-timescale of optical transients. Thus, novae span two orders of
magnitude in both luminosity and time. Perhaps, the MMRD, which is
characterized only by the white dwarf mass, was an over-simplification. Nova
physics appears to be characterized by quite a rich four-dimensional parameter
space in white dwarf mass, temperature, composition and accretion rate. | astro-ph_SR |
Sunspot umbra atmosphere from full Stokes inversion: Sunspots are prominent manifestations of the solar cycle and provide key
constraints for understanding its operation. Also, knowing internal structure
of sunspots allows us to gain insights on the energy transport in strong
magnetic fields and, thus, on the processes inside the convection zone, where
solar magnetic fields are generated and amplified before emerging at the
surface on various scales, even during solar minima. In this paper, we present
results of a spectropolarimetric analysis of a sunspot observed during the
declining phase of the solar cycle 23. By inversion of full Stokes spectra
observed in several spectral regions in the optical at the THEMIS facility we
infer the height dependence of physical quantities such as the temperature and
the magnetic field strength for different sunspot regions. The simultaneous use
of atomic (Fe{\sc i} 5250.2 and 5250.6 \AA) and highly temperature sensitive
molecular (TiO 7055 \AA and MgH 5200 \AA) lines allow us to improve a model of
the sunspot umbra. | astro-ph_SR |
A JWST project on 47 Tucanae. Overview, photometry and early
spectroscopic results of M dwarfs, and observation of brown dwarfs: The James Webb Space Telescope (JWST) observations have been demonstrated to
be efficient in detecting globular clusters' (GCs) multiple stellar populations
in the low mass regime of M dwarfs. We present an overview, and first results,
of different projects that can be explored by using the JWST observations
gathered under the GO2560 for 47 Tucanae, a first program entirely devoted to
the investigation of multiple populations in very low mass stars, which
includes spectroscopic data for the faintest GC stars for which spectra are
available. Our color-magnitude diagram (CMD) shows some substructures for
ultracool stars, including gaps and breaks in slope. In particular, we observe
both a gap and a minimum in the F322W2 luminosity function less than one
magnitude apart, and discuss which one could be associated with the H-burning
limit. We detect stars fainter than this minimum, very likely the brown dwarfs.
We corroborate the ubiquity of the multiple populations across different
masses, from ~0.1 solar masses up to red giants (~0.8 solar masses). The oxygen
range inferred from the M dwarfs, both from the CMD and from the spectra of two
M dwarfs associated with different populations, is similar to that observed in
giants. We have not detected any difference between the fractions of stars in
distinct populations across stellar masses >~0.1 solar masses. This work
demonstrates the JWST's capability in uncovering multiple populations within M
dwarfs and illustrates the possibility to analyse very low-mass stars in GCs
approaching the H-burning limit and the brown-dwarf sequence. | astro-ph_SR |
Using ForeCAT Deflections and Rotations to Constrain the Early Evolution
of CMEs: To accurately predict the space weather effects of coronal mass ejection
(CME) impacts at Earth one must know if and when a CME will impact Earth, and
the CME parameters upon impact. Kay et al. (2015b) presents Forecasting a CME's
Altered Trajectory (ForeCAT), a model for CME deflections based on the magnetic
forces from the background solar magnetic field. Knowing the deflection and
rotation of a CME enables prediction of Earth impacts, and the CME orientation
upon impact. We first reconstruct the positions of the 2008 April 10 and the
2012 July 12 CMEs from the observations. The first of these CMEs exhibits
significant deflection and rotation (34 degrees deflection and 58 degrees
rotation), while the second shows almost no deflection or rotation (<3 degrees
each). Using ForeCAT, we explore a range of initial parameters, such as the CME
location and size, and find parameters that can successfully reproduce the
behavior for each CME. Additionally, since the deflection depends strongly on
the behavior of a CME in the low corona (Kay et al. (2015a, 2015b)), we are
able to constrain the expansion and propagation of these CMEs in the low
corona. | astro-ph_SR |
Solar cycle variations of large frequency separations of acoustic modes:
Implications for asteroseismology: We have studied solar cycle changes in the large frequency separations that
can be observed in Birmingham Solar Oscillations Network (BiSON) data. The
large frequency separation is often one of the first outputs from asteroseismic
studies because it can help constrain stellar properties like mass and radius.
We have used three methods for estimating the large separations: use of
individual p-mode frequencies, computation of the autocorrelation of
frequency-power spectra, and computation of the power spectrum of the power
spectrum. The values of the large separations obtained by the different methods
are offset from each other and have differing sensitivities to the realization
noise. A simple model was used to predict solar cycle variations in the large
separations, indicating that the variations are due to the well-known solar
cycle changes to mode frequency. However, this model is only valid over a
restricted frequency range. We discuss the implications of these results for
asteroseismology. | astro-ph_SR |
Suzaku observation of the transient X-ray pulsar GRO J1008-57: We report the timing and broad-band spectral properties of the Be transient
high mass X-ray binary pulsar GRO J1008-57 using a Suzaku observation in the
declining phase of its 2007 November-December outburst. Pulsations with a
period of 93.737 s were clearly detected in the light curves of the pulsar up
to the 80-100 keV energy band. The pulse profile was found to be strongly
energy dependent, a double peaked profile at soft X-ray energy bands (< 8 keV)
and a single peaked smooth profile at hard X-rays. The broad-band energy
spectrum of the pulsar, reported for the first instance in this paper, is well
described with three different continuum models viz. (i) a high energy cut-off
power-law, (ii) a Negative and Positive power-law with EXponential cut-off
(NPEX), and (iii) a partial covering power-law with high energy cut-off.
Inspite of large value of absorption column density in the direction of the
pulsar, a blackbody component of temperature ~0.17 keV for the soft excess was
required for the first two continuum models. A narrow iron K_\alpha emission
line was detected in the pulsar spectrum. The partial covering model, however,
is found to explain the phase averaged and phase resolved spectra well. The dip
like feature in the pulse profile can be explained by the presence of an
additional absorption component with high column density and covering fraction
at the same pulse phase. The details of the results are described in the paper. | astro-ph_SR |
Twelve years of spectroscopic monitoring in the Galactic Center: the
closest look at S-stars near the black hole: We study the young S-stars within a distance of 0.04 pc from the supermassive
black hole in the center of our Galaxy. Given how inhospitable the region is
for star formation, their presence is more puzzling the younger we estimate
their ages. In this study, we analyse the result of 12 years of high resolution
spectroscopy within the central arcsecond of the Galactic Center (GC). By
co-adding between 55 and 105 hours of spectra we have obtained high signal to
noise H- and K-band spectra of eight stars orbiting the central supermassive
black hole. Using deep H-band spectra, we show that these stars must be high
surface gravity (dwarf) stars. We compare these deep spectra to detailed model
atmospheres and stellar evolution models to infer the stellar parameters. Our
analysis reveals an effective temperature of 21000-28500 K, a rotational
velocity of 60-170 km/s, and a surface gravity of 4.1-4.2. These parameters
imply a spectral type of B0-B3V for these stars. The inferred masses lie within
8-14 Msun. We derive an age of 6.6^{+3.4}{-4.7} Myr for the star S2, which is
compatible with the age of the clockwise rotating young stellar disk in the GC.
We estimate the age of all other studied S-stars to be less than 15 Myr, which
are compatible with the age of S2 within the uncertainties. The relatively low
ages for these S-stars favor a scenario in which the stars formed in a local
disk rather than the field-binary-disruption scenario throughout a longer
period of time. | astro-ph_SR |
Chemical Abundances in a Sample of Red Giants in the Open Cluster NGC
2420 from APOGEE: NGC 2420 is a $\sim$2 Gyr-old well-populated open cluster that lies about 2
kpc beyond the solar circle, in the general direction of the Galactic
anti-center. Most previous abundance studies have found this cluster to be
mildly metal-poor, but with a large scatter in the obtained metallicities for
this open cluster. Detailed chemical abundance distributions are derived for 12
red-giant members of NGC 2420 via a manual abundance analysis of
high-resolution (R = 22,500) near-infrared ($\lambda$1.5 - 1.7$\mu$m) spectra
obtained from the Apache Point Observatory Galactic Evolution Experiment
(APOGEE) survey. The sample analyzed contains 6 stars that are identified as
members of the first-ascent red giant branch (RGB), as well as 6 members of the
red clump (RC). We find small scatter in the star-to-star abundances in NGC
2420, with a mean cluster abundance of [Fe/H] = -0.16 $\pm$ 0.04 for the 12 red
giants. The internal abundance dispersion for all elements (C, N, O, Na, Mg,
Al, Si, K, Ca, Ti, V, Cr, Mn, Co and Ni) is also very small ($\sim$0.03 - 0.06
dex), indicating a uniform cluster abundance distribution within the
uncertainties. NGC 2420 is one of the clusters used to calibrate the APOGEE
Stellar Parameter and Chemical Abundance Pipeline (ASPCAP). The results from
this manual analysis compare well with ASPCAP abundances for most of the
elements studied, although for Na, Al and V there are more significant offsets.
No evidence of extra-mixing at the RGB luminosity bump is found in the $^{12}$C
and $^{14}$N abundances from the pre-luminosity-bump RGB stars in comparison to
the post-He core-flash RC stars. | astro-ph_SR |
The 26 December 2001 Solar Eruptive Event Responsible for GLE63. II.
Multi-Loop Structure of Microwave Sources in a Major Long-Duration flare: Analysis of the observations of the SOL2001-12-26 event related to
ground-level-event GLE63, including microwave spectra and images from NoRH at
17 and 34 GHz, SSRT at 5.7 GHz, and TRACE in 1600 \AA\ has led to the following
results. A flare ribbon overlapped with the sunspot umbra, which is typical of
large particle events. Atypical were: i) long duration of the flare of more
than one hour; ii) moderate intensity of a microwave burst, about $10^4$ sfu;
iii) low peak frequency of the gyrosynchrotron spectrum, around 6 GHz; and its
insensitivity to the flux increase by more than one order of magnitude. This
was accompanied by a nearly constant ratio of the flux emitted by the volume in
the high-frequency part of the spectrum to its elevated low-frequency part
determined by the area of the source. With the self-similarity of the spectrum,
a similarity was observed between the moving microwave sources and the
brightest parts of the flare ribbons in 1600 \AA. Comparison of the 17 GHz and
1600 \AA\ images has confirmed that the microwave sources were associated with
multiple flare loops, whose footpoints appeared in ultraviolet as intermittent
bright kernels. To understand the properties of the event, we simulated its
microwave emission using a system of several homogeneous gyrosynchrotron
sources above the ribbons. The scatter between the spectra and sizes of the
individual sources is determined by the inhomogeneity of the magnetic field
within the ribbons. The microwave flux is mainly governed by the magnetic flux
passing through the ribbons and the sources. An apparent simplicity of
microwave structures is caused by a poorer spatial resolution and dynamic range
of the microwave imaging. The results indicate that microwave manifestations of
accelerated electrons correspond to the structures observed in thermal
emissions, as well-known models predict. | astro-ph_SR |
NSE abundance data: Novel method of calculating Nuclear Statistical Equilibrium is presented.
Basic equations are carefully solved using arbitrary precision arithmetic.
Special interpolation procedure is then used to retrieve all abundances using
tabulated results for neutrons and protons, together with basic nuclear data.
Proton and neutron abundance tables, basic nuclear data and partition functions
for nuclides used in calculations are provided. Simple interpolation algorithm
using pre-calculated p and n abundances tabulated as a functions of kT, rho and
Ye is outlined. Unique properties of this method are: (1) ability to pick-up
out of NSE selected nuclei only (2) computational time scaling linearly with
number of re-calculated abundances (3) relatively small amount of stored data:
only two large tables (4) slightly faster than solving NSE equations using
traditional Newton-Raphson methods for small networks (few tens of species);
superior for huge (800-3000) networks (5) do not require initial guess; works
well on random input (6) can tailored to specific application (7) ability to
use third-party NSE solvers to obtain fully compatible tables (8) encapsulation
of the NSE code for bug-free calculations.
Range of applications for this approach is possible: coverage test of
traditional NSE Newton-Raphson codes, generating starting values, code-to-code
verification and possible replacement of the old legacy procedures in supernova
simulations. | astro-ph_SR |
Identification of red supergiants in nearby galaxies with mid-IR
photometry: The role of episodic mass loss in massive star evolution is one of the most
important open questions of current stellar evolution theory. Episodic mass
loss produces dust and therefore causes evolved massive stars to be very
luminous in the mid-infrared and dim at optical wavelengths. We aim to increase
the number of investigated luminous mid-IR sources to shed light on the late
stages of these objects. To achieve this we employed mid-IR selection criteria
to identity dusty evolved massive stars in two nearby galaxies. The method is
based on mid-IR colors, using 3.6 {\mu}m and 4.5 {\mu}m photometry from
archival Spitzer Space Telescope images of nearby galaxies and J-band
photometry from 2MASS. We applied our criteria to two nearby star-forming dwarf
irregular galaxies, Sextans A and IC 1613, selecting eight targets, which we
followed up with spectroscopy. Our spectral classification and analysis yielded
the discovery of two M-type supergiants in IC 1613, three K-type supergiants
and one candidate F-type giant in Sextans A, and two foreground M giants. We
show that the proposed criteria provide an independent way for identifying
dusty evolved massive stars, that can be extended to all nearby galaxies with
available Spitzer/IRAC images at 3.6 {\mu}m and 4.5 {\mu}m. | astro-ph_SR |
Fine structures of radio bursts from flare star AD Leo with FAST
observations: Radio bursts from nearby active M-dwarfs have been frequently reported and
extensively studied in solar or planetary paradigms. Whereas, their
sub-structures or fine structures remain rarely explored despite their
potential significance in diagnosing the plasma and magnetic field properties
of the star. Such studies in the past have been limited by the sensitivity of
radio telescopes. Here we report the inspiring results from the high
time-resolution observations of a known flare star AD Leo with the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). We detected many
radio bursts in the two days of observations with fine structures in the form
of numerous millisecond-scale sub-bursts. Sub-bursts on the first day display
stripe-like shapes with nearly uniform frequency drift rates, which are
possibly stellar analogs to Jovian S-bursts. Sub-bursts on the second day,
however, reveal a different blob-like shape with random occurrence patterns and
are akin to solar radio spikes. The new observational results suggest that the
intense emission from AD Leo is driven by electron cyclotron maser instability
which may be related to stellar flares or interactions with a planetary
companion. | astro-ph_SR |
Alfvén wave experiments with liquid rubidium in a pulsed magnetic
field: Magnetic fields are key ingredients for heating the solar corona to
temperatures of several million Kelvin. A particularly important region with
respect to this is the so-called magnetic canopy below the corona, where sound
and Alfv\'en waves have roughly the same speed and can, therefore, easily
transform into each other. We present the results of an Alfv\'en-wave
experiment with liquid rubidium carried out in a pulsed field of up to 63 T. At
the critical point of 54 T, where the speeds of Alfv\'en waves and sound
coincide, a new 4 kHz signal appears in addition to the externally excited 8
kHz torsional wave. This emergence of an Alfv\'en wave with a doubled period is
in agreement with the theoretical predictions of a parametric resonance between
the two wave types. We also present preliminary results from numerical
simulations of Alfv\'en and magneto-sonic waves using a compressible MHD code. | astro-ph_SR |
Empirical 2MASS-WFC3/IR filter transformations from synthetic photometry: Near-infrared bandpasses on spaceborne observatories diverge from their
ground-based counterparts as they are free of atmospheric telluric absorption.
Available transformations between respective filter systems in the literature
rely on theoretical stellar atmospheres, which are known to have difficulties
reproducing observed spectral energy distributions of cool giants. We present
new transformations between the 2MASS $JHK_S$ and HST WFC3/IR F110W, F125W, &
F160W photometric systems based on synthetic photometry of empirical stellar
spectra from four spectral libraries. This sample comprises over 1000
individual stars, which together span nearly the full HR diagram and sample
stellar populations from the solar neighborhood out to the Magellanic Clouds,
covering a broad range of ages, metallicities, and other relevant stellar
properties. In addition to global color-dependent transformations, we examine
band-to-band differences for cool, luminous giant stars in particular,
including multiple types of primary distance indicators. | astro-ph_SR |
Jeans Instability in a viscoelastic fluid: The well known Jeans instability is studied for a viscoelastic, gravitational
fluid using generalized hydrodynamic equations of motions. It is found that the
threshold for the onset of instability appears at higher wavelengths in a
viscoelastic medium. Elastic effects playing a role similar to thermal pressure
are found to lower the growth rate of the gravitational instability. Such
features may manifest themselves in matter constituting dense astrophysical
objects. | astro-ph_SR |
HD 51106 and HD 50747: an ellipsoidal binary and a triple system
observed with CoRoT: We present an analysis of the observations of HD 51106 and HD 50747 by the
satellite CoRoT, obtained during its initial run, and of the spectroscopic
preparatory observations.
AIMS: We complete an analysis of the light curve, extract the main
frequencies observed, and discuss some preliminary interpretations about the
stars.
Methods: We used standard Fourier transform and pre-whitening methods to
extract information about the periodicities of the stars.
Results: HD 51106 is an ellipsoidal binary, the light curve of which can be
completely explained by the tidal deformation of the star and smaller secondary
effects. HD 50747 is a triple system containing a variable star, which exhibits
many modes of oscillation with periods in the range of a few hours. On the
basis of this period range and the analysis of the physical parameters of the
star, we conclude that HD 50747 is a Gamma-Doradus star. | astro-ph_SR |
Luminosities of Carbon-rich Asymptotic Giant Branch stars in the Milky
Way: Stars evolving along the Asymptotic Giant Branch can become Carbon-rich in
the final part of their evolution. They replenish the inter-stellar medium with
nuclear processed material via strong radiative stellar winds. The
determination of the luminosity function of these stars, even if far from being
conclusive, is extremely important to test the reliability of theoretical
models. In particular, strong constraints on the mixing treatment and the
mass-loss rate can be derived.
We present an updated Luminosity Function of Galactic Carbon Stars obtained
from a re-analysis of available data already published in previous papers.
Starting from available near- and mid-infrared photometric data, we
re-determine the selection criteria. Moreover, we take advantage from updated
distance estimates and Period-Luminosity relations and we adopt a new
formulation for the computation of Bolometric Corrections. This leads us to
collect an improved sample of carbon-rich sources from which we construct an
updated Luminosity Function.
The Luminosity Function of Galactic Carbon Stars peaks at magnitudes around
-4.9, confirming the results obtained in a previous work. Nevertheless, the
Luminosity Function presents two symmetrical tails instead of the larger high
luminosity tail characterizing the former Luminosity Function. The derived
Luminosity Function of Galactic Carbon Stars matches the indications coming
from recent theoretical evolutionary Asymptotic Giant Branch models, thus
confirming the validity of the choices of mixing treatment and mass-loss
history. Moreover, we compare our new Luminosity Function with its counterpart
in the Large Magellanic Cloud finding that the two distributions are very
similar for dust-enshrouded sources, as expected from stellar evolutionary
models. Finally, we derive a new fitting formula aimed to better determine
Bolometric Corrections for C-stars. | astro-ph_SR |
First axion bounds from a pulsating helium-rich white dwarf star: The Peccei-Quinn mechanism proposed to solve the CP problem of Quantum
Chromodynamics has as consequence the existence of axions, hypothetical weakly
interacting particles whose mass is constrained to be on the sub-eV range. If
these particles exist and interact with electrons, they would be emitted from
the dense interior of white dwarfs, becoming an important energy sink for the
star. Due to their well known physics, white dwarfs are good laboratories to
study the properties of fundamental particles such as the axions. We study the
general effect of axion emission on the evolution of helium-rich white dwarfs
and on their pulsational properties. To this aim, we calculate evolutionary
helium-rich white dwarf models with axion emission, and asses the pulsational
properties of this models. Our results indicate that the rates of change of
pulsation periods are significantly affected by the existence of axions. We are
able for the first time to independently constrain the mass of the axion from
the study of pulsating helium-rich white dwarfs. To do this, we use an
estimation of the rate of change of period of the pulsating white dwarf PG
1351+489 corresponding to the dominant pulsation period. From an
asteroseismological model of PG 1351+489 we obtain $g_{ae}<3.3\times10^{-13}$
for the axion-electron coupling constant, or $m_a\cos^2{\beta}\lesssim$ 11.5
meV for the axion mass. This constraint is relaxed to
$g_{ae}<5.5\times10^{-13}$ ($m_a\cos^2{\beta}\lesssim$ 19.5 meV), when no
detailed asteroseismological model is adopted for the comparison with
observations. | astro-ph_SR |
On modeling ICME cross-sections as static MHD columns: Solar coronal mass ejections are well known to expand as they propagate
through the heliosphere. Despite this, their cross-sections are usually modeled
as static plasma columns within the magnetohydrodynamics (MHD) framework. We
test the validity of this approach using in-situ plasma data from 151 magnetic
clouds (MCs) observed by the WIND spacecraft and 45 observed by the Helios
spacecrafts. We find that the most probable cross-section expansion speeds for
the WIND events are only $\approx 0.06$ times the Alfv\'en speed inside the MCs
while the most probable cross-section expansion speeds for the Helios events is
$\approx 0.03$. MC cross-sections can thus be considered to be nearly static
over an Alfv\'en crossing timescale. Using estimates of electrical conductivity
arising from Coulomb collisions, we find that the Lundquist number inside MCs
is high ($\approx 10^{13}$), suggesting that the MHD description is well
justified. The Joule heating rates using our conductivity estimates are several
orders of magnitude lower than the requirement for plasma heating inside MCs
near the Earth. While the (low) heating rates we compute are consistent with
the MHD description, the discrepancy with the heating requirement points to
possible departures from MHD and the need for a better understanding of plasma
heating in MCs. | astro-ph_SR |
An abundance analysis from the STIS-HST UV spectrum of the non-magnetic
Bp star HR 6000: The sharp-line spectrum of the Bp star HR 6000 has peculiarities that
distinguish it from those of the HgMn stars with which it is sometimes
associated. The position of the star close to the center of the Lupus 3
molecular cloud, whose estimated age is on the order of 9.1 +/- 2.1 Myr, has
lead to the hypothesis that the anomalous peculiarities of HR 6000 can be
explained by the young age of the star. Observational material from HST
provides the opportunity to extend the abundance analysis previously performed
for the optical region and clarify the properties of this remarkable peculiar
star. Our aim was to obtain the atmospheric abundances for all the elements
observed in a broad region from 1250 to 10000 A. An LTE synthetic spectrum was
compared with a high-resolution spectrum observed with STIS equipment in the
1250-3040 A interval. The adopted model is an ATLAS12 model already used for
the abundance analysis of a high-resolution optical spectrum observed at ESO
with UVES. The stellar parameters are Teff=13450K, logg=4.3, and zero
microturbulent velocity. Abundances for 28 elements and 7 upper limits were
derived from the ultraviolet spectrum. Adding results from previous work, we
have now quantitative results for 37 elements, some of which show striking
contrasts with those of a broad sample of HgMn stars. The analysis has pointed
out ionization anomalies and line-to-line variation in the derived abundances,
in particular for silicon. The inferred discrepancies could be explained by
non-LTE effects and with the occurrence of diffusion and vertical abundance
stratification. In the framework of the last hypothesis, we obtained, by means
of trial and error, empirical step functions of abundance versus optical depth
log(tau_5000) for carbon, silicon, manganese, and gold, while we failed to find
such a function for phosphorous. | astro-ph_SR |
Observations of Reconnection Flows in a Flare on the Solar Disk: Magnetic reconnection is a well-accepted part of the theory of solar eruptive
events, though the evidence is still circumstantial. Intrinsic to the
reconnection picture of a solar eruptive event, particularly in the standard
model for two-ribbon flares ("CSHKP" model), are an advective flow of
magnetized plasma into the reconnection region, expansion of field above the
reconnection region as a flux rope erupts, retraction of heated
post-reconnection loops, and downflows of cooling plasma along those loops. We
report on a unique set of SDO/AIA imaging and Hinode/EIS spectroscopic
observations of the disk flare SOL2016-03-23T03:54 in which all four flows are
present simultaneously. This includes spectroscopic evidence for a plasma
upflow in association with large-scale expanding closed inflow field. The
reconnection inflows are symmetric, and consistent with fast reconnection, and
the post-reconnection loops show a clear cooling and deceleration as they
retract. Observations of coronal reconnection flows are still rare, and most
events are observed at the solar limb, obscured by complex foregrounds, making
their relationship to the flare ribbons, cusp field and arcades formed in the
lower atmosphere difficult to interpret. The disk location and favorable
perspective of this event have removed these ambiguities giving a clear picture
of the reconnection dynamics. | astro-ph_SR |
Evidence of a Mira-like tail and bow shock about the semi-regular
variable V CVn from four decades of polarization measurements: Polarization is a powerful tool for understanding stellar atmospheres and
circumstellar environments. Mira and semi-regular variable stars have been
observed for decades and some are known to be polarimetrically variable,
however, the semi-regular variable V Canes Venatici displays an unusually
large, unexplained amount of polarization. We present ten years of optical
polarization observations obtained with the HPOL instrument, supplemented by
published observations spanning a total interval of about forty years for V
CVn. We find that V CVn shows large polarization variations ranging from 1 -
6%. We also find that for the past forty years the position angle measured for
V CVn has been virtually constant suggesting a long-term, stable, asymmetric
structure about the star. We suggest that this asymmetry is caused by the
presence of a stellar wind bow shock and tail, consistent with the star's large
space velocity. | astro-ph_SR |
Far Ultraviolet Spectroscopy of Three Long Period Nova-Like Variables: We have selected three nova-like variables at the long period extreme of
nova-like orbital periods: V363 Aur, RZ Gru and AC Cnc, all with IUE archival
far ultraviolet spectra. All are UX UMa type nova-like variables and all have
$P_{orb} > 7$h. V363 Aur is a bona fide SW Sex star, and AC Cnc is a probable
one, while RZ Gru has not proven to be a member of the SW Sex subclass. We have
carried out the first synthetic spectral analysis of far ultraviolet spectra of
the three systems using state-of-the-art models both of accretion disks and
white dwarf photospheres. We find that the FUV spectral energy distribution of
both V363 Aur and RZ Gru are in agreement with optically thick steady state
accretion disk models in which the luminous disk accounts for 100% of the FUV
light. We present accretion rates and model-derived distances for V363 Aur and
RZ Gru. For AC Cnc, we find that a hot accreting white dwarf accounts for
$\sim$ 60% of the FUV light with an accretion disk providing the rest. We
compare our accretion rates and model-derived distances with estimates in the
literature. | astro-ph_SR |
Chemical Composition of the Atmospheres of Red Giants with High Space
Velocities: The results of a comparative analysis of the elemental abundances in the
atmospheres of 14 red giants with high Galactic space velocities are presented.
For almost all of the chemical elements considered, the their abundance trends
with metallicity correspond to those constructed for thick-disk dwarfs. In the
case of sodium, the main factor affecting the [Na/Fe] abundance in the stellar
atmosphere for red giants is the surface gravity that characterizes the degree
of development of the convective envelope. The difference between the [Na/Fe]
abundances in the atmospheres of thin-and thick-disk red giants has been
confirmed. | astro-ph_SR |
MESA Models of the Evolutionary State of the Interacting Binary epsilon
Aurigae: Using MESA code (Modules for Experiments in Stellar Astrophysics, version
9575), an evaluation was made of the evolutionary state of the epsilon Aurigae
binary system (HD 31964, F0Iap + disk). We sought to satisfy several
observational constraints: 1) requiring evolutionary tracks to pass close to
the current temperature and luminosity of the primary star; 2) obtaining a
period near the observed value of 27.1 years; 3) matching a mass function of
3.0; 4) concurrent Roche lobe overflow and mass transfer; 5) an isotopic ratio
12 C/ 13 C = 5 and, (6) matching the interferometrically determined angular
diameter. A MESA model starting with binary masses of 9.85 + 4.5 Msun , with a
100 day initial period, produces a 1.2 + 10.6 Msun result having a 547 day
period, and a single digit 12 C/ 13 C ratio. These values were reached near an
age of 20 Myr, when the donor star comes close to the observed luminosity and
temperature for epsilon Aurigae A, as a post-RGB/pre-AGB star.
Contemporaneously, the accretor then appears as an upper main sequence, early
B-type star. This benchmark model can provide a basis for further exploration
of this interacting binary, and other long period binary stars. | astro-ph_SR |
IRTF Observations of White Dwarfs with Possible Near-Infrared Excess: Near-infrared photometry and spectroscopy is obtained for a heterogeneous
sample of nearby white dwarfs with possible excess flux as identified primarily
in the Two Micron All Sky Survey. Among the sample of 43 stars are a number of
white dwarfs that are either metal-rich, magnetic, or binary suspects. With a
few notable exceptions in four (or possibly five) distinct categories, the
newly obtained JHK photometric data fail to corroborate the putative excesses,
with K_IRTF - K_2MASS = +0.31 mag. Where available, GALEX photometric data are
used to better constrain the overall spectral energy distribution of the white
dwarfs, enabling any excess near-infrared flux to stand out more readily
against the expected stellar photosphere.
With superior data, a near-infrared photometric excess is confirmed at three
metal-rich white dwarfs and ruled out at nine others. Several new binaries are
confirmed or suggested; five white dwarf - red dwarf pairs and five double
degenerates. Four apparently single magnetic white dwarfs -- two DA and two DQp
-- display modest to strong near-infrared excess (relative to non-magnetic
models), which may be better described as two effective temperatures owing to a
redistribution of energy in highly magnetic or peculiar atmospheres. | astro-ph_SR |
Astro2020 Science White Paper: gravity-wave asteroseismology of
intermediate- and high-mass stars: The evolution of a star is driven by the physical processes in its interior
making the theory of stellar structure and evolution the most crucial
ingredient for not only stellar evolution studies, but any field of astronomy
which relies on the yields along stellar evolution. High-precision time-series
photometric data assembled by recent space missions revealed that current
models of stellar structure and evolution show major shortcomings already in
the two earliest nuclear burning phases, impacting all subsequent phases prior
to the formation of the end-of-life remnant. This white paper focuses
specifically on the transport of chemical elements and of angular momentum in
the stellar structure and evolution models of stars born with convective core,
as revealed by their gravity-mode oscillations. | astro-ph_SR |
Magnetoclinicity Instability: In strongly compressible magnetohydrodynamic turbulence, obliqueness between
the large-scale density gradient and magnetic field gives an electromotive
force mediated by density variance (intensity of density fluctuation). This
effect is named ``magnetoclinicity'', and is expected to play an important role
in large-scale magnetic-field generation in astrophysical compressible
turbulent flows. Analysis of large-scale instability due to the
magnetoclinicity effect shows that the mean magnetic-field perturbation is
destabilised at large scales in the vicinity of strong mean density gradient in
the presence of density variance. | astro-ph_SR |
Early Time Light Curves of Type Ia Supernovae Observed with TESS: We present early time light curves of Type Ia supernovae observed in the
first six sectors of TESS data. Ten of these supernovae were discovered by
ASAS-SN, seven by ATLAS, six by ZTF, and one by \textit{Gaia}. For nine SNe
with sufficient dynamic range ($>$3.0 mag from detection to peak), we fit power
law models and search for signatures of companion stars. We find a diversity of
early time light curve shapes, although most of our sources are consistent with
fireball models where the flux increases $\propto t^2$. Three SN display a
flatter rise with flux $\propto t$. We do not find any evidence for additional
structure such as multiple power law components in the early rising light
curves. For assumptions about the SN properties and the observer viewing angle,
and further assuming that companion stars would be in Roche-lobe overflow, we
place limits on the radii of companions for six SNe with complete coverage of
the early time light curves. The upper limits are $\lesssim$\,32 R$_\odot$ for
these six supernovae, $\lesssim$\,20 R$_\odot$ for five of these six, and
$\lesssim$\,4 R$_\odot$ for two of these six. The small sample size does not
constrain occurrence rates of single degenerate Type Ia SN progenitors, but we
expect that TESS observed enough SNe in its primary mission (26 sectors) to
inform this measurement. We also show that TESS is capable of detecting
emission from a 1 \rsun\ companion for a Type Ia SN within 50 Mpc, and may do
so after about six years. | astro-ph_SR |
Keplerian frequency of uniformly rotating neutron stars and quark stars: We calculate Keplerian (mass shedding) configurations of rigidly rotating
neutron stars and quark stars with crusts. We check the validity of empirical
formula for Keplerian frequency, f_K, proposed by Lattimer & Prakash, f_K(M)=C
(M/M_sun)^1/2 (R/10km)^-3/2, where M is the (gravitational) mass of Keplerian
configuration, R is the (circumferential) radius of the non-rotating
configuration of the same gravitational mass, and C = 1.04 kHz. Numerical
calculations are performed using precise 2-D codes based on the multi-domain
spectral methods. We use a representative set of equations of state (EOSs) of
neutron stars and quark stars. We show that the empirical formula for f_K(M)
holds within a few percent for neutron stars with realistic EOSs, provided 0.5
M_sun < M < 0.9 M_max,stat, where M_max,stat is the maximum allowable mass of
non-rotating neutron stars for an EOS, and C=C_NS=1.08 kHz. Similar precision
is obtained for quark stars with 0.5 M_sun < M < 0.9 M_max,stat. For maximal
crust masses we obtain C_QS = 1.15 kHz, and the value of C_QS is not very
sensitive to the crust mass. All our C's are significantly larger than the
analytic value from the relativistic Roche model, C_Roche = 1.00 kHz. For 0.5
M_sun < M < 0.9 M_max,stat, the equatorial radius of Keplerian configuration of
mass M, R_K(M), is, to a very good approximation, proportional to the radius of
the non-rotating star of the same mass, R_K(M) = aR(M), with a_NS \approx a_QS
\approx 1.44. The value of a_QS is very weakly dependent on the mass of the
crust of the quark star. Both a's are smaller than the analytic value a_Roche =
1.5 from the relativistic Roche model. | astro-ph_SR |
Predicting the Magnetic Fields of a Stealth CME Detected by Parker Solar
Probe at 0.5 AU: Stealth coronal mass ejection (CMEs) are eruptions from the Sun that are not
associated with appreciable low-coronal signatures. Because they often cannot
be linked to a well-defined source region on the Sun, analysis of their initial
magnetic configuration and eruption dynamics is particularly problematic. In
this manuscript, we address this issue by undertaking the first attempt at
predicting the magnetic fields of a stealth CME that erupted in 2020 June from
the Earth-facing Sun. We estimate its source region with the aid of off-limb
observations from a secondary viewpoint and photospheric magnetic field
extrapolations. We then employ the Open Solar Physics Rapid Ensemble
Information (OSPREI) modelling suite to evaluate its early evolution and
forward-model its magnetic fields up to Parker Solar Probe, which detected the
CME in situ at a heliocentric distance of 0.5 AU. We compare our hindcast
prediction with in-situ measurements and a set of flux rope reconstructions,
obtaining encouraging agreement on arrival time, spacecraft crossing location,
and magnetic field profiles. This work represents a first step towards reliable
understanding and forecasting of the magnetic configuration of stealth CMEs and
slow, streamer-blowout events. | astro-ph_SR |
The Rich Circumstellar Chemistry of SMP LMC 11: Carbon-rich evolved stars from the asymptotic giant branch to the planetary
nebula phase are characterized by a rich and complex carbon chemistry in their
circumstellar envelopes. A peculiar object is the preplanetary nebula SMP LMC
11, whose Spitzer-IRS spectrum shows remarkable and diverse molecular
absorption bands. To study how the molecular composition in this object
compares to our current understanding of circumstellar carbon chemistry, we
modeled this molecular absorption. We find high abundances for a number of
molecules, perhaps most notably benzene. We also confirm the presence of
propyne (CH3C2H) in this spectrum. Of all the cyanopolyynes, only HC3N is
evident; we can detect at best a marginal presence of HCN. From comparisons to
various chemical models, we can conclude that SMP LMC 11 must have an unusual
circumstellar environment (a torus rather than an outflow). | astro-ph_SR |
Photometric and spectroscopic study of the intermediate-age open cluster
NGC 2355: In this paper we analyse the evolutionary status and properties of the old
open cluster NGC 2355, located in the Galactic anticentre direction, as a part
of the long term programme BOCCE. NGC 2355 was observed with LBC@LBT using the
Bessel $B$, $V$, and $I_c$ filters. The cluster parameters have been obtained
using the synthetic colour-magnitude diagram (CMD) method, as done in other
papers of this series. Additional spectroscopic observations with FIES@NOT of
three giant stars were used to determine the chemical properties of the
cluster. Our analysis shows that NGC 2355 has metallicity slightly less than
solar, with [Fe/H]$=-0.06$ dex, age between 0.8 and 1 Gyr, reddening $E(B-V)$
in the range 0.14 and 0.19 mag, and distance modulus $(m-M)_0$ of about 11 mag.
We also investigated the abundances of O, Na, Al, $\alpha$, iron-peak, and
neutron capture elements, showing that NGC 2355 falls within the abundance
distribution of similar clusters (same age and metallicity). The Galactocentric
distance of NGC~2355 places it at the border between two regimes of metallicity
distribution; this makes it an important cluster for the study of the chemical
properties and evolution of the disc. | astro-ph_SR |
Spectroscopic Studies of 30 Short-period Cataclysmic Variable Stars, and
Remarks on the Evolution and Population of Similar Objects: We present spectroscopy and orbital periods Porb for 30 apparently
non-magnetic cataclysmic binaries with periods below about 3 hours, nearly all
of which are dwarf novae, mostly of the SU Ursae Majoris subclass. We then turn
to the evidence supporting the prediction that short-period dwarf novae evolve
toward longer periods after passing through a minimum period -- the "period
bounce" phenomenon. Plotting data from the literature reveals that for
superhump period excess $\epsilon = (P_{\rm sh} - P_{\rm orb} )/P_{\rm orb}$
below about 0.015, the period appears to increase with decreasing epsilon,
agreeing at least qualitatively with the predicted behavior. Next, motivated by
the long (decadal) outburst intervals of the WZ Sagittae subclass of
short-period dwarf novae, we ask whether there could be a sizable population of
"lurkers" -- systems that resemble dwarf novae at minimum light, but do not
outburst over accessible timescales (or at all), and therefore do not draw
attention to themselves. By examining the outburst history of the Sloan Digital
Sky Survey sample of CVs, which were selected by color and not by outburst, we
find that a large majority of the color-selected dwarf-nova-like objects have
been observed to outburst, and conclude that "lurkers", if they exist, are a
relatively minor part of the CV population. | astro-ph_SR |
The formation of IRIS diagnostics VIII. IRIS observations in the C II
133.5 nm multiplet: The C II 133.5 nm multiplet has been observed by NASA's Interface Region
Imaging Spectrograph (IRIS) in unprecedented spatial resolution. The aims of
this work are to characterize these new observations of the C II lines, place
them in context with previous work, and to identify any additional value the C
II lines bring when compared with other spectral lines. We make use of wide,
long exposure IRIS rasters covering the quiet Sun and an active region. Line
properties such as velocity shift and width are extracted from individual
spectra and analyzed. The lines have a variety of shapes (mostly single-peak or
double-peak), are strongest in active regions and weaker in the quiet Sun. The
ratio between the 133.4 nm and 133.5 nm components is always less than 1.8,
indicating that their radiation is optically thick in all locations. Maps of
the C II line widths are a powerful new diagnostic of chromospheric structures,
and their line shifts are a robust velocity diagnostic. Compared with earlier
quiet Sun observations, we find similar absolute intensities and mean line
widths, but smaller red shifts; this difference can perhaps be attributed to
differences in spectral resolution and spatial coverage. The C II intensity
maps are somewhat similar to those of transition region lines, but also share
some features with chromospheric maps such as those from the Mg II k line,
indicating that they are formed between the upper chromosphere and transition
region. C II intensity, width, and velocity maps can therefore be used to
gather additional information about the upper chromosphere. | astro-ph_SR |
The orbital and superhump periods of the SU UMa-type dwarf nova V1212
Tauri: We report CCD photometry of the superoutburst of the dwarf nova V1212 Tau
obtained during 2011 January and February. The outburst amplitude was at least
6 magnitudes and it lasted at least 12 days. Three distinct superhump regimes
were observed. Initially low amplitude superhumps (0.03 to 0.05 magnitude
peak-to-peak) with Psh = 0.0782(52) d were present. The superhumps reached a
maximum amplitude of 0.31 magnitudes at the beginning of the plateau phase,
with Psh = 0.07031(96) d. Subsequently the star began to fade slowly. During
the first part of the decline, the period increased with dPsh/dt = +1.62(9) x
10-3 and the amplitude of the superhumps also declined. Mid way through the
slow decline, the superhumps partially regrew and this point coincided with a
change to a new superhump regime during which the period decreased with dPsh/dt
= -1.50(39) x 10-3. We determined the orbital period as Porb = 0.06818(64) d
and the superhump period excess as epsilon = 0.034(15) | astro-ph_SR |
3D Stellar Reddening Map from 2MASS Photometry: An Improved Version: An improved version of the 3D stellar reddening map in a space with a radius
of 1200 pc around the Sun and within 600 pc of the Galactic midplane is
presented. As in the previous 2010 and 2012 versions of the map, photometry
with an accuracy better than $0.05^m$ in the $J$ and $K_s$ bands for more than
70 million stars from the 2MASS catalogue is used in the new version. However,
the data reduction technique is considerably more complicated. As before, an
analysis of the distribution of stars near the main-sequence turnoff on the
$(J-K_s)$ - $K_s$ diagram, where they form a distribution maximum, provides a
basis for the method. The shift of this maximum, i.e., the $mode(J-K_s)$, along
$(J-K_s)$ and $K_s$, given the spatial variations of the mean de-reddened color
$(J-K_s)_0$ of these stars, is interpreted as a growth of the reddening with
increasing distance. The main distinction of the new method is that instead of
the fixed mean absolute magnitude, de-reddened color, distance, and reddening
for each cell, the individual values of these quantities are calculated for
each star by iterations when solving the system of equations relating them.
This has allowed one to increase the random accuracy of the map to $0.01^m$ and
its spatial resolution to 20 pc in coordinates and distance and to $1^{\circ}$
in longitude and latitude. Comparison with other reddening estimates for the
same spatial cells and Gaia DR1 TGAS stars shows that the constructed map is
one of the best maps for the space under consideration. Its systematic errors
have been estimated to be $\sigma(E(J-K_s))=0.025^m$, or
$\sigma(E(B-V))=0.04^m$. The main purpose of the map is to analyze the
characteristics of Galactic structures, clouds, and cloud complexes. For this
purpose, the reddening map within each spatial cell has also been computed by
analyzing the reddening along each line of sight. | astro-ph_SR |
Pulsating Components in Binary and Multiple Stellar Systems --- A
Catalog of Oscillating Binaries: We present an up-to-date catalog of pulsating binaries, i.e. the binary and
multiple stellar systems containing pulsating components, along with a
statistics on them. Compared to the earlier compilation by Soydugan et
al.(2006a) of 25 delta Scuti-type `oscillating Algol-type eclipsing binaries'
(oEA), the recent collection of 74 oEA by Liakos et al.(2012), and the
collection of Cepheids in binaries by Szabados (2003a), the numbers and types
of pulsating variables in binaries are now extended. The total numbers of
pulsating binary/multiple stellar systems have increased to be 515 as of 2014
October 26, among which 262+ are oscillating eclipsing binaries and the oEA
containing delta Scuti components are updated to be 96. The catalog is intended
to be a collection of various pulsating binary stars across the
Hertzsprung-Russell diagram. We reviewed the open questions, advances and
prospects connecting pulsation/oscillation and binarity. The observational
implication of binary systems with pulsating components, to stellar evolution
theories is also addressed. In addition, we have searched the Simbad database
for candidate pulsating binaries. As a result, 322 candidates were extracted.
Furthermore, a brief statistics on Algol-type eclipsing binaries (EA) based on
the existing catalogs is given. We got 5315 EA, of which there are 904 EA with
spectral types A and F. The present catalog has a sortable web version allowing
easy updating and maintenance at http://www.chjaa.org/COB/ . | astro-ph_SR |
The G-O Rule and Waldmeier Effect in the Variations of the Numbers of
Large and Small Sunspot Groups: We have analysed the combined Greenwich and Solar Optical Observing Network
(SOON) sunspot group data during the period of 1874-2011 and determined
variations in the annual numbers (counts) of the small, large and big sunspot
groups (these classifications are made on the basis of the maximum areas of the
sunspot groups). We found that the amplitude of an even-numbered cycle of the
number of large groups is smaller than that of its immediately following
odd-numbered cycle. This is consistent with the well known Gnevyshev and Ohl
rule or G-O rule of solar cycles, generally described by using the Zurich
sunspot number (Rz). During cycles 12-21 the G-O rule holds good for the
variation in the number of small groups also, but it is violated by cycle pair
(22, 23) as in the case of Rz. This behaviour of the variations in the small
groups is largely responsible for the anomalous behaviour of Rz in cycle pair
(22, 23). It is also found that the amplitude of an odd-numbered cycle of the
number of small groups is larger than that of its immediately following
even-numbered cycle. This can be called as `reverse G-O rule'. In the case of
the number of the big groups, both cycle pairs (12, 13) and (22, 23) violated
the G-O rule. In many cycles the positions of the peaks of the small, large,
and big groups are different and considerably differ with respect to the
corresponding positions of the Rz peaks. In the case of cycle 23, the
corresponding cycles of the small and large groups are largely symmetric/less
asymmetric (Waldmeier effect is weak/absent) with their maxima taking place two
years later than that of Rz. The corresponding cycle of the big groups is more
asymmetric (strong Waldmeier effect) with its maximum epoch taking place at the
same time as that of Rz. | astro-ph_SR |
On effects of surface bipolar magnetic regions on the convection zone
dynamo: We investigate the effect of the surface bipolar magnetic regions (BMR) on
the large-scale dynamo distributed in the bulk of the convection zone. The
study employs the nonlinear 3D mean-field dynamo model. We model the emergence
of the BMR on the surface through the nonaxisymmetric magnetic buoyancy effect,
which acts on the large-scale toroidal magnetic field in the upper half of the
convection zone. The nonaxisymmetric magnetic field which results from this
mechanism is shallow. On the surface, the effect of the BMR on the magnetic
field generation is dominant. {However, because of the shallow BMR
distribution, its effect on the global dynamo is less compared to the
convective zone dynamo.} We find that the mean-field $\alpha$ effect, which
acts on the nonaxisymmetric magnetic field of the BMRs, provides the greater
contribution to the dynamo process than the BMR's tilt does. Even so, the
fluctuations of the BMR's tilt lead to the parity braking in the global dynamo.
At the surface the nonaxisymmetric magnetic field, which are generated because
of the BMR's activity, shows a tendency for the bihelical spectrum with the
positive sign for the low $\ell$ modes during the maximum of the magnetic
activity cycle. | astro-ph_SR |
Discovery of Radial Spectral Hardening in the Hot Bubble of Planetary
Nebula BD+30 3639 with Median Energy Imaging: We introduce a new imaging analysis technique to study the spatial
distribution of the X-ray emission from the hot bubble of planetary nebula
BD+30 3639. Hot bubble emission is typically photon-starved, thus limiting the
methods for spatial-spectral analysis, however, this new technique uses the
statistics of photon energies across the nebula to identify spatial variations.
Using the median energy value of the X-ray photons, we identified a rise in
median energy values towards the projected edge of the nebula, which we refer
to as radial spectral hardening. We explored the origin of this radial spectral
hardening with X-ray spectral analysis of distinct regions of high- and
low-median energy values. Given that the hot bubble is embedded within a young,
dense, planetary nebula, we argue that the radial spectral hardening is due to
an increased column density at the projected nebular edge. Median energy
imaging provides a promising new methodology for exploring the spatial
variations in faint extended X-ray sources. | astro-ph_SR |
The data center for the Spectrometer and Telescope for Imaging X-rays
(STIX) onboard Solar Orbiter: The Spectrometer and Telescope for Imaging X-rays (STIX) on board Solar
Orbiter observes solar X-ray emission in the range of 4 – 150 keV and
produces spectra and images of solar flares over a wide range of flare
magnitudes. During nominal operation, STIX continuously generates data. A
constant data flow requires fully automated data-processing pipelines to
process and analyze the data, and a data platform to manage, visualize, and
distribute the data products to the scientific community. The STIX Data Center
has been built to fulfill these needs. In this paper, we outline its main
components to help the community better understand the tools and data it
provides. The STIX Data Center is operated at the University of Applied
Sciences and Arts Northwestern Switzerland (FHNW) and consists of automated
processing pipelines and a data platform. The pipelines process STIX telemetry
data, perform common analysis tasks, and generate data products at different
processing levels. They have been designed to operate fully automatically with
minimal human intervention. The data platform provides web-based user
interfaces and application programmable interfaces for searching and
downloading STIX data products. The STIX Data Center has been operating
successfully for more than two years. The platform facilitates instrument
operations and provides vital support to STIX data users. | astro-ph_SR |
Population of post-nova supersoft X-ray sources: Novae undergo a supersoft X-ray phase of varying duration after the optical
outburst. Such transient post-nova supersoft X-ray sources (SSSs) are the
majority of the observed SSSs in M31. In this paper, we use the post-nova
evolutionary models of Wolf et al. to compute the expected population of
post-nova SSSs in M31. We predict that depending on the assumptions about the
WD mass distribution in novae, at any instant there are about 250-600 post-nova
SSSs in M31 with (unabsorbed) 0.2-1.0 keV luminosity L_x>10^36 erg/s. Their
combined unabsorbed luminosity is of the order of ~10^39 erg/s. Their
luminosity distribution shows significant steepening around log(L_x)~37.7-38
and becomes zero at L_x~2x10^38 erg/s, the maximum L_x achieved in the
post-nova evolutionary tracks. Their effective temperature distribution has a
roughly power law shape with differential slope of ~4-6 up to the maximum
temperature of T_eff~1.5x10^6 K.
We compare our predictions with the results of the XMM-Newton monitoring of
the central field of M31 between 2006 and 2009. The predicted number of
post-nova SSSs exceed the observed number by a factor of ~2-5, depending on the
assumed WD mass distribution in novae. This is good agreement, considering the
number and magnitude of uncertainties involved in calculations of the post-nova
evolutionary models and their X-ray output. Furthermore, only a moderate
circumstellar absorption, with hydrogen column density of the order of ~10^21
cm^-2, will remove the discrepancy. | astro-ph_SR |
The planetary nebula population of M33 and its metallicity gradient: A
look into the galaxy's distant past: The Planetary Nebula (PN) population of M33 is studied via multi-fiber
spectroscopy with Hectospec at the MMT. In this paper we present the spectra of
102 PNe, whereas plasma diagnostic and chemical abundances were performed on
the 93 PNe where the necessary diagnostic lines were measured. About 20% of the
PNe are compatible with being Type I; the rest of the sample is the progeny of
an old disk stellar population, with main sequence masses M<3M${_\odot}$ and
ages t$>$0.3 Gyr.
By studying the elemental abundances of the PNe in the M33 disk we were able
to infer that: (1) there is a tight correlation between O/H and Ne/H, broadly
excluding the evolution of oxygen; (2) the average abundances of the
$\alpha$-elements are consistent with those of \hii regions, indicating a
negligible global enrichment in the disk of M33 from the epoch of the formation
of the PN progenitors to the present time; (3) the radial oxygen gradient
across the M33 disk has a slope of -0.031$\pm$0.013 dex kpc$^{-1}$, in
agreement, within the errors, with the corresponding gradient derived from HII
regions. Our observations do not seem to imply that the metallicity gradient
across the M33 disk has flattened considerably with time. We report also the
discovery of a PN with Wolf-Rayet features, PN039, belonging the class of late
[WC] stars | astro-ph_SR |
SN 2008gz - most likely a normal type IIP event: We present BV RI photometric and low-resolution spectroscopic investigation
of a type II core-collapse supernova (SN) 2008gz, which occurred in a star
forming arm and within a half-light radius (solar metallicity region) of a
nearby spiral galaxy NGC 3672. The SN event was detected late and a detailed
investigation of its light curves and spectra spanning 200 days suggest that it
is an event of type IIP similar to archetypal SNe 2004et and 1999em. However,
in contrast to other events of its class, the SN 2008gz exhibits rarely
observed V magnitude drop of 1.5 over the period of a month during plateau to
nebular phase. Using 0.21 mag of Av as a lower limit and a distance of 25.5
Mpc, we estimate synthesized $^{56}$Ni mass of 0.05 \pm 0.01 M* and a
mid-plateau Mv of -16.6 \pm 0.2 mag. The photospheric velocity is observed to
be higher than that was observed for SN 2004et at similar epochs, indicating
explosion energy was comparable to or higher than SN 2004et. Similar trend was
also seen for the expansion velocity of H-envelopes. By comparing its
properties with other well studied events as well as by using a recent
simulation of pre-SN models of Dessart, Livne & Waldman (2010), we infer an
explosion energy range of 2 - 3 x 10$^{51}$ erg and this coupled with the
observed width of the forbidden [O I] 6300-6364 {\AA} line at 275 days after
the explosion gives an upper limit for the main-sequence (non-rotating, solar
metallicity) progenitor mass of 17 M*. Our narrow-band H{\alpha} observation,
taken nearly 560 days after the explosion and the presence of an emission kink
at zero velocity in the Doppler corrected spectra of SN indicate that the event
took place in a low luminosity star forming H II region. | astro-ph_SR |
Distances to Recent Near-Earth Supernovae From Geological and Lunar 60Fe: Near-Earth supernova blasts which engulf the solar system have left traces of
their ejecta in the geological and lunar records. There is now a wealth of data
on live radioactive ${}^{60}$Fe pointing to a supernova at 3 Myr ago, as well
as the recent discovery of an event at 7 Myr ago. We use the available
measurements to evaluate the distances to these events. For the better analyzed
supernova at 3 Myr, samples include deep-sea sediments, ferromanganese crusts,
and lunar regolith; we explore the consistency among and across these
measurements, which depends sensitively on the uptake of iron in the samples as
well as possible anisotropies in the ${}^{60}$Fe fallout. There is also
significant uncertainty in the astronomical parameters needed for these
calculations. We take the opportunity to perform a parameter study on the
effects that the ejected ${}^{60}$Fe mass from a core-collapse supernova and
the fraction of dust that survives the remnant have on the resulting distance.
We find that with an ejected ${}^{60}$Fe mass of $3\times10^{-5} M_\odot$ and a
dust fraction of 10%, the distance range for the supernova 3 Myr ago is $D \sim
20 - 140$ pc, with the most likely range between $50 - 65$ pc. Using the same
astrophysical parameters, the distance for the supernova at 7 Myr ago is $D
\sim 110$ pc. We close with a brief discussion of geological and astronomical
measurements that can improve these results. | astro-ph_SR |
Power-law statistics and stellar rotational velocities in the Pleiades: In this paper we will show that, the non-gaussian statistics framework based
on the Kaniadakis statistics is more appropriate to fit the observed
distributions of projected rotational velocity measurements of stars in the
Pleiades open cluster. To this end, we compare the results from the $\kappa$
and $q$-distributions with the Maxwellian. | astro-ph_SR |
GYRE: A New Open-Source Stellar Oscillation Code: We introduce GYRE, a new open-source stellar oscillation code which solves
the adiabatic/non-adiabatic pulsation equations using a novel Magnus Multiple
Shooting (MMS) numerical scheme. The code has a global error scaling of up to
6th order in the grid spacing, and can therefore achieve high accuracy with few
grid points. It is moreover robust and efficiently makes use of multiple
processor cores and/or nodes. We present an example calculation using GYRE, and
discuss recent work to integrate GYRE into the asteroseismic optimization
module of the MESA stellar evolution code. | astro-ph_SR |
Local Helioseismology of Sunspots: Current Status and Perspectives
(Invited Review): Mechanisms of the formation and stability of sunspots are among the
longest-standing and intriguing puzzles of solar physics and astrophysics.
Sunspots are controlled by subsurface dynamics hidden from direct observations.
Recently, substantial progress in our understanding of the physics of the
turbulent magnetized plasma in strong-field regions has been made by using
numerical simulations and local helioseismology. Both the simulations and
helioseismic measurements are extremely challenging, but it becomes clear that
the key to understanding the enigma of sunspots is a synergy between models and
observations. Recent observations and radiative MHD numerical models have
provided a convincing explanation to the Evershed flows in sunspot penumbrae.
Also, they lead to the understanding of sunspots as self-organized magnetic
structures in the turbulent plasma of the upper convection zone, which are
maintained by a large-scale dynamics. Local helioseismic diagnostics of
sunspots still have many uncertainties, some of which are discussed in this
review. However, there have been significant achievements in resolving these
uncertainties, verifying the basic results by new high-resolution observations,
testing the helioseismic techniques by numerical simulations, and comparing
results obtained by different methods. For instance, a recent analysis of
helioseismology data from the Hinode space mission has successfully resolved
several uncertainties and concerns (such as the inclined-field and phase-speed
filtering effects) that might affect the inferences of the subsurface
wave-speed structure of sunspots and the flow pattern. It becomes clear that
for the understanding of the phenomenon of sunspots it is important to further
improve the helioseismology methods and investigate the whole life cycle of
active regions, from magnetic-flux emergence to dissipation. | astro-ph_SR |
Magnetic imprints of eruptive and non-eruptive Solar flares as observed
by Solar Dynamics Observatory: The abrupt and permanent changes of photospheric magnetic field in the
localized regions of active regions during solar flares called magnetic
imprints (MIs), have been observed for the past nearly three decades. The well
known "coronal implosion" model is assumed to explain such flare associated
changes but the complete physical understanding is still missing and debatable.
In this study, we made a systematic analysis of flare-related changes of
photospheric magnetic field during 21 flares (14 eruptive and 7 non-eruptive)
using the high-cadence (\texttt{135s}) vector-magnetogram data obtained from
Helioseismic and Magnetic Imager. The MI regions for eruptive flares are found
to be strongly localised, whereas the majority of non-eruptive events
($>70~\%$) have scattered imprint regions. To quantify the strength of the MIs,
we derived the integrated change of horizontal field and total change of
Lorentz force over an area. These quantities correlate well with the flare
strength, irrespective of whether flares being eruptive or not, short or long
duration. Further, the free-energy (FE), determined from virial-theorem
estimates, exhibits statistically significant downward trend which starts
around the flare time is observed in majority of flares. The change of FE
during flares do not depend on eruptivity but have a strong positive
correlation ($\approx 0.8$) with the Lorentz force change, indicating that the
part of FE released would penetrate into the photosphere. While these results
strongly favor the idea of significant feedback from corona on the photospheric
magnetic field, the characteristics of MIs are quite indistinguishable for
flares being eruptive or not. | astro-ph_SR |
Characterisation of the turbulent electromotive force and its
magnetically-mediated quenching in a global EULAG-MHD simulation of solar
convection: We perform a mean-field analysis of the EULAG-MHD millenium simulation of
global magnetohydrodynamical convection presented in Passos et al. 2014. The
turbulent electromotive force operating in the simulation is assumed to be
linearly related to the cyclic axisymmetric mean magnetic field and its first
spatial derivatives. At every grid point in the simulation's meridional plane,
this assumed relationship involves 27 independent tensorial coefficients.
Expanding on Racine et al. 2011, we extract these coefficients from the
simulation data through a least-squares minimization procedure based on
singular value decomposition. The reconstructed alpha-tensor shows good
agreement with that obtained by Racine et al. 2011, who did not include
derivatives of the mean-field in their fit, as well as with the alpha-tensor
extracted by Augustson et al. 2015 from a distinct ASH MHD simulation. The
isotropic part of the turbulent magnetic diffusivity tensor beta is positive
definite and reaches values of 5.0x10^7 m2s-1 in the middle of the convecting
fluid layers. The spatial variations of both alpha_phiphi and beta_phiphi
component are well reproduced by expressions obtained under the SOCA, with a
good matching of amplitude requiring a turbulent correlation time about five
times smaller than the estimated turnover time of the small-scale turbulent
flow. We find the magnetic quenching of the alpha-effect to be driven primarily
by a reduction of the small-scale flow's kinetic helicity, with variations of
the current helicity playing a lesser role in most locations in the simulation
domain. Our measurements of turbulent diffusivity quenching are restricted to
the beta_phiphi component, but indicate a weaker quenching, by a factor of
1.36, than of the alpha effect, which in our simulation drops by a factor of
three between the minimum and maximum phases of the magnetic cycle. | astro-ph_SR |
Heavy element abundances in planetary nebulae: A theorist's perspective: The determination of heavy element abundances from planetary nebula (PN)
spectra provides an exciting opportunity to study the nucleosynthesis occurring
in the progenitor asymptotic giant branch (AGB) star. We perform
post-processing calculations on AGB models of a large range of mass and
metallicity to obtain predictions for the production of neutron-capture
elements up to the first s-process peak at strontium. We find that solar
metallicity intermediate-mass AGB models provide a reasonable match to the
heavy element composition of Type I PNe. Likewise, many of the Se and Kr
enriched PNe are well fitted by lower mass models with solar or close-to-solar
metallicities. However the most Kr-enriched objects, and the PN with sub-solar
Se/O ratios are difficult to explain with AGB nucleosynthesis models.
Furthermore, we compute s-process abundance predictions for low-mass AGB models
of very low metallicity ([Fe/H] =-2.3) using both scaled solar and an
alpha-enhanced initial composition. For these models, O is dredged to the
surface, which means that abundance ratios measured relative to this element
(e.g., [X/O]) do not provide a reliable measure of initial abundance ratios, or
of production within the star owing to internal nucleosynthesis. | astro-ph_SR |
Numerical methods for solution of the stochastic differential equations
equivalent to the non-stationary Parker's transport equation: We derive the numerical schemes for the strong order integration of the set
of the stochastic differential equations (SDEs) corresponding to the
non-stationary Parker transport equation (PTE). PTE is 5-dimensional (3 spatial
coordinates, particles energy and time) Fokker- Planck type equation describing
the non-stationary the galactic cosmic ray (GCR) particles transport in the
heliosphere. We present the formulas for the numerical solution of the obtained
set of SDEs driven by a Wiener process in the case of the full
three-dimensional diffusion tensor. We introduce the solution applying the
strong order Euler-Maruyama, Milstein and stochastic Runge-Kutta methods. We
discuss the advantages and disadvantages of the presented numerical methods in
the context of increasing the accuracy of the solution of the PTE. | astro-ph_SR |
Fundamental parameters of Ap-star HD 108662: We present the results of a self-consistent spectroscopic analysis of the
atmosphere of Ap-star HD 108662 based on high resolution spectrum and low
resolution spectrophotometric observations. Magnetic field effects, such as
Zeeman broadening and polarized line formation, were taking into account in the
analysis of spectral line profiles. We derived abundances of 24 chemical
elements as well as the stratification of Fe -- an element which is the main
contributor to the line absorption in the visible wavelengths. Another abundant
chemical element -- Chromium -- was found to be distributed homogeneously in
the atmosphere of the star. From our analysis we determined the following
fundamental parameters of HD~108662: $T_{eff}$ = 10212 K, log$g$ = 4.0,
$R/R_{\odot}$ = 2.09 and log$(L/L_{\odot})$ = 1.63. The estimated rotational
velocity of the star is $v\,sini$ = 20.4 km/s and the strength of the surface
magnetic field is $B_s$ = 3300 G. | astro-ph_SR |
Disk-Driven Rotating Bipolar Outflow in Orion Source I: One of the outstanding problems in star-formation theory concerns the
transfer of angular momentum such that mass can accrete onto a newly born young
stellar object (YSO). From a theoretical standpoint, outflows and jets are
predicted to play an essential role in angular momentum transfer and their
rotation motions have been reported for both low- and high-mass YSOs. However,
little quantitative discussion on outflow launching mechanisms have been
presented for high-mass YSOs due to a lack of observational data. Here we
present a clear signature of rotation in the bipolar outflow driven by Orion
Source I, a high-mass YSO candidate, using the Atacama Large
Millimeter/Submillimeter Array (ALMA). A rotational transition of silicon
monoxide (Si18O) reveals a velocity gradient perpendicular to the outflow axis
which is consistent with that of the circumstellar disk traced by a
high-excitation water (H2O) line. The launching radii and outward velocity of
the outflow are estimated to be >10 au and 10 km s-1, respectively. These
parameters rule out a possibility that the observed outflow is produced by
entrainment of a high-velocity jet, and that contribution from stellar-wind or
X-wind which have smaller launching radii are significant in the case of Source
I. Thus, present results provide a convincing evidence of a rotating outflow
directly driven by the magneto-centrifugal disk wind launched by a high-mass
YSO candidate. | astro-ph_SR |
Constraining the amplitude of turbulence in solar corona using
observations of angular broadening of radio sources: The angular broadening of compact radio sources observed through a medium
having turbulent density irregularities is usually estimated using the phase
structure function. We employ an exact formulation for the phase structure
function that helps in obtaining an accurate estimate of angular broadening
when the baseline lengths are comparable to the inner scale of the turbulent
spectrum. | astro-ph_SR |
Flare Rates, Rotation Periods. and Spectroscopic Activity Indicators of
a Volume-Complete Sample of Mid-to-Late M dwarfs within 15 Parsecs: We present a study of flare rates, rotation periods, and spectroscopic
activity indicators of 125 single stars within 15 parsecs and with masses
between 0.1$-$0.3 $M_\odot$ observed during the first year of the TESS mission,
with the goal of elucidating the relationship between these various
magnetically connected phenomena. We gathered multi-epoch high resolution
spectra of each target and we measured equivalent widths of the activity
indicators Helium I D$_3$, $H\alpha$, and the Calcium infrared triplet line at
8542.09 angstroms. We present 18 new rotation periods from MEarth photometry
and 19 new rotation periods from TESS photometry. We present a catalog of 1392
flares. After correcting for sensitivity, we find the slope of the flare
frequency distribution for all stars to have a standard value of $\alpha$ =
1.98 $\pm$ 0.02. We determine R$_{31.5}$, the rate of flares per day with
energies above E = 3.16$\times$10$^{31}$ ergs in the TESS bandpass. We find
that below a critical value of $H\alpha$ EW = -0.71 angstroms, log R$_{31.5}$
increases linearly with increasing $H\alpha$ emission; above this value, log
R$_{31.5}$ declines rapidly. The stars divide into two groups: 26% have
$H\alpha$ in emission, high flare rates with typical values of log R$_{31.5}$ =
-1.30 $\pm$ 0.08, and have Rossby numbers $<$ 0.50. The remaining 74% show
little to no $H\alpha$ in emission and exhibit log R$_{31.5}$ $<$ -3.86, with
the majority of these stars not showing a single flare during the TESS
observations. | astro-ph_SR |
Monotonicity of the cores of massive stars: Massive stars are linked with diverse astronomical processes and objects
including star formation, supernovae and their remnants, cosmic rays,
interstellar media, and galaxy evolution. Understanding their properties is of
primary importance for modern astronomy, and finding simple rules that
characterize them is especially useful. However, theoretical simulations have
not yet realized such relations, instead finding that the late evolutionary
phases are significantly affected by a complicated interplay between nuclear
reactions, chemical mixing, and neutrino radiation, leading to non-monotonic
initial mass dependencies of the iron core mass and the compactness parameter.
We conduct a set of stellar evolution simulations, in which evolutions of He
star models are followed until their central densities uniformly reach
10$^{10}$ g cm$^{-3}$, and analyze their final structures as well as their
evolutionary properties including the lifetime, surface radius change, and
presumable fates after core collapse. Based on the homogeneous data set, we
have found that monotonicity is inherent in the cores of massive stars. We show
that not only the density, entropy, and chemical distributions, but also their
lifetimes and explosion properties such as the proto-neutron-star mass and the
explosion energy can be simultaneously ordered into a monotonic sequence. This
monotonicity can be regarded as an empirical principle that characterizes the
cores of massive stars. | astro-ph_SR |
Shaken and stirred: the effects of turbulence and rotation on disc and
outflow formation during the collapse of magnetised molecular cloud cores: We present the results of eighteen magnetohydrodynamical calculations of the
collapse of a molecular cloud core to form a protostar. Some calculations
include radiative transfer in the flux-limited diffusion approximation while
others employ a barotropic equation of state. We cover a wide parameter space,
with mass-to-flux ratios ranging from $\mu = 5$ to $20$; initial turbulent
amplitudes ranging from a laminar calculation (i.e. where the Mach number,
$\mathscr{M} = 0$) to transonic $\mathscr{M} = 1$; and initial rotation rates
from $\beta_\mathrm{rot} = 0.005$ to $0.02$. We first show that using a
radiative transfer scheme produces warmer pseudo-discs than the barotropic
equation of state, making them more stable. We then `shake' the core by
increasing the initial turbulent velocity field, and find that at all three
mass-to-flux ratios transonic cores are weakly bound and do not produce
pseudo-discs; $\mathscr{M} = 0.3$ cores produce very disrupted discs; and
$\mathscr{M} = 0.1$ cores produce discs broadly comparable to a laminar core.
In our previous paper (arXiv:1701.08741), we showed that a pseudo-disc coupled
with sufficient magnetic field is necessary to form a bipolar outflow. Here we
show that only weakly turbulent cores exhibit collimated jets. We finally take
the $\mathscr{M} = 1.0$, $\mu = 5$ core and `stir' it by increasing the initial
angular momentum, finding that once the degree of rotational energy exceeds the
turbulent energy in the core the disc returns, with a corresponding (though
slower), outflow. These conclusions place constraints on the initial mixtures
of rotation and turbulence in molecular cloud cores which are conducive to the
formation of bipolar outflows early in the star formation process. | astro-ph_SR |
A Closer Look at the Alpha Persei Coronal Conundrum: A ROSAT survey of the Alpha Per open cluster in 1993 detected its brightest
star, mid-F supergiant Alpha Persei: the X-ray luminosity and spectral hardness
were similar to coronally active late-type dwarf members. Later, in 2010, a
Hubble Cosmic Origins Spectrograph SNAPshot of Alpha Persei found
far-ultraviolet coronal proxy SiIV unexpectedly weak. This, and a suspicious
offset of the ROSAT source, suggested that a late-type companion might be
responsible for the X-rays. Recently, a multi-faceted program tested that
premise. Groundbased optical coronography, and near-UV imaging with HST Wide
Field Camera 3, searched for any close-in faint candidate coronal objects, but
without success. Then, a Chandra pointing found the X-ray source single and
coincident with the bright star. Significantly, the SiIV emissions of Alpha
Persei, in a deeper FUV spectrum collected by HST COS as part of the joint
program, aligned well with chromospheric atomic oxygen (which must be intrinsic
to the luminous star), within the context of cooler late-F and early-G
supergiants, including Cepheid variables. This pointed to the X-rays as the
fundamental anomaly. The over-luminous X-rays still support the case for a
hyperactive dwarf secondary, albeit now spatially unresolved. However, an
alternative is that Alpha Persei represents a novel class of coronal source.
Resolving the first possibility now has become more difficult, because the easy
solution -- a well separated companion -- has been eliminated. Testing the
other possibility will require a broader high-energy census of the early-F
supergiants. | astro-ph_SR |
A comprehensive study of young B stars in NGC 2264: I. Space photometry
and asteroseismology: Space photometric time series of the most massive members of the young open
cluster NGC 2264 allow us to study their different sources of variability down
to the millimagnitude level and permits a search for Slowly Pulsating B (SPB)
type pulsation among objects that are only a few million years old. Our goal is
to conduct a homogeneous study of young B type stars in the cluster NGC 2264
using photometric time series from space in combination with high-resolution
spectroscopy and spectropolarimetry obtained from the ground. The latter will
be presented in a separate follow-up article. We performed frequency analyses
for eleven B stars in the field of the young cluster NGC 2264 using photometric
time series from the MOST, CoRoT and Spitzer space telescopes and the routines
Period04 and SigSpec. We employ the MESA stellar evolution code in combination
with the oscillation code GYRE to identify the pulsation modes for two SPB
stars which exhibit short period spacing series. From our analysis we identify
four objects that show SPB pulsations, five stars that show rotational
modulation of their light curves caused by spots, one star that is identified
to be a binary, and one object in the field of the cluster that is found to be
a non-member Be star. In two SPB stars we detect a number of regularly spaced
pulsation modes that are compatible with being members of a g mode period
series. Despite NGC 2264's young age, our analysis illustrates that its B type
members have already arrived on the zero-age main sequence (ZAMS). Our
asteroseismic analysis yields masses between 4 and 6 Msun and ages between 1
and 6 million years, which agree well to the overall cluster age. | astro-ph_SR |
Diagnostics of non-thermal distributions in solar flare spectra observed
by RESIK and RHESSI: We focus on the non-thermal components of the electron distribution in the
keV range and analyse high-energy resolution X-ray spectra detected by RESIK
and RHESSI for three solar flares.In the 2-4 keV range we assume that the
electron distribution can be modelled by an n-distribution. Using a method of
line-intensity ratios, we analyse allowed and satellite lines of Si observed by
RESIK and estimate the parameters of this n-distribution. At higher energies we
explore RHESSI bremsstrahlung spectra. Adopting a forward-fitting approach and
thick-target approximation, we determine the characteristics of injected
electron beams. RHESSI non-thermal component associated with the electron beam
is correlated well with presence of the non-thermal n-distribution obtained
from the RESIK spectra. In addition, such an n-distribution occurs during radio
bursts observed in the 0.61-15.4 GHz range. Furthermore, we show that the
n-distribution could also explain RHESSI emission below ~5 keV. Therefore, two
independent diagnostics methods indicate the flare plasma being affected by the
electron beam can have a non-thermal component in the ~2-5 keV range, which is
described by the n-distribution well. Finally, spectral line analysis reveals
that the n-distribution does not occupy the same location as the thermal
component detected by RHESSI at ~10 keV. | astro-ph_SR |
The floor in the interplanetary magnetic field: Estimation on the basis
of relative duration of ICME observations in solar wind during 1976-2000: To measure the floor in interplanetary magnetic field and estimate the time-
invariant open magnetic flux of Sun, it is necessary to know a part of magnetic
field of Sun carried away by CMEs. In contrast with previous papers, we did not
use global solar parameters: we identified different large-scale types of solar
wind for 1976-2000 interval, obtained a fraction of interplanetary CMEs (ICMEs)
and calculated magnitude of interplanetary magnetic field B averaged over 2
Carrington rotations. The floor of magnetic field is estimated as B value at
solar cycle minimum when the ICMEs were not observed and it was calculated to
be 4,65 \pm 6,0 nT. Obtained value is in a good agreement with previous
results. | astro-ph_SR |
Determination of the inclination of the multi-planet hosting star HR8799
using asteroseismology: Direct imaging of the HR8799 system was a major achievement in the study of
exoplanets. HR8799 is a $\gamma$\,Doradus variable and asteroseismology can
provide an independent constraint on the inclination. Using 650 high
signal-to-noise, high resolution, full visual wavelength spectroscopic
observations obtained over two weeks at Observatoire de Haute Provence (OHP)
with the SOPHIE spectrograph we find that the main frequency in the radial
velocity data is 1.9875 d$^{-1}$. This frequency corresponds to the main
frequency as found in previous photometric observations. Using the FAMIAS
software to identify the pulsation modes, we find this frequency is a prograde
$\ell$=1 sectoral mode and obtain the constraint that inclination
$i\gtrsim$40$^{\circ}$. | astro-ph_SR |
Two-step evolution of a rising flux rope resulting in a confined solar
flare: Combining the Solar Dynamics Observatory and the New Vacuum Solar Telescope
observations, we study a confined flare triggered by a rising flux rope within
the trailing sunspots of active region 12733. The flux rope lying above the
sheared polarity inversion line can be constructed through magnetic
extrapolation but could not be detected in multi-wavelength images at the
pre-flare stage. The conspicuous shearing motions between the opposite-polarity
fields in the photosphere are considered to be responsible for the flux rope
formation. The maximum twist of the flux rope is as high as -1.76, and then the
flux rope rises due to the kink instability. Only when the flare starts can the
flux rope be observed in high-temperature wavelengths. The differential
emission measure results confirm that this flux rope is a high-temperature
structure. Associated with the rising flux rope, there appear many post-flare
loops and a pair of flare ribbons. When the rising flux rope meets and
reconnects with the large-scale overlying field lines, a set of large-scale
twisted loops are formed, and two flare ribbons propagating in opposite
directions appear on the outskirts of the former ribbons, indicating that the
twist of the flux rope is transferred to a much larger system. These results
imply that the external reconnection between the rising flux rope and the
large-scale overlying loops plays an important role in the confined flare
formation. | astro-ph_SR |
Radiation-pressure-driven dust waves inside bursting interstellar
bubbles: Massive stars drive the evolution of the interstellar medium through their
radiative and mechanical energy input. After their birth, they form bubbles of
hot gas surrounded by a dense shell. Traditionally, the formation of bubbles is
explained through the input of a powerful stellar wind, even though direct
evidence supporting this scenario is lacking. Here we explore the possibility
that interstellar bubbles seen by the Spitzer- and Herschel space telescopes,
blown by stars with log(L/L_sun) < 5.2, form and expand due to the thermal
pressure accompanying ionization of the surrounding gas. We show that density
gradients in the natal cloud or a puncture in the swept up shell lead to an
ionized gas flow through the bubble into the general interstellar medium, which
is traced by a dust wave near the star, demonstrating the importance of
radiation pressure during this phase. Dust waves provide a natural explanation
for the presence of dust inside H II bubbles, offer a novel method to study
dust in H II regions and provide direct evidence that bubbles are relieving
their pressure into the ISM through a champagne flow, acting as a probe of the
radiative interaction of a massive star with its surroundings. We create a
parameter space connecting the ambient density, the ionizing source luminosity,
and the position of the dust wave, while using the well-studied H II bubbles
RCW 120 and RCW 82 as benchmarks of our model. Finally, we briefly examine the
implications of our study for the environments of super star clusters formed in
UltraLuminous InfraRed Galaxies (ULIRGs), merging galaxies and the early
Universe, which occur in very luminous and dense environments and where
radiation pressure is expected to dominate the dynamical evolution. | astro-ph_SR |
The physics of neutron stars: Topical problems in the physics of and basic facts about neutron stars are
reviewed. The observational manifestations of neutron stars, their core and
envelope structure, magnetic fields, thermal evolution, and masses and radii
are briefly discussed, along with the underlying microphysics. | astro-ph_SR |
Statistics of the two-point cross-covariance function of solar
oscillations: Context: The cross-covariance of solar oscillations observed at pairs of
points on the solar surface is a fundamental ingredient in time-distance
helioseismology. Wave travel times are extracted from the cross-covariance
function and are used to infer the physical conditions in the solar interior.
Aims: Understanding the statistics of the two-point cross-covariance function
is a necessary step towards optimizing the measurement of travel times.
Methods: By modeling stochastic solar oscillations, we evaluate the variance of
the cross-covariance function as function of time-lag and distance between the
two points. Results: We show that the variance of the cross-covariance is
independent of both time-lag and distance in the far field, i.e., when they are
large compared to the coherence scales of the solar oscillations. Conclusions:
The constant noise level for the cross-covariance means that the
signal-to-noise ratio for the cross-covariance is proportional to the amplitude
of the expectation value of the cross-covariance. This observation is important
for planning data analysis efforts. | astro-ph_SR |
An interesting candidate for isolated massive star formation in the
Small Magellanic Cloud: The region of the Small Magellanic Cloud (SMC) with which this paper is
concerned contains the highest concentration of IRAS/Spitzer sources, H I
emission, and molecular clouds in this neighboring galaxy. However very few
studies have been devoted to it, despite these signs of star formation. We
present the first detailed study of the compact H II region N33 in the SMC by
placing it in a wider context of massive star formation. Moreover, we show that
N33 is a particularly interesting candidate for isolated massive star
formation. This analysis is based mainly on optical ESO NTT observations, both
imaging and spectroscopy, coupled with other archive data, notably Spitzer
images (IRAC 3.6, 4.5, 5.8, and 8.0 mic) and 2MASS observations. We derive a
number of physical characteristics of the compact H II region N33 for the first
time. This gas and dust formation of 7".4 (2.2 pc) in diameter is powered by a
massive star of spectral type O6.5-O7 V. The compact H II region belongs to a
rare class of H II regions in the Magellanic Clouds, called high-excitation
blobs (HEBs). We show that this H II region is not related to any star cluster.
Specifically, we do not find any traces of clustering around N33 on scales
larger than 10" (~ 3 pc). On smaller scales, there is a marginal stellar
concentration, the low density of which, below the 3 sigma level, does not
classify it as a real cluster. We also verify that N33 is not a member of any
large stellar association. Under these circumstances, N33 is also therefore
attractive because it represents a remarkable case of isolated massive-star
formation in the SMC. Various aspects of the relevance of N33 to the topic of
massive-star formation in isolation are discussed. | astro-ph_SR |
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