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Evolutionary models of the optical component of the LMC X-1/Star 32
binary system: Calculations carried out to model the evolution of Star 32 under different
assumptions about the stellar wind mass-loss rate provide robust limits on the
present mass of the star. The obtained range is 31 to 35.5 Msun, which is in
very good agreement with the orbital solution of Orosz et al., namely 28.3 to
35.3 Msun. The initial mass of Star 32 had to be in the range 35 to 40 Msun and
the present age of the system is 3.7 to 4.0 Myr. | astro-ph_SR |
An overall view of temperature oscillations in the solar chromosphere
with ALMA: By direct measurements of the gas temperature, the Atacama Large
Millimeter/sub-millimeter Array (ALMA) has yielded a new diagnostic tool to
study the solar chromosphere. Here we present an overview of the
brightness-temperature fluctuations from several high-quality and
high-temporal-resolution (i.e., 1 and 2 sec cadence) time series of images
obtained during the first two years of solar observations with ALMA, in Band 3
and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz),
respectively. The various datasets represent solar regions with different
levels of magnetic flux. We perform Fast Fourier and Lomb-Scargle transforms to
measure both the spatial structuring of dominant frequencies and the average
global frequency distributions of the oscillations (i.e., averaged over the
entire field of view). We find that the observed frequencies significantly vary
from one dataset to another, which is discussed in terms of the solar regions
captured by the observations (i.e., linked to their underlying magnetic
topology). While the presence of enhanced power within the frequency range 3-5
mHz is found for the most magnetically quiescent datasets, lower frequencies
dominate when there is significant influence from strong underlying magnetic
field concentrations (present inside and/or in the immediate vicinity of the
observed field of view). We discuss here a number of reasons which could
possibly contribute to the power suppression at around 5.5 mHz in the ALMA
observations. However, it remains unclear how other chromospheric diagnostics
(with an exception of Halpha line-core intensity) are unaffected by similar
effects, i.e., they show very pronounced 3-min oscillations dominating the
dynamics of the chromosphere, whereas only a very small fraction of all the
pixels in the ten ALMA data sets analysed here show peak power near 5.5 mHz. | astro-ph_SR |
Millimeter imaging of MWC 758: probing the disk structure and kinematics: We investigate the structure and kinematics of the circumstellar disk around
the Herbig Ae star MWC 758 using high resolution observations of the 12CO (3-2)
and dust continuum emission at the wavelengths of 0.87 and 3.3 mm. We find that
the dust emission peaks at an orbital radius of about 100 AU, while the CO
intensity has a central peak coincident with the position of the star. The CO
emission is in agreement with a disk in keplerian rotation around a 2.0 Msun
star, confirming that MWC758 is indeed an intermediate mass star. By comparing
the observation with theoretical disk models, we derive that the disk surface
density Sigma(r) steeply increases from 40 to 100 AU, and decreases
exponentially outward. Within 40 AU, the disk has to be optically thin in the
continuum emission at millimeter wavelengths to explain the observed dust
morphology, though our observations lack the angular resolution and sensitivity
required to constrain the surface density on these spatial scales. The surface
density distribution in MWC 758 disk is similar to that of ``transition''
disks, though no disk clearing has been previously inferred from the analysis
of the spectral energy distribution (SED). Moreover, the asymmetries observed
in the dust and CO emission suggest that the disk may be gravitationally
perturbed by a low mass companion orbiting within a radius of 30 AU. Our
results emphasize that SEDs alone do not provide a complete picture of disk
structure and that high resolution millimeter-wave images are essential to
reveal the structure of the cool disk mid plane. | astro-ph_SR |
New R Coronae Borealis and DY Persei Candidates in the SMC: We report 3 new R Coronae Borealis and 63 new DY Persei candidates in the
Small Magellanic Cloud. Our analysis, based on data published by the OGLE team,
consisted in a search for the characteristic drops in brightness that define
these classes. All candidates had been previously classified as semi-regular or
Mira variables. We briefly remark upon the possible existence of a "borderline"
DY Per-like star and a "transitional" DY Per/RCB star. Follow-up observations
are needed to conclusively establish the nature of our candidates. | astro-ph_SR |
Measurement of the radial velocity of the Sun as a star by means of a
reflecting solar system body. The effect of the body rotation: Minor bodies of the solar system can be used to measure the spectrum of the
Sun as a star by observing sunlight reflected by their surfaces. To perform an
accurate measurement of the radial velocity of the Sun as a star by this
method, it is necessary to take into account the Doppler shifts introduced by
the motion of the reflecting body. Here we discuss the effect of its rotation.
It gives a vanishing contribution only when the inclinations of the body
rotation axis to the directions of the Sun and of the Earth observer are the
same. When this is not the case, the perturbation of the radial velocity does
not vanish and can reach up to about 2.4 m/s for an asteroid such as 2 Pallas
that has an inclination of the spin axis to the plane of the ecliptic of about
30 degrees. We introduce a geometric model to compute the perturbation in the
case of a uniformly reflecting body of spherical or triaxial ellipsoidal shape
and provide general results to easily estimate the magnitude of the effect. | astro-ph_SR |
Supergranules as Probes of the Sun's Meridional Circulation: Recent analysis revealed that supergranules (convection cells seen at the
Sun's surface) are advected by the zonal flows at depths equal to the widths of
the cells themselves. Here we probe the structure of the meridional circulation
by cross-correlating maps of the Doppler velocity signal using a series of
successively longer time lags between maps. We find that the poleward
meridional flow decreases in amplitude with time lag and reverses direction to
become an equatorward return flow at time lags > 24 hours. These
cross-correlation results are dominated by larger and deeper cells at longer
time lags. (The smaller cells have shorter lifetimes and do not contribute to
the correlated signal at longer time lags.) We determine the characteristic
cell size associated with each time lag by comparing the equatorial zonal flows
measured at different time lags with the zonal flows associated with different
cell sizes from a Fourier analysis. This association gives a characteristic
cell size of ~50 Mm at a 24 hour time lag. This indicates that the poleward
meridional flow returns equatorward at depths > 50 Mm -- just below the base of
the surface shear layer. A substantial and highly significant equatorward flow
(4.6 +/- 0.4 m/s) is found at a time lag of 28 hours corresponding to a depth
of ~70 Mm. This represents one of the first positive detections of the Sun's
meridional return flow and illustrates the power of using supergranules to
probe the Sun's internal dynamics. | astro-ph_SR |
Chemical composition of stars with massive planets: Stellar parameters of 25 planet-hosting stars and abundances of Li, C, O, Na,
Mg, Al, S, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Zn, Y, Zr, Ba, Ce, Pr, Nd, Sm and
Eu, were studied based on homogeneous high resolution spectra and uniform
techniques. The iron abundance [Fe/H] and key elements (Li, C, O, Mg, Si)
indicative of the planet formation, as well as the dependencies of [El/Fe] on
$T_{cond}$, were analyzed. The iron abundances determined in our sample stars
with detected massive planets range within -0.3<[Fe/H]<0.4. The behaviour of
[C/Fe], [O/Fe], [Mg/Fe] and [Si/Fe] relative to [Fe/H] is consistent with the
Galactic Chemical Evolution trends. The mean values of C/O and [C/O] are <C/O>=
0.48 +/-0.07 and <[C/O]>=-0.07 +/-0.07, which are slightly lower than solar
ones. The Mg/Si ratios range from 0.83 to 0.95 for four stars in our sample and
from 1.0 to 1.86 for the remaining 21 stars. Various slopes of [El/Fe] vs.
Tcond were found. The dependencies of the planetary mass on metallicity, the
lithium abundance, the C/O and Mg/Si ratios, and also on the [El/Fe]-Tcond
slopes were considered. | astro-ph_SR |
SuperWASP observations of pulsating Am stars: We have studied over 1600 Am stars at a photometric precision of 1 mmag with
SuperWASP photometric data. Contrary to previous belief, we find that around
200 Am stars are pulsating delta Sct and gamma Dor stars, with low amplitudes
that have been missed in previous, less extensive studies. While the amplitudes
are generally low, the presence of pulsation in Am stars places a strong
constraint on atmospheric convection, and may require the pulsation to be
laminar. While some pulsating Am stars have been previously found to be delta
Sct stars, the vast majority of Am stars known to pulsate are presented in this
paper. They will form the basis of future statistical studies of pulsation in
the presence of atomic diffusion. | astro-ph_SR |
Towards a consistent model of the hot quadruple system HD 93206 = QZ
Carinæ: II. N-body model: HD 93206 is early-type massive stellar system, composed of components
resolved by direct imaging (Ab, Ad, B, C, D) as well as a compact sub-system
(Aa1, Aa2, Ac1, Ac2). Its geometry was already determined on the basis of
extensive photometric, spectroscopic and interferometric observations. However,
the fundamental absolute parameters are still not known precisely enough. We
use an advanced N-body model to account for all mutual gravitational
perturbations among the four close components, and all observational data
types, including: astrometry, radial velocities, eclipse timing variations,
squared visibilities, closure phases, triple products, normalized spectra, and
spectral-energy distribution (SED). The respective model has 38 free
parameters, namely three sets of orbital elements, component masses, and their
basic radiative properties ($T$, $\log g$, $v_{\rm rot}$). We revised the
fundamental parameters of QZ Car as follows. For a model with the nominal
extinction coefficient $R_V \equiv A_V/E(B-V) = 3.1$, the best-fit masses are
$m_1 = 26.1\,M_{\rm S}$, $m_2 = 32.3\,M_{\rm S}$, $m_3 = 70.3\,M_{\rm S}$, $m_4
= 8.8\,M_{\rm S}$, with uncertainties of the order of $2\,M_{\rm S}$, and the
system distance $d = (2800\pm 100)\,{\rm pc}$. In an alternative model, where
we increased the weights of RV and TTV observations and relaxed the SED
constraints, because extinction can be anomalous with $R_V \sim 3.4$, the
distance is smaller, $d = (2450\pm 100)\,{\rm pc}$. This would correspond to
that of Collinder 228 cluster. Independently, this is confirmed by dereddening
of the SED, which is only then consistent with the early-type classification
(O9.7Ib for Aa1, O8III for Ac1). Future modelling should also account for an
accretion disk around Ac2 component. | astro-ph_SR |
Collisional- and photo-excitations of Ca IV including strong 3.2 $μ$m
emission line: We report a detailed study of features of electron-impact excitation (EIE) of
Ca IV for the first time using the relativistic Breit-Pauli R-Matrix method
with a large close coupling wavefunction expansion of 54 fine structure levels
belonging to n=2,3,4 complexes. Our study predicts presence of a strong 3.2
$\mu$m emission line in IR. The EIE collision strength ($\Omega$) shows
extensive resonances with enhanced background resulting in an effective
collision strength ($\gamma$) of 2.2 at about 10,000 K that increases to 9.66
around 300,000 K. The present results include collision strength of all 1431
excitations among the 54 levels and effective collision strength for a limited
number of transitions of possible interest. We have found extensive resonances
in the low energy region, convergence of the resonances, and of the partial
waves with the 54 levels wavefunction. At higher energy, the collision strength
decreases beyond the resonance region for forbidden transitions, is almost
constant or decreases slowly for dipole-allowed transitions with low oscillator
strengths, and rises with Bethe-Coulomb behavior of ln(E)to almost a plateau
for transitions with high f-values. | astro-ph_SR |
Bright Spectroscopic Binaries: II. A study of five systems with orbital
periods of P < 500 days: We present a detailed analysis of five bright spectroscopic binary systems
(HD 18665, HD 27131, HD 171852, HD 215550, HD 217427) that have orbital periods
of P < 500 days. We determined the complete set of orbital parameters using the
toolkit RadVel by analyzing the observed radial velocity curves. To study the
properties of the five systems, we also analyzed the intermediate resolution
spectra (R = 20,000) observed with the TIGRE telescope and determined the
stellar parameters of the primary stars using the toolkit iSpec. With Gaia
Early Data Release 3 parallaxes, a correction for interstellar extinction using
the 3D dust map, and bolometric corrections, we placed the stars in the
Hertzsprung-Russell diagram and compared the positions with stellar evolution
tracks calculated with the Eggleton code to determine the masses and ages of
the primary stars. They have all evolved to the giant phase. Finally, we were
able to determine the masses of the secondary stars and to estimate the orbital
inclinations i of the binary systems. | astro-ph_SR |
Temperature and Distance Dependence of Tidal Circularization in Close
Binaries: A Catalog of Eclipsing Binaries in the Southern Hemisphere Observed
by the TESS Satellite: Tidal forces are important for understanding how close binary stars and
compact exoplanetary systems form and evolve. However, tides are difficult to
model and significant uncertainties exist about the strength of tides. Here, we
investigate tidal circularization in close binaries using a large sample of
well-characterised eclipsing systems. We searched TESS photometry from the
southern hemisphere for eclipsing binaries. We derive best-fit orbital and
stellar parameters by jointly modelling light curves and spectral energy
distributions. To determine the eccentricity distribution of eclipsing binaries
over a wide range of stellar temperatures ($3\,000-50\,000\,$K) and orbital
separations $a/R_1$ ($2-300$), we combine our newly obtained TESS sample with
eclipsing binaries observed from the ground and by the Kepler mission. We find
a clear dependency of stellar temperature and orbital separation in the
eccentricities of close binaries. We compare our observations with predictions
of the equilibrium and dynamical tides. We find that while cool binaries agree
with the predictions of the equilibrium tide, a large fraction of binaries with
temperatures between $6\,250\,$K and $10\,000\,$K and orbital separations
between $a/R_1 \sim 4$ and $10$ are found on circular orbits contrary to the
predictions of the dynamical tide. This suggests that some binaries with
radiative envelopes may be tidally circularised significantly more efficiently
than usually assumed. Our findings on orbital circularization have important
implications also in the context of hot Jupiters where tides have been invoked
to explain the observed difference in the spin-orbit alignment between hot and
cool host stars. | astro-ph_SR |
Oxygen abundances in G- and F-type stars from HARPS: We present a detailed and uniform study of oxygen abundance from two
different oxygen lines at 6158$\AA$ and 6300$\AA$ in a large sample of
solar-type stars. The results are used to check the behaviour of these spectral
lines as oxygen abundance indicators and to study the evolution of oxygen in
thick and thin disk populations of the Galaxy. Equivalent width measurements
were carried out for the [OI]~6158$\AA$ and OI~6300$\AA$ lines. LTE abundances
were obtained from these two lines in 610 and 535 stars, respectively. We were
able to measure oxygen abundance from both indicators in 447 stars, enabling
us, for the first time, to compare them in a uniform way. Careful error
analysis has been performed. We found that oxygen abundances derived from the
6158$\AA$ and 6300$\AA$ lines agree to within 0.1dex in 58\% of the stars in
our sample, and this result improves for higher signal-to-noise values. We
confirm an oxygen enhancement in stars of the thick disk, as has also been seen
for other $\alpha$-elements. The new oxygen abundances confirm previous
findings for a progressive linear rise in the oxygen-to-iron ratio with a slope
equal to 0.78 from solar metallicity to [Fe/H]$\sim$-1. However, the slope we
measured is steeper than the one found in previous studies based on the oxygen
triplet. Below [Fe/H]=$-$0.6 our stars show [O/Fe] ratios as high as $\sim$0.8,
which can be interpreted as evidence for oxygen overproduction in the Galactic
thick disk. These high oxygen abundances do not pose a problem to
chemodynamical models since there is a range of parameters that can accommodate
our results. | astro-ph_SR |
Connecting the Wilson depression to the magnetic field of sunspots: Context: In sunspots, the geometric height of continuum optical depth unity
is depressed compared to the quiet Sun. This so-called Wilson depression is
caused by the Lorentz force of the strong magnetic field inside the spots.
However, it is not understood in detail yet, how the Wilson depression is
related to the strength and geometry of the magnetic field or to other
properties of the sunspot.
Aims: We aim to study the dependence of the Wilson depression on the
properties of the magnetic field of the sunspots and how exactly the magnetic
field contributes to balancing the Wilson depression with respect to the gas
pressure of the surroundings of the spots.
Methods: Our study is based on 24 spectropolarimetric scans of 12 individual
sunspots performed with Hinode. We derived the Wilson depression for each spot
using both, a recently developed method that is based on minimizing the
divergence of the magnetic field, and an approach developed earlier that
enforces an equilibrium between the gas pressure and the magnetic pressure
inside the spot and the gas pressure in the quiet Sun, thus neglecting the
influence of the curvature force. We then performed a statistical analysis by
comparing the Wilson depression resulting from the two techniques with each
other and by relating them to various parameters of the sunspots, such as their
size or the strength of the magnetic field.
Results: We find that the Wilson depression becomes larger for spots with a
stronger magnetic field, but not as much as one would expect from the increased
magnetic pressure. This suggests that the curvature integral provides an
important contribution to the Wilson depression, particularly for spots with a
weak magnetic field. Our results indicate that the geometry of the magnetic
field in the penumbra is different between spots with different strengths of
the average umbral magnetic field. | astro-ph_SR |
Low-temperature chemistry between water and hydroxyl radicals: H/D
isotopic effects: Sets of systematic laboratory experiments are presented -- combining Ultra
High Vacuum cryogenic and plasma-line deposition techniques -- that allow us to
compare H/D isotopic effects in the reaction of H2O (D2O) ice with the hydroxyl
radical OD (OH). The latter is known to play a key role as intermediate species
in the solid-state formation of water on icy grains in space. The main finding
of our work is that the reaction H2O + OD --> OH + HDO occurs and that this may
affect the HDO/H2O abundances in space. The opposite reaction D2O + OH --> OD +
HDO is much less effective, and also given the lower D2O abundances in space
not expected to be of astronomical relevance. The experimental results are
extended to the other four possible reactions between hydroxyl and water
isotopes and are subsequently used as input for Kinetic Monte Carlo
simulations. This way we interpret our findings in an astronomical context,
qualitatively testing the influence of the reaction rates. | astro-ph_SR |
LMC Blue Supergiant Stars and the Calibration of the Flux-weighted
Gravity--Luminosity Relationship: High quality spectra of 90 blue supergiant stars in the Large Magellanic
Cloud are analyzed with respect to effective temperature, gravity, metallicity,
reddening, extinction and extinction law. An average metallicity, based on Fe
and Mg abundances, relative to the Sun of [Z] = -0.35 +/- 0.09 dex is obtained.
The reddening distribution peaks at E(B-V) = 0.08 mag, but significantly larger
values are also encountered. A wide distribution of the ratio of extinction to
reddening is found ranging from Rv = 2 to 6. The results are used to
investigate the blue supergiant relationship between flux-weighted gravity, and
absolute bolometric magnitude. The existence of a tight relationship, the FGLR,
is confirmed. However, in contrast to previous work the observations reveal
that the FGLR is divided into two parts with a different slope. For
flux-weighted gravities larger than 1.30 dex the slope is similar as found in
previous work, but the relationship becomes significantly steeper for smaller
values of the flux-weighted gravity. A new calibration of the FGLR for
extragalactic distance determinations is provided. | astro-ph_SR |
The CARMENES search for exoplanets around M dwarfs: Convective shift and
starspot constraints from chromatic radial velocities: Context. Variability caused by stellar activity represents a challenge to the
discovery and characterization of terrestrial exoplanets and complicates the
interpretation of atmospheric planetary signals.
Aims. We aim to use a detailed modeling tool to reproduce the effect of
active regions on radial velocity measurements, which aids the identification
of the key parameters that have an impact on the induced variability.
Methods. We analyzed the effect of stellar activity on radial velocities as a
function of wavelength by simulating the impact of the properties of spots,
shifts induced by convective motions, and rotation. We focused our modeling
effort on the active star YZ CMi (GJ 285), which was photometrically and
spectroscopically monitored with CARMENES and the Telescopi Joan Or\'o.
Results. We demonstrate that radial velocity curves at different wavelengths
yield determinations of key properties of active regions, including spot
filling factor, temperature contrast, and location, thus solving the degeneracy
between them. Most notably, our model is also sensitive to convective motions.
Results indicate a reduced convective shift for M dwarfs when compared to
solar-type stars (in agreement with theoretical extrapolations) and points to a
small global convective redshift instead of blueshift.
Conclusions. Using a novel approach based on simultaneous chromatic radial
velocities and light curves, we can set strong constraints on stellar activity,
including an elusive parameter such as the net convective motion effect. | astro-ph_SR |
Spectroscopic Observations of Selected Planetary Nebulae from the HASH
Database: We conducted research on the classification and physical properties of 10
objects from the HASH (Hong Kong/Australian Astronomical Observatory/Strasbourg
Observatory H-alpha Planetary Nebula (PN)) database with small angular sizes (<
8\arcsec) in the northern hemisphere. The sample consisted of 6 Likely PNe, 2
new candidates, one emission-line star, and one object of unknown nature. Among
them, we observed 4 objects for the first time using the medium-resolution
TFOSC spectrograph located on the RTT150 cm of the T\"UB\.ITAK National
Observatory (TUG). To investigate the classification of the observed objects,
we utilized the emission line ratios of [O III]/H$_{\gamma}$, [O
III]/H$_{\beta}$, [N II]/H$_{\alpha}$ and [S II]/H$_{\alpha}$ and diagnostic
diagrams such as the Sabbadin-Minello-Bianchini (SMB) and
Baldwin-Phillips-Terlevich (BPT). When considering a broader range of
diagnostic criteria compared to those provided in the literature, our analyses
resulted in the reclassification of 4 objects from Likely PNe to True PNe and
the retention of the previous classification for the remaining 6 objects. In
addition, we obtained various physical conditions such as electron
temperatures, electron densities, logarithmic extinction coefficients, and
excitation classes for the 10 objects under study. Our analysis revealed that
the ionic abundances of the majority of these objects were in agreement with
Galactic PNe. Our spectral observations have led to the updating of 10 PNe in
the HASH database. | astro-ph_SR |
Solar extreme events: Solar flares and CMEs have a broad range of magnitudes. This review discusses
the possibility of "extreme events," defined as those with magnitudes greater
than have been seen in the existing historical record. For most quantitative
measures, this direct information does not extend more than a century and a
half into the recent past. The magnitude distributions (occurrence frequencies)
of solar events (flares/CMEs) typically decrease with the parameter measured or
inferred (peak flux, mass, energy etc. Flare radiation fluxes tend to follow a
power law slightly flatter than $S^{-2}$, where S represents a peak flux; solar
particle events (SPEs) follow a still flatter power law up to a limiting
magnitude, and then appear to roll over to a steeper distribution, which may
take an exponential form or follow a broken power law. This inference comes
from the terrestrial $^{14}$C record and from the depth dependence of various
radioisotope proxies in the lunar regolith and in meteorites. Recently major
new observational results have impacted our use of the relatively limited
historical record in new ways: the detection of actual events in the $^{14}$C
tree-ring records, and the systematic observations of flares and "superflares"
by the Kepler spacecraft. I discuss how these new findings may affect our
understanding of the distribution function expected for extreme solar events. | astro-ph_SR |
Field Blue Stragglers and Related Mass Transfer Issues: This chapter contains my impressions and perspectives about the current state
of knowledge about field blue stragglers (FBS) stars, drawn from an extensive
literature that I searched. I conclude my review of issues that attend FBS and
mass transfer, by a brief enumeration of a few mildly disquieting observational
facts. | astro-ph_SR |
The data center for the Spectrometer and Telescope for Imaging X-rays
(STIX) onboard Solar Orbiter: The Spectrometer and Telescope for Imaging X-rays (STIX) on board Solar
Orbiter observes solar X-ray emission in the range of 4 – 150 keV and
produces spectra and images of solar flares over a wide range of flare
magnitudes. During nominal operation, STIX continuously generates data. A
constant data flow requires fully automated data-processing pipelines to
process and analyze the data, and a data platform to manage, visualize, and
distribute the data products to the scientific community. The STIX Data Center
has been built to fulfill these needs. In this paper, we outline its main
components to help the community better understand the tools and data it
provides. The STIX Data Center is operated at the University of Applied
Sciences and Arts Northwestern Switzerland (FHNW) and consists of automated
processing pipelines and a data platform. The pipelines process STIX telemetry
data, perform common analysis tasks, and generate data products at different
processing levels. They have been designed to operate fully automatically with
minimal human intervention. The data platform provides web-based user
interfaces and application programmable interfaces for searching and
downloading STIX data products. The STIX Data Center has been operating
successfully for more than two years. The platform facilitates instrument
operations and provides vital support to STIX data users. | astro-ph_SR |
Eclipsing Binary Stars: the Royal Road to Stellar Astrophysics: Russell (1948) famously described eclipses as the "royal road" to stellar
astrophysics. From photometric and spectroscopic observations it is possible to
measure the masses and radii (to 1% or better!), and thus surface gravities and
mean densities, of stars in eclipsing binary systems using nothing more than
geometry. Adding an effective temperature subsequently yields luminosity and
then distance (or vice versa) to high precision. This wealth of directly
measurable quantities makes eclipsing binaries the primary source of empirical
information on the properties of stars, and therefore a cornerstone of stellar
astrophysics. In this review paper I summarise the current standing of
eclipsing binary research, present an overview of useful analysis techniques,
and conclude with a glance to the future. | astro-ph_SR |
Identifying Footpoints of Pre-eruptive and Coronal Mass Ejection Flux
Ropes with Sunspot Scars: The properties of pre-eruptive structures and coronal mass ejections (CMEs)
are characterized by those of their footpoints, the latter of which thus
attract great interest. However, how to identify the footpoints of pre-eruptive
structures and how to identify the footpoints with ground-based instruments,
still remain elusive. In this work, we study an arc-shaped structure intruding
in the sunspot umbra. It is located close to the (pre-)eruptive flux rope
footpoint and is thus expected to help identify the footpoint. We analyse this
arc-shaped structure, which we name as "sunspot scar", in a CME event on 2012
July 12 and in two CME events in observationally-inspired MHD simulations
performed by OHM and MPI-AMRVAC. The sunspot scar has a more inclined magnetic
field with a weaker vertical component and a stronger horizontal component
relative to that in the surrounding umbra and manifests as a light bridge in
the white light passband. The hot field lines anchored in the sunspot scar are
spatially at the transition between the flux rope and the background coronal
loops, and temporally in the process of the slipping reconnection which builds
up the flux rope. The sunspot scar and its related light bridge mark the edge
of the CME flux rope footpoint, and especially, the edge of the pre-eruptive
flux rope footpoint in the framework of "pre-eruptive structures being flux
ropes". Therefore, they provide a new perspective for the identification of
pre-eruptive and CME flux rope footpoints, and also new methods for studying
the properties and evolution of pre-eruptive structures and CMEs with
photospheric observations only. | astro-ph_SR |
Elemental Abundance Analysis of Single and Binary Late-B Stars Using
Sub-meter Class Telescopes: HR 342, HR 769, HR 1284, and HR 8705: We test the capabilities of 0.4 m telescopes equipped with an \'{e}chelle
spectrograph to derive fundamental parameters and elemental abundances of four
late-B type stars: HR 342, HR 769, HR 1284, and HR 8705. The medium resolution
(R~14000) spectra covering the wavelength range of 4380-7350 {\AA} of the four
stars have been obtained using the 40-cm-telescope in Ankara University Kreiken
Observatory (AUKR). Using spectrum synthesis, we were able to derive the
abundances of eleven chemical elements. We find that these stars do not show
remarkable departures from the solar abundances, except for HR 8705 and the
primary component of HR 1284, which exhibit slight underabundances of a few
elements, i.e., O, Mg, Al, Si, and Fe. We also find that HR 1284 is probably a
new spectroscopic binary star. In order to model the spectrum of this object,
one of us (TK) has developed a new graphic interface which allows us to
synthesize the composite spectrum of binary stars. | astro-ph_SR |
The Age-Rotation-Activity Relation: From Myrs to Gyrs: Over the past 40 years, observational surveys have established the existence
of a tight relationship between a star's age, rotation period, and magnetic
activity. This age-rotation-activity relation documents the interplay between a
star's magnetic dynamo and angular momentum evolution, and provides a valuable
age estimator for isolated field stars. While the age-rotation-activity
relation has been studied extensively in clusters younger than 500 Myr,
empirically measured rotation periods are scarce for older ages. Using the
Palomar Transient Factory (PTF), we have begun a survey of stellar rotation to
map out the late-stage evolution of the age-rotation-activity relation: the
Columbia/Cornell/Caltech PTF (CCCP) survey of open clusters. The first CCCP
target is the nearby ~600 Myr Hyades-analog Praesepe, where PTF has produced
light curves spanning more than 3 months and containing >150 measurements for
~650 cluster members. Analyzing these light curves, we have measured rotation
periods for 40 K & M cluster members, filling the gap between the periods
previously reported for solar-type Hyads (Radick et al. 1987, Prosser et al.
1995) and for a handful of low-mass Praesepe members (Scholz et al. 2007). Our
measurements indicate that Praesepe's period-color relation undergoes at
transition at a characteristic spectral type of ~M1 --- from a well-defined
singular relation at higher mass, to a more scattered distribution of both fast
and slow-rotators at lower masses. The location of this transition is broadly
consistent with expectations based on observations of younger clusters and the
assumption that stellar-spin down is the dominant mechanism influencing angular
momentum evolution at ~600 Myr. In addition to presenting the results of our
photometric monitoring of Praesepe, we summarize the status and future of the
CCCP survey. | astro-ph_SR |
Spectropolarimetric multi line analysis of stellar magnetic fields: In this paper we study the feasibility of inferring the magnetic field from
polarized multi-line spectra using two methods: The pseudo line approach and
The PCA-ZDI approach. We use multi-line techniques, meaning that all the lines
of a stellar spectrum contribute to obtain a polarization signature. The use of
multiple lines dramatically increases the signal to noise ratio of these
polarizations signatures. Using one technique, the pseudo-line approach, we
construct the pseudo-line as the mean profile of all the individual lines. The
other technique, the PCA-ZDI approach proposed recently by Semel et al. (2006)
for the detection of polarized signals, combines Principle Components Analysis
(PCA) and the Zeeman Do ppler Imaging technique (ZDI). This new method has a
main advantage: the polarized signature is extracted using cross correlations
between the stellar spectra nd functions containing the polarization properties
of each line. These functions are the principal components of a database of
synthetic spectra. The synthesis of the spectra of the database are obtained
using the radiative transfer equations in LTE. The profiles built with the
PCA-ZDI technique are denominated Multi-Zeeman-Signatures. The construction of
the pseudo line as well as the Multi-Zeeman-Signatures is a powerful tool in
the study of stellar and solar magnetic fields. The information of the physical
parameters that governs the line formation is contained in the final polarized
profiles. In particular, using inversion codes, we have shown that the magnetic
field vector can be properly inferred with both approaches despite the magnetic
field regime. | astro-ph_SR |
HST hot-Jupiter transmission spectral survey: Haze in the atmosphere of
WASP-6b: We report Hubble Space Telescope (HST) optical to near-infrared transmission
spectroscopy of the hot Jupiter WASP-6b, measured with the Space Telescope
Imaging Spectrograph (STIS) and Spitzer's InfraRed Array Camera (IRAC). The
resulting spectrum covers the range $0.29-4.5\,\mu$m. We find evidence for
modest stellar activity of WASP-6b and take it into account in the transmission
spectrum. The overall main characteristic of the spectrum is an increasing
radius as a function of decreasing wavelength corresponding to a change of
$\Delta (R_p/R_{\ast})=0.0071$ from 0.33 to $4.5\,\mu$m. The spectrum suggests
an effective extinction cross-section with a power law of index consistent with
Rayleigh scattering, with temperatures of $973\pm144$ K at the planetary
terminator. We compare the transmission spectrum with hot-Jupiter atmospheric
models including condensate-free and aerosol-dominated models incorporating Mie
theory. While none of the clear-atmosphere models is found to be in good
agreement with the data, we find that the complete spectrum can be described by
models that include significant opacity from aerosols including Fe-poor
Mg$_2$SiO$_4$, MgSiO$_3$, KCl and Na$_2$S dust condensates. WASP-6b is the
second planet after HD189733b which has equilibrium temperatures near
$\sim1200$ K and shows prominent atmospheric scattering in the optical. | astro-ph_SR |
Binary Central Stars of Planetary Nebulae Identified With Kepler/K2: We present the identification of 34 likely binary central stars (CSs) of
planetary nebulae (PNe) from {\it Kepler/K2} data, seven of which show
eclipses. Of these, 29 are new discoveries. Two additional CSs with more
complicated variability are also presented. We examined the light curves of all
`possible', `likely' and `true' PNe in every {\it Kepler/K2} campaign (0
through 19) to identify CS variability that may indicate a binary CS. For
Campaigns 0, 2, 7, 15, and 16 we find 6 likely or confirmed variables among 21
PNe. Our primary effort, though, was focused on Campaign 11 which targeted a
Galactic bulge field containing approximately 183 PNe, in which we identified
30 candidate variable CSs. The periods of these variables range from 2.3~h to
30~d, and based on our analysis, most are likely to be close binary star
systems. We present periods and preliminary classifications (eclipsing, double
degenerate, or irradiated systems) for the likely binaries based on light curve
shape. From our total sample of 204 target PNe, with a correction for
incompleteness due to magnitude limits, we calculate a binary fraction of PN
central stars to be 20.7 percent for all the observed PNe, or 23.5 percent if
we limit our sample only to `true' PNe. However these fractions are almost
certainly lower limits due to the large angular size of the \emph{Kepler}
pixels, which leads to reduced sensitivity in detecting variability, primarily
as a result of dilution and noise from the nebula and neighbouring stars. We
discuss the binary population of CSs based on these results as part of the
total known sample of close binary CSs. | astro-ph_SR |
PHL 417: a zirconium-rich pulsating hot subdwarf (V366 Aquarid)
discovered in K2 data: The Kepler spacecraft observed the hot subdwarf star PHL 417 during its
extended K2 mission, and the high-precision photometric lightcurve reveals the
presence of 17 pulsation modes with periods between 38 and 105 minutes. From
follow-up ground-based spectroscopy we find that the object has a relatively
high temperature of 35 600 K, a surface gravity of $\log g / {\rm
cm\,s^{-2}}\,=\,5.75$ and a super-solar helium abundance. Remarkably, it also
shows strong zirconium lines corresponding to an apparent +3.9 dex
overabundance compared with the Sun. These properties clearly identify this
object as the third member of the rare group of pulsating heavy-metal stars,
the V366 Aquarii pulsators. These stars are intriguing in that the pulsations
are inconsistent with the standard models for pulsations in hot subdwarfs,
which predicts that they should display short-period pulsations rather than the
observed longer periods. We perform a stability analysis of the pulsation modes
based on data from two campaigns with K2. The highest amplitude mode is found
to be stable with a period drift, $\dot{P}$, of less than $1.1\cdot10^{-9}$
s/s. This result rules out pulsations driven during the rapid stages of helium
flash ignition. | astro-ph_SR |
New hydrodynamic solutions for line-driven winds of hot massive stars
using Lambert $W$-function: Hot massive stars present strong stellar winds that are driven by absorption,
scattering and re\-emission of photons by the ions of the atmosphere
(\textit{line-driven winds}). A better comprehension of this phenomenon, and a
more accurate calculation of hydrodynamics and radiative acceleration is
required to reduce the number of free parameters in spectral fitting, to
determine accurate wind parameters such as mass-loss rates and velocity
profiles.
We use the non-LTE model-atmosphere code CMFGEN to numerically solve the
radiative transfer equation in the stellar atmosphere and to calculate the
radiative acceleration $g_\text{rad}(r)$. Under the assumption that the
radiative acceleration depends only on the radial coordinate, we solve
analytically the equation of motion by means of the Lambert $W$-function. An
iterative procedure between the solution of the radiative transfer and the
equation of motion is executed in order to obtain a final self-consistent
velocity field that is no longer based on any $\beta$-law.
We apply the Lambert-procedure to three O supergiant stars ($\zeta$-Puppis,
HD~165763 and $\alpha$-Cam) and discuss the Lambert-solutions for the velocity
profiles. It is found that, even without recalculation of the mass-loss rate,
the Lambert-procedure allows the calculation of consistent velocity profiles
that reduce the number of free parameters when a spectral fitting using CMFGEN
is performed. Synthetic spectra calculated from our Lambert-solutions show
significant differences compared to the initial $\beta$-law CMFGEN models. The
results indicate the importance of consistent velocity profile calculation in
the CMFGEN code and its usage in a fitting procedure and interpretation of
observed spectra. | astro-ph_SR |
V5856 Sagittarii/2016: Broad Multi-Epoch Spectral Coverage of a
Sustained High Luminosity Nova: Nova V5856 Sagittarii is unique for having remained more than nine magnitudes
above its pre-outburst brightness for more than six years. Extensive visible
and IR spectra from the time of outburst to the present epoch reveal separate
emitting regions with distinct spectral characteristics. Permitted emission
lines have both broad and narrow components, whereas the forbidden line
profiles are almost entirely broad. The permitted line components frequently
display P Cygni profiles indicating high optical depth, whereas the broad
components do not show detectable absorption. The densities and velocities
deduced from the spectra, including differences in the O I 7773 and 8446 lines,
are not consistent with an on-going wind. Instead, the prolonged high
luminosity and spectral characteristics are indicative of a post-outburst
common envelope that enshrouds the binary, and is likely the primary source of
the visible and IR emission. | astro-ph_SR |
On the variation of the scaling exponent of the flare fluence with
temperature: Solar flares result in an increase of the solar irradiance at all
wavelengths. While the distribution of the flare fluence observed in coronal
emission has been widely studied and found to scale as f(E) ~ E^{-\alpha}, with
\alpha slightly below 2, the distribution of the flare fluence in chromospheric
lines is poorly known. We used the solar irradiance measurements observed by
the SDO/EVE instrument at a 10s-cadence to investigate if there is a dependency
of the scaling exponent on the formation region of the lines (or temperature).
We analyzed all flares above the C1 level since the start of the EVE
observation (May 2010) to determine the flare fluence distribution in 16 lines
covering a large range of temperature, several of which were not studied
before. Our results show a small downward trend with the temperature of the
scaling exponent of the PDF, going from above 2 at lower temperature (a few
10^4 K) to about1.8 for hot coronal emission (several 10^6 K). However, because
colder lines also have smaller contrast, we could not exclude that this
behavior is caused by including more noise for smaller flare for these lines.
We discuss the method and its limits and tentatively associate this possible
trend to the different mechanisms responsible for the heating of the
chromosphere and corona during flares. | astro-ph_SR |
Radiation Magnetohydrodynamics Simulation of Proto-Stellar Collapse:
Two-Component Molecular Outflow: We perform a three-dimensional nested-grid radiation magneto-hydrodynamics
(RMHD) simulation with self-gravity to study the early phase of the low-mass
star formation process from a rotating molecular cloud core to a first
adiabatic core just before the second collapse begins. Radiation transfer is
handled with the flux-limited diffusion approximation, operator-splitting and
implicit time-integrator. In the RMHD simulation, the outer region of the first
core attains a higher entropy and the size of first core is larger than that in
the magnetohydrodynamics simulations with the barotropic approximation. Bipolar
molecular outflow consisting of two components is driven by magnetic Lorentz
force via different mechanisms, and shock heating by the outflow is observed.
Using the RMHD simulation we can predict and interpret the observed properties
of star-forming clouds, first cores and outflows with millimeter/submillimeter
radio interferometers, especially the Atacama Large Millimeter/submillimeter
Array (ALMA). | astro-ph_SR |
Eleven Exoplanet Host Star Angular Diameters from the CHARA Array: We directly measured the angular diameters for 11 exoplanet host stars using
Georgia State University's CHARA Array interferometer and calculated their
linear radii and effective temperatures. The sample tends towards evolving or
evolved stars and includes one dwarf, four subgiants, and six giants. We then
estimated masses and ages for the stars using our effective temperatures
combined with metallicity measurements from the literature. | astro-ph_SR |
The Heating and Pulsations of V386 Serpentis after its 2019 Dwarf Nova
Outburst: Following the pulsation spectrum of a white dwarf through the heating and
cooling involved in a dwarf nova outburst cycle provides a unique view of the
changes to convective driving that take place on timescales of months versus
millenia for non-accreting white dwarfs. In 2019 January the dwarf nova V386
Ser (one of a small number containing an accreting, pulsating white dwarf),
underwent a large amplitude outburst. Hubble Space Telescope ultraviolet
spectra were obtained 7 and 13 months after outburst along with optical
ground-based photometry during this interval and high-speed photometry at 5.5
and 17 months after outburst. The resulting spectral and pulsational analysis
shows a cooling of the white dwarf from 21,020 K to 18,750 K (with a gravity
log(g) = 8.1) between the two UV observations, along with the presence of
strong pulsations evident in both UV and optical at a much shorter period after
outburst than at quiescence. The pulsation periods consistently lengthened
during the year following outburst, in agreement with pulsation theory.
However, it remains to be seen if the behavior at longer times past outburst
will mimic the unusual non-monotonic cooling and long periods evident in the
similar system GW Lib. | astro-ph_SR |
Chromospheric cannonballs on the Sun: In the highly dynamic chromosphere, there exist many kinds of small-scale
activities, such as spicules, surges, and Ellerman bombs. Here, we report the
discovery of a new phenomenon in the chromosphere observed with the New Vacuum
Solar Telescope at the Fuxian Solar Observatory. In the high tempo-spatial
resolution H$\alpha$ images, some dark or bright structures are found to fly
along the curved trajectory, looking like cannonballs. Their average size,
mass, and velocity are about 1.5 $\times$ 10$^{9}$ km$^{3}$, 1.5 $\times$
10$^{8}$ kg, and 56 km s$^{-1}$, respectively. In the simultaneous
(extreme-)ultraviolet images obtained by the Solar Dynamics Observatory, these
cannonballs appear as brighter features compared to the surrounding area,
implying that there exists some kind of heating during this process. The
photospheric magnetograms show the magnetic flux emergence and interaction with
the pre-existing fields. These observations reveal that the cannonballs are
chromospheric material blobs launched due to the magnetic reconnection between
emerging magnetic flux and the pre-existing loops. | astro-ph_SR |
Prandtl number dependence of compressible convection: Flow statistics
and convective energy transport: (abridged) Context: The ratio of kinematic viscosity to thermal diffusivity,
the Prandtl number, is much smaller than unity in stellar convection zones.
Aims: To study the statistics of convective flows and energy transport as
functions of the Prandtl number. Methods: Three-dimensional numerical
simulations convection in Cartesian geometry are used. The convection zone (CZ)
is embedded between two stably stratified layers. Statistics and transport
properties of up- and downflows are studied separately. Results: The rms
velocity increases with decreasing Prandtl number. At the same time the filling
factor of downflows decreases and leads to stronger downflows at lower Prandtl
numbers, and to a strong dependence of overshooting on the Prandtl number.
Velocity power spectra do not show marked changes as a function of Prandtl
number. At the highest Reynolds numbers the velocity power spectra are
compatible with the Bolgiano-Obukhov $k^{-11/5}$ scaling. The horizontally
averaged convected energy flux ($\overline{F}_{\rm conv}$) is independent of
the Prandtl number within the CZ. However, the upflows (downflows) are the
dominant contribution to the convected flux at low (high) Prandtl number. These
results are similar to those from Rayleigh-Ben\'ard convection in the low
Prandtl number regime where convection is vigorously turbulent but inefficient
at transporting energy. Conclusions: The current results indicate a strong
dependence of convective overshooting and energy flux on the Prandtl number.
Numerical simulations of astrophysical convection often use Prandtl number of
unity. The current results suggest that this can lead to misleading results and
that the astrophysically relevant low Prandtl number regime is qualitatively
different from the parameters regimes explored in typical simulations. | astro-ph_SR |
The shape of sunspots and solar activity cycles: The paper presents the results of the analysis of the geometric
characteristics of sunspots for the period of 19-24 cycles of activity. The
shape of sunspots was studied on the basis of the method of normalization of
images of sunspots to study the average profile of the spot. The deviation of
the shape of sunspots from the axisymmetric configuration is investigated. It
was found that the spots, as a rule, have an ellipsoid shape, and the major
axis of the ellipse has a predominant inclination to the equator, opposite in
the Northern and Southern hemispheres. The angle of inclination of the sunspot
axis corresponds to the angle of inclination of the bipoles in the activity
cycles. The relationship between the shape of sunspots in the current cycle and
the amplitude of the next cycle of activity is found. The greater the
elongation along the longitude of the current cycle of spots, the higher the
next cycle of activity will be. | astro-ph_SR |
ALMA Observations of the Water Fountain Pre-Planetary Nebula IRAS
16342-3814: High-velocity bipolar jets and an Expanding Torus: We have mapped 12CO J=3-2 and other molecular lines from the "water-fountain"
bipolar pre-planetary nebula (PPN) IRAS 16342-3814 with ~0."35 resolution using
ALMA. We find (i) two very high-speed knotty, jet-like molecular outflows, (ii)
a central high-density (> few x 10^6 cm^{-3}), expanding torus of diameter 1300
AU, and (iii) the circumstellar envelope of the progenitor AGB, generated by a
sudden, very large increase in the mass-loss rate to >3.5 x 10^{-4} Msun/yr in
the past ~455 yr. Strong continuum emission at 0.89 mm from a central source
(690 mJy), if due to thermally-emitting dust, implies a substantial mass (0.017
Msun) of very large (~mm-sized) grains. The measured expansion ages of the
above structural components imply that the torus (age~160 yr) and the younger
high-velocity outflow (age~110 yr) were formed soon after the sharp increase in
the AGB mass-loss rate. Assuming a binary model for the jets in IRAS 16342, the
high momentum rate for the dominant jet-outflow in IRAS 16342 implies a high
minimum accretion rate, ruling out standard Bondi-Hoyle-Lyttleton wind
accretion and wind Roche lobe overflow (RLOF) models with white-dwarf or
main-sequence companions. Most likely, enhanced RLOF from the primary or
accretion modes operating within common envelope evolution are needed. | astro-ph_SR |
The orbital periods of subdwarf B binaries produced by the first stable
Roche overflow channel: Long-orbital-period subdwarf B (sdB) stars with main-sequence companions are
believed to be the product of stable Roche Lobe overflow (RLOF), a scenario
challenged by recent observations. Here we represent the results of a
systematic study of the orbital-period distribution of sdB binaries in this
channel using detailed binary evolution calculations. We show that the observed
orbital-period distribution of long-period sdB binaries can be well explained
by this scenario. Furthermore, we find that, if the progenitors of the sdB
stars have initial masses below the helium flash mass, the sdB binaries
produced from stable RLOF follow a unique mass -- orbital period relation for a
given metallicity $Z$; increasing the orbital period from $\sim 400$ to $\sim
1100$\,d corresponds to increasing the mass of the sdB star from $\sim 0.40$ to
$\sim 0.49\,M_\odot$ for $Z=0.02$. We suggest that the longest sdB binaries
(with orbital period $> 1100$\,d) could be the result of atmospheric RLOF. The
mass -- orbital period relation can be tested observationally if the mass of
the sdB star can be determined precisely, e.g.\ from asteroseismology. Using
this relation, we revise the orbital period distribution of sdB binaries
produced by the first stable RLOF channel for the best fitting model of Han et
al (2003), and show that the orbital period has a peak around 830\,d. | astro-ph_SR |
On the maximum black hole mass at solar metallicity: In high metallicity environments the mass that black holes (BHs) can reach
just after core-collapse widely depends on how much mass their progenitor stars
lose via winds. On one hand new theoretical and observational insights suggest
that early-stage winds should be weaker than what many canonical models
prescribe. On the other hand the proximity to the Eddington limit should affect
the formation of optically thick envelopes already during the earliest stages
of stars with initial masses $M_{\rm ZAMS}\gtrsim 100$ M$_\odot$, hence
resulting in higher mass-loss rates during the main sequence. We use the
evolutionary codes MESA and Genec to calculate a suite of tracks for massive
stars at solar metallicity Z$_\odot=0.014$ which incorporate these changes in
our wind mass loss prescription. In our calculations we employ moderate
rotation, high overshooting and magnetic angular momentum transport. We find a
maximum BH mass $M_{\rm BH, max}=28.3$ M$_\odot$ at Z$_\odot$. The most massive
BHs are predicted to form from stars with $M_{\rm ZAMS}\gtrsim 250$ M$_\odot$,
with the BH mass directly proportional to its progenitor's $M_{\rm ZAMS}$. We
also find in our models that at Z$_\odot$ almost any BH progenitor naturally
evolves into a Wolf-Rayet star due to the combined effect of internal mixing
and wind mass loss. These results are considerably different from most recent
studies regarding the final mass of stars before their collapse into BHs. While
we acknowledge the inherent uncertainties in stellar evolution modelling, our
study underscores the importance of employing the most up-to-date physics in BH
mass predictions. | astro-ph_SR |
Quasilinear Approach of the Whistler Heat-Flux Instability in the Solar
Wind: The hot beaming (or strahl) electrons responsible for the main electron
heat-flux in the solar wind are believed to be self-regulated by the
electromagnetic beaming instabilities, also known as the heat-flux
instabilities. Here we report the first quasi-linear theoretical approach of
the whistler unstable branch able to characterize the long-term saturation of
the instability as well as the relaxation of the electron velocity
distributions. The instability saturation is not solely determined by the drift
velocities, which undergo only a minor relaxation, but mainly from a concurrent
interaction of electrons with whistlers that induces (opposite) temperature
anisotropies of the core and beam populations and reduces the effective
anisotropy. These results might be able to (i) explain the low intensity of the
whistler heat-flux fluctuations in the solar wind (although other explanations
remain possible and need further investigation), and (ii) confirm a reduced
effectiveness of these fluctuations in the relaxation and isotropization of the
electron strahl and in the regulation of the electron heat-flux. | astro-ph_SR |
The EBLM Project I-Physical and orbital parameters, including spin-orbit
angles, of two low-mass eclipsing binaries on opposite sides of the Brown
Dwarf limit: This paper introduces a series of papers aiming to study the dozens of low
mass eclipsing binaries (EBLM), with F, G, K primaries, that have been
discovered in the course of the WASP survey. Our objects are mostly single-line
binaries whose eclipses have been detected by WASP and were initially followed
up as potential planetary transit candidates. These have bright primaries,
which facilitates spectroscopic observations during transit and allows the
study of the spin-orbit distribution of F, G, K+M eclipsing binaries through
the Rossiter-McLaughlin effect. Here we report on the spin-orbit angle of
WASP-30b, a transiting brown dwarf, and improve its orbital parameters. We also
present the mass, radius, spin-orbit angle and orbital parameters of a new
eclipsing binary, J1219-39b (1SWAPJ121921.03-395125.6, TYC 7760-484-1), which,
with a mass of 95 +/- 2 Mjup, is close to the limit between brown dwarfs and
stars. We find that both objects orbit in planes that appear aligned with their
primaries' equatorial planes. Neither primaries are synchronous. J1219-39b has
a modestly eccentric orbit and is in agreement with the theoretical
mass--radius relationship, whereas WASP-30b lies above it. | astro-ph_SR |
Very Long Baseline Interferometry imaging of the advancing ejecta in the
first gamma-ray nova V407 Cyg: In 2010/3, the Large Area Telescope on board Fermi revealed a transient
gamma-ray source, positionally coincident with the optical nova in the
symbiotic binary, V407Cyg. This event marked the first discovery of gamma-ray
emission from a nova. We aimed to obtain resolved radio imaging of the material
involved in the nova event; to determine the ejecta geometry and advance
velocity directly in the image plane; to constrain the physical conditions of
the system. We observed the source with the EVN and the VLBA over 16 epochs,
between 20 days and 6 months after the optical discovery. The source is
initially very dim but it later shows a substantial increase in brightness and
a resolved shell-like structure 40 to 90 days after the optical event. The
shell has a projected elliptical shape and is asymmetric in brightness and
spectral index, being brighter and characterised by a rising spectrum at the
S-E edge. We determine a projected velocity of ~3500 km/s in the initial phase,
and ~2100 km/s between day 20 and 91. We also found an emitting feature about
350 mas (940 AU) to the N-W, advancing at a projected velocity of ~700 km/s
along the polar axis of the binary. The total flux density in the VLBI images
is significantly lower than that previously reported at similar epochs and over
much wider angular scales with the VLA. Optical spectra demonstrated that in
2010 we were viewing V407Cyg along the equatorial plane and from behind the
Mira. Our radio observations image the bipolar flow of the ejecta perpendicular
to the orbital plane, where deceleration is much lower than through the
equatorial plane probed by the truncated profile of optical emission lines. The
separated polar knot at 350 mas and the bipolar flow strictly resemble the
similar arrangement seen in Hen 2-104. The observed ~700 km/s expansion
constrains the launch-date of the polar knot around 2004. [Abridged] | astro-ph_SR |
Dissipation of magnetic fields in star-forming clouds with different
metallicities: We study dissipation process of magnetic fields in the metallicity range $0-1
Z_{\odot}$ for contracting prestellar cloud cores. By solving non-equilibrium
chemistry for important charged species including charged grains, we evaluate
the drift velocity of the magnetic-field lines with respect to the gas. We find
that the magnetic flux dissipates in the density range $10^{12}{\rm cm^{-3}}
\lesssim n_{\rm H} \lesssim 10^{17}{\rm cm^{-3}}$ for the solar-metallicity
case at the scale of the core, which is assumed to be the Jeans scale. The
dissipation density range becomes narrower for lower metallicity. The magnetic
field is always frozen to the gas below metallicity $\lesssim
10^{-7}-10^{-6}Z_\odot$, depending on the ionization rate by cosmic rays and/or
radioactivity. With the same metallicity, the dissipation density range becomes
wider for lower ionization rate. The presence of such a dissipative regime is
expected to cause various dynamical phenomena in protostellar evolution such as
the suppression of jet/outflow launching and fragmentation of the circumstellar
disks depending on the metallicity. | astro-ph_SR |
The effect of the Solar wind on low-frequency observations of pulsars: We operate the six German stations of the LOw Frequency ARray as standalone
telescopes to observe more than 100 pulsars every week. To date, we have
collected almost four years of high-quality data at an unprecedented weekly
cadence. This allows us to perform a wide variety of analyses aimed at
characterising the magnetoionic plasma crossed by pulsar radiation. In
particular, our studies are focused on electron density variations in the
interstellar and interplanetary media, the Galactic and interplanetary magnetic
field, scintillation, and extreme scattering events. Here we report the first
results from our Solar wind monitoring campaign. | astro-ph_SR |
The chemical signatures of planetary engulfment events in binary systems: Planetary engulfment events involve the chemical assimilation of a planet
into a star's external layer. This can cause a change in the chemical pattern
of the stellar atmosphere in a way that mirrors the composition of the rocky
object engulfed, with the refractory elements being more abundant than the
volatiles. Due to these stellar chemical changes, planetary engulfment events
can render the process of chemical tagging potentially inaccurate. A
line-by-line differential analysis of twin stars in wide binary systems allows
us to test the chemical homogeneity of these associations with typical
individual stellar Fe I uncertainties of 0.01 dex and eventually unveil
chemical anomalies that could be attributed to planetary engulfment events. Out
of the 14 systems analysed here, we report the discovery of the most chemically
inhomogeneous system to date (HIP34407/HIP34426). The median difference in
abundances of refractory elements within the pair is 0.19 dex and the trend
between the differential abundances and condensation temperature suggests that
the anomaly is likely due to a planetary engulfment event. Within our sample,
five other chemically anomalous systems are found. | astro-ph_SR |
Multiwavelength observation of a large-scale flux rope eruption above
kinked mini-filament: We analyse multiwavelength observations of a western limb flare (C3.9)
occurred in AR NOAA 111465 on 30 April 2012. The high resolution images
recorded by SDO/AIA 304, 1600 \AA\ and Hinode/SOT H$\alpha$ show the activation
of a mini-filament (rising speed$\sim$40 km s$^{-1}$) associated with kink
instability and the onset of a C-class flare near the southern leg of the
filament. The first magnetic reconnection occurred at one of the footpoints of
the filament causing the breaking of its southern leg. The filament shows
unwinding motion of the northern leg and apex in the counterclockwise direction
and failed to erupt. A flux-rope (visible only in hot channels, i.e., AIA 131
and 94 \AA\ channels and Hinode/SXT) structure was appeared along the neutral
line during the second magnetic reconnection taking place above the kinked
filament. Formation of the RHESSI hard X-ray source (12-25 keV) above the
kinked filament and simultaneous appearance of the hot 131 \AA\ loops
associated with photospheric brightenings (AIA 1700 \AA) suggest the particle
acceleration along these loops from the top of the filament. In addition, EUV
disturbances/waves observed above the filament in 171 \AA\ also show a close
association with magnetic reconnection. The flux rope rises slowly ($\sim$100
km s$^{-1}$) producing a rather big twisted structure possibly by reconnection
with the surrounding sheared magnetic fields within $\sim$15-20 minutes, and
showed an impulsive acceleration reaching a height of about 80--100 Mm. AIA 171
and SWAP 174 \AA\ images reveal a cool compression front (or CME frontal loop)
surrounding the hot flux rope structure. | astro-ph_SR |
13C17O suggests gravitational instability in the HL Tau disc: We present the first detection of the 13C17O J=3-2 transition toward the HL
Tau protoplanetary disc. We find significantly more gas mass (at least a factor
of ten higher) than has been previously reported using C18O emission. This
brings the observed total disc mass to 0.2 M, which we consider to be a
conservative lower limit. Our analysis of the Toomre Q profile suggests that
this brings the disc into the regime of gravitational instability. The radial
region of instability (50-110 au) coincides with the location of a proposed
planet-carved gap in the dust disc and a spiral in the gas. We, therefore,
propose that if the origin of the gap is confirmed to be due to a forming giant
planet, then it is likely to have formed via the gravitational fragmentation of
the protoplanetary disc. | astro-ph_SR |
High-precision abundances of Sc, Mn, Cu, and Ba in solar twins. Trends
of element ratios with stellar age: A previous study of correlations between element abundance ratios, [X/Fe],
and ages of solar twin stars is extended to include Sc, Mn, Cu, and Ba. HARPS
spectra with S/N > 600 are used to derive very precise (+/- 0.01 dex)
differential abundances, and stellar ages with internal errors less than 1 Gyr
are obtained by interpolation in the logg - Teff diagram between isochrones
calculated with the Aarhus Stellar Evolution Code. For stars younger than 6
Gyr, [X/Fe] is tightly correlated with stellar age for all elements. For ages
between 6 and 9 Gyr, the [X/Fe] - age correlations break down and the stars
split up into two groups having respectively high and low [X/Fe] for the odd-Z
elements. It is concluded that while stars in the solar neighborhood younger
than about 6 Gyr were formed from interstellar gas with a smooth chemical
evolution, older stars have originated from regions enriched by supernovae with
different neutron excesses. Furthermore, the correlations between abundance
ratios and stellar age suggest that: i) Sc is made in Type II supernovae along
with the alpha-capture elements, ii) the Type II to Ia SNe yield ratio is about
the same for Mn and Fe, iii) Cu is mainly made by the weak s-process in massive
stars, iv) the Ba/Y yield ratio for AGB stars increases with decreasing stellar
mass, v) [Y/Mg] and [Y/Al] can be used as "chemical clocks" when determining
ages of solar metallicity stars. | astro-ph_SR |
Trends and Characteristics of High-Frequency Type II Bursts Detected by
CALLISTO Spectrometers: Solar radio type II bursts serve as early indicators of incoming
geo-effective space weather events such as coronal mass ejections (CMEs). In
order to investigate the origin of high-frequency type II bursts (HF type II
bursts), we have identified 51 of them (among 180 type II bursts from SWPC
reports) that are observed by ground-based Compound Astronomical Low-cost
Low-frequency Instrument for Spectroscopy and Transportable Observatory
(CALLISTO) spectrometers and whose upper-frequency cutoff (of either
fundamental or harmonic emission) lies in between 150 MHz-450 MHz during
2010-2019. We found that 60% of HF type II bursts, whose upper-frequency cutoff
$\geq$ 300 MHz originate from the western longitudes. Further, our study finds
a good correlation $\sim $ 0.73 between the average shock speed derived from
the radio dynamic spectra and the corresponding speed from CME data. Also, we
found that analyzed HF type II bursts are associated with wide and fast CMEs
located near the solar disk. In addition, we have analyzed the spatio-temporal
characteristics of two of these high-frequency type II bursts and compared the
derived from radio observations with those derived from multi-spacecraft CME
observations from SOHO/LASCO and STEREO coronagraphs. | astro-ph_SR |
On the Variation of Fourier Parameters for Galactic and LMC Cepheids at
Optical, Near-Infrared and Mid-Infrared Wavelengths: We present a light curve analysis of fundamental-mode Galactic and Large
Magellanic Cloud (LMC) Cepheids based on the Fourier decomposition technique.
We have compiled light curve data for Galactic and LMC Cepheids in optical
({\it VI}), near-infrared ({\it JHK}$_s$) and mid-infrared (3.6 $\&$
4.5-$\mu$m) bands from the literature and determined the variation of their
Fourier parameters as a function of period and wavelength. We observed a
decrease in Fourier amplitude parameters and an increase in Fourier phase
parameters with increasing wavelengths at a given period. We also found a
decrease in the skewness and acuteness parameters as a function of wavelength
at a fixed period. We applied a binning method to analyze the progression of
the mean Fourier parameters with period and wavelength. We found that for
periods longer than about 20 days, the values of the Fourier amplitude
parameters increase sharply for shorter wavelengths as compared to wavelengths
longer than the $J$-band. We observed the variation of the Hertzsprung
progression with wavelength. The central period of the Hertzsprung progression
was found to increase with wavelength in the case of the Fourier amplitude
parameters and decrease with increasing wavelength in the case of phase
parameters. We also observed a small variation of the central period of the
progression between the Galaxy and LMC, presumably related to metallicity
effects. These results will provide useful constraints for stellar pulsation
codes that incorporate stellar atmosphere models to produce Cepheid light
curves in various bands. | astro-ph_SR |
Binary Neutron Stars in Quasi-equilibrium: Quasi-equilibrium sequences of binary neutron stars are constructed for a
variety of equations of state in general relativity. Einstein's constraint
equations in the Isenberg-Wilson-Mathews approximation are solved together with
the relativistic equations of hydrostationary equilibrium under the assumption
of irrotational flow. We focus on unequal-mass sequences as well as equal-mass
sequences, and compare those results. We investigate the behavior of the
binding energy and total angular momentum along a quasi-equilibrium sequence,
the endpoint of sequences, and the orbital angular velocity as a function of
time, changing the mass ratio, the total mass of the binary system, and the
equation of state of a neutron star. It is found that the orbital angular
velocity at the mass-shedding limit can be determined by an empirical formula
derived from an analytic estimation. We also provide tables for 160 sequences
which will be useful as a guideline of numerical simulations for the inspiral
and merger performed in the near future. | astro-ph_SR |
The search of the stellar clusters in vicinity of YSOs with high and
middle masses: The results of the searching on the bases of GPS UKIDSS survey's data of
dense compact stellar clusters in the vicinity of 20 YSOs with high and middle
masses are presented. Totally we have revealed clusters in 13 areas. Around 5
objects (IRAS 18151-1208, IRAS 18316-0602, IRAS 19110+1045, IRAS 19213+1723,
IRAS 20056+3350) they are newly detected. The radii and stellar density have
significant gradient: from 0.2 to 2.7 pc and from 3 to 1000 stars/arcmin^2
respectively. | astro-ph_SR |
Scattering line polarization in rotating, optically thick disks: To interpret observations of astrophysical disks it is essential to
understand the formation process of the emitted light. If the disk is optically
thick, scattering dominated and permeated by a Keplerian velocity field,
Non-Local Thermodynamic Equilibrium radiative transfer modeling must be done to
compute the emergent spectrum from a given disk model. We investigate Non-local
thermodynamic equilibrium polarized line formation in different simple disk
models and aim to demonstrate the importance of both radiative transfer effects
and scattering as well as the effects of velocity fields. We self-consistently
solve the coupled equations of radiative transfer and statistical equilibrium
for a two level atom model by means of Jacobi iteration. We compute scattering
polarization, that is Q/I and U/I line profiles. The degree of scattering
polarization is significantly influenced by the inclination of the disk with
respect to observer, but also by the optical thickness of the disk and the
presence of rotation. Stokes U shows double-lobed profiles with amplitude which
increases with the disk rotation. Our results suggest that the line profiles,
especially the polarized ones, emerging from gaseous disks differ significantly
from the profiles predicted by simple approximations. The profiles are diverse
in shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A
clear indicator of disk rotation is the presence of Stokes U, which might prove
to be a useful diagnostic tool. We also demonstrate that, for moderate
rotational velocities, an approximate treatment can be used, where non-local
thermodynamic equilibrium radiative transfer is done in the velocity field-free
approximation and Doppler shift is applied in the process of spatial
integration over the whole emitting surface. | astro-ph_SR |
First ALMA Observation of a Solar Plasmoid Ejection from an X-ray Bright
Point: Eruptive phenomena such as plasmoid ejections or jets are an important
feature of solar activity with the potential for improving our understanding of
the dynamics of the solar atmosphere. Such ejections are often thought to be
signatures of the outflows expected in regions of fast magnetic reconnection.
The 304 A EUV line of Helium, formed at around 10^5 K, is found to be a
reliable tracer of such phenomena, but the determination of physical parameters
from such observations is not straightforward. We have observed a plasmoid
ejection from an X-ray bright point simultaneously at millimeter wavelengths
with ALMA, at EUV wavelengths with AIA, in soft X-rays with Hinode/XRT. This
paper reports the physical parameters of the plasmoid obtained by combining the
radio, EUV and X-ray data. As a result, we conclude that the plasmoid can
consist either of (approximately) isothermal 10^5 K plasma that is optically
thin at 100 GHz, or else a 10^4 K core with a hot envelope. The analysis
demonstrates the value of the additional temperature and density constraints
that ALMA provides, and future science observations with ALMA will be able to
match the spatial resolution of space-borne and other high-resolution
telescopes. | astro-ph_SR |
Nuclear constraints on non-Newtonian gravity at femtometer scale: Effects of the non-Newtonian gravity on properties of finite nuclei are
studied by consistently incorporating both the direct and exchange contribution
of the Yukawa potential in the Hartree-Fock approach using a well-tested Skyrme
force for the strong interaction. It is shown for the first time that the
strength of the Yukawa term in the non-Newtonian gravity is limited to
$\log(|\alpha|)<1.75/[\lambda(\rm fm)]^{0.54} + 33.6$ within the length scale
of $\lambda=1-10$ fm in order for the calculated properties of finite nuclei
not to be in conflict with accurate experimental data available. | astro-ph_SR |
The Periodic Spectroscopic Variability of FU Orionis: FU Orionis systems are young stars undergoing outbursts of disc accretion and
where the optical spectrum contains lines associated with both the disc
photosphere and a wind component. Previous observations of the prototype FU
Orionis have suggested that the wind lines and the photospheric lines are
modulated with periods of 14.54 and 3.54 days respectively (Herbig et al.
2003). We have re-observed the system at higher spectral resolution, by
monitoring variations of optical line profiles over 21 nights in 2007 and have
found periods of 13.48 and 3.6 days in the wind and disc components consistent
with the above: this implies variability mechanisms that are stable over at
least a decade. In addition we have found: i) that the variations in the
photospheric absorption lines are confined to the blue wing of the line (around
-9km/s): we tentatively ascribe this to an orbiting hotspot in the disc which
is obscured by a disc warp during its receding phase. ii) The wind period is
manifested not only in blue-shifted Halpha absorption, but also in red-shifted
emission of Halpha and Hbeta, as well as in blue-shifted absorption of Na I D,
Li I and Fe II. iii) We find that the periodic modulation of blue-shifted
Halpha absorption at around -100km/s, is phase lagged with respect to
variations in the other lines by ~1.8days. This is consistent with a picture in
which variations at the wind base first affect chromospheric emission and then
low velocity blue-shifted absorption, followed - after a lag equal to the
propagation time of disturbances across the wind's acceleration region - by a
response in high velocity blue-shifted absorption. Such arguments constrain the
size of the acceleration region to ~10^12cm. We discuss possible mechanisms for
periodic variations within the innermost 0.1AU of the disc, including the
possibility that these variations indicate the presence of an embedded hot
Jupiter. | astro-ph_SR |
Undetected Binary Stars Cause an Observed Mass Dependent Age Gradient in
Upper Scorpius: Young stellar associations represent a key site for the study of star
formation, but to accurately compare observations to models of stellar
evolution, the age of an association must be determined. The Upper Scorpius
region is the youngest section of the Scorpius-Centaurus OB association, which
is the largest collection of nearby, young, low-mass stars. The true age of
Upper Scorpius is not clear, and an observed mass-dependent age gradient in
Upper Scorpius, as well as in other star-forming regions, complicates age
measurements. The age gradient may indicate a genuine astrophysical feature or
may be an artifact of unrecognized systematic effects in stellar age
measurements. We have conducted a synthetic red-optical low-resolution
spectroscopic survey of a simulated analog to the Upper Scorpius star-forming
region to investigate the effects of unresolved binary stars (which have
mass-dependent demographics) on age measurements of a stellar population. We
find that the observed mass-dependent age gradient in Upper Scorpius can be
explained by a population of undetected binary stars. For a simulated
population with an age of 10 (RMS = 2) Myr, we measure an age of 10.5 (RMS =
3.5) Myr for F stars, and 7.5 (RMS = 5.8) Myr for M stars. This discrepancy is
caused by the mass-dependent mass ratio distribution and the variable steepness
of the mass-luminosity relation. Our results support the previously suggested
10 Myr age for Upper Scorpius, with a small intrinsic age spread. | astro-ph_SR |
A novel investigation of the small-scale magnetic activity of the quiet
Sun via the Hanle effect in the Sr I 4607 Å line: One of the key research problems in stellar physics is to decipher the
small-scale magnetic activity of the quiet solar atmosphere. Recent
magneto-convection simulations that account for small-scale dynamo action have
provided three-dimensional (3D) models of the solar photosphere characterized
by a high degree of small-scale magnetic activity, similar to that found
through theoretical interpretation of the scattering polarization observed in
the Sr I 4607 \AA\ line. Here we present the results of a novel investigation
of the Hanle effect in this resonance line, based on 3D radiative transfer
calculations in a high-resolution magneto-convection model having most of the
convection zone magnetized close to the equipartition and a surface mean field
strength ${\langle B \rangle}{\approx}170$ G. The Hanle effect produced by the
model's magnetic field depolarizes the zero-field scattering polarization
signals significantly, to the extent that the center-to-limb variation of the
calculated spatially-averaged polarization amplitudes is compatible with the
observations. The standard deviation of the horizontal fluctuations of the
calculated scattering polarization signals is very sensitive to the model's
magnetic field and we find that the predicted spatial variations are
sufficiently sizable so as to be able to detect them, especially with the next
generation of solar telescopes. We find that at all on-disk positions the
theoretical scattering polarization signals are anti-correlated with the
continuum intensity. To facilitate reaching new observational breakthroughs, we
show how the theoretically predicted polarization signals and spatial
variations are modified when deteriorating the signal-to-noise ratio and the
spectral and spatial resolutions of the simulated observations. | astro-ph_SR |
Some glimpses from helioseismology at the dynamics of the deep solar
interior: Helioseismology has taught us a great deal about the stratification and
kinematics of the solar interior, sufficient for us to embark upon dynamical
studies more detailed than have been possible before. The most sophisticated
studies to date have been the very impressive numerical simulations of the
convection zone, from which, especially in recent years, a great deal has been
learnt. Those simulations, and the seismological evidence with which they are
being confronted, are reviewed elsewhere in this volume. Our understanding of
the global dynamics of the radiative interior of the Sun is in a much more
primitive state. Nevertheless, some progress has been made, and seismological
inference has provided us with evidence of more to come. Some of that I
summarize here, mentioning in passing hints that are pointing the way to the
future. | astro-ph_SR |
Probing the shape of the mixing profile and of the thermal structure at
the convective core boundary through asteroseismology: Aims: We investigate from a theoretical perspective if space asteroseismology
can be used to distinguish between different thermal structures and shapes of
the near-core mixing profiles for different types of coherent oscillation modes
in massive stars with convective cores, and if this capacity depends on the
evolutionary stage of the models along the main sequence. Methods: We compute
1D stellar structure and evolution models for four different prescriptions of
the mixing and temperature gradient in the near-core region. Their effect on
the frequencies of dipole prograde gravity modes in both Slowly Pulsating B and
$\beta$ Cep stars is investigated, as well as for pressure modes in $\beta$ Cep
stars. Results: A comparison between the mode frequencies of the different
models at various stages during the main sequence evolution reveals that they
are more sensitive to a change in temperature gradient than to the exact shape
of the mixing profile in the near-core region. Depending on the duration of the
observed light curve, one can distinguish between either just the temperature
gradient, or also between the shapes of the mixing coefficient. The relative
frequency differences are in general larger for more evolved models, and are
largest for the higher-frequency pressure modes in $\beta$ Cep stars.
Conclusions:In order to unravel the core boundary mixing and thermal structure
of the near-core region, one must have asteroseismic masses and radii with
$\sim 1\%$ relative precision for hundreds of stars. | astro-ph_SR |
Horizontal-branch morphology and multiple stellar populations in the
anomalous globular cluster M22: M22 is an anomalous globular cluster that hosts two groups of stars with
different metallicity and s-element abundance. The star-to-star light-element
variations in both groups, with the presence of individual Na-O and C-N
anticorrelations, demonstrates that this Milky-Way satellite has experienced a
complex star-formation history. We have analysed FLAMES/UVES spectra for seven
stars covering a small color interval, on the reddest horizontal-branch (HB)
portion of this cluster and investigated possible relations between the
chemical composition of a star and its location along the HB. Our chemical
abundance analysis takes into account effects introduced by deviations from the
local-thermodynamic equilibrium (NLTE effects), that are significant for the
measured spectral lines in the atmospheric parameters range spanned by our
stars. We find that all the analysed stars are barium-poor and sodium-poor,
thus supporting the idea that the position of a star along the HB is strictly
related to the chemical composition, and that the HB-morphology is influenced
by the presence of different stellar populations. | astro-ph_SR |
Automatic Recognition of Sunspots in HSOS Full-Disk Solar Images: A procedure is introduced to recognise sunspots automatically in solar
full-disk photosphere images obtained from Huairou Solar Observing Station,
National Astronomical Observatories of China. The images are first
pre-processed through Gaussian algorithm. Sunspots are then recognised by the
morphological Bot-hat operation and Otsu threshold. Wrong selection of sunspots
is eliminated by a criterion of sunspot properties. Besides, in order to
calculate the sunspots areas and the solar centre, the solar limb is extracted
by a procedure using morphological closing and erosion operations and setting
an adaptive threshold. Results of sunspot recognition reveal that the number of
the sunspots detected by our procedure has a quite good agreement with the
manual method. The sunspot recognition rate is 95% and error rate is 1.2%. The
sunspot areas calculated by our method have high correlation (95%) with the
area data from USAF/NOAA. | astro-ph_SR |
On Socially Distant Neighbors: Using Binaries to Constrain the Density
of Objects in the Galactic Center: Stars often reside in binary configurations. The nuclear star cluster
surrounding the supermassive black hole (SMBH) in the Galactic Center (GC) is
expected to include a binary population. In this dense environment, a binary
frequently encounters and interacts with neighboring stars. These interactions
vary from small perturbations to violent collisions. In the former case, weak
gravitational interactions unbind a soft binary over the evaporation timescale,
which depends on the binary properties as well as the density of surrounding
objects and velocity dispersion. Similarly, collisions can also unbind a
binary, and the collision rate depends on the density. Thus, the detection of a
binary with known properties can constrain the density profile in the GC with
implications for the number of compact objects, which are otherwise challenging
to detect. We estimate the density necessary to unbind a binary within its
lifetime for an orbit of arbitrary eccentricity about the SMBH. We find that
the eccentricity has a minimal impact on the density constraint. In this proof
of concept, we demonstrate that this procedure can probe the density in the GC
using hypothetical young and old binaries as examples. Similarly, a known
density profile provides constraints on the binary orbital separation. Our
results highlight the need to consider multiple dynamical processes in tandem.
In certain cases, often closer to the SMBH, the collision timescale rather than
the evaporation timescale gives the more stringent density constraint, while
other binaries farther from the SMBH provide unreliable density constraints
because they migrate inwards due to mass segregation. | astro-ph_SR |
Study of the Blazhko type RRc stars in the Stripe 82 region using SDSS
and ZTF: RR Lyrae stars are pulsating stars, many of which also show a long-term
variation called the Blazhko effect which is a modulation of amplitude and
phase of the lightcurve. In this work, we searched for the incidence rate of
the Blazhko effect in the first-overtone pulsating RR Lyrae (RRc) stars of the
Galactic halo. The focus was on the Stripe 82 region in the Galactic halo which
was studied by Sesar et al using the Sloan Digital Sky Survey (SDSS) data. In
their work, 104 RR Lyrae stars were classified as RRc type. We combined their
SDSS light curves with Zwicky Transient Facility (ZTF) data, and use them to
document the Blazhko properties of these RRc stars. Our analysis showed that
among the 104 RRc stars, 8 were rather RRd stars, and were excluded from the
study. Out of remaining 96, 34 were Blazhko type, 62 were non-Blazhko type,
giving the incidence rate of 35.42% for Blazhko RRc stars. The shortest Blazhko
period found was 12.808 +/- 0.001 d for SDSS 747380, while the longest was 3100
+/- 126 d for SDSS 3585856. Combining SDSS and ZTF data sets increased the
probability of detecting the small variations due to the Blazhko effect, and
thus provided a unique opportunity to find longer Blazhko periods. We found
that 85% of RRc stars had the Blazhko period longer than 200 d. | astro-ph_SR |
Direct formation of millisecond pulsars from rotationally delayed
accretion-induced collapse of massive white dwarfs: Millisecond pulsars (MSPs) are believed to be old neutron stars, formed via
Type Ib/c core-collapse supernovae, which have subsequently been spun up to
high rotation rates via accretion from a companion star in a highly
circularised low-mass X-ray binary. The recent discoveries of Galactic field
binary MSPs in eccentric orbits, and mass functions compatible with that
expected for helium white dwarf companions, PSR J2234+06 and PSR J1946+3417,
therefore challenge this picture. Here we present a hypothesis for producing
this new class of systems, where the MSPs are formed directly from a
rotationally-delayed accretion-induced collapse of a super-Chandrasekhar mass
white dwarf. We compute the orbital properties of the MSPs formed in such
events and demonstrate that our hypothesis can reproduce the observed
eccentricities, masses and orbital periods of the white dwarfs, as well as
forecasting the pulsar masses and velocities. Finally, we compare this
hypothesis to a triple star scenario. | astro-ph_SR |
Imaging from STIX visibility amplitudes: Aims: To provide the first demonstration of STIX Fourier-transform X-ray
imaging using semi-calibrated (amplitude-only) visibility data acquired during
the Solar Orbiter's cruise phase. Methods: We use a parametric imaging approach
by which STIX visibility amplitudes are fitted by means of two non-linear
optimization methods: a fast meta-heuristic technique inspired by social
behavior, and a Bayesian Monte Carlo sampling method, which, although slower,
provides better quantification of uncertainties. Results: When applied to a set
of solar flare visibility amplitudes recorded by STIX on November 18, 2020 the
two parametric methods provide very coherent results. The analysis also
demonstrates the ability of STIX to reconstruct high time resolution
information and, from a spectral viewpoint, shows the reliability of a
double-source scenario consistent with a thermal versus nonthermal
interpretation. Conclusions: In this preliminary analysis of STIX imaging based
only on visibility amplitudes, we formulate the imaging problem as a non-linear
parametric issue we addressed by means of two high-performance optimization
techniques that both showed the ability to sample the parametric space in an
effective fashion, thus avoiding local minima. | astro-ph_SR |
Angular Expansion of Nova Shells: Nova shells can provide us with important information on their distance,
their interactions with the circumstellar and interstellar media, and the
evolution in morphology of the ejecta. We have obtained narrow-band images of a
sample of five nova shells, namely DQHer, FHSer, TAur, V476Cyg, and V533Her,
with ages in the range from 50 to 130 years. These images have been compared
with suitable available archival images to derive their angular expansion
rates. We find that all the nova shells in our sample are still in the free
expansion phase, which can be expected, as the mass of the ejecta is 7-45 times
larger than the mass of the swept-up circumstellar medium. The nova shells will
keep expanding freely for time periods up to a few hundred years, reducing
their time dispersal into the interstellar medium. | astro-ph_SR |
Accretion Kinematics in the T Tauri Binary TWA 3A: Evidence for
Preferential Accretion onto the TWA 3A Primary: We present time-series, high-resolution optical spectroscopy of the eccentric
T Tauri binary TWA 3A. Our analysis focuses on variability in the strength and
structure of the accretion tracing emission lines H alpha and He I 5876A. We
find emission line strengths to display the same orbital-phase dependent
behavior found with time-series photometry, namely, bursts of accretion near
periastron passages. Such bursts are in good agreement with numerical
simulations of young eccentric binaries. During accretion bursts, the emission
of He I 5876A consistently traces the velocity of the primary star. After
removing a model for the system's chromospheric emission, we find the primary
star typically emits ~70% of the He I accretion flux. We interpret this result
as evidence for circumbinary accretion streams that preferentially feed the TWA
3A primary. This finding is in contrast to most numerical simulations, which
predict the secondary should be the dominant accretor in a binary system. Our
results may be consistent with a model in which the precession of an eccentric
circumbinary disk gap alternates between preferentially supplying mass to the
primary and secondary. | astro-ph_SR |
Properties of quasi-periodic pulsations in solar flares from a single
active region: We investigate the properties of a set of solar flares originating from a
single active region (AR) that exhibit QPPs, and look for signs of the QPP
periods relating to AR properties. The AR studied, best known as NOAA 12192,
was unusually long-lived and produced 181 flares. Data from the GOES, EVE,
Fermi, Vernov and NoRH observatories were used to determine if QPPs were
present in the flares. For the soft X-ray GOES and EVE data, the time
derivative of the signal was used. Power spectra of the time series data
(without any form of detrending) were inspected, and flares with a peak above
the 95% confidence level in the spectrum were labelled as having candidate
QPPs. The confidence levels were determined taking account of uncertainties and
the possible presence of red noise. AR properties were determined using HMI
line of sight magnetograms. A total of 37 flares (20% of the sample) show good
evidence of having QPPs, and some of the pulsations can be seen in data from
multiple instruments and in different wavebands. The QPP periods show a weak
correlation with the flare amplitude and duration, but this may be due to an
observational bias. A stronger correlation was found between the QPP period and
duration of the QPP signal, which can be partially but not entirely explained
by observational constraints. No correlations were found with the AR area,
bipole separation, or average magnetic field strength. The fact that a
substantial fraction of the flare sample showed evidence of QPPs using a strict
detection method with minimal processing of the data demonstrates that these
QPPs are a real phenomenon, which cannot be explained by the presence of red
noise or the superposition of multiple unrelated flares. The lack of
correlation between the QPP periods and AR properties implies that the
small-scale structure of the AR is important, and/or that different QPP
mechanisms act in different cases. | astro-ph_SR |
Detailed optical spectroscopy of the B[e] star MWC 17: Based on the data of multiple high-resolution R=60 000 observations obtained
at the 6-meter telescope BTA in combination with the NES spectrograph, we
studied the features of the optical spectrum of the star MWC 17 with the B[e]
phenomenon. In the wavelength interval 4050-6750 A we identified numerous
permitted and forbidden emissions, interstellar NaI lines, and diffuse
interstellar bands (DIBs). Radial velocities were estimated from lines of
various origin. As the systemic velocity, Vsys, the velocity from the forbidden
emissions can be accepted: Vr=-47 km/s (relative to the local standard Vlsr=-42
km/s). Comparison of the obtained data with the ealier measurements allows us
to conclude on the absence of considerable variability of spectral details. | astro-ph_SR |
Mass and orbit constraints of the gamma-ray binary LS 5039: We present the results of space-based photometric and ground-based
spectroscopic observing campaigns on the gamma-ray binary LS 5039. The new
orbital and physical parameters of the system are similar to former results,
except we found a lower eccentricity. Our MOST-data show that any broad-band
optical photometric variability at the orbital period is below the 2 mmag
level. Light curve simulations support the lower value of eccentricity and
imply that the mass of the compact object is higher than 1.8 solar masses. | astro-ph_SR |
Embedded protostellar disks around (sub-)solar stars. II. Disk masses,
sizes, densities, temperatures and the planet formation perspective: We present basic properties of protostellar disks in the embedded phase of
star formation (EPSF), which is difficult to probe observationally using
available observational facilities. We use numerical hydrodynamics simulations
of cloud core collapse and focus on disks formed around stars in the 0.03-1.0
Msun mass range. Our obtained disk masses scale near-linearly with the stellar
mass. The mean and median disk masses in the Class 0 and I phases
(M_{d,C0}^{mean}=0.12 Msun, M_{d,C0}^{mdn}=0.09 Msun and M_{d,CI}^{mean}=0.18
Msun, M_{d,CI}^{mdn}=0.15 Msun, respectively) are greater than those inferred
from observations by (at least) a factor of 2--3. We demonstrate that this
disagreement may (in part) be caused by the optically thick inner regions of
protostellar disks, which do not contribute to millimeter dust flux. We find
that disk masses and surface densities start to systematically exceed that of
the minimum mass solar nebular for objects with stellar mass as low as
M_st=0.05-0.1 Msun. Concurrently, disk radii start to grow beyond 100 AU,
making gravitational fragmentation in the disk outer regions possible. Large
disk masses, surface densities, and sizes suggest that giant planets may start
forming as early as in the EPSF, either by means of core accretion (inner disk
regions) or direct gravitational instability (outer disk regions), thus
breaking a longstanding stereotype that the planet formation process begins in
the Class II phase. | astro-ph_SR |
Calculating energy storage due to topological changes in emerging active
region NOAA AR 11112: The Minimum Current Corona (MCC) model provides a way to estimate stored
coronal energy using the number of field lines connecting regions of positive
and negative photospheric flux. This information is quantified by the net flux
connecting pairs of opposing regions in a connectivity matrix. Changes in the
coronal magnetic field, due to processes such as magnetic reconnection,
manifest themselves as changes in the connectivity matrix. However, the
connectivity matrix will also change when flux sources emerge or submerge
through the photosphere, as often happens in active regions. We have developed
an algorithm to estimate the changes in flux due to emergence and submergence
of magnetic flux sources. These estimated changes must be accounted for in
order to quantify storage and release of magnetic energy in the corona. To
perform this calculation over extended periods of time, we must additionally
have a consistently labeled connectivity matrix over the entire observational
time span. We have therefore developed an automated tracking algorithm to
generate a consistent connectivity matrix as the photospheric source regions
evolve over time. We have applied this method to NOAA Active Region 11112,
which underwent a GOES M2.9 class flare around 19:00 on Oct.16th, 2010, and
calculated a lower bound on the free magnetic energy buildup of ~8.25 x 10^30
ergs over 3 days. | astro-ph_SR |
Modelling the occurrence of grand minima in sun-like stars using a
dynamo model: In this work, we have studied the variability and frequency of occurrence of
the grand minima using kinematic dynamo models of one solar mass star with
different rotation rates and depths of convection zones. We specify the
large-scale flows (differential rotations and meridional circulations) from
corresponding hydrodynamic models. We include stochastic fluctuations in the
Babcock-Leighton source for the poloidal field to produce variable stellar
cycles. We observe that the rapidly rotating stars produce highly irregular
cycles with strong magnetic fields and rarely produce Maunder-like grand
minima, whereas the slowly rotating stars (Sun and longer rotation period)
produce smooth cycles of weaker strength and occasional grand minima. In
general, the number of the grand minima increases with the decrease in rotation
rate. These results can be explained by the fact that with the increase of
rotation period, the supercriticality of the dynamo decreases, and the dynamo
is more prone to produce extended grand minima in this regime. | astro-ph_SR |
A Search for Strongly Mg-enhanced Stars from the Sloan Digital Sky
Survey: Strongly Mg-enhanced stars with [Mg/Fe] $>$ 1.0 show peculiar abundance
patterns and hence are of great interest for our understanding of stellar
formation and chemical evolution of the Galaxy. A systematical search for
strongly Mg-enhanced stars based on the low-resolution $(R\simeq2000)$ spectra
of the Sloan Digital Sky Survey (SDSS) is carried out by finding the best
matched synthetic spectrum to the observed one in the region of Mg I b lines
around $\lambda$5170\AA$~$via a profile matching method. The advantage of our
method is that fitting parameters are refined by reproducing the [Mg/Fe] ratios
of 47 stars from very precise high-resolution spectroscopic (HRS) analysis by
Nissen and Schuster (2010); and these parameters are crucial to the precision
and validity of the derived Mg abundances. As a further check of our method, Mg
abundances are estimated with our method for member stars in four Galactic
globular clusters (M92, M13, M3, M71) which cover the same metallicity range as
our sample, and the results are in good agreement with those of HRS analysis in
the literature. The validation of our method is also proved by the agreement of
[Mg/Fe] between our values and those of HRS analysis by Aoki et al.(2013).
Finally, 33 candidates of strongly Mg-enhanced stars with [Mg/Fe]$>$1.0 are
selected from 14850 F and G stars. Follow-up observations will be carried out
on these candidates with high-resolution spectroscopy by large telescopes in
the near future, so as to check our selection procedure and to perform a
precise and detailed abundance analysis and to explore the origins of these
stars. | astro-ph_SR |
Helium abundances and the helium isotope anomaly of sdB stars: Helium abundances and atmospheric parameters have been determined from high
resolution spectra for a new sample of 46 bright hot subdwarf B (sdB) stars.
The helium abundances have been measured with high accuracy. We confirm the
correlation of helium abundance with temperature and the existence of two
distinct sequences in helium abundance found previously. We focused on isotopic
shifts of helium lines and found helium-3 to be strongly enriched in 8 of our
programme stars. Most of these stars cluster in a small temperature range
between 27000 K and 31000 K very similar to the known helium-3-rich main
sequence B stars, which cluster at somewhat lower temperatures. This phenomenon
is most probably related to diffusion processes in the atmosphere, but poses a
challenge to diffusion models. | astro-ph_SR |
On the optically-thick winds of Wolf-Rayet stars: (abridged) The strong winds of Wolf-Rayet (WR) stars are important for the
mechanical and chemical feedback of the most massive stars and determine
whether they end their lives as neutron stars or black holes. In this work we
investigate theoretically the mass-loss properties of H-free WR stars of the
nitrogen sequence (WN stars). We connect stellar structure models for He stars
with wind models for optically-thick winds and assess how both types of models
can simultaneously fulfill their respective sonic-point conditions. Fixing the
outer wind law and terminal wind velocity, we obtain unique solutions for the
mass-loss rates of optically-thick, radiatively-driven winds of WR stars in the
phase of core He-burning. The resulting mass-loss relations as a function of
stellar parameters, agree well with previous empirical relations. Furthermore,
we encounter stellar mass limits below which no continuous solutions exist.
While these mass limits agree with observations of WR stars in the Galaxy, they
are in conflict with observations in the LMC. While our results confirm in
particular the slope of oft-used empirical mass-loss relations, they imply that
only part of the observed WN population can be understood in the framework of
the standard assumptions of a smooth transonic flow and compact stellar core.
This means that alternative approaches, such as a clumped and inflated wind
structure, or deviations from the diffusion limit at the sonic point may have
to be invoked. Qualitatively, the existence of mass limits for the formation of
WR-type winds may be relevant for the non-detection of low-mass WR stars in
binary systems, which are believed to be progenitors of Type Ib/c supernovae.
The sonic-point conditions derived in this work may provide a possibility to
include optically-thick winds in stellar evolution models in a more physically
motivated form than in current models. | astro-ph_SR |
MOST Observations of our Nearest Neighbor: Flares on Proxima Centauri: We present a study of white light flares from the active M5.5 dwarf Proxima
Centauri using the Canadian microsatellite MOST. Using 37.6 days of monitoring
data from 2014 and 2015, we have detected 66 individual flare events, the
largest number of white light flares observed to date on Proxima Cen. Flare
energies in our sample range from $10^{29}$-$10^{31.5}$ erg. The flare rate is
lower than that of other classic flare stars of similar spectral type, such as
UV Ceti, which may indicate Proxima Cen had a higher flare rate in its youth.
Proxima Cen does have an unusually high flare rate given its slow rotation
period, however. Extending the observed power-law occurrence distribution down
to $10^{28}$ erg, we show that flares with flux amplitudes of 0.5% occur 63
times per day, while superflares with energies of $10^{33}$ erg occur ~8 times
per year. Small flares may therefore pose a great difficulty in searches for
transits from the recently announced 1.27 M_earth Proxima b, while frequent
large flares could have significant impact on the planetary atmosphere. | astro-ph_SR |
The formation of long-period eccentric binaries with a helium white
dwarf: The recent discovery of long-period eccentric binaries hosting a He-WD or a
sdB star has been challenging binary-star modelling. Based on accurate
determinations of the stellar and orbital parameters for IP Eri, a K0 + He-WD
system, we propose an evolutionary path that is able to explain the
observational properties of this system and, in particular, to account for its
high eccentricity (0.25). Our scenario invokes an enhanced-wind mass loss on
the first red giant branch (RGB) in order to avoid mass transfer by Roche-lobe
overflow, where tides systematically circularize the orbit. We explore how the
evolution of the orbital parameters depends on the initial conditions and show
that eccentricity can be preserved and even increased if the initial separation
is large enough. The low spin velocity of the K0 giant implies that accretion
of angular momentum from a (tidally-enhanced) RGB wind should not be efficient. | astro-ph_SR |
Reconnaissance of the HR 8799 Exosolar System II: Astrometry and Orbital
Motion: We present an analysis of the orbital motion of the four sub-stellar objects
orbiting HR8799. Our study relies on the published astrometric history of this
system augmented with an epoch obtained with the Project 1640 coronagraph +
Integral Field Spectrograph (IFS) installed at the Palomar Hale telescope. We
first focus on the intricacies associated with astrometric estimation using the
combination of an Extreme Adaptive Optics system (PALM-3000), a coronagraph and
an IFS. We introduce two new algorithms. The first one retrieves the stellar
focal plane position when the star is occulted by a coronagraphic stop. The
second one yields precise astrometric and spectro-photometric estimates of
faint point sources even when they are initially buried in the speckle noise.
The second part of our paper is devoted to studying orbital motion in this
system. In order to complement the orbital architectures discussed in the
literature, we determine an ensemble of likely Keplerian orbits for HR8799bcde,
using a Bayesian analysis with maximally vague priors regarding the overall
configuration of the system. While the astrometric history is currently too
scarce to formally rule out coplanarity, HR8799d appears to be misaligned with
respect to the most likely planes of HR8799bce orbits. This misalignment is
sufficient to question the strictly coplanar assumption made by various authors
when identifying a Laplace resonance as a potential architecture. Finally, we
establish a high likelihood that HR8799de have dynamical masses below 13 M_Jup
using a loose dynamical survival argument based on geometric close encounters.
We illustrate how future dynamical analyses will further constrain dynamical
masses in the entire system. | astro-ph_SR |
Sub-Chandrasekhar White Dwarf Mergers as the Progenitors of Type Ia
Supernovae: Type Ia supernovae are generally thought to be due to the thermonuclear
explosions of carbon-oxygen white dwarfs with masses near the Chandrasekhar
mass. This scenario, however, has two long-standing problems. First, the
explosions do not naturally produce the correct mix of elements, but have to be
finely tuned to proceed from sub-sonic deflagration to super-sonic detonation.
Second, population models and observations give formation rates of
near-Chandrasekhar white dwarfs that are far too small. Here, we suggest that
type Ia supernovae instead result from mergers of roughly equal-mass
carbon-oxygen white dwarfs, including those that produce sub-Chandrasekhar mass
remnants. Numerical studies of such mergers have shown that the remnants
consist of rapidly rotating cores that contain most of the mass and are hottest
in the center, surrounded by dense, small disks. We argue that the disks
accrete quickly, and that the resulting compressional heating likely leads to
central carbon ignition. This ignition occurs at densities for which pure
detonations lead to events similar to type Ia supernovae. With this merger
scenario, we can understand the type Ia rates, and have plausible reasons for
the observed range in luminosity and for the bias of more luminous supernovae
towards younger populations. We speculate that explosions of white dwarfs
slowly brought to the Chandrasekhar limit---which should also occur---are
responsible for some of the "atypical" type Ia supernovae. | astro-ph_SR |
AGB and post-AGB objects in the outer Galaxy: We present the results of our search for low- and intermediate mass evolved
stars in the outer Galaxy using AllWISE catalogue photometry. We show that the
[3.4]-[12] versus [4.6]-[22] colour-colour diagram is most suitable for
separating C-rich/O-rich AGB and post- AGB star candidates. We are able to
select 2,510 AGB and 24,821 post-AGB star candidates. However, the latter are
severely mixed with the known young stellar objects in this diagram. | astro-ph_SR |
New mass-loss rates of B supergiants from global wind models: Massive stars lose a significant fraction of mass during their evolution.
However, the corresponding mass-loss rates are rather uncertain. To improve
this, we calculated global line-driven wind models for Galactic B supergiants.
Our models predict radial wind structure directly from basic stellar
parameters. The hydrodynamic structure of the flow is consistently determined
from the photosphere in nearly hydrostatic equilibrium to supersonically
expanding wind. The radiative force is derived from the solution of the
radiative transfer equation in the comoving frame. We provide a simple formula
that predicts theoretical mass-loss rates as a function of stellar luminosity
and effective temperature. The mass-loss rate of B supergiants slightly
decreases with temperature down to about 22.5 kK, where the region of
recombination of Fe IV to Fe III starts to appear. In this region, which is
about 5 kK wide, the mass-loss rate gradually increases by a factor of about 6.
The increase of the mass-loss rate is associated with a gradual decrease of
terminal velocities by a factor of about 2. We compared the predicted wind
parameters with observations. While the observed wind terminal velocities are
reasonably reproduced by the models, the situation with mass-loss rates is less
clear. The mass-loss rates derived from observations that are uncorrected for
clumping are by a factor of 3 to 9 higher than our predictions on cool and hot
sides of the studied sample, respectively. These observations can be reconciled
with theory assuming a temperature-dependent clumping factor. On the other
hand, the mass-loss rate estimates that are not sensitive to clumping agree
with our predictions much better. Our predictions are by a factor of about 10
lower than the values currently used in evolutionary models appealing for
reconsideration of the role of winds in the stellar evolution. | astro-ph_SR |
A helium-flash-induced mixing event can explain the lithium abundances
of red clump stars: Observations demonstrate that the surface abundance of $^7{\rm Li}$ in
low-mass stars changes dramatically between the tip of the red giant branch and
the red clump. This naturally suggests an association with the helium core
flash, which occurs between these two stages. Using stellar evolution models
and a simple, ad hoc mixing prescription, we demonstrate that the $^7{\rm Li}$
enhancement can be explained by a brief chemical mixing event that occurs at
the time of the first, strongest He sub-flash. The amount of $^7{\rm Be}$
already present above the H-burning shell just before the flash, once it mixes
into the cooler envelope and undergoes an electron capture converting it to
$^7{\rm Li}$, is sufficient to explain the observed abundance at the red clump.
We suggest that the excitation of internal gravity waves by the vigorous
turbulent convection during the flash may provide a physical mechanism that can
induce such mixing. | astro-ph_SR |
Externally heated protostellar cores in the Ophiuchus star-forming
region: We present APEX 218 GHz observations of molecular emission in a complete
sample of embedded protostars in the Ophiuchus star-forming region. To study
the physical properties of the cores, we calculate H$_2$CO and c-C$_3$H$_2$
rotational temperatures, both of which are good tracers of the kinetic
temperature of the molecular gas. We find that the H$_2$CO temperatures range
between 16 K and 124 K, with the highest H$_2$CO temperatures toward the hot
corino source IRAS 16293-2422 (69-124 K) and the sources in the $\rho$ Oph A
cloud (23-49 K) located close to the luminous Herbig Be star S 1, which
externally irradiates the $\rho$ Oph A cores. On the other hand, the
c-C$_3$H$_2$ rotational temperature is consistently low (7-17 K) in all
sources. Our results indicate that the c-C$_3$H$_2$ emission is primarily
tracing more shielded parts of the envelope whereas the H$_2$CO emission (at
the angular scale of the APEX beam; 3600 au in Ophiuchus) mainly traces the
outer irradiated envelopes, apart from in IRAS 16293-2422, where the hot corino
emission dominates. In some sources, a secondary velocity component is also
seen, possibly tracing the molecular outflow. | astro-ph_SR |
$f$-mode interaction with models of sunspot : near-field scattering and
multi-frequency effects: We use numerical simulations to investigate the interaction of an $f$-mode
wave packet with small and large models of a sunspot in a stratified
atmosphere. While a loose cluster model has been largely studied before, we
focus in this study on the scattering from an ensemble of tightly compact
tubes. We showed that the small compact cluster produces a slight distorted
scattered wave field in the transverse direction, which can be attributed to
the simultaneous oscillations of the pairs of tubes within the cluster aligned
in a perpendicular direction to the incoming wave. However, no signature of a
multiple-scattering regime has been observed from this model, while it has been
clearly observable for the large compact cluster model. Furthermore, we pointed
out the importance of the geometrical shape of the monolithic model on the
interaction of $f$-mode waves with a sunspot in a high frequency range ($\nu =$
5 mHz). These results are a contribution to the observational effort to
distinguish seismically between different configurations of magnetic flux tubes
within sunspots and plage. | astro-ph_SR |
What is the relationship between photospheric flow fields and solar
flares?: We estimated photospheric velocities by separately applying the Fourier Local
Correlation Tracking (FLCT) and Differential Affine Velocity Estimator (DAVE)
methods to 2708 co-registered pairs of SOHO/MDI magnetograms, with nominal
96-minute cadence and ~2" pixels, from 46 active regions (ARs) from 1996-1998
over the time interval t45 when each AR was within 45^o of disk center. For
each magnetogram pair, we computed the average estimated radial magnetic field,
B; and each tracking method produced an independently estimated flow field, u.
We then quantitatively characterized these magnetic and flow fields by
computing several extensive and intensive properties of each; extensive
properties scale with AR size, while intensive properties do not depend
directly on AR size. Intensive flow properties included moments of speeds,
horizontal divergences, and radial curls; extensive flow properties included
sums of these properties over each AR, and a crude proxy for the ideal Poynting
flux, the total |u| B^2. Several magnetic quantities were also computed,
including: total unsigned flux; a measure of the amount of unsigned flux near
strong-field polarity inversion lines, R; and the total B^2. Next, using
correlation and discriminant analysis, we investigated the associations between
these properties and flares from the GOES flare catalog, when averaged over
both t45 and shorter time windows, of 6 and 24 hours. We found R and total |u|
B^2 to be most strongly associated with flares; no intensive flow properties
were strongly associated with flares. | astro-ph_SR |
The Origin of Sequential Chromospheric Brightenings: Sequential chromospheric brightenings (SCBs) are often observed in the
immediate vicinity of erupting flares and are associated with coronal mass
ejections. Since their initial discovery in 2005, there have been several
subsequent investigations of SCBs. These studies have used differing detection
and analysis techniques, making it difficult to compare results between
studies. This work employs the automated detection algorithm of Kirk et al.
(Solar Phys. 283, 97, 2013) to extract the physical characteristics of SCBs in
11 flares of varying size and intensity. We demonstrate that the magnetic
substructure within the SCB appears to have a significantly smaller area than
the corresponding H-alpha emission. We conclude that SCBs originate in the
lower corona around 0.1 R_sun above the photosphere, propagate away from the
flare center at speeds of 35 - 85 km/s, and have peak photosphere magnetic
intensities of 148 +/- 2.9 G. In light of these measurements, we infer SCBs to
be distinctive chromospheric signatures of erupting coronal mass ejections. | astro-ph_SR |
The origin of the planetary nebula M 1-16. A morphokinematic and
chemical analysis: We investigated the origin of the Planetary Nebula (PN) M 1-16 using
narrow-band optical imaging, and high- and low-resolution optical spectra to
perform a detailed morpho-kinematic and chemical studies. M 1-16 is revealed to
be a multipolar PN that predominantly emits in [O III] in the inner part of the
nebula and [N II] in the lobes. A novel spectral unsharp masking technique was
applied to the position-velocity (PV) maps to reveal a set of multiple
structures at the centre of M 1-16 spanning radial velocities from
$-$40km$\,$s$^{-1}$ to 20km$\,$s$^{-1}$, with respect to the systemic velocity.
The morpho-kinematic model indicates that the deprojected velocity of the lobe
outflows are $\geq$100km$\,$s$^{-1}$, and particularly the larger lobes and
knots have a deprojected velocity of $\simeq$350km$\,$s$^{-1}$; the inner
ellipsoidal component has a deprojected velocity of $\simeq$29km$\,$s$^{-1}$. A
kinematical age of $\sim$8700yr has been obtained from the model assuming a
homologous velocity expansion law and a distance of 6.2$\pm$1.9kpc. The
chemical analysis indicates that M 1-16 is a Type I PN with a central star of
PN (CSPN) mass in the range of $\simeq$0.618-0.713M$_\odot$ and an initial mass
for the progenitor star between 2.0 and 3.0M$_\odot$ (depending on
metallicity). An $T_\mathrm{eff}\simeq$140$\,$000K and log$(L/L_{\odot})$=2.3
was estimated using the 3MdB photoionisation models to reproduce the ionisation
stage of the PN. All of these results have led us to suggest that M 1-16 is an
evolved PN, contrary to the scenario of proto-PN suggested in previous studies.
We propose that the mechanism responsible for the morphology of M 1-16 is
related to the binary (or multiple star) evolution scenario. | astro-ph_SR |
Simultaneous Multiwavelength Observations of Magnetic Activity in
Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS
J13142039+1320011): We present multi-epoch simultaneous radio, optical, H{\alpha}, UV, and X-ray
observations of the active, young, low-mass binary NLTT 33370 AB (blended
spectral type M7e). This system is remarkable for its extreme levels of
magnetic activity: it is the most radio-luminous ultracool dwarf (UCD) known,
and here we show that it is also one of the most X-ray luminous UCDs known. We
detect the system in all bands and find a complex phenomenology of both flaring
and periodic variability. Analysis of the optical light curve reveals the
simultaneous presence of two periodicities, 3.7859 $\pm$ 0.0001 and 3.7130
$\pm$ 0.0002 hr. While these differ by only ~2%, studies of differential
rotation in the UCD regime suggest that it cannot be responsible for the two
signals. The system's radio emission consists of at least three components:
rapid 100% polarized flares, bright emission modulating periodically in phase
with the optical emission, and an additional periodic component that appears
only in the 2013 observational campaign. We interpret the last of these as a
gyrosynchrotron feature associated with large-scale magnetic fields and a cool,
equatorial plasma torus. However, the persistent rapid flares at all rotational
phases imply that small-scale magnetic loops are also present and reconnect
nearly continuously. We present an SED of the blended system spanning more than
9 orders of magnitude in wavelength. The significant magnetism present in NLTT
33370 AB will affect its fundamental parameters, with the components' radii and
temperatures potentially altered by ~+20% and ~-10%, respectively. Finally, we
suggest spatially resolved observations that could clarify many aspects of this
system's nature. | astro-ph_SR |
Bayesian Asteroseismology of 23 Solar-Like Kepler Targets: We study 23 previously published Kepler targets to perform a consistent
grid-based Bayesian asteroseismic analysis and compare our results to those
obtained via the Asteroseismic Modelling Portal (AMP). We find differences in
the derived stellar parameters of many targets and their uncertainties. While
some of these differences can be attributed to systematic effects between
stellar evolutionary models, we show that the different methodologies deliver
incompatible uncertainties for some parameters. Using non-adiabatic models and
our capability to measure surface effects, we also investigate the dependency
of these surface effects on the stellar parameters. Our results suggest a
dependence of the magnitude of the surface effect on the mixing length
parameter which also, but only minimally, affects the determination of stellar
parameters. While some stars in our sample show no surface effect at all, the
most significant surface effects are found for stars that are close to the
Sun's position in the HR diagram. | astro-ph_SR |
X-ray Irradiation of the LkCa 15 Protoplanetary Disk: LkCa 15 in the Taurus star-forming region has recently gained attention as
the first accreting T Tauri star likely to host a young protoplanet. High
spatial resolution infrared observations have detected the suspected
protoplanet within a dust-depleted inner gap of the LkCa 15 transition disk at
a distance of 15 AU from the star. If this object's status as a protoplanet is
confirmed, LkCa 15 will serve as a unique laboratory for constraining physical
conditions within a planet-forming disk. Previous models of the LkCa 15 disk
have accounted for disk heating by the stellar photosphere but have ignored the
potential importance of X-ray ionization and heating. We report here the
detection of LkCa 15 as a bright X-ray source with Chandra. The X-ray emission
is characterized by a cool heavily-absorbed plasma component at kT_cool ~0.3
keV and a harder component at kT_hot ~5 keV. We use the observed X-ray
properties to provide initial estimates of the X-ray ionization and heating
rates within the tenuous inner disk. These estimates and the observed X-ray
properties of LkCa 15 can be used as a starting point for developing more
realistic disk models of this benchmark system. | astro-ph_SR |
Impact of Type II Spicules in the Corona: Simulations and Synthetic
Observables: The role of type II spicules in the corona has been a much debated topic in
recent years. This paper aims to shed light on the impact of type II spicules
in the corona using novel 2.5D radiative MHD simulations including ion-neutral
interaction effects with the Bifrost code. We find that the formation of
simulated type II spicules, driven by the release of magnetic tension, impacts
the corona in various manners. Associated with the formation of spicules, the
corona exhibits 1) magneto-acoustic shocks and flows which supply mass to
coronal loops, and 2) transversal magnetic waves and electric currents that
propagate at Alfv\'en speeds. The transversal waves and electric currents,
generated by the spicule's driver and lasting for many minutes, are dissipated
and heat the associated loop. These complex interactions in the corona can be
connected with blue shifted secondary components in coronal spectral lines
(Red-Blue asymmetries) observed with Hinode/EIS and SOHO/SUMER, as well as the
EUV counterpart of type II spicules and propagating coronal disturbances (PCDs)
observed with the 171~\AA\ and 193~\AA\ SDO/AIA channels. | astro-ph_SR |
Coronal temperature profiles obtained from kinetic models and from
coronal brightness measurements obtained during solar eclipses: Coronal density, temperature and heat flux distributions for the equatorial
and polar corona have been deduced by Lemaire [2012] from Saito's model of
averaged coronal white light (WL) brightness and polarization observations.
They are compared with those determined from a kinetic collisionless/exospheric
model of the solar corona. This comparison indicates rather similar
distributions at large radial distances (> 7 Rs) in the collisionless region.
However, rather important differences are found close to the Sun in the
acceleration region of the solar wind. The exospheric heat flux is directed
away from the Sun, while that inferred from all WL coronal observations is in
the opposite direction, i.e., conducting heat from the inner corona toward the
chromosphere. This could indicate that the source of coronal heating rate
extends up into the inner corona where it maximizes at r > 1.5 Rs well above
the transition region. | astro-ph_SR |
Cannibals in the thick disk II -- Radial-velocity monitoring of the
young alpha-rich stars: Determining ages of stars for reconstructing the history of the Milky Way
remains one of the most difficult tasks in astrophysics. This involves knowing
when it is possible to relate the stellar mass with its age and when it is not.
The young $\alpha-$rich (YAR) stars present such a case in which we are still
not sure about their ages because they are relatively massive, implying young
ages, but their abundances are $\alpha-$enhanced, which implies old ages. We
report the results from new observations from a long-term
radial-velocity-monitoring campaign complemented with high-resolution
spectroscopy, as well as new astrometry and seismology of a sample of 41 red
giants from the third version of APOKASC, which includes YAR stars. The aim is
to better characterize the YAR stars in terms of binarity, mass, abundance
trends, and kinematic properties.The radial velocities of HERMES, APOGEE, and
Gaia were combined to determine the binary fraction among YAR stars. In
combination with their mass estimate, evolutionary status, chemical
composition, and kinematic properties, it allowed us to better constrain the
nature of these objects. We found that stars with $\mathrm{M} < 1
\mathrm{M}_\odot$ were all single, whereas stars with $\mathrm{M} > 1
\mathrm{M}_\odot$ could be either single or binary. This is in agreement with
theoretical predictions of population synthesis models. Studying their [C/N],
[C/Fe], and [N/Fe], trends with mass, it became clear that many YAR stars do
not follow the APOKASC stars, favoring the scenario that most of them are the
product of mass transfer. Abr. | astro-ph_SR |
UVMag: stellar formation, evolution, structure and environment with
space UV and visible spectropolarimetry: Important insights into the formation, structure, evolution and environment
of all types of stars can be obtained through the measurement of their winds
and possible magnetospheres. However, this has hardly been done up to now
mainly because of the lack of UV instrumentation available for long periods of
time. To reach this aim, we have designed UVMag, an M-size space mission
equipped with a high-resolution spectropolarimeter working in the UV and
visible spectral range. The UV domain is crucial in stellar physics as it is
very rich in atomic and molecular lines and contains most of the flux of hot
stars. Moreover, covering the UV and visible spectral domains at the same time
will allow us to study the star and its environment simultaneously. Adding
polarimetric power to the spectrograph will multiply tenfold the capabilities
of extracting information on stellar magnetospheres, winds, disks, and magnetic
fields. Examples of science objectives that can be reached with UVMag are
presented for pre-main sequence, main sequence and evolved stars. They will
cast new light onto stellar physics by addressing many exciting and important
questions. UVMag is currently undergoing a Research and Technology study and
will be proposed at the forthcoming ESA call for M-size missions. This
spectropolarimeter could also be installed on a large UV and visible
observatory (e.g. NASA's LUVOIR project) within a suite of instruments. | astro-ph_SR |
The hybrid CONe WD + He star scenario for the progenitors of type Ia
supernovae: The hybrid CONe white dwarfs (WDs) have been suggested to be possible
progenitors of type Ia supernovae (SNe Ia). In this article, we systematically
studied the hybrid CONe WD + He star scenario for the progenitors of SNe Ia, in
which a hybrid CONe WD increases its mass to the Chandrasekhar mass limit by
accreting He-rich material from a non-degenerate He star. According to a series
of detailed binary population synthesis simulations, we obtained the SN Ia
birthrates and delay times for this scenario. The SN Ia birthrates for this
scenario are ~0.033-0.539*10^(-3)yr^(-1), which roughly accounts for 1-18% of
all SNe Ia. The estimated delay times are ~28Myr-178Myr, which are the youngest
SNe Ia predicted by any progenitor model so far. We suggest that SNe Ia from
this scenario may provide an alternative explanation of type Iax SNe. We also
presented some properties of the donors at the point when the WDs reach the
Chandrasekhar mass. These properties may be a good starting point for
investigating the surviving companions of SNe Ia, and for constraining the
progenitor scenario studied in this work. | astro-ph_SR |
Case AD, AR, and AS binary evolution and their possible connections with
W UMa binaries: Close detached binaries were theoretically predicted to evolve into contact
by three subtypes of case A binary evolution: case AD, AR, and AS, which
correspond to the formation of contact during dynamic-, thermal-, and
nuclear-timescale mass transfer phase, respectively. It is unclear, however,
what is the difference between contact binaries in these subtypes, and whether
all of these subtypes can account for the formation of observed W UMa binaries.
Using Eggleton's stellar evolution code with the nonconservative assumption, I
obtained the low-mass contact binaries produced by case AD, AR, and AS at the
moment of contact, and their parameter spaces. The results support that the
progenitors of low-mass contact binaries are detached binaries with orbital
periods shorter than $\sim2-5\,$d, and their borderlines depend strongly on the
primary mass. In addition, the period-colour relations for case AR and AS can
be in better agreement with that for observed W UMa candidates, but case AD
shows a significantly worse agreement. Moreover, case AR and AS can produce a
short-period limit (corresponding to a low-mass limit) at almost any age, e.g.
from young age ($\sim0.2\,$Gyr) to old age ($\sim13\,$Gyr), agreeing with
observed W UMa binaries in star clusters, but no such limit occurs for case AD
at any age. These results support that case AR and AS, as opposed to case AD,
can lead to W UMa binaries (including young W UMa binaries). | astro-ph_SR |
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