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Abundances in the Herbig Ae star HD 101412: Abundance anomalies; Lambda
Boo-Vega characteristics?: Context: Recent attention has been directed to abundance variations among
very young stars.
Aims: To perform a detailed abundance study of the Herbig Ae star HD 101412,
taking advantage of its unusually sharp spectral lines.
Methods: High-resolution spectra are measured for accurate wavelengths and
equivalent widths. Balmer-line fits and ionization equlibria give a relation
between Teff, and log(g). Abundance anomalies and uncertain reddening preclude
the use of spectral type or photometry to fix Teff. Excitation temperatures are
used to break the degeneracy between Teff and log(g).
Results: Strong lines are subject to an anomalous saturation that cannot be
removed by assuming a low microturbulence. By restricting the analysis to weak
(<= 20 m[A]) lines, we find consistent results for neutral and ionized species,
based on a model with Teff = 8300K, and log(g)=3.8. The photosphere is depleted
in the most refractory elements, while volatiles are normal or, in the case of
nitrogen, overabundant with respect to the sun. The anomalies are unlike those
of Ap or Am stars.
Conclusions: We suggest the anomalous saturation of strong lines arises from
heating of the upper atmospheric layers by infalling material from a disk. The
overall abundance pattern may be related to those found for the Lambda Boo
stars, though the depletions of the refractory elements are milder, more like
those of Vega. However, the intermediate volatile zinc is depleted, precluding
a straightforward interpretation of the abundance pattern in terms of gas-grain
separation. | astro-ph_SR |
Six New Recycled Globular Cluster Pulsars Discovered with the Green Bank
Telescope: We have completed sensitive searches for new pulsars in seven globular
clusters using the Robert C. Byrd Green Bank Telescope, and have discovered six
new recycled pulsars (four in NGC 6517 and two in M22), five of which are fully
recycled millisecond pulsars with P < 10 ms. We report full timing solutions
for all six new pulsars and provide estimates of their flux densities and
spectral indices. None of the pulsars are detected in archival Chandra data
down to L_X~10^32 erg/s for NGC 6517 and L_X~10^31 erg/s for M22. One of the
millisecond pulsars in M22 appears to have a very low mass companion, and is
likely a new "black widow". A second binary pulsar in NGC 6517 is in a
long-period, mildly eccentric orbit. We are able to set some lower limits on
the age of the system, and find that it may be less than a few hundred million
years old, which would indicate recent pulsar recycling in NGC 6517. An
isolated pulsar in NGC 6517 that lies about 20 core radii from the cluster
center appears to have been ejected from the core by interacting with a massive
binary. By analyzing the luminosity function of the pulsars in NGC 6517, we
predict the cluster to harbor roughly a dozen pulsars. We use the observed
period derivatives of three pulsars to set lower limits on the mass-to-light
ratios in the cores of their host clusters, and find no evidence for large
amounts of low-luminosity matter. We also discuss reasons for non-detections in
some of the clusters we searched. | astro-ph_SR |
Partially-erupting prominences: a comparison between observations and
model-predicted observables: AIM: To investigate several partially-erupting prominences to study their
relationship with other CME-associated phenomena and to compare these
observations with observables predicted by a model of partially-expelled flux
ropes (Gibson & Fan, 2006a, b).
METHODS: We have studied 6 selected events with partially-erupting
prominences using multi wavelength observations recorded by the
Extreme-ultraviolet Imaging Telescope (EIT), Transition Region and Coronal
Explorer (TRACE), Mauna Loa Solar Observatory (MLSO), Big Bear Solar
Observatory (BBSO) and soft X-ray telescope (SXT). The observational features
associated with partially-erupting prominences were then compared with the
predicted observables from the model.
RESULTS: The partially-expelled-flux-rope (PEFR) model of Gibson & Fan
(2006a, b) can explain the partial eruption of these prominences, and in
addition predicts a variety of other CME-related observables that provide
evidence for internal reconnection during eruption. We find that all of the
partially-erupting prominences studied in this paper exhibit indirect evidence
for internal reconnection. Moreover, all cases showed evidence of at least one
observable unique to the PEFR model, e.g., dimmings external to the source
region, and/or a soft X-ray cusp overlying a reformed sigmoid.
CONCLUSIONS: The PEFR model provides a plausible mechanism to explain the
observed evolution of partially-erupting-prominence-associated CMEs in our
study. | astro-ph_SR |
The Surface Magnetic Activity of the Weak-Line T Tauri Stars TWA 7 and
TWA 25: We present an analysis of spectropolarimetric observations of the low-mass
weak-line T Tauri stars TWA 25 and TWA 7. The large-scale surface magnetic
fields have been reconstructed for both stars using the technique of Zeeman
Doppler imaging. Our surface maps reveal predominantly toroidal and
non-axisymmetric fields for both stars. These maps reinforce the wide range of
surface magnetic fields that have been recovered, particularly in pre-main
sequence stars that have stopped accreting from the (now depleted) central
regions of their discs. We reconstruct the large scale surface brightness
distributions for both stars, and use these reconstructions to filter out the
activity-induced radial velocity jitter, reducing the RMS of the radial
velocity variations from 495 m/s to 32 m/s for TWA 25, and from 127 m/s to 36
m/s for TWA 7, ruling out the presence of close-in giant planets for both
stars. The TWA 7 radial velocities provide an example of a case where the
activity-induced radial velocity variations mimic a Keplerian signal that is
uncorrelated with the spectral activity indices. This shows the usefulness of
longitudinal magnetic field measurements in identifying activity-induced radial
velocity variations. | astro-ph_SR |
Hydrodynamical instabilities induced by atomic diffusion in A stars and
their consequences: Aims. Atomic diffusion, including the effect of radiative accelerations on
individual elements, leads to important variations of the chemical composition
inside the stars. The accumulation in specific layers of the elements, which
are the main contributors of the local opacity, leads to hydrodynamical
instabilities that modify the internal stellar structure and surface
abundances. Our aim is to study these effects and compare the resulting surface
abundances with spectroscopic observations Methods. We computed the detailed
structure of A-type stars including these effects. We used the Toulouse-Geneva
Evolution Code (TGEC), where radiative accelerations are computed using the
Single Valued Parameter (SVP) method, and we added double-diffusive convection
with mixing coefficients deduced from three-dimensional (3D) simulations.
Results. We show that the modification of the initial chemical composition has
important effects on the internal stellar mixing and leads to different surface
abundances of the elements. The results fit the observed surface chemical
composition well if the layers, which are individually mixed by
double-diffusive convection, are connected. | astro-ph_SR |
The Closest View of a Fast Coronal Mass Ejection: How Faulty Assumptions
near Perihelion Lead to Unrealistic Interpretations of PSP/WISPR Observations: We report on the closest view of a coronal mass ejection observed by the
Parker Solar Probe (PSP)/Wide-field Imager for {Parker} Solar PRobe (WISPR)
instrument on September 05, 2022, when PSP was traversing from a distance of
15.3~to~13.5~R$_\odot$ from the Sun. The CME leading edge and an arc-shaped
{\emph{concave-up} structure near the core} was tracked in WISPR~field of view
using the polar coordinate system, for the first time. Using the impact
distance on Thomson surface, we measured average speeds of CME leading edge and
concave-up structure as $\approx$2500~$\pm$~270\,km\,s$^{-1}$ and
$\approx$400~$\pm$~70\,km\,s$^{-1}$ with a deceleration of
$\approx$20~m~s$^{-2}$ for the later. {The use of the plane-of-sky approach
yielded an unrealistic speed of more than three times of this estimate.} We
also used single viewpoint STEREO/COR-2A images to fit the Graduated
Cylindrical Shell (GCS) model to the CME while incorporating the source region
location from EUI of Solar Orbiter and estimated a 3D speed of
$\approx$2700\,km\,s$^{-1}$. We conclude that this CME exhibits the highest
speed during the ascending phase of solar cycle 25. This places it in the
category of extreme speed CMEs, which account for only 0.15\% of all CMEs
listed in the CDAW CME catalog. | astro-ph_SR |
Modeling Magnetically Channeled Winds in 3D: I. Isothermal Simulations
of a Magnetic O Supergiant: In this paper we present the first set of 3D magnetohydrodynamic (MHD)
simulations performed with the riemann geomesh code. We study the dynamics of
the magnetically channeled winds of magnetic massive stars in full three
dimensions using a code that is uniquely suited to spherical problems.
Specifically, we perform isothermal simulations of a smooth wind on a rotating
star with a tilted, initially dipolar field. We compare the mass-loss, angular
momentum loss, and magnetospheric dynamics of a template star (with the
properties that are reminiscent of the O4 supergiant {\zeta} Pup) over a range
of rotation rates, magnetic field strengths, and magnetic tilt angles. The
simulations are run up to a quasi-steady state and the results are observed to
be consistent with the existing literature, showing the episodic centrifugal
breakout events of the mass outflow, confined by the magnetic field loops that
form the closed magnetosphere of the star. The catalogued results provide
perspective on how angular-momentum loss varies for different configurations of
rotation rate, magnetic field strength, and large magnetic tilt angles. In
agreement with previous 2D MHD studies, we find that high magnetic confinement
reduces the overall mass-loss rate, and higher rotation increases the mass-loss
rate. This and future studies will be used to estimate the angular-momentum
evolution, spin-down time, and mass-loss evolution of magnetic massive stars as
a function of magnetic field strength, rotation rate, and dipole tilt. | astro-ph_SR |
Continuum and line emission of flares on red dwarf stars: The emission spectrum has been calculated of a homogeneous pure hydrogen
layer, which parameters are typical for a flare on a red dwarf. The ionization
and excitation states were determined by the solution of steady-state equations
taking into account the continuum and all discrete hydrogen levels. We consider
the following elementary processes: electron-impact transitions, spontaneous
and induced radiative transitions, and ionization by the bremsstrahlung and
recombination radiation of the layer itself. The Biberman--Holstein
approximation was used to calculate the scattering of line radiation.
Asymptotic formulae for the escape probability are obtained for a symmetric
line profile taking into account the Stark and Doppler effects. The
approximation for the core of the H$-\alpha$ line by a gaussian curve has been
substantiated.
The spectral intensity of the continuous spectrum, the intensity of the lines
of the Balmer series and the magnitude of the Balmer jump have been calculated.
The conditions have been determined for which the Balmer jump and the emission
line intensities above the continuum decrease to such low values that the
emission spectrum can be assumed to be continuum as well as the conditions at
which the emission spectrum becomes close to the blackbody. | astro-ph_SR |
Arrival time calculation for interplanetary coronal mass ejections with
circular fronts and application to STEREO observations of the 2009 February
13 eruption: A goal of the NASA STEREO mission is to study the feasibility of forecasting
the direction, arrival time and internal structure of solar coronal mass
ejections (CMEs) from a vantage point outside the Sun-Earth line. Through a
case study, we discuss the arrival time calculation of interplanetary CMEs
(ICMEs) in the ecliptic plane using data from STEREO/SECCHI at large
elongations from the Sun in combination with different geometric assumptions
about the ICME front shape (Fixed-\Phi (FP): a point and harmonic Mean (HM): a
circle). These forecasting techniques use single-spacecraft imaging data and
are based on the assumptions of constant velocity and direction. We show that
for the slow (350 km/s) ICME on 2009 February 13-18, observed at quadrature by
the two STEREO spacecraft, the results for the arrival time given by the HM
approximation are more accurate by 12 hours than those for FP in comparison to
in situ observations of solar wind plasma and magnetic field parameters by
STEREO/IMPACT/PLASTIC, and by 6 hours for the arrival time at Venus Express
(MAG). We propose that the improvement is directly related to the ICME front
shape being more accurately described by HM for an ICME with a low inclination
of its symmetry axis to the ecliptic. In this case the ICME has to be tracked
to > 30{\deg} elongation to get arrival time errors < 5 hours. A newly derived
formula for calculating arrival times with the HM method is also useful for a
triangulation technique assuming the same geometry. | astro-ph_SR |
Extended radio emission associated with a breakout eruption from the
back side of the Sun: Context. Coronal mass ejections (CMEs) on the Sun are the largest explosions
in the Solar System that can drive powerful plasma shocks. The eruptions,
shocks, and other processes associated to CMEs are efficient particle
accelerators and the accelerated electrons in particular can produce radio
bursts through the plasma emission mechanism. Aims. Coronal mass ejections and
associated radio bursts have been well studied in cases where the CME
originates close to the solar limb or within the frontside disc. Here, we study
the radio emission associated with a CME eruption on the back side of the Sun
on 22 July 2012. Methods. Using radio imaging from the Nan\c{c}ay
Radioheliograph, spectroscopic data from the Nan\c{c}ay Decametric Array, and
extreme-ultraviolet observations from the Solar Dynamics Observatory and Solar
Terrestrial Relations Observatory spacecraft, we determine the nature of the
observed radio emission as well as the location and propagation of the CME.
Results. We show that the observed low-intensity radio emission corresponds to
a type II radio burst or a short-duration type IV radio burst associated with a
CME eruption due to breakout reconnection on the back side of the Sun, as
suggested by the pre-eruptive magnetic field configuration. The radio emission
consists of a large, extended structure, initially located ahead of the CME,
that corresponds to various electron acceleration locations. Conclusions. The
observations presented here are consistent with the breakout model of CME
eruptions. The extended radio emission coincides with the location of the
current sheet and quasi-separatrix boundary of the CME flux and the overlying
helmet streamer and also with that of a large shock expected to form ahead of
the CME in this configuration. | astro-ph_SR |
Uncovering new white dwarf - open cluster associations using Gaia DR3: Context: Open clusters (OCs) provide homogeneous samples of white dwarfs
(WDs) with known distances, extinctions, and total ages. The unprecedented
astrometric precision of \textit{\textit{Gaia}} allows us to identify many
novel OC--WD pairs. Studying WDs in the context of their parent OCs makes it
possible to determine the properties of WD progenitors and study the
initial-final mass relation (IFMR). Aims: We seek to find potential new WD
members of OCs in the solar vicinity. The analysis of OC members' parallaxes
allows us to determine the OC distances to a high precision, which in turn
enables us to calculate WD masses and cooling ages and to constrain the IFMR.
Methods: We searched for new potential WD members of nearby OCs using the
density-based machine learning clustering algorithm \texttt{HDBSCAN}. The
clustering analysis was applied in five astrometric dimensions -- positions in
the sky, proper motions and parallaxes -- and in three dimensions where the
positional information was not considered in the clustering analysis. The
identified candidate OC WDs were further filtered using the photometric
criteria and properties of their putative host OCs. The masses and cooling ages
of the WDs were calculated via a photometric method using all available
\textit{\textit{Gaia}}, Pan-STARRS, SDSS, and GALEX photometry. The WD
progenitor masses were determined using the ages and metallicities of their
host OCs. Results: Altogether, 63 OC WD candidates were recovered, 27 of which
are already known in the literature. We provide characterization for 36 novel
WDs that have significant OC membership probabilities. Six of them fall into
relatively unconstrained sections of the IFMR where the relation seems to
exhibit nonlinear behavior. We were not able to identify any WDs originating
from massive progenitors that would even remotely approach the widely adopted
WD progenitor mass limit. (abridged) | astro-ph_SR |
Very fast variations of SiO maser emission in evolved stars: Context. Stars on the asymptotic giant branch (AGB) are long-period variables
that present strong flux variations at almost all wavelengths, including the
SiO maser lines. The periods of these variations are of 300-500 days in
Mira-type stars and somewhat shorter in semi-regular variables. The variability
of the SiO lines on short timescales has been investigated, but the data are
inconclusive. Aims. We aim to study the time evolution of the SiO maser lines
in Mira-type and semi-regular variables at short timescales. We also discuss
the origin of the observed fast variations. Methods. We observed the SiO maser
lines at 7 mm (28SiO v=1,2 J=1-0) and 3 mm (28SiO v=1 J=2-1) using the 40 m
Yebes antenna and the 30 m IRAM telescope, respectively, with a minimum spacing
of 1 day. We studied the semi-regular variables RX Boo and RT Vir and the
Mira-type variables U Her, R LMi, R Leo, and $\chi$ Cyg. We performed a
detailed statistical analysis of the variations on different timescales.
Results. RX Boo shows strong and fast variations in the intensity of the
different spectral features of the SiO lines at 7 mm and 3 mm. On a timescale
of one day, we find variations of >10% in 25% of the cases. Variations of
greater than $\sim$50% are often found when the observations are separated by 2
or 3 days. A similar variation rate of the SiO lines at 7 mm is found for RT
Vir, but the observations of this object are less complete. On the contrary,
the variations of the SiO maser line intensity in the Mira-type variables are
moderate, with typical variation rates around <10% in 7 days. This phenomenon
can be explained by the presence of particularly small maser-emitting clumps in
semi-regular variables, which would lead to a strong dependence of the
intensity on the density variations due to the passage of shocks. | astro-ph_SR |
Peeking Between the Pulses: The Far-UV Spectrum of the Previously Unseen
White Dwarf in AR Scorpii: The compact object in the interacting binary AR Sco has widely been presumed
to be a rapidly rotating, magnetized white dwarf (WD), but it has never been
detected directly. Isolating its spectrum has proven difficult because the
spin-down of the WD generates pulsed synchrotron radiation that far outshines
the WD's photosphere. As a result, a previous study of AR Sco was unable to
detect the WD in the averaged far-ultraviolet spectrum from a Hubble Space
Telescope (HST) observation. In an effort to unveil the WD's spectrum, we
reanalyze these HST observations by calculating the average spectrum in the
troughs between synchrotron pulses. We identify weak spectral features from the
previously unseen WD and estimate its surface temperature to be 11500$\pm$500K.
Additionally, during the synchrotron pulses, we detect broad Lyman-$\alpha$
absorption consistent with hot WD spectral models. We infer the presence of a
pair of hotspots, with temperatures between 23000K and 28000K, near the
magnetic poles of the WD. As the WD is not expected to be accreting from its
companion, we describe two possible mechanisms for heating the magnetic poles.
The Lyman-$\alpha$ absorption of the hotspots appears relatively undistorted by
Zeeman splitting, constraining the WD's field strength to be 100 MG, but the
data are insufficient to search for the subtle Zeeman splits expected at lower
field strengths. | astro-ph_SR |
WISE Y Dwarfs As Probes of the Brown Dwarf-Exoplanet Connection: We have determined astrometric positions for 15 WISE-discovered late-type
brown dwarfs (6 T8-9 and 9 Y dwarfs) using the Keck II telescope, the Spitzer
Space Telescope, and the Hubble Space Telescope. Combining data from 8 to 20
epochs we derive parallactic and proper motions for these objects which put the
majority within 15 parsecs. For ages greater than a few Gyr, as suggested from
kinematic considerations, we find masses of 10-30 M$_{Jup}$ based on standard
models for the evolution of low mass objects with a range of mass estimates for
individual objects depending on the model in question. Three of the coolest
objects have effective temperatures $\sim$ 350 K and inferred masses of 10-15
M$_{Jup}$. Our parallactic distances confirm earlier photometric estimates
(Kirkpatrick et al. 2012) and direct measurements (Marsh et al. 2013, Beichman
et al. 2013, Dupuy and Krauss 2013) and suggest that the number of objects with
masses below about 15 M$_{Jup}$ must be flat or declining relative to higher
mass objects. The masses of the coldest Y dwarfs may be similar to those
inferred for recently imaged planet-mass companions to nearby young stars.
Objects in this mass range, which appear to be rare in both the interstellar
and proto-planetary environments, may both have formed via gravitational
fragmentation: the brown dwarfs in interstellar clouds and companion objects in
a protoplanetary disk. In both cases, however, the fact that objects in this
mass range are relatively infrequent, suggests that this mechanism must be
inefficient in both environments. | astro-ph_SR |
Close Binary Companions to APOGEE DR16 Stars: 20,000 Binary-star Systems
Across the Color-Magnitude Diagram: Many problems in contemporary astrophysics---from understanding the formation
of black holes to untangling the chemical evolution of galaxies---rely on
knowledge about binary stars. This, in turn, depends on discovery and
characterization of binary companions for large numbers of different kinds of
stars in different chemical and dynamical environments. Current stellar
spectroscopic surveys observe hundreds of thousands to millions of stars with
(typically) few observational epochs, which allows binary discovery but makes
orbital characterization challenging. We use a custom Monte Carlo sampler (The
Joker) to perform discovery and characterization of binary systems through
radial-velocities, in the regime of sparse, noisy, and poorly sampled
multi-epoch data. We use it to generate posterior samplings in Keplerian
parameters for 232,531 sources released in APOGEE Data Release 16. Our final
catalog contains 19,635 high-confidence close-binary (P < few years, a < few
AU) systems that show interesting relationships between binary occurrence rate
and location in the color-magnitude diagram. We find notable faint companions
at high masses (black-hole candidates), at low masses (substellar candidates),
and at very close separations (mass-transfer candidates). We also use the
posterior samplings in a (toy) hierarchical inference to measure the
long-period binary-star eccentricity distribution. We release the full set of
posterior samplings for the entire parent sample of 232,531 stars. This set of
samplings involves no heuristic "discovery" threshold and therefore can be used
for myriad statistical purposes, including hierarchical inferences about
binary-star populations and sub-threshold searches. | astro-ph_SR |
The timescale of low-mass proto-helium white dwarf evolution: A large number of low-mass (< 0.20 M_sun) helium white dwarfs (He WDs) have
recently been discovered. The majority of these are orbiting another WD or a
millisecond pulsar (MSP) in a close binary system; a few examples are found to
show pulsations or to have a main-sequence star companion. There appear to be
discrepancies between the current theoretical modelling of such low-mass He WDs
and a number of key observed cases, indicating that their formation scenario
remains to be fully understood. Here we investigate the formation of detached
proto-He WDs in close-orbit low-mass X-ray binaries (LMXBs). Our prime focus is
to examine the thermal evolution and the contraction phase towards the WD
cooling track and investigate how this evolution depends on the WD mass. Our
calculations are then compared to the most recent observational data. Numerical
calculations with a detailed stellar evolution code were used to trace the
mass-transfer phase in a large number of close-orbit LMXBs. Subsequently, we
followed the evolution of the detached low-mass proto-He WDs, including stages
with residual shell hydrogen burning and vigorous flashes caused by unstable
CNO burning. We find that the time between Roche-lobe detachment until the
low-mass proto-He WD reaches the WD cooling track is typically Delta_t_proto =
0.5 - 2 Gyr, depending systematically on the WD mass and therefore on its
luminosity. The lowest WD mass for developing shell flashes is ~0.21 M_sun for
progenitor stars of mass M2 <= 1.5 M_sun (and ~0.18 M_sun for M2 = 1.6 M_sun).
The long timescale of low-mass proto-He WD evolution can explain a number of
recent observations, including some MSP systems hosting He WD companions with
very low surface gravities and high effective temperatures. We find no evidence
for Delta_t_proto to depend on the occurrence of flashes and thus question the
suggested dichotomy in thermal evolution of proto-WDs. | astro-ph_SR |
Revealing the Two `Horns' of Taurus with GAIA DR2: We investigate the spatial properties of sources from the GAIA catalogue
previously identified as being members of the Taurus star forming region and
which appear in the Spitzer catalogue. We study an area of sky of 10x15 deg,
centred on Right Ascension (2000.0) = 68.5 deg and Declination (2000.0)= 27.0
deg, this being an area surrounding the Taurus molecular cloud. We use data
obtained from the GAIA DR2 release. By using an inversion of GAIA parallax
measurements to obtain distance values and by defining limits to the proper
motions of the Taurus moving group, we are able to show that there are
substantial differences in depth within the Taurus complex. Our results suggest
that the Taurus cloud has significant depth and that there are two main
associations centred at ~130.6+/-0.7 pc and ~160.2+/-0.9 pc at 1{\sigma}. These
two associations also have different proper motions, of 24.5+/-2.8 and
20.1+/-2.4 mas/yr respectively. We here label them the 'Two Horns' of Taurus. | astro-ph_SR |
A cavity and further radial substructures in the disk around HD~97048: Context: Gaps, cavities and rings in circumstellar disks are signposts of
disk evolution and planet-disk interactions. We follow the recent suggestion
that Herbig Ae/Be disks with a flared disk harbour a cavity, and investigate
the disk around HD~97048.
Aims: We aim to resolve the 34$\pm$ 4 au central cavity predicted by Maaskant
et al. (2013) and to investigate the structure of the disk.
Methods: We image the disk around HD~97048 using ALMA at 0.85~mm and 2.94~mm,
and ATCA (multiple frequencies) observations. Our observations also include the
12CO J=1-0, 12CO J=3-2 and HCO+ J=4-3 emission lines.
Results: A central cavity in the disk around HD~97048 is resolved with a
40-46 au radius. Additional radial structure present in the surface brightness
profile can be accounted for either by an opacity gap at ~90 au or by an extra
emitting ring at ~150 au. The continuum emission tracing the dust in the disk
is detected out to 355 au. The 12CO J=3-2 disk is detected 2.4 times farther
out. The 12CO emission can be traced down to $\approx$ 10 au scales.
Non-Keplerian kinematics are detected inside the cavity via the HCO+ J=4-3
velocity map. The mm spectral index measured from ATCA observations suggests
that grain growth has occurred in the HD~97048 disk. Finally, we resolve a
highly inclined disk out to 150 au around the nearby 0.5~$M_{\odot}$ binary
ISO-ChaI 126.
Conclusions: The data presented here reveal a cavity in the disk of HD 97048,
and prominent radial structure in the surface brightness. The cavity size
varies for different continuum frequencies and gas tracers. The gas inside the
cavity follows non-Keplerian kinematics seen in HCO+ emission. The variable
cavity size along with the kinematical signature suggests the presence of a
substellar companion or massive planet inside the cavity. | astro-ph_SR |
Seismological determination of the Alfvén speed and plasma-beta in
solar photospheric bright points: The Alfv\'{e}n speed and plasma beta in photospheric bright points observed
by the Broadband Filter Imager (BFI) of the Solar Optical Telescope (SOT) on
board the \textit{Hinode} satellite, are estimated seismologically. The
diagnostics is based on the theory of slow magnetoacoustic waves in a
non-isothermally stratified photosphere with a uniform vertical magnetic field.
We identify and track bright points in a G-band movie by using the 3D region
growing method, and align them with blue continuum images to derive their
brightness temperatures. From the Fourier power spectra of 118 continuum light
curves made in the bright points, we find that light curves of 91 bright points
have oscillations with properties which are significantly different from
oscillation in quiet regions, with the periods ranging 2.2--16.2~min. We find
that the model gives a moderate value of the plasma beta when $\gamma$ lies at
around 5/3. The calculated Alfv\'{e}n speed is 9.68$\pm$2.02~km~s$^{-1}$,
ranging in 6.3--17.4~km~s$^{-1}$. The plasma beta is estimated to be of
0.93$\pm$0.36, ranging in 0.2--1.9. | astro-ph_SR |
The L dwarf/T dwarf transition: multiplicity, magnetic activity and
mineral meteorology across the hydrogen burning limit: The transition between the L dwarf and T dwarf spectral classes is one of the
most remarkable along the stellar/brown dwarf Main Sequence, separating sources
with photospheres containing mineral condensate clouds from those containing
methane and ammonia gases. Unusual characteristics of this transition include a
1 micron brightening between late L and early T dwarfs observed in both
parallax samples and coeval binaries; a spike in the multiplicity fraction;
evidence of increased photometric variability, possibly arising from patchy
cloud structures; and a delayed transition for young, planetary-mass objects.
All of these features can be explained if this transition is governed by the
"rapid" (nonequlibrium) rainout of clouds from the photosphere, triggered by
temperature, surface gravity, metallicity and (perhaps) rotational effects.
While the underlying mechanism of this rainout remains under debate, the
transition is now being exploited to discover and precisely characterize tight
(< 1 AU) very low-mass binaries that can be used to test brown dwarf
evolutionary and atmospheric theories, and resolved binaries that further
constrain the properties of this remarkable transition. | astro-ph_SR |
Spectral analysis of 3D MHD models of coronal structures: We study extreme-ultraviolet emission line spectra derived from
three-dimensional magnetohydrodynamic models of structures in the corona. In
order to investigate the effects of increased magnetic activity at photospheric
levels in a numerical experiment, a much higher magnetic flux density is
applied at photospheric levels as compared to the Sun. Thus, we can expect our
results to highlight the differences between the Sun and more active, but still
solar-like stars. We discuss signatures seen in extreme-ultraviolet emission
lines synthesized from these models and compare them to signatures found in the
spatial distribution and temporal evolution of Doppler shifts in lines formed
in the transition region and corona. This is of major interest to test the
quality of the underlying magnetohydrodynamic model to heat the corona, i.e.
currents in the corona driven by photospheric motions (flux braiding). | astro-ph_SR |
The Cepheid Impostor HD 18391 and its Anonymous Parent Cluster: New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in
order to confirm the nature of the variablity previously detected in the star,
which lies off the hot edge of the Cepheid instability strip. Small-amplitude
variability at a level of \Delta V = 0.016+-0.002 is indicated, with a period
of P=123.04+-0.06 d. A weaker second signal may be present at P=177.84+-0.18
with \Delta V = 0.007+-0.002, likely corresponding to fundamental mode
pulsation if the primary signal represents overtone pulsation (123.04/177.84 =
0.69). The star, with a spectroscopic reddening of E(B-V) = 1.02, is associated
with heavily-reddened B-type stars in its immediate vicinity that appear to be
outlying members of an anonymous young cluster centered ~10 arcmin to the west
and 1661+-73 pc distant. The cluster has nuclear and coronal radii of r_n=3.5
arcmin and R_c=14 arcmin, respectively, while the parameters for HD 18391
derived from membership in the cluster with its outlying B stars are consistent
with those implied by its Cepheid-like pulsation, provided that it follows the
semi-period-luminosity relation expected of such objects. Its inferred
luminosity as a cluster member is M_V=-7.76+-0.10, its age (9+-1)x10^6 years,
and its evolutionary mass ~19 M_{\sun}. HD 18391 is not a classical Cepheid,
yet it follows the Cepheid period-luminosity relation closely, much like
another Cepheid impostor, V810 Cen. | astro-ph_SR |
The EBLM project -- IX. Five fully convective M-dwarfs, precisely
measured with CHEOPS and TESS light curves: Eclipsing binaries are important benchmark objects to test and calibrate
stellar structure and evolution models. This is especially true for binaries
with a fully convective M-dwarf component for which direct measurements of
these stars' masses and radii are difficult using other techniques. Within the
potential of M-dwarfs to be exoplanet host stars, the accuracy of theoretical
predictions of their radius and effective temperature as a function of their
mass is an active topic of discussion. Not only the parameters of transiting
exoplanets but also the success of future atmospheric characterisation rely on
accurate theoretical predictions. We present the analysis of five eclipsing
binaries with low-mass stellar companions out of a sub-sample of 23, for which
we obtained ultra high-precision light curves using the CHEOPS satellite. The
observation of their primary and secondary eclipses are combined with
spectroscopic measurements to precisely model the primary parameters and derive
the M-dwarfs mass, radius, surface gravity, and effective temperature estimates
using the PYCHEOPS data analysis software. Combining these results to the same
set of parameters derived from TESS light curves, we find very good agreement
(better than 1\% for radius and better than 0.2% for surface gravity). We also
analyse the importance of precise orbits from radial velocity measurements and
find them to be crucial to derive M-dwarf radii in a regime below 5% accuracy.
These results add five valuable data points to the mass-radius diagram of
fully-convective M-dwarfs. | astro-ph_SR |
High Level Magnetic Activity Nature of an Eclipsing Binary KIC 12418816: We present comprehensive spectroscopic and photometric analysis of the
detached eclipsing binary KIC 12418816, which is composed of two very similar
and young main sequence stars of spectral type K0 on a circular orbit.
Combining spectroscopic and photometric modelling, we find masses and radii of
the components as 0.88+-0.06 Msun and 0.84+-0.05 Msun, and 0.85+-0.02 Rsun, and
0.84+-0.02 Rsun for the primary and the secondary, respectively. Both
components exhibit narrow emission features superposed on the cores of the Ca
II H&K lines, while H alpha and H beta photospheric absoprtion is more
completely infilled by broader emission. Very high precision Kepler photometry
reveals remarkable sinusoidal light variation at out-of-eclipse phases,
indicating strong spot activity, presumably on the surface of the secondary
component. Spots on the secondary component appear to migrate towards
decreasing orbital phases with a migration period of 0.72+-0.05 year. Besides
the sinusoidal variation, we detect 81 flares, and find that both components
possess flare activity. Our analysis shows that 25 flares among 81 come exhibit
very high energies with lower frequency, while the rest of them are very
frequent with lower energies. | astro-ph_SR |
The Initial Magnetic Field Distribution in AB Stars: Stars are born with magnetic fields, but the distribution of their initial
field strengths remains uncertain. We combine observations with theoretical
models of magnetic field evolution to infer the initial distribution of
magnetic fields for AB stars in the mass range of 1.6 - 3.4 M$_{\odot}$. We
tested a variety of distributions with different shapes and found that a
distribution with a mean of $\sim$800 G and a full width of $\sim$600 G is most
consistent with the observed fraction of strongly magnetized stars as a
function of mass. Our most-favored distribution is a Gaussian with a mean of
$\mu$ = 770 G and standard deviation of $\sigma$ = 146 G. Independent
approaches to measure the typical field strength suggest values closer to 2 - 3
kG, a discrepancy that could suggest a mass-dependent and bimodal initial field
distribution, or an alternative theoretical picture for the origin of these
magnetic fields. | astro-ph_SR |
Updated High-Temperature Opacities for the Dartmouth Stellar Evolution
Program and their Effect on the Jao Gap Location: The Jao Gap, a 17 percent decrease in stellar density at $M_G \sim10$
identified in both Gaia DR2 and EDR3 data, presents a new method to probe the
interior structure of stars near the fully convective transition mass. The Gap
is believed to originate from convective kissing instability wherein asymmetric
production of $^{3}$He causes the core convective zone of a star to
periodically expand and contract and consequently the stars' luminosity to
vary. Modeling of the Gap has revealed a sensitivity in its magnitude to a
population's metallicity primarily through opacity. Thus far, models of the Jao
Gap have relied on OPAL high-temperature radiative opacities. Here we present
updated synthetic population models tracing the Gap location modeled with the
Dartmouth stellar evolution code using the OPLIB high-temperature radiative
opacities. Use of these updated opacities changes the predicted location of the
Jao Gap by $\sim0.05$ mag as compared to models which use the OPAL opacities.
This difference is likely too small to be detectable in empirical data. | astro-ph_SR |
On the circumstellar envelopes of semi-regular long-period variables: The mass loss process along the AGB phase is crucial for the formation of
circumstellar envelopes (CSEs), which in the post-AGB phase will evolve into
planetary nebulae (PNe). There are still important issues that need to be
further explored in this field; in particular, the formation of axially
symmetric PNe from spherical CSEs. To address the problem, we have conducted
high S/N IRAM 30m observations of 12CO J=1-0 and J=2-1, and 13CO J=1-0 in a
volume-limited unbiased sample of semi-regular variables (SRs). We also
conducted Yebes 40m SiO J=1-0 observations in 1/2 of the sample in order to
complement our 12CO observations. We report a moderate correlation between mass
loss rates and the 12CO(1-0)-to-12CO(2-1) intensity ratio, introducing a
possible new method for determining mass loss rates of SRs with short analysis
time. We also find that for several stars the SiO profiles are very similar to
the 12CO profiles, a totally unexpected result unless these are non-standard
envelopes. | astro-ph_SR |
Energetic consequences of flux emergence: When magnetic field in the solar convection zone buoyantly rises to pierce
the visible solar surface (photosphere), the atmosphere (corona) above this
surface must respond in some way. One response of the coronal field to
photospheric forcing is the creation of stress in the magnetic field,
generating large currents and storing magnetic free energy. Using a topological
model of the coronal magnetic field we will quantify this free energy. We find
the free energy just prior to major flares in active regions to be between 30%
and 50% of the potential field energy. In a second way, the coronal field may
topologically restructure to form new magnetic connections with newly emerged
fields. We use our topological model to quantify the rapid restructuring in the
case of solar flare and coronal mass ejections, finding that between 1% and 10%
of total active region flux is exchanged. Finally, we use observational data to
quantify the slow, quiescent reconnection with preexisting field, and find that
for small active regions between 20% and 40% of the total emerged flux may have
reconnected at any given time. | astro-ph_SR |
The CIDA Variability Survey of Orion OB1 II: demographics of the young,
low-mass stellar populations: We present results of our large scale, optical, multi-epoch photometric
survey of ~180 square degrees across the Orion OB1 association, complemented
with extensive follow up spectroscopy. We map and characterize in an uniform
way the off-cloud, low-mass, pre-main sequence populations. We report 2064,
mostly K and M-type, confirmed T Tauri members. Most (59%) are located in the
OB1a subassociation, 27% in OB1b, and 14% within the confines of the A and B
molecular clouds. There is significant structure in the spatial distribution of
the young stars. We characterize two new clusterings of T Tauri stars, HD 35762
and HR 1833 in the OB1a subassociation, and two stellar overdensities in OB1b.
There is indication of two populations of young stars in the OB1b region,
located at two different distances, which may be due to the OB1a subassociation
overlapping on front of OB1b. The various groups and regions can be ordered in
an age sequence that agrees with the long standing picture of star formation
starting in Orion OB1a some 10-15 Myr ago.
We define a new type of T Tauri star, the C/W class, objects we propose may
be nearing the end of their accretion phase. We detect the observational
signature of Li depletion in young K and M stars with a timescale of 8.5 Myr.
The decline of the accretion fraction from ~2 - 10 Myr, implies an accretion
e-folding timescale of 2.1 Myr. Finally, the median amplitude of the V-band
variability shows the decline of stellar activity, from accreting Classical T
Tauri stars to the least active field dwarfs. | astro-ph_SR |
Abundances of Suprathermal Heavy Ions in CIRs on STEREO during the
Minimum of Solar Cycle 23: We examine the composition of the 0.1 - 1 MeV/n interplanetary heavy ions
from H to Fe in corotating interaction regions (CIRs) measured by the SIT
(Suprathermal Ion Telescope) instrument. We use observations taken on board the
two STEREO spacecraft during the unusually long minimum of Solar Cycle 23 from
January 2007 through December 2010. During this period instruments on STEREO
observed more than 50 CIR events making it possible to investigate CIR ion
abundances during solar minimum conditions with unprecedentedly high
statistics. The observations reveal annual variations of relative ion
abundances in the CIRs during the 2007 - 2008 period. In 2010 the elemental
composition in CIRs were influenced by solar energetic particle events. | astro-ph_SR |
Coronal radiation belts: The magnetic field of the solar corona has a large-scale dipole character,
which maps into the bipolar field in the solar wind. Using standard
representations of the coronal field, we show that high-energy ions can be
trapped stably in these large-scale closed fields. The drift shells that
describe the conservation of the third adiabatic invariant may have complicated
geometries. Particles trapped in these zones would resemble the Van Allen Belts
and could have detectable consequences. We discuss potential sources of trapped
particles. | astro-ph_SR |
Hydrodynamic models of pulsation period evolution in R Hydrae: Pulsation period evolution during the helium-shell flash in the Mira variable
R Hya is investigated using consistent stellar evolution and non-linear stellar
pulsation computations. The initial and time-dependent inner boundary
conditions for the equations of radiation hydrodynamics describing non-linear
stellar oscillations were determined using a grid of TP-AGB model sequences
with initial masses on the main sequence $1.5M_\odot\le M_\mathrm{ZAMS}\le
5.0M_\odot$ and the initial metallicity $Z=0.014$. The setup of initial
conditions for hydrodynamic models corresponds to $\approx 100$ yr prior to the
maximum of the helium-shell luminosity and ensures that the stellar envelope of
the evolution model is under both hydrostatic and thermal equilibrium. Solution
of the equations of hydrodynamics allowed us to determine the temporal
variation of the pulsation period $\Pi(t)$ during $\approx 500$~yr. Within this
time interval R Hya is a fundamental mode pulsator. The period temporal
dependencies $\Pi(t)$ calculated for the AGB star models at the beginning of
the third dredge-up phase and with masses $4.4M_\odot\le M\le 4.5M_\odot$ are
in agreement with observational estimates of the period of R Hya obtained
during last two centuries. The mean radius of R Hya pulsation models at the end
of the XX century ($470 R_\odot < \bar{R} < 490 R_\odot$) agrees with
observational estimates obtained using the interferometric angular diameter
measurements. | astro-ph_SR |
Parameters of rotating neutron stars with and without hyperons: The discovery of a 2 Msun neutron star provided a robust constraint for the
theory of exotic dense matter, bringing into question the existence of strange
baryons in the interiors of neutron stars. Although many theories fail to
reproduce this observational result, several equations of state containing
hyperons are consistent with it. We study global properties of stars using
equations of state containing hyperons, and compare them to those without
hyperons to find similarities, differences, and limits that can be compared
with the astrophysical observations. Rotating, axisymmetric, and stationary
stellar configurations in general relativity are obtained, and their global
parameters are studied. Approximate formulae describing the behavior of the
maximum and minimum stellar mass, compactness, surface redshifts, and moments
of inertia as functions of spin frequency are provided. We also study the thin
disk accretion and compare the spin-up evolution of stars with different
moments of inertia. | astro-ph_SR |
Observations and numerical models of solar coronal heating associated
with spicules: Spicules have been proposed as significant contributors to the mass and
energy balance of the corona. While previous observations have provided a
glimpse of short-lived transient brightenings in the corona that are associated
with spicules, these observations have been contested and are the subject of a
vigorous debate both on the modeling and the observational side. Therefore, it
remains unclear whether plasma is heated to coronal temperatures in association
with spicules. We use high-resolution observations of the chromosphere and
transition region with the Interface Region Imaging Spectrograph (IRIS) and of
the corona with the Atmospheric Imaging Assembly (AIA) onboard the Solar
Dynamics Observatory (SDO) to show evidence of the formation of coronal
structures associated with spicular mass ejections and heating of plasma to
transition region and coronal temperatures. Our observations suggest that a
significant fraction of the highly dynamic loop fan environment associated with
plage regions may be the result of the formation of such new coronal strands, a
process that previously had been interpreted as the propagation of transient
propagating coronal disturbances (PCD)s. Our observations are supported by 2.5D
radiative MHD simulations that show heating to coronal temperatures in
association with spicules. Our results suggest that heating and strong flows
play an important role in maintaining the substructure of loop fans, in
addition to the waves that permeate this low coronal environment. | astro-ph_SR |
The intermediate neutron-capture process and carbon-enhanced metal-poor
stars: Carbon-enhanced metal-poor (CEMP) stars in the Galactic Halo display
enrichments in heavy elements associated with either the s (slow) or the r
(rapid) neutron-capture process (e.g., barium and europium respectively), and
in some cases they display evidence of both. The abundance patterns of these
CEMP-s/r stars, which show both Ba and Eu enrichment, are particularly puzzling
since the s and the r processes require neutron densities that are more than
ten orders of magnitude apart, and hence are thought to occur in very different
stellar sites with very different physical conditions. We investigate whether
the abundance patterns of CEMP-s/r stars can arise from the nucleosynthesis of
the intermediate neutron-capture process (the i process), which is
characterised by neutron densities between those of the s and the r processes.
Using nuclear network calculations, we study neutron capture nucleosynthesis at
different constant neutron densities n ranging from $10^7$ to $10^{15}$
cm$^{-3}$. With respect to the classical s process resulting from neutron
densities on the lowest side of this range, neutron densities on the highest
side result in abundance patterns that show an increased production of heavy
s-process and r-process elements but similar abundances of the light s-process
elements. Such high values of n may occur in the thermal pulses of asymptotic
giant branch (AGB) stars due to proton ingestion episodes. Comparison to the
surface abundances of 20 CEMP-s/r stars show that our modelled i-process
abundances successfully reproduce observed abundance patterns that could not be
previously explained by s-process nucleosynthesis. Because the i-process models
fit the abundances of CEMP-s/r stars so well, we propose that this class should
be renamed as CEMP-i. | astro-ph_SR |
Episodic modulations in supernova radio light curves from luminous blue
variable supernova progenitor models: <Context> Ideally, one would like to know which type of core-collapse SNe is
produced by different progenitors and the channels of stellar evolution leading
to these progenitors. These links have to be very well known to use the
observed frequency of different types of SN events for probing the star
formation rate and massive star evolution in different types of galaxies.
<Aims> We investigate the link between LBV as SN progenitors and the appearance
of episodic radio light curve modulations of the SN event. <Methods> We use the
20Msun and 25Msun models with rotation at solar metallicity, part of an
extended grid of stellar models computed by the Geneva team. At their pre-SN
stage, these two models have recently been shown to have spectra similar to
those of LBV stars and possibly explode as Type IIb SNe. Based on the wind
properties before the explosion, we derive the density structure of their
circumstellar medium. This structure is used as input for computing the SN
radio light curve. <Results> We find that the 20Msun model shows radio light
curves with episodic luminosity modulations, similar to those observed in some
Type IIb SNe. This occurs because the evolution of the 20Msun model terminates
in a region of the HR diagram where radiative stellar winds present strong
density variations, caused by the bistability limit. The 25Msun model, ending
its evolution in a zone of the HR diagram where no change of the mass-loss
rates is expected, presents no such modulations in its radio SN light curve.
<Conclusions> Our results reinforce the link between SN progenitors and LBV
stars. We also confirm the existence of a physical mechanism for a single star
to have episodic radio light curve modulations. In the case of the 25Msun
progenitors, we do not obtain modulations in the radio light curve, but our
models may miss some outbursting behavior in the late stages of massive stars. | astro-ph_SR |
Determination of stellar atmospheric parameters for a sample of the
post-AGB stars: We report for the first time the stellar atmospheric parameters for a a set
of post-AGB stars classified by Suarez et al. (2006). The stellar spectra were
taken from optical region, with low-resolution and have different spectral
ranges. We select a sample of 70 objects with A-K spectral types and
luminosities I and Ie. The large majority of these objects have been scarcely
studied and are located toward the galactic south pole region. We employ a set
of empirical relationships that use pseudo-equivalent widths like spectral
feature to estimate effective temperature, surface gravity and metallicity. The
criteria chosen for selection of absorption are similar to employed by MK
classification system. | astro-ph_SR |
Preliminary results of giant pulse investigations from Crab pulsar with
Radioastron: Giant pulses from Crab pulsar were observed together with Radioastron space
radiotelescope, Global EVN radio telescopes and Kvazar-KVO VLBI stations to
study the scattering effects in the ISM. Five observing sessions were conducted
at 18 cm (EVN codes: EG060A, EG060B, EG067B, EG075) and one at 92 cm (EVN code:
GS033A). We have estimated distance to the scattering screen, angular size of
scattering disk, scattering time. All estimations were done in an assumption of
single thin scattering screen. Also, a significant change in the shape of
cross-correlation functions for space-ground baselines was found (starting from
4 up to 12 Earth diameters). | astro-ph_SR |
Oscillator Strength Measurements of Atomic Absorption Lines from Stellar
Spectra: We develop a new method to determine oscillator strength values of atomic
absorption lines with state-of-the-art detailed spectral synthesis calculations
of the optical spectrum of the Sun and of standard spectral reference stars. We
update the log(gf)-values of 911 neutral lines observed in the KPNO-FTS flux
spectrum of the Sun and high-resolution echelle spectra (R=80,000) of Procyon
(F5 IV-V) and {\epsilon} Eri (K2 V) observed with large signal-to-noise (S/N)
ratios of ~2,000 using the new Mercator-Hermes spectrograph at La Palma
Observatory (Spain). We find for 483 Fe I, 85 Ni I, and 51 Si I absorption
lines in the sample a systematic over-estimation of the literature
log(gf)-values with central line depths below 15 %. We employ a curve-of-growth
analysis technique to test the accuracy of the new oscillator strength values
and compare calculated equivalent line widths to the Moore, Minnaert, &
Houtgast atlas of the Sun. The online SpectroWeb database at
http://spectra.freeshell.org interactively displays the observed and synthetic
spectra and provides the new log(gf)-values together with important atomic line
data. The graphical database is under development for stellar reference spectra
of every spectral sub-class observed with large spectral resolution and S/N
ratios. | astro-ph_SR |
Search for aluminium monoxide in the winds of oxygen-rich AGB stars: Aluminium monoxide, AlO, is likely efficiently depleted from the gas around
oxygen-rich evolved stars to form alumina clusters and dust seeds. Its presence
in the extended atmospheres of evolved stars has been derived from optical
spectroscopy. More recently, AlO gas was also detected at long wavelengths
around the supergiant VY CMa and the oxygen-rich asymptotic giant branch (AGB)
star o Cet (Mira A). In search of AlO, we mined data obtained with APEX, the
IRAM 30m telescope, Herschel/HIFI, SMA, and ALMA, which were primarily aimed at
studying other molecular species. We report here on observations of AlO towards
a sample of eight oxygen-rich AGB stars in different rotational transitions, up
to seven for some stars. We present definite detections of one rotational
transition of AlO for o Cet and R Aqr, and tentative detections of one
transition for R Dor and o Cet, and two for IK Tau and W Hya. The presented
spectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R
Aqr, and IK Tau, we find that the AlO(N=9-8) emission likely traces the inner
parts of the wind, out to only a few tens of AU, where the gas has not yet
reached its terminal velocity. The conclusive detections of AlO emission in the
case of o Cet and R Aqr confirm the presence of AlO gas in outflows of AGB
stars. The tentative detections further support this. Since most of the
observations presented in this study were obtained with stronger emission from
other species than AlO in mind, observations with higher sensitivity in
combination with high angular resolution will improve our understanding of the
presence and behaviour of AlO. From the current data sets we cannot firmly
conclude whether there is a direct correlation between the wind properties and
the detection rate of AlO emission. We hope that this study can serve as a
stimulus to perform sample studies in search of AlO in oxygen-rich outflows. | astro-ph_SR |
Chemical Abundance Signature of J0023+0307 -- A Second-Generation
Main-Sequence Star with [Fe/H]<-6: We present a chemical abundance analysis of the faint halo metal-poor
main-sequence star J0023+0307, with [Fe/H]<-6.3, based on a high-resolution
(R~35,000) Magellan/MIKE spectrum. The star was originally found to have
[Fe/H]< -6.6 based on a Ca II K measurement in an R~2,500 spectrum. No iron
lines could be detected in our MIKE spectrum. Spectral lines of Li, C, Na, Mg,
Al, Si, and Ca were detected. The Li abundance is close to the Spite Plateau,
A(Li) = 1.7, not unlike that of other metal-poor stars although in stark
contrast to the extremely low value found e.g., in HE~1327-2326 at a similar
[Fe/H] value. The carbon G-band is detected and indicates strong C-enhancement,
as is typical for stars with low Fe abundances. Elements from Na through Si
show a strong odd-even effect, and J0023+0307 displays the second-lowest known
[Ca/H] abundance. Overall, the abundance pattern of J0023+0307 suggests that it
is a second-generation star that formed from gas enriched by a massive
Population III first star exploding as a fall-back supernova The inferred
dilution mass of the ejecta is 10^(5+-0.5) Msun of hydrogen, strongly
suggesting J0023+0307 formed in a recollapsed minihalo. J0023+0307 is likely
very old because it has a very eccentric orbit with a pericenter in the
Galactic bulge. | astro-ph_SR |
Protostellar Evolution in Serpens Main: Possible Origin of Disk-Size
Diversity: We have observed the submillimeter continuum condensations SMM2, SMM4, SMM9,
and SMM11 in the star forming cluster Serpens Main using the Atacama Large
Millimeter/submillimeter Array during Cycle 3 in the 1.3 mm continuum, 12CO
J=2-1, SO J_N=6_5-5_4, and C18O J=2-1 lines at an angular resolution of ~0.55
(240 au). Sixteen sources have been detected in the 1.3 mm continuum, which can
be classified into three groups. Group 1 consists of six sources showing
extended continuum emission and bipolar/monopolar 12CO outflows. Although all
the Group 1 members are classified as Class 0 protostars, our observations
suggest evolutionary trends among them in terms of 12CO outflow dynamical time,
SO emission distribution, C18O fractional abundance, and continuum morphology.
Group 2 consists of four sources associated with a continuum filamentary
structure and no 12CO outflows. Central densities estimated from the 1.3 mm
continuum intensity suggest that they are prestellar sources in a marginally
Jeans unstable state. Group 3 consists of six Spitzer sources showing
point-like 1.3 mm continuum emission and clumpy 12CO outflows. These features
of Group 3 suggest envelope dissipation, preventing disk growth from the
present size, r <~ 60 au. The Group 3 members are protostars that may be
precursors to the T Tauri stars associated with small disks at tens-au radii
identified in recent surveys. | astro-ph_SR |
3-D Kinematics of the near-IR HH 223 outflow in L723: In this work we derive the full 3-D kinematics of the near-infrared outflow
HH 223, located in the dark cloud Lynds 723 (L723), where a well-defined
quadrupolar CO outflow is found. HH 223 appears projected onto the two lobes of
the east-west CO outflow. The radio continuum source VLA 2, towards the centre
of the CO outflow, harbours a multiple system of low-mass young stellar
objects. One of the components has been proposed to be the exciting source of
the east-west CO outflow.
From the analisys of the kinematics, we get further evidence on the
relationship between the near-infrared and CO outflows and on the location of
their exciting source. The proper motions were derived using multi-epoch,
narrow-band H$_2$ (2.122 $\mu$m line) images. Radial velocities were derived
from the 2.122 $\mu$m line of the spectra. Because of the extended (~5 arcmin),
S-shaped morphology of the target, the spectra were obtained with the
Multi-Object-Spectroscopy (MOS) observing mode using the instrument LIRIS at
the 4.2m William Herschel Telescope. To our knowledge, this work is the first
time that MOS observing mode has been successfully used in the near infrared
range for an extended target. | astro-ph_SR |
Mind the Gap I: H$α$ Activity of M Dwarfs Near the Partially/Fully
Convective Boundary and a New H$α$ Emission Deficiency Zone on the Main
Sequence: Since identifying the gap in the H-R Diagram (HRD) marking the transition
between partially and fully convective interiors, a unique type of slowly
pulsating M dwarf has been proposed. These unstable M dwarfs provide new
laboratories in which to understand how changing interior structures can
produce potentially observable activity at the surface. In this work, we report
the results of the largest high-resolution spectroscopic H$\alpha$ emission
survey to date spanning this transition region, including 480 M dwarfs observed
using the CHIRON spectrograph at CTIO/SMARTS 1.5-m. We find that M dwarfs with
H$\alpha$ in emission are almost entirely found 0 to 0.5 magnitude above the
top edge of the gap in the HRD, whereas effectively no stars in and below the
gap show emission. Thus, the top edge of the gap marks a relatively sharp
activity transition, and there is no anomalous H$\alpha$ activity for stars in
the gap. We also identify a new region at 10.3 $<M_{G}<$ 10.8 on the main
sequence where fewer M dwarfs exhibit H$\alpha$ emission compared to M dwarfs
above and below this magnitude range. Careful evaluation of literature results
indicates that 1) rotation and H$\alpha$ activity distributions on the main
sequence are closely related, and 2) fewer stars in this absolute magnitude
range rotate in less than $\sim$13 days than populations surrounding this
region. This result suggests that the most massive fully convective stars lose
their angular momentum faster than both partially convective stars and less
massive fully convective stars. | astro-ph_SR |
Non-Ideal Magnetohydrodynamic Simulations of the Two-Stage Fragmentation
Model for Cluster Formation: We model molecular cloud fragmentation with thin disk non-ideal
magnetohydrodynamic simulations that include ambipolar diffusion and partial
ionization that transitions from primarily ultraviolet dominated to cosmic ray
dominated regimes. These simulations are used to determine the conditions
required for star clusters to form through a two-stage fragmentation scenario.
Recent linear analyses have shown that the fragmentation length and time scales
can undergo a dramatic drop across the column density boundary that separates
the ultraviolet and cosmic ray dominated ionization regimes. As found in
earlier studies, the absence of an ionization drop and regular perturbations
leads to a single-stage fragmentation on parsec scales in transcritical clouds,
so that the nonlinear evolution yields the same fragment sizes as predicted by
linear theory. However, we find that a combination of initial transcritical
mass-to-flux ratio, evolution through a column density regime in which the
ionization drop takes place, and regular small perturbations to the
mass-to-flux ratio are sufficient to cause a second stage of fragmentation
during the nonlinear evolution. Cores of size ~0.1 pc are formed within an
initial fragment of ~ pc size. Regular perturbations to the mass-to-flux ratio
also accelerate the onset of runaway collapse. | astro-ph_SR |
Correlated Strontium and Barium Isotopic Compositions of Acid-Cleaned
Single Silicon Carbides from Murchison: We present strontium, barium, carbon, and silicon isotopic compositions of 61
acid-cleaned presolar SiC grains from Murchison. Comparison with previous data
shows that acid washing is highly effective in removing both strontium and
barium contamination. For the first time, by using correlated
$^{88}Sr$/$^{86}Sr$ and $^{138}Ba$/$^{136}Ba$ ratios in mainstream SiC grains,
we are able to resolve the effect of $^{13}C$ concentration from that of
$^{13}C$-pocket mass on s-process nucleosynthesis, which points towards the
existence of large $^{13}C$-pockets with low $^{13}C$ concentration in AGB
stars. The presence of such large $^{13}$R-pockets with a variety of relatively
low $^{13}C$ concentrations seems to require multiple mixing processes in
parent AGB stars of mainstream SiC grains. | astro-ph_SR |
Gravitational waves from pulsars in the context of magnetic ellipticity: In one of our previous articles we have considered the role of a time
dependent magnetic ellipticity on pulsars' braking indices and on the putative
gravitational waves these objects can emit. Since only nine of more than 2000
known pulsars have accurately measured braking indices, it is of interest to
extend this study to all known pulsars, in particular to what concerns the
gravitational waves generation. To do so, as shown in our previous article, we
need to know some pulsars' observable quantities such as: periods and their
time derivatives, and estimated distances to the Earth. Moreover, we also need
to know the pulsars' masses and radii, for which, in here we are adopting
current fiducial values. Our results show that the gravitational wave amplitude
is at best $h \sim 10^{-28}$. This leads to a pessimistic prospect for the
detection of gravitational waves generated by these pulsars, even for Advanced
LIGO and Advanced Virgo, and the planned Einstein Telescope, whether the
ellipticity has magnetic origin. | astro-ph_SR |
HST Low Resolution Stellar Library: Hubble Space Telescope's (HST) Space Telescope Imaging Spectrograph (STIS)
targeted 556 stars in a long-running program called Next Generation Spectral
Library (NGSL) via proposals GO9088, GO9786, GO10222, and GO13776. Exposures
through three low resolution gratings provide wavelength coverage from 0.2 $<
\lambda <$ 1 $\mu$m at $\lambda/\Delta\lambda\sim$ 1000, providing unique
coverage in the ultraviolet (UV). The UV grating (G230LB) scatters red light
and this results in unwanted flux that becomes especially troubling for cool
stars. We applied scattered light corrections based on
\cite{2022stis.rept....5W} and flux corrections arising from pointing errors
relative to the center of the 0\farcs2 slit. We present 514 fully reduced
spectra, fluxed, dereddened, and cross-correlated to zero velocity. Because of
the broad spectral range, we can simultaneously study H$\alpha$ and Mg II
$\lambda$2800, indicators of chromospheric activity. Their behaviours are
decoupled. Besides three cool dwarfs and one giant with mild flares in
H$\alpha$, only Be stars show strong H$\alpha$ emission. Mg2800 emission,
however, strongly anti-correlates with temperature such that warm stars show
absorption and stars cooler than $5000 \: \! \rm{K}$ universally show
chromospheric emission regardless of dwarf/giant status or metallicity.
Transformed to Mg2800 flux emerging from the stellar surface, we find a
correlation with temperature with approximately symmetric astrophysical
scatter, in contrast to other workers who find a basal level with asymmetric
scatter to strong values. Unsurprisingly, we confirm that Mg2800 activity is
variable. | astro-ph_SR |
HAZMAT VI: The Evolution of Extreme Ultraviolet Radiation Emitted from
Early M Star: Quantifying the evolution of stellar extreme ultraviolet (EUV, 100 -- 1000
$\overset{\circ}{A}$) emission is critical for assessing the evolution of
planetary atmospheres and the habitability of M dwarf systems. Previous studies
from the HAbitable Zones and M dwarf Activity across Time (HAZMAT) program
showed the far- and near-UV (FUV, NUV) emission from M stars at various stages
of a stellar lifetime through photometric measurements from the Galaxy
Evolution Explorer (GALEX). The results revealed increased levels of
short-wavelength emission that remain elevated for hundreds of millions of
years. The trend for EUV flux as a function of age could not be determined
empirically because absorption by the interstellar medium prevents access to
the EUV wavelengths for the vast majority of stars. In this paper, we model the
evolution of EUV flux from early M stars to address this observational gap. We
present synthetic spectra spanning EUV to infrared wavelengths of 0.4 $\pm$
0.05 M$_{\odot}$ stars at five distinct ages between 10 and 5000 Myr, computed
with the PHOENIX atmosphere code and guided by the GALEX photometry. We model a
range of EUV fluxes spanning two orders of magnitude, consistent with the
observed spread in X-ray, FUV, and NUV flux at each epoch. Our results show
that the stellar EUV emission from young M stars is 100 times stronger than
field age M stars, and decreases as t$^{-1}$ after remaining constant for a few
hundred million years. This decline stems from changes in the chromospheric
temperature structure, which steadily shifts outward with time. Our models
reconstruct the full spectrally and temporally resolved history of an M star's
UV radiation, including the unobservable EUV radiation, which drives planetary
atmospheric escape, directly impacting a planet's potential for habitability. | astro-ph_SR |
The (non-)variability of magnetic chemically peculiar candidates in the
Large Magellanic Cloud: The galactic magnetic chemically peculiar (mCP) stars of the upper main
sequence are well known as periodic spectral and light variables. The observed
variability is obviously caused by the uneven distribution of overabundant
chemical elements on the surfaces of rigidly rotating stars. The mechanism
causing the clustering of some chemical elements into disparate structures on
mCP stars has not been fully understood up to now. The observations of light
changes of mCP candidates recently revealed in the nearby Large Magellanic
Cloud (LMC) should provide us with information about their rotational periods
and about the distribution of optically active elements on mCP stars born in
other galaxies. We queried for photometry at the Optical Gravitational Lensing
Experiment (OGLE)-III survey of published mCP candidates selected because of
the presence of the characteristic 5200A flux depression. In total, the
intersection of both sources resulted in twelve stars. We performed our own and
standard periodogram time series analyses of all available data. The final
results are, amongst others, the frequency of the maximum peak and the
bootstrap probability of its reality. We detected that only two mCP candidates,
190.1 1581 and 190.1 15527, may show some weak rotationally modulated light
variations with periods of 1.23 and 0.49 days; however, the 49% and 32%
probabilities of their reality are not very satisfying. The variability of the
other 10 mCP candidates is too low to be detectable by their V and I OGLE
photometry. The relatively low amplitude variability of the studied LMC mCP
candidates sample can be explained by the absence of photometric spots of
overabundant optically active chemical elements. The unexpected LMC mCPs
behaviour is probably caused by different conditions during the star formation
in the LMC and the Galaxy. | astro-ph_SR |
Gaia18aen: First symbiotic star discovered by Gaia: Besides the astrometric mission of the Gaia satellite, its repeated and
high-precision measurements serve also as an all-sky photometric transient
survey. The sudden brightenings of the sources are published as Gaia
Photometric Science Alerts and are made publicly available allowing the
community to photometrically and spectroscopically follow-up the object. The
goal of this paper was to analyze the nature and derive the basic parameters of
Gaia18aen, transient detected at the beginning of 2018. It coincides with the
position of the emission line star WRAY 15-136. The brightening was classified
as a "nova?" on the basis of subsequent spectroscopic observation. We have
analyzed two spectra of Gaia18aen and collected the available photometry of the
object covering the brightenings in 2018 and also the preceding and following
periods of quiescence. Based on this observational data, we have derived the
parameters of Gaia18aen and discussed the nature of the object. Gaia18aen is
the first symbiotic star discovered by the Gaia satellite. The system is an
S-type symbiotic star and consists of an M giant of a slightly super-solar
metallicity, with Teff ~3500 K, a radius of ~230 R$\odot$, and a high
luminosity L ~7400 L$\odot$. The hot component is a hot white dwarf. We
tentatively determined the orbital period of the system ~487 days. The main
outburst of Gaia18aen in 2018 was accompanied by a decrease in the temperature
of the hot component. The first phase of the outburst was characterized by the
high luminosity L ~27000 L$\odot$, which remained constant for about three
weeks after the optical maximum, later followed by the gradual decline of
luminosity and increase of temperature. Several re-brightenings have been
detected on the timescales of hundreds of days. | astro-ph_SR |
Transit Timing Observations of the Extrasolar Hot-Neptune Planet GL 436b: Gliese 436 is an M dwarf with a mass of 0.45 Msun and hosts the extrasolar
planet GL 436b [3, 6, 7, 2], which is currently the least massive transiting
planet with a mass of ~23.17 Mearth [10], and the only planet known to transit
an M dwarf. GL 436b represents the first transiting detection of the class of
extrasolar planets known as "Hot Neptunes" that have masses within a few times
that of Neptune's mass (~17 Mearth) and orbital semimajor axis <0.1 AU about
the host star. Unlike most other known transiting extrasolar planets, GL 436b
has a high eccentricity (e~0.16). This brings to light a new parameter space
for habitability zones of extrasolar planets with host star masses much smaller
than typical stars of roughly a solar mass. This unique system is an ideal
candidate for orbital perturbation and transit-time variation (TTV) studies to
detect smaller, possibly Earth-mass planets in the system. In April 2008 we
began a long-term intensive campaign to obtain complete high-precision light
curves using the Apache Point Observatory's 3.5-meter telescope, NMSU's 1-meter
telescope (located at APO), and Sommers Bausch Observatory's 24" telescope.
These light curves are being analyzed together, along with amateur and other
professional astronomer observations. Results of our analysis are discussed.
Continued measurements over the next few years are needed to determine if
additional planets reside in the system, and to study the impact of other
manifestations on the light curves, such as star spots and active regions. | astro-ph_SR |
Young Stars near Cometary Globule CG 30 in the Tumultuous Gum Nebula: We have conducted a high-dispersion ($R \sim$ 34,000) optical spectroscopic
study of 10 young stars near the cometary gloule CG 30 in the Gum Nebula, a
diffuse H \textsc{ii} region home to at least 32 cometary globules. All 10
spectroscopically observed stars at the nebula's northern edge are of low mass
(spectral types M4.5 - K5), have broad H$\alpha$ emission, and show spectral
veiling. Eight of the 10 are classical T Tauri stars. We spectroscopically
measure the photospheric properties of CG 30 IRS 4 inside CG 30. Though
embedded, CG 30 IRS 4 is T Tauri-like, with relatively slow projected rotation
and moderate veiling. Undepleted Li absorptions, strong H$\alpha$ emissions,
and positions well above the main sequence on an HR diagram suggest the 10
stars are $\lesssim$1 Myr old. Using our measurements, previous spectroscopy,
and previous photometry of 11 other young stars in the area, we determine
stellar, kinematic, and accretion properties of a total of 21 young stars.
Shared radial velocities, proper motions, distances, and ages suggest 14 of the
young stars (including CG 30 IRS 4) are kinematically related to CG 30. From
\textit{Gaia} DR2 distances to 6 of these stars, we derive a distance of
$358.1\pm2.2$ pc to the cometary globule complex CG 30/31/38. The CG 30
association has an accretor fraction of $29\pm14$\%, low for quiescent clusters
of similar age but consistent with other irradiated clusters. The Gum Nebula's
moderate radiation environment ($G_0=6.6^{+3.2}_{-2.7}$ at CG 30) may be strong
enough to shorten disk lifetimes. | astro-ph_SR |
RR Lyrae stars in the globular cluster Palomar 2: A CCD VI imaging time-series over 11-year is employed to explore the light
curves of stars in the field of Palomar 2. We discovered 20 RRab and 1 RRc
variables. A revision of Gaia-DR3 data enabled us to identify 10 more variables
and confirm the RRab nature of 6 of them and one RGB. The cluster membership is
discussed and 18 variables are most likely cluster members. The Fourier light
curve decomposition for the 11 best quality light curves of cluster member
stars leads to independent estimates of the cluster distance 27.2 +- 1.8 kpc
and [Fe/H]ZW=-1.39 +- 0.55. We confirm the cluster as of the Oo I type. | astro-ph_SR |
Observed luminosity spread in young clusters and Fu Ori stars: a unified
picture: The idea that non steady accretion during the embedded phase of protostar
evolution can produce the observed luminosity spread in the Herzsprung-Russell
diagram (HRD) of young clusters has recently been called into question.
Observations of Fu Ori, for instance, suggest an expansion of the star during
strong accretion events whereas the luminosity spread implies a contraction of
the accreting objects, decreasing their radiating surface. In this paper, we
present a global scenario based on calculations coupling episodic accretion
histories derived from numerical simulations of collapsing cloud prestellar
cores of various masses and subsequent protostar evolution. Our calculations
show that, assuming an initial protostar mass $\mi \sim 1\,\mjup$, typical of
the second Larson's core, both the luminosity spread in the HRD and the
inferred properties of Fu Ori events (mass, radius, accretion rate) can be
explained by this scenario, providing two conditions. First, there must be some
variation within the fraction of accretion energy absorbed by the protostar
during the accretion process. Second the range of this variation should
increase with increasing accretion burst intensity, and thus with the initial
core mass and final star mass. The numerical hydrodynamics simulations of
collapsing cloud prestellar cores indeed show that the intensity of the
accretion bursts correlates with the mass and initial angular momentum of the
prestellar core. Massive prestellar cores with high initial angular momentum
are found to produce intense bursts characteristic of Fu Ori like events. Our
results thus suggest a link between the burst intensities and the fraction of
accretion energy absorbed by the protostar, with some threshold in the
accretion rate, of the order of $10^{-5}\msolyr$, delimitating the transition
from "cold" to "hot" accretion. [Abridged] | astro-ph_SR |
Large-amplitude longitudinal oscillations in solar prominences simulated
with different resolutions: Large-amplitude longitudinal oscillations (LALOs) in solar prominences have
been widely studied in the last decades. However, their damping and
amplification mechanisms are not well understood. In this study, we investigate
the attenuation and amplification of LALOs using high-resolution numerical
simulations with progressively increasing spatial resolutions. We performed
time-dependent numerical simulations of LALOs using the 2D magnetic
configuration that contains a dipped region. After the prominence mass loading
in the magnetic dips, we triggered LALOs by perturbing the prominence mass
along the magnetic field. We performed the experiments with four values of
spatial resolution. In the simulations with the highest resolution, the period
shows a good agreement with the pendulum model. The convergence experiment
revealed that the damping time saturates at the bottom prominence region with
improving the resolution, indicating the existence of a physical reason for the
damping of oscillations. At the prominence top, the oscillations are amplified
during the first minutes and then are slowly attenuated. The characteristic
time suggests more significant amplification in the experiments with the
highest spatial resolution. The analysis revealed that the energy exchange
between the bottom and top prominence regions is responsible for the
attenuation and amplification of LALOs. The high-resolution experiments are
crucial for the study of the periods and the damping mechanism of LALOs. The
period agrees with the pendulum model only when using high enough spatial
resolution. The results suggest that numerical diffusion in simulations with
insufficient spatial resolution can hide important physical mechanisms, such as
amplification of oscillations. | astro-ph_SR |
Optical and X-ray studies of three polars: RX J0859.1+0537, RX
J0749.1-0549, and RX J0649.8-0737: We present optical photometric and spectroscopic observations, and an
analysis of archival X-ray data of three polars: RX J0859.1+0537, RX
J0749.1-0549, and RX J0649.8-0737. Optical light curves of these three polars
reveal eclipse features that are deep, total, and variable in shape. The
optical and X-ray modulations of RX J0859.1+0537, RX J0749.1-0549, and RX
J0649.8-0737 are both found to occur at the orbital periods of 2.393$\pm$0.003
hrs, 3.672$\pm$0.001 hrs, and 4.347$\pm$0.001 hrs, respectively. RX
J0859.1+0537 is found to be an eclipsing polar which lies in the region of the
period gap, whereas RX J0749.1-0549 and RX J0649.8-0737 are found to be
long-period eclipsing polars above the period gap. The eclipse length is found
to be 61 min for RX J0749.1-0549 in the R-band, which is the highest among the
long period eclipsing polars. The radius of the eclipsed light source is found
to be more than the actual size of the white dwarf for these three systems,
indicating that the eclipsed component is not only the white dwarf but also
appears to include the presence of an extended accretion region. Optical
spectra of these systems show the presence of high ionization emission lines
along with the strong Balmer emission lines with an inverted Balmer decrement.
Cyclotron harmonics are also detected in the optical spectra from which we
infer magnetic field strength of the surface of the white dwarf to be 49$\pm$2
MG, 43.5$\pm$1.4 MG, and 44$\pm$1 MG for RX J0859.1+0537, RX J0749.1-0549, and
RX J0649.8-0737, respectively. | astro-ph_SR |
On the Performance of Multi-Instrument Solar Flare Observations During
Solar Cycle 24: The current fleet of space-based solar observatories offers us a wealth of
opportunities to study solar flares over a range of wavelengths. Significant
advances in our understanding of flare physics often come from coordinated
observations between multiple instruments. Consequently, considerable efforts
have been, and continue to be made to coordinate observations among instruments
(e.g. through the Max Millennium Program of Solar Flare Research). However,
there has been no study to date that quantifies how many flares have been
observed by combinations of various instruments. Here we describe a technique
that retrospectively searches archival databases for flares jointly observed by
RHESSI, SDO/EVE (MEGS-A and -B), Hinode/(EIS, SOT, and XRT), and IRIS. Out of
the 6953 flares of GOES magnitude C1 or greater that we consider over the 6.5
years after the launch of SDO, 40 have been observed by six or more instruments
simultaneously. Using each instrument's individual rate of success in observing
flares, we show that the numbers of flares co-observed by three or more
instruments are higher than the number expected under the assumption that the
instruments operated independently of one another. In particular, the number of
flares observed by larger numbers of instruments is much higher than expected.
Our study illustrates that these missions often acted in cooperation, or at
least had aligned goals. We also provide details on an interactive widget now
available in SSWIDL that allows a user to search for flaring events that have
been observed by a chosen set of instruments. This provides access to a broader
range of events in order to answer specific science questions. The difficulty
in scheduling coordinated observations for solar-flare research is discussed
with respect to instruments projected to begin operations during Solar Cycle
25, such as DKIST, Solar Orbiter, and Parker Solar Probe. | astro-ph_SR |
Polarization due to rotational distortion in the bright star Regulus: Polarization in stars was first predicted by Chandrasekhar [1] who calculated
a substantial linear polarization at the stellar limb for a pure
electron-scattering atmosphere. This polarization will average to zero when
integrated over a spherical star but could be detected if the symmetry is
broken, for example by the eclipse of a binary companion. Nearly 50 years ago,
Harrington and Collins [2] modeled another way of breaking the symmetry and
producing net polarization - the distortion of a rapidly rotating hot star.
Here we report the first detection of this effect. Observations of the linear
polarization of Regulus, with two different high-precision polarimeters, range
from +42 parts-per-million (ppm) at a wavelength of 741 nm to -22 ppm at 395
nm. The reversal from red to blue is a distinctive feature of rotation-induced
polarization. Using a new set of models for the polarization of rapidly
rotating stars we find that Regulus is rotating at 96.5$\substack{+0.6-0.8}$%
of its critical angular velocity for breakup, and has an inclination greater
than 76.5 degrees. The rotation axis of the star is at a position angle of
79.5$\pm$0.7 degrees. The conclusions are independent of, but in good agreement
with, the results of previously published interferometric observations of
Regulus [3]. The accurate measurement of rotation in early-type stars is
important for understanding their stellar environments [4], and course of their
evolution [5]. | astro-ph_SR |
Sunspot Rotation as a Driver of Major Solar Eruptions in NOAA Active
Region 12158: We studied the developing conditions of sigmoid structure under the influence
of magnetic non-potential characteristics of a rotating sunspot in the active
region (AR) 12158. Vector magnetic field measurements from Helioseismic
Magnetic Imager and coronal EUV observations from Atmospheric Imaging Assembly
reveal that the erupting inverse-S sigmoid had roots in the location of the
rotating sunspot. Sunspot rotates at a rate of 0-5deg/h with increasing trend
in the first half followed by a decrease. Time evolution of many non-potential
parameters had a well correspondence with the sunspot rotation. The evolution
of the AR magnetic structure is approximated by a time series of force free
equilibria. The NLFFF magnetic structure around the sunspot manifests the
observed sigmoid structure. Field lines from the sunspot periphery constitute
the body of the sigmoid and those from interior overly the sigmoid similar to a
fluxrope structure. While the sunspot is being rotating, two major CME
eruptions occurred in the AR. During the first (second) event, the coronal
current concentrations enhanced (degraded) consistent with the photospheric net
vertical current, however the magnetic energy is released during both the
cases. The analysis results suggest that the magnetic connections of the
sigmoid are driven by slow motion of sunspot rotation, which transforms to a
highly twisted flux rope structure in a dynamical scenario. An exceeding
critical twist in the flux rope probably leads to the loss of equilibrium and
thus triggering the onset of two eruptions. | astro-ph_SR |
Formation and Eruption of Hot Channels during an M6.5 Class Solar Flare: We investigate the formation and eruption of hot channels associated with the
M6.5 class flare (SOL2015-06-22T18:23) occurring in NOAA AR 12371 on 2015 June
22. Two flare precursors are observed before the flare main phase. Observations
in 94 {\AA} and 131 {\AA} by SDO/AIA have revealed the early morphology of the
first hot channel as a group of hot loops, which is termed as seed hot channel.
A few seed hot channels are formed above the polarity inversion line (PIL) and
the formation is associated with footpoint brightenings' parallel motion along
the PIL, which proceeds into the early stage of the flare main phase. During
this process, seed hot channels build up and rise slowly, being accelerated at
the peak of the second precursor. They merge in the process of acceleration
forming a larger hot channel, which then forms an "inverted {\gamma}" shape
kinking structure. Before the flare peak, the second kinking hot channel with
negative crossing appears near the first kinking hot channel that has erupted.
The eruption of these two hot channels produce two peaks on the main flare's
GOES light curve. The footpoint brightenings' propagation along the PIL
indicate that the first kinking hot channel may be formed due to zipper
reconnection. The occurrence of merging between seed hot channels observed by
AIA is supported by the extrapolated nonlinear force-free field models. The
observed writhing motion of the first kinking hot channel may be driven by the
Lorentz force. | astro-ph_SR |
MIDI interferometry of massive YSOs: Updates on the MPIA programme: Information about the inner structure of young stellar objects is crucial for
understanding how the central forming stars gain their mass. However,
especially for observations of (usually further away) high-mass young stellar
objects, conventional imaging has limitations in spatial resolution. By means
of mid-infrared interferometry, we can peek deeply into the strongly
extinguished central 100 AU of such objects. Here, we report on new data we
obtained within our programme using MIDI at the VLTI. Shown are preliminary
results on the known outflow sources AFGL 2136 IRS 1 and Mon R2 IRS 3A. In
particular, we describe how quantities like differential phases give additional
geometrical structure information. We demonstrate how the combined
interpretation of single-telescope and interferometric data at different
wavelength regimes can lead to a more complete picture of the nature of such
MYSOs. | astro-ph_SR |
Evolutionary tracks of massive stars with different rotation and
metallicity in neutrino H-R diagram: Neutrino losses play a crucial role in the evolution of massive stars. We
study the neutrino luminosity of stars ranging from 20 to 90 M_{\odot} from
Zero Age Main Sequence (ZAMS) to Fe Core Collapse (FeCC) with different
rotation and metallicity in a neutrino Hertzsprung-Russell diagram. In our
simulations, we consider {\omega}/{\omega}crit = 0 and 0.7 to represent
non-rotation and high rotation, respectively, and set the metallicities to
0.014, 0.001, and 0.0001. During hydrogen burning stages, neutrino luminosity
primarily originates from CNO cycle, and increases with higher stellar mass
while decreasing with increasing metallicity. For the high metallicity models
(Z = 0.014) during the helium burning stage, the reduction of the hydrogen
envelope caused by a larger mass loss rate leads to a gradual decrease in
neutrino luminosity. The rapid rotation results in extra mixing inside massive
stars, which increases the neutrino luminosity during main sequence (MS), while
decreases the neutrino luminosity during helium burning phase. Simultaneously,
the rapid rotation also increases CO core mass, which enhances the neutrino
luminosity during C and O burning phase. We also investigate the effect of
neutrino magnetic moment (NMM) on the massive stars. We find that the energy
loss caused by the NMM does not have effects on the evolutionary destiny of
massive stars, and it does not significant change the compactness at the time
of Fe core collapse. | astro-ph_SR |
A New Multi-wavelength Solar Telescope: Optical and Near-infrared Solar
Eruption Tracer (ONSET): A new multi-wavelength solar telescope, Optical and Near-infrared Solar
Eruption Tracer (ONSET) of Nanjing University, was constructed, being
fabricated by Nanjing Institute of Astronomical Optics & Technology and run in
cooperation with Yunnan Astronomical Observatory. ONSET is able to observe the
Sun in three wavelength windows: He {\small I} 10830 \AA, H$\alpha$, and
white-light at 3600 \AA and 4250 \AA, which are selected in order to obtain the
dynamics in the corona, chromosphere, and the photosphere simultaneously.
Full-disk or partial-disk solar images with a field of 10 arcmin at three
wavelengths can be obtained nearly simultaneously. It is designed to trace
solar eruptions with high spatial and temporal resolutions. This telescope was
installed at a new solar observing site near Fuxian Lake in Yunnan Province,
southwest China. The site is located at E102N24, with an altitude of 1722 m.
The seeing is stable and of high quality. We give a brief description of the
scientific objectives and the basic structure of the ONSET. Some preliminary
results are also presented. | astro-ph_SR |
PEPSI deep spectra. III. A chemical analysis of the ancient planet-host
star Kepler-444: We obtained an LBT/PEPSI spectrum with very high resolution and high
signal-to-noise ratio (S/N) of the K0V host Kepler-444, which is known to host
5 sub-Earth size rocky planets. The spectrum has a resolution of R=250,000, a
continuous wavelength coverage from 4230 to 9120A, and S/N between 150 and
550:1 (blue to red). We performed a detailed chemical analysis to determine the
photospheric abundances of 18 chemical elements, in order to use the abundances
to place constraints on the bulk composition of the five rocky planets. Our
spectral analysis employs the equivalent width method for most of our spectral
lines, but we used spectral synthesis to fit a small number of lines that
require special care. In both cases, we derived our abundances using the MOOG
spectral analysis package and Kurucz model atmospheres. We find no correlation
between elemental abundance and condensation temperature among the refractory
elements. In addition, using our spectroscopic stellar parameters and isochrone
fitting, we find an age of 10+/-1.5 Gyr, which is consistent with the
asteroseismic age of 11+/-1 Gyr. Finally, from the photospheric abundances of
Mg, Si, and Fe, we estimate that the typical Fe-core mass fraction for the
rocky planets in the Kepler-444 system is approximately 24 per cent. If our
estimate of the Fe-core mass fraction is confirmed by more detailed modeling of
the disk chemistry and simulations of planet formation and evolution in the
Kepler-444 system, then this would suggest that rocky planets in more
metal-poor and alpha-enhanced systems may tend to be less dense than their
counterparts of comparable size in more metal-rich systems. | astro-ph_SR |
New outburst of the symbiotic nova AG Peg after 165 years: AG Peg is known as the slowest symbiotic nova, which experienced its
nova-like outburst around 1850. After 165 years, during June of 2015, it
erupted again showing characteristics of the Z And-type outburst. The primary
objective is to determine basic characteristics, the nature and type of the
2015 outburst of AG Peg. We achieved this aim by modelling the spectral energy
distribution using low-resolution spectroscopy (330-750 nm), medium-resolution
spectroscopy (420-720 nm; R=11000), and $UBVR_{\rm C}I_{\rm C}$ photometry
covering the 2015 outburst with a high cadence. Optical observations were
complemented with the archival HST and FUSE spectra from the preceding
quiescence. During the outburst, the luminosity of the hot component was in the
range of 2-11$\times 10^{37}(d/0.8{\rm kpc})^2$ erg/s. To generate the maximum
luminosity the white dwarf (WD) had to accrete at $\sim 3\times 10^{-7}$
M$_{\odot}yr^{-1}$, which exceeds the stable-burning limit and thus led to
blowing optically thick wind from the WD. We determined its mass-loss rate to a
few $\times 10^{-6}$ M$_{\odot}yr^{-1}$. At the high temperature of the
ionising source, $1.5-2.3\times 10^5$ K, the wind converted a fraction of the
WD's photospheric radiation into the nebular emission that dominated the
optical. A one order of magnitude increase of the emission measure, from a few
$\times 10^{59}(d/0.8 {\rm kpc})^2$ cm$^{-3}$ during quiescence, to a few
$\times 10^{60}(d/0.8\,{\rm kpc})^2$ cm$^{-3}$ during the outburst, caused a 2
mag brightening in the LC, which is classified as the Z And-type of the
outburst. The very high nebular emission and the presence of a disk-like HI
region encompassing the WD, as indicated by a significant broadening and high
flux of the Raman-scattered OVI 6825 \AA\ line during the outburst, is
consistent with the ionisation structure of hot components in symbiotic stars
during active phases. | astro-ph_SR |
Extraordinary Solar Modulation Effects On Galactic Cosmic Rays Observed
By V1 Near The Heliopause: We discuss two extraordinary increases of cosmic ray intensity observed by
Voyager 1 in the last 1.1 AU before heliopause crossing, Aug 2012 at 121.7 AU.
The two increases are roughly similar in amplitude and result in a total
increase of cosmic ray nuclei around 1 GV of over 50 percent and of 0.01 GV
electrons of a factor about 2. During the first increase, the changes in the
magnetic, B, field are small. After the first increase, the B field variations
and cosmic ray changes become large and during the second increase the B field
variations and the cosmic ray changes are correlated to within a day. The
intensity variations of H and He nuclei during these time intervals are
measured from 0.1 to over 1 GV. The total GCR increse over the two events
resemble those expected from a simple force-field 'like' solar modulation model
with a modulation potential of about 80MV. This is nearly one third of the
total modulation potential of about 250 GV required to produce the modulation
of these particles observed at the earth at the 2009 sunspot minimum and adds a
new aspect to ideas about heliospheric modulation. | astro-ph_SR |
SN 2017ens: The Metamorphosis of a Luminous Broad-lined Type Ic
Supernova into an SN IIn: We present observations of supernova (SN) 2017ens, discovered by the ATLAS
survey and identified as a hot blue object through the GREAT program. The
redshift z=0.1086 implies a peak brightness of M_g=-21.1 mag, placing the
object within the regime of superluminous supernovae. We observe a dramatic
spectral evolution, from initially being blue and featureless, to later
developing features similar to those of the broadlined Type Ic SN 1998bw, and
finally showing ~2000 km s^-1 wide H-alpha and H-beta emission. Relatively
narrow Balmer emission (reminiscent of a SN IIn) is present at all times. We
also detect coronal lines, indicative of a dense circumstellar medium. We
constrain the progenitor wind velocity to ~50-60 km s^-1 based on P-Cygni
profiles, which is far slower than those present in Wolf-Rayet stars. This may
suggest that the progenitor passed through a luminous blue variable phase, or
that the wind is instead from a binary companion red supergiant star. At late
times we see the ~2000 km s^-1 wide H-alpha emission persisting at high
luminosity (~3x10^40 erg s^-1) for at least 100 day, perhaps indicative of
additional mass loss at high velocities that could have been ejected by a
pulsational pair instability. | astro-ph_SR |
Rotating massive O stars with non-spherical 2D winds: We present solutions for the velocity field and mass-loss rates for 2D
axisymmetric outflows, as well as for the case of mass accretion through the
use of the Lambert W-function. For the case of a rotating radiation-driven wind
the velocity field is obtained analytically using a parameterised description
of the line acceleration that only depends on radius r at any given latitude
$\theta$. The line acceleration g(r) is obtained from Monte-Carlo multi-line
radiative transfer calculations. The critical/sonic point of our equation of
motion varies with latitude $\theta$. Furthermore, an approximate analytical
solution for the supersonic flow of a rotating wind is derived, which is found
to closely resemble the exact solution. For the simultaneous solution of the
mass-loss rate and velocity field, we use the iterative method of our 1D method
extended to the non-spherical 2D case. We apply the new theoretical expressions
with our iterative method to the stellar wind from a differentially rotating 40
$M_{sun}$ O5-V main sequence star as well as to a 60 $M_{sun}$ O-giant star,
and we compare our results to previous studies that are extensions of the
Castor et al. (1975, ApJ, 195, 157) CAK formalism. Next, we account for the
effects of oblateness and gravity darkening. Our numerical results predict an
equatorial decrease of the mass-loss rate, which would imply that
(surface-averaged) total mass-loss rates are lower than for the spherical 1D
case, in contradiction to the Maeder & Meynet (2000, A&A, 361, 159) formalism
that is oftentimes employed in stellar evolution calculations for rotating
massive stars. To clarify the situation in nature we discuss observational
tests to constrain the shapes of large-scale 2D stellar winds. | astro-ph_SR |
Influence of magnetic activity on the determination of stellar
parameters through asteroseismology: Magnetic activity changes the gravito-acoustic modes of solar-like stars and
in particular their frequencies. There is an angular-degree dependence that is
believed to be caused by the non-spherical nature of the magnetic activity in
the stellar convective envelope. These changes in the mode frequencies could
modify the small separation of low-degree modes (i.e. frequency difference
between consecutive quadrupole and radial modes), which is sensitive to the
core structure and hence to the evolutionary stage of the star. Determining
global stellar parameters such as the age using mode frequencies at a given
moment of the magnetic activity cycle could lead to biased results. Our
estimations show that in general these errors are lower than other systematic
uncertainties, but in some circumstances they can be as high as 10% in age and
of a few percent in mass and radius. In addition, the frequency shifts caused
by the magnetic activity are also frequency dependent. In the solar case this
is a smooth function that will mostly be masked by the filtering of the
so-called surface effects. However the observations of other stars suggest that
there is an oscillatory component with a period close to the one corresponding
to the acoustic depth of the He II zone. This could give rise to a
misdetermination of some global stellar parameters, such as the helium
abundance. Our computations show that the uncertainties introduced by this
effect are lower than the 3% level. | astro-ph_SR |
Testing Disk-Locking in NGC 2264: We test analytic predictions from different models of magnetospheric
accretion, which invoke disk-locking, using stellar and accretion parameters
derived from models of low resolution optical spectra of 36 T Tauri stars
(TTSs) in NGC 2264 (age~3 Myrs). Little evidence is found for models that
assume purely dipolar field geometries; however, strong support is found in the
data for a modified version of the X-wind model (Shu et al. 1994) which allows
for non-dipolar field geometries. The trapped flux concept in the X-wind model
is key to making the analytic predictions which appear supported in the data.
By extension, our analysis provides support for the outflows predicted by the
X-wind as these also originate in the trapped flux region. In addition, we find
no support in the data for accretion powered stellar winds from young stars. By
comparing the analysis presented here of NGC 2264 with a similar analysis of
stars in Taurus (age~1-2 Myr), we find evidence that the equilibrium
interaction between the magnetic field and accretion disk in TTS systems
evolves as the stars grow older, perhaps as the result of evolution of the
stellar magnetic field geometry. We compare the accretion rates we derive with
accretion rates based on U-band excess, finding good agreement. In addition, we
use our accretion parameters to determine the relationship between accretion
and H-beta luminosity, again finding good agreement with previously published
results; however, we also find that care must be used when applying this
relationship due to strong chromospheric emission in young stars which can lead
to erroneous results in some cases. | astro-ph_SR |
Oscillations above sunspots from the temperature minimum to the corona: Context. An analysis of the oscillations above sunspots was carried out using
simultaneous ground-based and Solar Dynamics Observatory (SDO) observations
(SiI 10827A, HeI 10830A, FeI 6173A, 1700A, HeII 304A, FeIX 171A).
Aims. Investigation of the spatial distribution of oscillation power in the
frequency range 1-8 mHz for the different height levels of the solar
atmosphere. Measuring the time lags between the oscillations at the different
layers.
Methods. We used frequency filtration of the intensity and Doppler velocity
variations with Morlet wavelet to trace the wave propagation from the
photosphere to the chromosphere and the corona.
Results. The 15 min oscillations are concentrated near the outer penumbra in
the upper photosphere (1700 A), forming a ring, that expands in the transition
zone. These oscillations propagate upward and reach the corona level, where
their spatial distribution resembles a fan structure. The spatial distribution
of the 5 min oscillation power looks like a circle-shape structure matching the
sunspot umbra border at the photospheric level. The circle expands at the
higher levels (HeII 304A and FeIX 171A). This indicates that the low-frequency
oscillations propagate along the inclined magnetic tubes in the spot. We found
that the inclination of the tubes reaches 50--60 degrees in the upper
chromosphere and the transition zone. The main oscillation power in the 5-8 mHz
range concentrates within the umbra boundaries at all the levels. The highest
frequency oscillations (8 mHz) are located in the peculiar points inside the
umbra. These points probably coincide with umbral dots. We deduced the
propagation velocities to be 28+-15 km/s, 26+-15 km/s, and 55+-10 km/s for the
SiI 10827A-HeI 10830A, 1700A-HeII 304A, and HeII 304A-FeIX 171A height levels,
respectively. | astro-ph_SR |
Calibrating the surface brightness - color relation for late-type red
giants stars in the visible domain using VEGA/CHARA interferometric
observations: The surface brightness - color relationship (SBCR) is a poweful tool for
determining the angular diameter of stars from photometry. It was for instance
used to derive the distance of eclipsing binaries in the Large Magellanic Cloud
(LMC), which led to its distance determination with an accuracy of 1%. We
calibrate the SBCR for red giant stars in the 2.1 < V-K < 2.5 color range using
homogeneous VEGA/CHARA interferometric data secured in the visible domain, and
compare it to the relation based on infrared interferometric observations,
which were used to derive the distance to the LMC. Observations of eight G-K
giants were obtained with the VEGA/CHARA instrument. The derived limb-darkened
angular diameters were combined with a homogeneous set of infrared magnitudes
in order to constrain the SBCR. The average precision we obtain on the
limb-darkened angular diameters of the eight stars in our sample is 2.4%. For
the four stars in common observed by both VEGA/CHARA and PIONIER/VLTI, we find
a 1 sigma agreement for the angular diameters. The SBCR we obtain in the
visible has a dispersion of 0.04 magnitude and is consistent with the one
derived in the infrared (0.018 magnitude). The consistency of the infrared and
visible angular diameters and SBCR reinforces the result of 1% precision and
accuracy recently achieved on the distance of the LMC using the
eclipsing-binary technique. It also indicates that it is possible to combine
interferometric observations at different wavelengths when the SBCR is
calibrated. | astro-ph_SR |
Wind mass-loss rates of stripped stars inferred from Cygnus X-1: Recent observations of the high-mass X-ray binary Cygnus X-1 have shown that
both the companion star (41 solar masses) and the black hole (21 solar masses)
are more massive than previously estimated. Furthermore, the black hole appears
to be nearly maximally spinning. Here we present a possible formation channel
for the Cygnus X-1 system that matches the observed system properties. In this
formation channel, we find that the orbital parameters of Cygnus X-1, combined
with the observed metallicity of the companion, imply a significant reduction
in mass loss through winds relative to commonly used prescriptions for stripped
stars. | astro-ph_SR |
Three-dimensional delayed-detonation models with nucleosynthesis for
Type Ia supernovae: We present results for a suite of fourteen three-dimensional, high resolution
hydrodynamical simulations of delayed-detonation modelsof Type Ia supernova (SN
Ia) explosions. This model suite comprises the first set of three-dimensional
SN Ia simulations with detailed isotopic yield information. As such, it may
serve as a database for Chandrasekhar-mass delayed-detonation model
nucleosynthetic yields and for deriving synthetic observables such as spectra
and light curves. We employ a physically motivated, stochastic model based on
turbulent velocity fluctuations and fuel density to calculate in situ the
deflagration to detonation transition (DDT) probabilities. To obtain different
strengths of the deflagration phase and thereby different degrees of
pre-expansion, we have chosen a sequence of initial models with 1, 3, 5, 10,
20, 40, 100, 150, 200, 300, and 1600 (two different realizations) ignition
kernels in a hydrostatic white dwarf with central density of 2.9 x 10^9 gcc,
plus in addition one high central density (5.5 x 10^9 gcc), and one low central
density (1.0 x 10^9 gcc) rendition of the 100 ignition kernel configuration.
For each simulation we determined detailed nucleosynthetic yields by
post-processing 10^6 tracer particles with a 384 nuclide reaction network. All
delayed detonation models result in explosions unbinding the white dwarf,
producing a range of 56Ni masses from 0.32 to 1.11 solar masses. As a general
trend, the models predict that the stable neutron-rich iron group isotopes are
not found at the lowest velocities, but rather at intermediate velocities
(~3,000 - 10,000 km/s) in a shell surrounding a 56Ni-rich core. The models
further predict relatively low velocity oxygen and carbon, with typical minimum
velocities around 4,000 and 10,000 km/s, respectively. | astro-ph_SR |
The (non-)variability of magnetic chemically peculiar candidates in the
Large Magellanic Cloud: The galactic magnetic chemically peculiar (mCP) stars of the upper main
sequence are well known as periodic spectral and light variables. The observed
variability is obviously caused by the uneven distribution of overabundant
chemical elements on the surfaces of rigidly rotating stars. The mechanism
causing the clustering of some chemical elements into disparate structures on
mCP stars has not been fully understood up to now. The observations of light
changes of mCP candidates recently revealed in the nearby Large Magellanic
Cloud (LMC) should provide us with information about their rotational periods
and about the distribution of optically active elements on mCP stars born in
other galaxies. We queried for photometry at the Optical Gravitational Lensing
Experiment (OGLE)-III survey of published mCP candidates selected because of
the presence of the characteristic 5200A flux depression. In total, the
intersection of both sources resulted in twelve stars. We performed our own and
standard periodogram time series analyses of all available data. The final
results are, amongst others, the frequency of the maximum peak and the
bootstrap probability of its reality. We detected that only two mCP candidates,
190.1 1581 and 190.1 15527, may show some weak rotationally modulated light
variations with periods of 1.23 and 0.49 days; however, the 49% and 32%
probabilities of their reality are not very satisfying. The variability of the
other 10 mCP candidates is too low to be detectable by their V and I OGLE
photometry. The relatively low amplitude variability of the studied LMC mCP
candidates sample can be explained by the absence of photometric spots of
overabundant optically active chemical elements. The unexpected LMC mCPs
behaviour is probably caused by different conditions during the star formation
in the LMC and the Galaxy. | astro-ph_SR |
Imaging sub-milliarcsecond stellar features with intensity
interferometry using air Cherenkov telescope arrays: Recent proposals have been advanced to apply imaging air Cherenkov telescope
arrays to stellar intensity interferometry (SII). Of particular interest is the
possibility of model-independent image recovery afforded by the good (u,
v)-plane coverage of these arrays, as well as recent developments in phase
retrieval techniques. The capabilities of these instruments used as SII
receivers have already been explored for simple stellar objects, and here the
focus is on reconstructing stellar images with non-uniform radiance
distributions. We find that hot stars (T > 6000 K) containing hot and/or cool
localized regions (T \sim 500 K) as small as \sim 0.1 mas can be imaged at
short wavelengths ({\lambda} = 400 nm). | astro-ph_SR |
Beryllium detection in the very fast nova ASASSN-16kt (V407 Lupi): We present high-resolution spectroscopic observations of the fast nova
ASASSN-16kt (V407 Lup). A close inspection of spectra obtained at early stages
has revealed the presence of low-ionization lines, and among the others we have
identified the presence of the ionised $^7$Be doublet in a region relatively
free from possible contaminants. After studying their intensities, we have
inferred that ASASSN-16kt has produced (5.9 - 7.7)$ \times 10^{-9}$ M$_{\odot}$
of $^7$Be. The identification of bright Ne lines may suggest that the nova
progenitor is a massive (1.2 M$_{\odot}$) oxygen-neon white dwarf. The high
outburst frequency of oxygen-neon novae implies that they likely produce an
amount of Be similar, if not larger, to that produced by carbon-oxygen novae,
then confirming that classical novae are among the main factories of lithium in
the Galaxy. | astro-ph_SR |
Direct Imaging Explorations for Companions around Mid-Late M Stars from
the Subaru/IRD Strategic Program: The Subaru telescope is currently performing a strategic program (SSP) using
the high-precision near-infrared (NIR) spectrometer IRD to search for
exoplanets around nearby mid/late-M~dwarfs via radial velocity (RV) monitoring.
As part of the observing strategy for the exoplanet survey, signatures of
massive companions such as RV trends are used to reduce the priority of those
stars. However, this RV information remains useful for studying the stellar
multiplicity of nearby M~dwarfs. To search for companions around such
``deprioritized" M~dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2
observations with pyramid wavefront sensing at NIR wavelengths, leading to high
sensitivity to substellar-mass companions within a few arcseconds. We detected
two new companions (LSPM~J1002+1459~B and LSPM~J2204+1505~B) and two new
candidates that are likely companions (LSPM~J0825+6902~B and LSPM~J1645+0444~B)
as well as one known companion. Including two known companions resolved by the
IRD fiber injection module camera, we detected seven (four new) companions at
projected separations between $\sim2-20$~au in total. A comparison of the
colors with the spectral library suggests that LSPM~J2204+1505~B and
LSPM~J0825+6902~B are located at the boundary between late-M and early-L
spectral types. Our deep high-contrast imaging for targets where no bright
companions were resolved did not reveal any additional companion candidates.
The NIRC2 detection limits could constrain potential substellar-mass companions
($\sim10-75\ M_{\rm Jup}$) at 10~au or further. The failure with Keck/NIRC2
around the IRD-SSP stars having significant RV trends makes these objects
promising targets for further RV monitoring or deeper imaging with JWST to
search for smaller-mass companions below the NIRC2 detection limits. | astro-ph_SR |
Asteroseismology of the $δ$ Scuti star HD 50844: Aims. We aim to probe the internal structure and investigate more detailed
information of the $\delta$ Scuti star HD 50844 with asteroseismology. Methods.
We analyse the observed frequencies of the $\delta$ Scuti star HD 50844
obtained by Balona (2014), and search for possible multiplets based on the
rotational splitting law of g-mode. We tried to disentangle the frequency
spectra of HD 50844 by means of the rotational splitting only. We then compare
them with theoretical pulsation modes, which correspond to stellar evolutionary
models with various sets of initial metallicity and stellar mass, to find the
best-fitting model. Results. There are three multiplets including two complete
triplets and one incomplete quintuplet, in which mode identifications for
spherical harmonic degree $l$ and azimuthal number $m$ are unique. The
corresponding rotational period of HD 50844 is found to be
2.44$^{+0.13}_{-0.08}$ days. The physical parameters of HD 50844 are well
limited in a small region by three modes identified as nonradial ones
($f_{11}$, $f_{22}$, and $f_{29}$) and by the fundamental radial mode
($f_{4}$). Our results show that the three nonradial modes ($f_{11}$, $f_{22}$,
and $f_{29}$) are all mixed modes, which mainly represent the property of the
helium core. The fundamental radial mode ($f_{4}$) mainly represents the
property of the stellar envelope. In order to fit these four pulsation modes,
both the helium core and the stellar envelope must be matched to the actual
structure of HD 50844. Finally, the mass of the helium core of HD 50844 is
estimated to be 0.173 $\pm$ 0.004 $M_{\odot}$ for the first time. The physical
parameters of HD 50844 are determined to be $M=$ 1.81 $\pm$ 0.01 $M_{\odot}$,
$Z=$ 0.008 $\pm$ 0.001. $T_{\rm eff}=$ 7508 $\pm$ 125 K, log$g=$ 3.658 $\pm$
0.004, $R=$ 3.300 $\pm$ 0.023 $R_{\odot}$, $L=$ 30.98 $\pm$ 2.39 $L_{\odot}$. | astro-ph_SR |
Simulations of Protoplanetary Disk Dispersal: Stellar Mass Dependence of
the Disk Lifetime: Recent infrared and submillimeter observations suggest that the
protoplanetary disk lifetime depends on the central stellar mass. The disk
dispersal is thought to be driven by viscous accretion, magneto-hydrodynamics
(MHD) winds, and photoevaporation by the central star. We perform a set of
one-dimensional simulations of long-term disk evolution that include all the
three processes. We vary the stellar mass in the range of 0.5-7M$_{\odot}$, and
study the mass dependence of the disk evolution. We show that a significant
fraction of the disk gas is lost by MHD winds in the early stage, but the later
disk evolution is mainly governed by photoevaporation. The disk radius
decreases as photoevaporation clears out the gas in the outer disk efficiently.
The qualitative evolutionary trends of the disk mass are remarkably similar for
the wide range of the central stellar mass we consider, and the time evolution
of the disk mass can be well fitted by a simple function. The dispersal time is
approximately ten million years for low mass stars with weak mass dependence,
but gets as short as two million years around a 7M$_{\odot}$ star. In the
latter case, a prominent inner hole is formed by the combined effect of
accretion and MHD winds within about one million years. The strength of the MHD
wind and viscous accretion controls the overall mass-loss rate, but does not
alter the dependence of the dispersal timescale on the central stellar mass. | astro-ph_SR |
From the atmosphere to the circumstellar environment in cool evolved
stars: We discuss and illustrate contributions that optical interferometry has made
on our current understanding of cool evolved stars. We include red giant branch
(RGB) stars, asymptotic giant branch (AGB) stars, and red supergiants (RSGs).
Studies using optical interferometry from visual to mid-infrared wavelengths
have greatly increased our knowledge of their atmospheres, extended molecular
shells, dust formation, and winds. These processes and the morphology of the
circumstellar environment are important for the further evolution of these
stars toward planetary nebulae (PNe) and core-collapse supernovae (SNe), and
for the return of material to the interstellar medium. | astro-ph_SR |
ALMA Imaging of Millimeter/Submillimeter Continuum Emission in Orion KL: We have carried out high resolution observations with Atacama Large
Millimeter/Submillimeter Array (ALMA) of continuum emission from Orion KL
region. We identify 11 compact sources at ALMA band 6 (245 GHz) and band 7 (339
GHz), including Hot Core, Compact Ridge, SMA1, IRc4, IRc7, and a radio source I
(Source I). Spectral energy distribution (SED) of each source is determined by
using previous 3 mm continuum emission data. Physical properties such as size,
mass, hydrogen number density and column density are discussed based on the
dust graybody SED. Among 11 identified sources, Source I, a massive protostar
candidate, is a dominant energy source in Orion KL. We extensively investigate
its SED from centimeter to submillimeter wavelengths. The SED of Source I can
be fitted with a single power-law index of 1.97 suggesting an optically thick
emission. We employ the H$^{-}$ free-free emission as an opacity source of this
optically thick emission. The temperature, density, and mass of the
circumstellar disk associated with Source I are constrained by the SED of
H$^{-}$ free-free emission. Still the fitting result shows a significant
deviation from the observed flux densities. Combined with the thermal dust
graybody SED to explain excess emission at higher frequency, a smaller
power-law index of 1.60 for the H$^{-}$ free-free emission is obtained in the
SED fitting. The power-law index smaller than 2 would suggest a compact source
size or a clumpy structure unresolved with the present study. Future higher
resolution observations with ALMA are essential to reveal more detailed spatial
structure and physical properties of Source I. | astro-ph_SR |
Magnetic activities on two single-lined RS Canum Venaticorum binaries IM
Pegasi and $σ$ Geminorum: We present the study on continuous high-resolution spectroscopic observations
of two long-period single-lined RS Canum Venaticorum (RS CVn) binary stars IM
Pegasi (IM Peg) and $\sigma$ Geminorum ($\sigma$ Gem), obtained with the
Hertzsprung SONG telescope during the 2015-2016 season. Chromospheric activity
indicators H$_{\alpha}$, Na I D$_{1}$, D$_{2}$ doublet, He I D$_{3}$, and
H$_{\beta}$ lines have been analyzed by using the spectral subtraction
technique. The expected chromospheric emission features in the H$_{\alpha}$, Na
I D$_{1}$, D$_{2}$ doublet, and H$_{\beta}$ lines confirm that both of two
stars are very active systems. In the spectra, the He I D$_{3}$ line had been
always detected in absorption feature. Although the behavior of chromospheric
activity indicators is very similar for both stars, the activity level of IM
Peg is much stronger than that of $\sigma$ Gem. Moreover, the EW variations of
the H$_{\alpha}$, He I D$_{3}$, and H$_{\beta}$ line subtractions correlate
well and show different behavior among different orbital cycles, which
indicates the presence and evolution of activity longitudes over the surface of
two stars. Furthermore, the subtracted H$_{\alpha}$ line profile is usually
asymmetric. The red-shifted excess absorption features could be interpreted as
a strong down-flow of cool absorbing material, while the blue-shifted emission
component is probably caused by up-flow of hot materials through microflare
events. | astro-ph_SR |
A very young, compact bipolar H$_2$O maser outflow in the
intermediate-mass star-forming LkH$α$ 234 region: We report multi-epoch VLBI H$_2$O maser observations towards the compact
cluster of YSOs close to the Herbig Be star LkH$\alpha$ 234. This cluster
includes LkH$\alpha$ 234 and at least nine more YSOs that are formed within
projected distances of $\sim$10 arcsec ($\sim$9,000 au). We detect H$_2$O maser
emission towards four of these YSOs. In particular, our VLBI observations
(including proper motion measurements) reveal a remarkable very compact
($\sim$0.2 arcsec = $\sim$180 au), bipolar H$_2$O maser outflow emerging from
the embedded YSO VLA 2. We estimate a kinematic age of $\sim$40 yr for this
bipolar outflow, with expanding velocities of $\sim$20 km s$^{-1}$ and momentum
rate $\dot M_w V_w$ $\simeq$ $10^{-4}-10^{-3}$ M$_{\odot}$ yr$^{-1}$ km
s$^{-1}$$\times (\Omega$/$4\pi)$, powered by a YSO of a few solar masses. We
propose that the outflow is produced by recurrent episodic jet ejections
associated with the formation of this YSO. Short-lived episodic ejection events
have previously been found towards high-mass YSOs. We show now that this
behaviour is also present in intermediate-mass YSOs. These short-lived episodic
ejections are probably related to episodic increases in the accretion rate, as
observed in low-mass YSOs. We predict the presence of an accretion disk
associated with VLA 2. If detected, this would represent one of the few known
examples of intermediate-mass stars with a disk-YSO-jet system at scales of a
few hundred au. | astro-ph_SR |
Emergence of magnetic flux generated in a solar convective dynamo. I:
Formation of Sunspots and Active regions, and Origin of Their Asymmetries: We present a realistic numerical model of sunspot and active region formation
based on the emergence of flux bundles generated in a solar convective dynamo.
To this end we use the magnetic and velocity fields in a horizontal layer near
the top boundary of the solar convective dynamo simulation to drive realistic
radiative-magnetohydrodynamic simulations of the upper most layers of the
convection zone. The main results are: (1) The emerging flux bundles rise with
the mean speed of convective upflows, and fragment into small-scale magnetic
elements that further rise to the photosphere, where bipolar sunspot pairs are
formed through the coalescence of the small-scale magnetic elements. (2)
Filamentary penumbral structures form when the sunspot is still growing through
ongoing flux emergence. In contrast to the classical Evershed effect, the
inflow seems to prevail over the outflow in a large part of the penumbra. (3) A
well formed sunspot is a mostly monolithic magnetic structures that is anchored
in a persistent deep-seated downdraft lane. The flow field outside the spot
shows a giant vortex ring that comprises of an inflow below 15 Mm depth and an
outflow above 15 Mm depth. (4) The sunspots successfully reproduce the
fundamental properties of the observed solar active regions, including the more
coherent leading spots with a stronger field strength, and the correct tilts of
bipolar sunspot pairs. These asymmetries can be linked to the intrinsic
asymmetries in the magnetic and flow fields adapted from the convective dynamo
simulation. | astro-ph_SR |
Solid-state production of complex organic molecules: H-atom addition
versus UV irradiation: Complex organic molecules (COMs) have been observed in comets, hot cores and
cold dense regions of the interstellar medium. It is generally accepted that
these COMs form on icy dust grain through the recombination reaction of
radicals triggered by either energetic UV- photon or non-energetic H-atom
addition processing. In this work, we present for the first time laboratory
studies that allow for quantitative comparison of hydrogenation and UV-induced
reactions as well as their cumulative effect in astronomically relevant
CO:CH3OH=4:1 ice analogues. The formation of glycolaldehyde (GA) and ethylene
glycol (EG) is confirmed in pure hydrogenation experiments at 14 K, except
methyl formate (MF), which is only clearly observed in photolysis. The
fractions for MF:GA:EG are 0 : (0.2-0.4) : (0.8-0.6) for pure hydrogenation,
and 0.2 : 0.3 : 0.5 for UV involving experiments and can offer a diagnostic
tool to derive the chemical origin of these species. The GA/EG ratios in the
laboratory (0.3-1.5) compare well with observations toward different objects. | astro-ph_SR |
Envelope Convection, Surface Magnetism and Spots in A- and late B-type
Stars: Weak magnetic fields have recently been detected in a number of A-type stars,
including Vega and Sirius. At the same time, space photometry observations of
A- and late B-type stars from Kepler and TESS have highlighted the existence of
rotational modulation of surface features akin to stellar spots. Here we
explore the possibility that surface magnetic spots might be caused by the
presence of small envelope convective layers at or just below the stellar
surface, caused by recombination of H and He. Using 1D stellar evolution
calculations and assuming an equipartition dynamo, we make simple estimates of
field strength at the photosphere. For most models the largest effects are
caused by a convective layer driven by second helium ionization. While it is
difficult to predict the geometry of the magnetic field, we conclude that the
majority of intermediate-mass stars should have dynamo-generated magnetic
fields of order a few gauss at the surface. These magnetic fields can appear at
the surface as bright spots, and cause photometric variability via rotational
modulation, which could also be wide-spread in A-stars. The amplitude of
surface magnetic fields and their associated photometric variability is
expected to decrease with increasing stellar mass and surface temperature, so
that magnetic spots and their observational effects should be much harder to
detect in late B-type stars. | astro-ph_SR |
The Cluster AgeS Experiment (CASE). Variable stars in the field of the
globular cluster M10: The field of the globular cluster M10 (NGC 6254) was monitored between 1998
and 2015 in a search for variable stars. V -light curves were derived for 40
variables or likely variables, most of which are new detections. Proper motions
obtained within the CASE project indicate that 18 newly detected variables and
14 previously known ones are members or likely members of the cluster,
including one RRc-type, three type II Cepheids, and 14 SX Phe-type pulsators,
one contact binary, and six semi-regular red giants. As a byproduct of the
search we discovered a candidate binary comprised of main sequence stars with
the record-short orbital period of 0.042 d. We also confirmed the photometric
variability of the red straggler M10-VLA1 hinted at by Shishkovsky et al.
(2018), who discovered this object spectroscopically. In Appendix 1 we show
that CASE proper motion measurements are in a good agreement with those
retrieved from the Gaia archive, while Appendix 2 presents evidence for low
frequency {\gamma} Doradus-type oscillations in SX Phe stars belonging to M10. | astro-ph_SR |
Off-disk straylight measurements for the Swedish 1-meter Solar Telescope: Context. Accurate photometry with ground based solar telescopes requires
characterization of straylight. Scattering in Earth's atmosphere and in the
telescope optics are potentially significant sources of straylight, for which
the point spread function (PSF) has wings that reach very far. This kind of
straylight produces an aureola, extending several solar radii off the solar
disk. Aims. We want to measure such straylight using the ordinary science
instrumentation. Methods. We scanned the intensity on and far off the solar
disk by use of the science cameras in several different wavelength bands on a
day with low-dust conditions. We characterized the far wing straylight by
fitting a model to the recorded intensities involving a multi-component
straylight PSF and the limb darkening of the disk. Results. The measured
scattered light adds an approximately constant fraction of the local
granulation intensity to science images at any position on the disk. The
fraction varied over the day but never exceeded a few percent. The PSFs have
weak tails that extend to several solar radii but most of the scattered light
originates within ~1'. Conclusions. Far-wing scattered light contributes only a
small amount of straylight in SST data. Other sources of straylight are
primarily responsible for the reduced contrast in SST images. | astro-ph_SR |
Measuring the Magnetic Field Strength of the Quiet Solar Corona Using
"EIT Waves": Variations in the propagation of globally-propagating disturbances (commonly
called "EIT waves") through the low solar corona offer a unique opportunity to
probe the plasma parameters of the solar atmosphere. Here, high-cadence
observations of two "EIT wave" events taken using the Atmospheric Imaging
Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) are
combined with spectroscopic measurements from the Extreme ultraviolet Imaging
Spectrometer (EIS) onboard the Hinode spacecraft and used to examine the
variability of the quiet coronal magnetic-field strength. The combination of
pulse kinematics from SDO/AIA and plasma density from Hinode/EIS is used to
show that the magnetic-field strength is in the range ~2-6 G in the quiet
corona. The magnetic-field estimates are then used to determine the height of
the pulse, allowing a direct comparison with theoretical values obtained from
magnetic-field measurements from the Helioseismic and Magnetic Imager (HMI)
onboard SDO using PFSS and local-domain extrapolations. While local-scale
extrapolations predict heights inconsistent with prior measurements, the
agreement between observations and the PFSS model indicates that "EIT waves"
are a global phenomenon influenced by global-scale magnetic field. | astro-ph_SR |
A Statistical Survey of Hard X-ray Spectral Characteristics of Solar
Flares with Two Footpoints: Using RHESSI data, we have analyzed some 172 hard X-ray peaks during 53 solar
flares which exhibited a double-footpoint structure. Fitting both footpoints
with power-laws, we find that spectral index differences range mostly between 0
to 0.6, and only rarely go beyond. Asymmetries between footpoints were not
observed to be significantly dependent on their mean heliographic position,
their relative position with respect to each other, nor their orientation with
respect to the solar equator. Assuming a symmetric acceleration process, it is
also clear that differences in footpoint spectral indices and footpoint flux
ratios can seldom be attributed to a difference in column densities between the
two legs of a coronal loop. Our results corroborate better the magnetic mirror
trap scenario. Moreover, footpoint asymmetries are more marked during times of
peak HXR flux than when averaging over the whole HXR burst, suggesting that the
magnetic configuration evolves during individual HXR bursts. We observed also a
linear correlation between the peak 50-keV flux and the peak GOES 1-8A channel
flux, and that HXR burst duration seem correlated with loop length. | astro-ph_SR |
Exploring the Nature of EUV Waves in a Radiative Magnetohydrodynamic
Simulation: Coronal extreme-ultraviolet (EUV) waves are large-scale disturbances
propagating in the corona, whose physical nature and origin have been discussed
for decades. We report the first three dimensional (3D) radiative
magneto-hydrodynamic (RMHD) simulation of a coronal EUV wave and the
accompanying quasi-periodic wave trains. The numerical experiment is conducted
with the MURaM code and simulates the formation of solar active regions through
magnetic flux emergence from the convection zone to the corona. The coronal EUV
wave is driven by the eruption of a magnetic flux rope that also gives rise to
a C-class flare. It propagates in a semi-circular shape with an initial speed
ranging from about 550 to 700 km s$^{-1}$, which corresponds to an average Mach
number (relative to fast magnetoacoustic waves) of about 1.2. Furthermore, the
abrupt increase of the plasma density, pressure and tangential magnetic field
at the wavefront confirms fast-mode shock nature of the coronal EUV wave.
Quasi-periodic wave trains with a period of about 30 s are found as multiple
secondary wavefronts propagating behind the leading wavefront and ahead of the
erupting magnetic flux rope. We also note that the true wavefront in the 3D
space can be very inhomogeneous, however, the line-of-sight integration of EUV
emission significantly smoothes the sharp structures in 3D and leads to a more
diffuse wavefront. | astro-ph_SR |
Interpretation of CEMP(s) and CEMP(s + r) Stars with AGB Models: Asymptotic Giant Branch (AGB) stars play a fundamental role in the s-process
nucleosynthesis during their thermal pulsing phase. The theoretical predictions
obtained by AGB models at different masses, s-process efficiencies, dilution
factors and initial r-enrichment, are compared with spectroscopic observations
of Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s),
collected from the literature. We discuss here five stars as example, CS
22880-074, CS 22942-019, CS 29526-110, HE 0202-2204, and LP 625-44. All these
objects lie on the main-sequence or on the giant phase, clearly before the
TP-AGB stage: the hypothesis of mass transfer from an AGB companion, would
explain the observed s-process enhancement. CS 29526-110 and LP 625-44 are
CEMP(s+r) objects, and are interpreted assuming that the molecular cloud, from
which the binary system formed, was already enriched in r-process elements by
SNII pollution. In several cases, the observed s-process distribution may be
accounted for AGB models of different initial masses with proper 13C-pocket
efficiency and dilution factor. Na (and Mg), produced via the neutron capture
chain starting from 22Ne, may provide an indicator of the initial AGB mass. | astro-ph_SR |
Stellar Yields of Rotating First Stars. II. Pair Instability Supernovae
and Comparison with Observations: Recent theory predicts that a first star is born with a massive initial mass
of $\gtrsim$ 100 $M_\odot$. Pair instability supernova (PISN) is a common fate
for such a massive star. Our final goal is to prove the existence of PISN and
thus the high mass nature of the initial mass function in the early universe by
conducting {\it abundance profiling}, in which properties of a hypothetical
first star is constrained by metal-poor star abundances. In order to determine
reliable and useful abundances, we investigate the PISN nucleosynthesis taking
both rotating and non-rotating progenitors for the first time. We show that the
initial and CO core mass ranges for PISNe depend on the envelope structures:
non-magnetic rotating models developing inflated envelopes have a lower-shifted
CO mass range of $\sim$ 70--125 $M_\odot$, while non-rotating and magnetic
rotating models with deflated envelopes have a range of $\sim$ 80--135
$M_\odot$. However, we find no significant difference in explosive yields from
rotating and non-rotating progenitors, except for large nitrogen production in
non-magnetic rotating models. Furthermore, we conduct the first systematic
comparison between theoretical yields and a large sample of metal-poor star
abundances. We find that the predicted low [Na/Mg] $\sim$ $-1.5$ and high
[Ca/Mg] $\sim$ $0.5$--$1.3$ abundance ratios are the most important to
discriminate PISN signatures from normal metal-poor star abundances, and
confirm that no currently observed metal-poor star matches with the PISN
abundance. Extensive discussion on the non-detection is finally made. | astro-ph_SR |
A New Look at the YY CrB Binary System: This study presented a new analysis for the TESS-observed W Ursae Majoris (W
UMa) binary star YY Coronea Borealis (YY CrB). The light curve was analyzed by
the PHysics Of Eclipsing BinariEs (PHOEBE) Python version together with the
Markov chain Monte Carlo (MCMC) method. The light curve solutions required a
hot spot and l3. New eclipse times from the TESS observations were extracted,
and the O-C curve of primary and secondary minima showed an anti-correlated
manner. In order to study the O-C curve of minima, minima times between 1991
and 2023 were collected. This investigation reported a new linear ephemeris and
by fitting a quadratic function to the O-C curve of minima, calculated the
orbital period rate of \mathop P\limits^.\approx 5.786*{10^{-8}} day/year.
Assuming mass conservation, a mass exchange rate of
\mathop{{M_2}}\limits^.=2.472*{10^{-8}} calculated from the more massive
component to the less massive one. Then, by using the light travel time
function, the possible third body was determined in the binary and derived the
mass of the third body as 0.498M_Sun with a period of \simeq 7351.018 days. The
O-C curve analysis and the quantity of mass indicate that the presence of a
third body is unlikely. This binary is expected to evolve into a broken-contact
phase and is a good case to support the thermal relaxation oscillation model. | astro-ph_SR |
Searching For Wide Binary Stars with Non-coeval Components in the
Southern Sky: We have completed our observational program to search for wide binary systems
with non-coeval components in the southern sky and report our results here. The
final set of four systems was spectroscopically investigated in this paper. No
binary systems with components of different ages were found among them. Taking
into account our previous studies, we estimate the fraction of such binaries
(i.e., binaries formed, presumably, by capture) not higher than 0.06~\%. The
study will be continued on the northern sky. | astro-ph_SR |
BD +44 493: A Ninth Magnitude Messenger from the Early Universe; Carbon
Enhanced and Beryllium Poor: We present a 1D LTE chemical abundance analysis of the very bright (V=9.1)
Carbon-Enhanced Metal-Poor (CEMP) star BD +44 493, based on high-resolution,
high signal-to-noise spectra obtained with Subaru/HDS. The star is shown to be
a subgiant with an extremely low iron abundance ([Fe/H]=-3.7), while it is rich
in C ([C/Fe]=+1.3) and O ([O/Fe]=+1.6). Although astronomers have been
searching for extremely metal-poor stars for decades, this is the first star
found with [Fe/H]<-3.5 and an apparent magnitude V<12. Based on its low
abundances of neutron-capture elements (e.g., [Ba/Fe]=-0.59), BD +44 493 is
classified as a "CEMP-no" star. Its abundance pattern implies that a
first-generation faint supernova is the most likely origin of its carbon
excess, while scenarios related to mass loss from rapidly-rotating massive
stars or mass transfer from an AGB companion star are not favored. From a
high-quality spectrum in the near-UV region, we set an very low upper limit on
this star's beryllium abundance (A(Be)=log(Be/H)+12<-2.0), which indicates that
the decreasing trend of Be abundances with lower [Fe/H] still holds at
[Fe/H]<-3.5. This is the first attempt to measure a Be abundance for a CEMP
star, and demonstrates that high C and O abundances do not necessarily imply
high Be abundances. | astro-ph_SR |
The 12C + 12C reaction and the impact on nucleosynthesis in massive
stars: Despite much effort in the past decades, the C-burning reaction rate is
uncertain by several orders of magnitude, and the relative strength between the
different channels 12C(12C,alpha)20Ne, 12C(12C,p)23Na and 12C(12C,n)23Mg is
poorly determined. Additionally, in C-burning conditions a high 12C+12C rate
may lead to lower central C-burning temperatures and to 13C(alpha,n)16O
emerging as a more dominant neutron source than 22Ne(alpha,n)25Mg, increasing
significantly the s-process production. This is due to the rapid decrease of
the 13N(gamma,p)12C with decreasing temperature, causing the 13C production via
13N(beta+)13C. Presented here is the impact of the 12C+12C reaction
uncertainties on the s-process and on explosive p-process nucleosynthesis in
massive stars, including also fast rotating massive stars at low metallicity.
Using various 12C+12C rates, in particular an upper and lower rate limit of ~
50000 higher and ~ 20 lower than the standard rate at 5*10^8 K, five 25 Msun
stellar models are calculated. The enhanced s-process signature due to
13C(alpha,n)16O activation is considered, taking into account the impact of the
uncertainty of all three C-burning reaction branches. Consequently, we show
that the p-process abundances have an average production factor increased up to
about a factor of 8 compared to the standard case, efficiently producing the
elusive Mo and Ru proton-rich isotopes. We also show that an s-process being
driven by 13C(alpha,n)16O is a secondary process, even though the abundance of
13C does not depend on the initial metal content. Finally, implications for the
Sr-peak elements inventory in the Solar System and at low metallicity are
discussed. | astro-ph_SR |
The VLT-FLAMES Tarantula Survey XXV. Surface nitrogen abundances of
O-type giants and supergiants: Theoretically, rotation-induced chemical mixing in massive stars has far
reaching evolutionary consequences, affecting the sequence of morphological
phases, lifetimes, nucleosynthesis, and supernova characteristics. Using a
sample of 72 presumably single O-type giants to supergiants observed in the
context of the VLT-FLAMES Tarantula Survey (VFTS), we aim to investigate
rotational mixing in evolved core-hydrogen burning stars initially more massive
than $15\,M_\odot$ by analysing their surface nitrogen abundances. Using
stellar and wind properties derived in a previous VFTS study, we constrained
the nitrogen abundance by fitting the equivalent widths of relatively strong
lines that are sensitive to changes in the abundance of this element. Given the
quality of the data, we constrained the nitrogen abundance in 38 cases; for 34
stars only upper limits could be derived, which includes almost all stars
rotating at $v_\mathrm{e}\sin i >200\,\mathrm{km s^{-1}}$. We analysed the
nitrogen abundance as a function of projected rotation rate $v_\mathrm{e}\sin
i$ and confronted it with predictions of rotational mixing. The upper limits on
the nitrogen abundance of the rapidly rotating stars are not in apparent
violation with theoretical expectations. However, we found a group of
N-enhanced slowly-spinning stars that is not in accordance with predictions of
rotational mixing in single stars. Among O-type stars with (rotation-corrected)
gravities less than $\log\,g_c = 3.75$ this group constitutes 30$-$40 percent
of the population. We found a correlation between nitrogen and helium abundance
which is consistent with expectations, suggesting that, whatever the mechanism
that brings N to the surface, it displays CNO-processed material. | astro-ph_SR |
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