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AstroSat study of the globular cluster NGC 2298: probable evolutionary
scenarios of hot HB stars: We present the far-UV (FUV) photometry of images acquired with UVIT on
AstroSat to probe the horizontal branch (HB) population of the Galactic
globular cluster NGC 2298. UV-optical color-magnitude diagrams (CMDs) are
constructed for member stars in combination with HST UV Globular Cluster Survey
(HUGS) data for the central region and Gaia and ground-based photometric data
for the outer region. Blue HB (BHB) sequence with a spread and four hot HB
stars are detected in all FUV-optical CMDs and are compared with theoretical
updated BaSTI isochrones and synthetic HB models with a range in helium
abundance, suggesting that the hot HB stars are helium enhanced when compared
to the BHB. The estimated effective temperature, radius, and luminosity of HB
stars, using best SED fits, were compared with various HB models. BHB stars
span a temperature range from 7,500-12,250 K. The three hot HB stars have
35,000-40,000 K, whereas one star has around 100,000K. We suggest the following
evolutionary scenarios: two stars are likely to be the progeny of extreme HB
(EHB) stars formed through an early hot-flasher scenario; one is likely to be
an EHB star with probable helium enrichment, the hottest HB star is about to
enter the WD cooling phase, could have evolved from BHB phase. Nevertheless,
these are interesting spectroscopic targets to understand the late stages of
evolution. | astro-ph_SR |
The dispersion of period spacing for DAV stars: Three groups of DAV star models are evolved with time-dependent element
diffusion by \texttt{WDEC}. The core compositions of these models are directly
from white dwarf models evolved by \texttt{MESA}, which are results of
thermonuclear burning. Based on these DAV star models, we try to study the
dispersion of period spacing. The thickness of hydrogen atmosphere can
seriously affect the deviation degree of minimal period spacings. The minimal
period spacings dominate the dispersion of period spacing. The thinner the
hydrogen atmosphere, basically, the more dispersive the period spacing.
Standard deviations are used to analyze the dispersion of period spacing.
Studying the dispersion of period spacing on a DAV star KUV03442+0719
preliminarily, we suggest that log($M_{\rm H}/M_{*}$) is from -8.5 to -5.5. In
addition, modes partly trapped in C/O core are found based on those DAV star
models. The identified modes and average period spacings indicate that
KUV03442+0719 may be the first star to 'observe' modes partly trapped in C/O
core. | astro-ph_SR |
Surrogate forward models for population inference on compact binary
mergers: Rapidly growing catalogs of compact binary mergers from advanced
gravitational-wave detectors allow us to explore the astrophysics of massive
stellar binaries. Merger observations can constrain the uncertain parameters
that describe the underlying processes in the evolution of stars and binary
systems in population models. In this paper, we demonstrate that binary black
hole populations - namely, detection rates, chirp masses, and redshifts - can
be used to measure cosmological parameters describing the redshift-dependent
star formation rate and metallicity distribution. We present a method that uses
artificial neural networks to emulate binary population synthesis computer
models, and construct a fast, flexible, parallelisable surrogate model that we
use for inference. | astro-ph_SR |
The disappearance of the helium-giant progenitor of the type Ib
supernova iPTF13bvn and constraints on its companion: We report and discuss post-explosion observations of supernova iPTF13bvn. We
find that the brightness of the SN at +740 days is below the level of the
pre-explosion source and thus confirm that the progenitor star has gone. We
estimate that the late-time brightness is still dominated by the supernova,
which constrains the magnitude and thus mass of a possible companion star to
below approximately 10Msun. In turn this implies that the progenitor's initial
mass is constrained to a narrow range of between 10 to 12Msun. The progenitor
of iPTF13bvn would have been a helium giant rather than a Wolf-Rayet star. In
addition, we suggest that sufficiently deep observations acquired in 2016 would
now stand a chance to directly observe the companion star. | astro-ph_SR |
A characterization of ASAS-SN core-collapse supernova environments with
VLT+MUSE: I. Sample selection, analysis of local environments, and
correlations with light curve properties: The analysis of core-collapse supernova (CCSN) environments can provide
important information on the life cycle of massive stars and constrain the
progenitor properties of these powerful explosions. The MUSE instrument at the
VLT enables detailed local environment constraints of the progenitors of large
samples of CCSNe. Using a homogeneous SN sample from the ASAS-SN survey has
enabled us to perform a minimally biased statistical analysis of CCSN
environments. We analyze 111 galaxies observed by MUSE that hosted 112 CCSNe
detected or discovered by the ASAS-SN survey between 2014 and 2018. The
majority of the galaxies were observed by the the AMUSING survey. Here we
analyze the immediate environment around the SN locations and compare the
properties between the different CCSN types and their light curves. We used
stellar population synthesis and spectral fitting techniques to derive physical
parameters for all HII regions detected within each galaxy, including the star
formation rate (SFR), H$\alpha$ equivalent width (EW), oxygen abundance, and
extinction. We found that stripped-envelope (SE) SNe occur in environments with
a higher median SFR, H$\alpha$ EW, and oxygen abundances than SNe II and SNe
IIn/Ibn. The distributions of SNe II and IIn are very similar, indicating that
these events explode in similar environments. For the SESNe, SNe Ic have higher
median SFRs, H$\alpha$ EWs, and oxygen abundances than SNe Ib. SNe IIb have
environments with similar SFRs and H$\alpha$ EWs to SNe Ib, and similar oxygen
abundances to SNe Ic. We also show that the postmaximum decline rate, $s$, of
SNe II correlates with the H$\alpha$ EW, and that the luminosity and the
$\Delta m_{15}$ parameter of SESNe correlate with the oxygen abundance,
H$\alpha$ EW, and SFR at their environments. This suggests a connection between
the explosion mechanisms of these events to their environment properties. | astro-ph_SR |
The sunspot observations by Toaldo and Comparetti at November 1779: There is a low frequency of sunspot observations in some years of the 17th
and 18th century. In particular, the interval 1777-1795 is one of the periods
with scarce observations in the databases. In this note, we report a sunspot
observation carried out the 3rd November 1779 by Giuseppe Toaldo and Andrea
Comparetti. | astro-ph_SR |
Plasma sloshing in pulse-heated solar and stellar coronal loops: There is evidence that coronal heating is highly intermittent, and flares are
the high energy extreme. The properties of the heat pulses are difficult to
constrain. Here hydrodynamic loop modeling shows that several large amplitude
oscillations (~ 20% in density) are triggered in flare light curves if the
duration of the heat pulse is shorter that the sound crossing time of the
flaring loop. The reason is that the plasma has not enough time to reach
pressure equilibrium during the heating and traveling pressure fronts develop.
The period is a few minutes for typical solar coronal loops, dictated by the
sound crossing time in the decay phase. The long period and large amplitude
make these oscillations different from typical MHD waves. This diagnostic can
be applied both to observations of solar and stellar flares and to future
observations of non-flaring loops at high resolution. | astro-ph_SR |
Measuring mean densities of delta Scuti stars with asteroseismology.
Theoretical properties of large separations using TOUCAN: We aim at studying the theoretical properties of the regular spacings found
in the oscillation spectra of delta Scuti stars. We performed a multi-variable
analysis covering a wide range of stellar structure and seismic properties and
model parameters representative of intermediate-mass, main sequence stars. The
work-flow is entirely done using a new Virtual Observatory tool: TOUCAN (the VO
gateway for asteroseismic models), which is presented in this paper. A linear
relation between the large separation and the mean density is predicted to be
found in the low frequency frequency domain (i.e. radial orders spanning from 1
to 8, approximately) of the main-sequence, delta Scuti stars' oscillation
spectrum. We found that such a linear behavior stands whatever the mass,
metallicity, mixing length, and overshooting parameters considered in this
work. The intrinsic error of the method is discussed. This includes the
uncertainty in the large separation determination and the role of rotation. The
validity of the relation found is only guaranteed for stars rotating up to 40
percent of their break-up velocity. Finally, we applied the diagnostic method
presented in this work to five stars for which regular patterns have been
found. Our estimates for the mean density and the frequency of the fundamental
radial mode match with those given in the literature within a 20 percent of
deviation. Asteroseismology has thus revealed an independent direct measure of
the average density of delta Scuti stars, analogous to that of the Sun. This
places tight constraints on the mode identification and hence on the stellar
internal structure and dynamics, and allows a determination the radius of
planets orbiting around delta Scuti stars with unprecedented precision. This
opens the way for studying the evolution of regular patterns in pulsating
stars, and its relation with stellar structure and evolution. | astro-ph_SR |
The Non-Linear Growth of the Magnetic Rayleigh-Taylor Instability: This work examines the effect of the embedded magnetic field strength on the
non-linear development of the magnetic Rayleigh-Taylor Instability (RTI) (with
a field-aligned interface) in an ideal gas close to the incompressible limit in
three dimensions. Numerical experiments are conducted in a domain sufficiently
large so as to allow the predicted critical modes to develop in a physically
realistic manner. The ratio between gravity, which drives the instability in
this case (as well as in several of the corresponding observations), and
magnetic field strength is taken up to a ratio which accurately reflects that
of observed astrophysical plasma, in order to allow comparison between the
results of the simulations and the observational data which served as
inspiration for this work. This study finds reduced non-linear growth of the
rising bubbles of the RTI for stronger magnetic fields, and that this is
directly due to the change in magnetic field strength, rather than the indirect
effect of altering characteristic length scales with respect to domain size. By
examining the growth of the falling spikes, the growth rate appears to be
enhanced for the strongest magnetic field strengths, suggesting that rather
than affecting the development of the system as a whole, increased magnetic
field strengths in fact introduce an asymmetry to the system. Further
investigation of this effect also revealed that the greater this asymmetry, the
less efficiently the gravitational energy is released. By better understanding
the under-studied regime of such a major phenomenon in astrophysics, deeper
explanations for observations may be sought, and this work illustrates that the
strength of magnetic fields in astrophysical plasmas influences observed RTI in
subtle and complex ways. | astro-ph_SR |
A multi-wavelength study of the young star V1118 Orionis in outburst: Abriged version for astroph: The young late-type star V1118 Orionis was in
outburst from 2005 to 2006. We followed the outburst with optical and
near-infrared photometry; the X-ray emission was further probed with
observations taken with XMM-Newton and Chandra during and after the outburst.
In addition, we obtained mid-infrared photometry and spectroscopy with Spitzer
at the peak of the outburst and in the post-outburst phase. The spectral energy
distribution of V1118 Ori varied significantly over the course of the outburst.
The optical flux showed the largest variations, most likely due to enhanced
emission by a hot spot. The latter dominated the optical and near-infrared
emission at the peak of the outburst, while the disk emission dominated in the
mid-infrared. The X-ray flux correlated with the optical and infrared fluxes,
indicating that accretion affected the magnetically active corona and the
stellar magnetosphere. The thermal structure of the corona was variable with
some indication of a cooling of the coronal temperature in the early phase of
the outburst with a gradual return to normal values. Color-color diagrams in
the optical and infrared showed variations during the outburst, with no obvious
signature of reddening due to circumstellar matter. Using MC realizations of
star+disk+hotspot models to fit the SED in ``quiescence'' and at the peak of
the outburst, we determined that the mass accretion rate varied from about
2.5E-7 Msun/yr to 1E-6 Msun/yr; in addition the fractional area of the hotspot
increased significantly as well. The multi-wavelength study of the V1118 Ori
outburst helped us to understand the variations in spectral energy
distributions and demonstrated the interplay between the disk and the stellar
magnetosphere in a young, strongly accreting star. | astro-ph_SR |
Binarity and Accretion in AGB Stars: HST/STIS Observations of UV
Flickering in Y Gem: Binarity is believed to dramatically affect the history and geometry of mass
loss in AGB and post-AGB stars, but observational evidence of binarity is
sorely lacking. As part of a project to search for hot binary companions to
cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to
be a source of strong and variable UV and X-ray emission. Here we report UV
spectroscopic observations of Y Gem obtained with the Hubble Space Telescope
that show strong flickering in the UV continuum on time-scales of <~20 s,
characteristic of an active accretion disk. Several UV lines with P-Cygni-type
profiles from species such as Si IV and C IV are also observed, with emission
and absorption features that are red- and blue- shifted by velocities of ~500
km/s from the systemic velocity. Our model for these (and previous)
observations is that material from the primary star is gravitationally captured
by a companion, producing a hot accretion disk. The latter powers a fast
outflow that produces blue-shifted features due to absorption of UV continuum
emitted by the disk, whereas the red-shifted emission features arise in heated
infalling material from the primary. The outflow velocities support a previous
inference by Sahai et al. (2015) that Y Gem's companion is a low-mass
main-sequence star. Blackbody fitting of the UV continuum implies an accretion
luminosity of about 13 Lsun and thus a mass-accretion rate >5e-7 Msun/yr; we
infer that Roche lobe overflow is the most likely binary accretion mode for Y
Gem. | astro-ph_SR |
Spectroscopic factor of the 1+, 25Al(p,g)26Si resonance at Ex=5.68 MeV: Nuclear shell model predictions for the proton spectroscopic factor of the
1+, Ex = 5.68 MeV level in 26Si are about fifty times smaller than the value
suggested by the measured (a,3He) cross section for the Ex = 5.69 MeV mirror
level in 26Mg, assuming purely single-particle transfer. Given that the 5.69
MeV level has been very weakly, if it all, populated in previous studies of the
simpler 25Mg(d,p) reaction, it is unclear if the (a,3He) result is a true
single-particle spectroscopic factor. If we assume the (a,3He) result, the
thermonuclear rate of the 25Al(p,g)26Si reaction would increase by factors of 6
- 50 over stellar temperatures of T = 0.05 - 0.2 GK. We examine the
implications of this enhanced rate for model predictions of nucleosynthesis in
classical nova explosions. | astro-ph_SR |
Multicolour photometry of pulsating stars in the Galactic Bulge fields: We present a study of photometric properties of very crowded stellar fields
toward the Galactic Bulge. We performed a search for pulsating stars among
thousands of variable stars from the OGLE-II survey supplementing the
variability study with photometric measurements in four Johnson-Cousins
$UBVI_{\rm C}$ passbands. Using these data, we analysed the properties of
objects located at different distances and, whenever possible, classified them. | astro-ph_SR |
Parallax of the L4.5 dwarf 2M1821$+$14 from high-precision astrometry
with OSIRIS at GTC: We used the OSIRIS camera at the 10.4 m Gran Telescopio Canarias (GTC) to
monitor the astrometric motion of the L4.5 dwarf 2M1821$+$14 over 17 months.
The astrometric residuals of eleven epochs have a r.m.s. dispersion of 0.4 mas,
which is larger than the average precision of 0.23 mas per epoch and hints
towards an additional signal or excess noise. Comparison of the
point-spread-functions in OSIRIS and FORS2/VLT images reveals no differences
critical for high-precision astrometry, despite the GTC's segmented primary
mirror. We attribute the excess noise to an unknown effect that may be
uncovered with additional data. For 2M1821$+$14, we measured a relative
parallax of $106.15 \pm 0.18$ mas and determined a correction of $0.50\pm0.05$
mas to absolute parallax, leading to a distance of $9.38 \pm0.03$ pc. We
excluded at 3-$\sigma$ confidence the presence of a companion to 2M1821$+$14
down to a mass ratio of 0.1 ($\approx 5\, M_\mathrm{Jupiter}$) with a period of
50--1000 days and a separation of 0.1--0.7 au. The accurate parallax allowed us
to estimate the age and mass of 2M1821$+$14 of 120--700 Myr and
0.049$^{+0.014}_{-0.024}$ M$_\odot$, thus confirming its intermediate age and
substellar mass. We complement our study with a parallax and proper motion
catalogue of 587 stars ($i'\simeq15.5-22$) close to 2M1821$+$14, used as
astrometric references. This study demonstrates sub-mas astrometry with the
GTC, a capability applicable for a variety of science cases including the
search for extrasolar planets and relevant for future astrometric observations
with E-ELT and TMT. | astro-ph_SR |
Rotational periods and evolutionary models for subgiant stars observed
by CoRoT: We present rotation period measurements for subgiants observed by CoRoT.
Interpreting the modulation of stellar light that is caused by star-spots on
the time scale of the rotational period depends on knowing the fundamental
stellar parameters. Constraints on the angular momentum distribution can be
extracted from the true stellar rotational period. By using models with an
internal angular momentum distribution and comparing these with measurements of
rotation periods of subgiant stars we investigate the agreement between
theoretical predictions and observational results. With this comparison we can
also reduce the global stellar parameter space compatible with the rotational
period measurements from subgiant light curves. We can prove that an evolution
assuming solid body rotation is incompatible with the direct measurement of the
rotational periods of subgiant stars. Measuring the rotation periods relies on
two different periodogram procedures, the Lomb-Scargle algorithm and the
Plavchan periodogram. Angular momentum evolution models were computed to give
us the expected rotation periods for subgiants, which we compared with measured
rotational periods. We find evidence of a sinusoidal signal that is compatible
in terms of both phase and amplitude with rotational modulation. Rotation
periods were directly measured from light curves for 30 subgiant stars and
indicate a range of 30 to 100 d for their rotational periods. Our models
reproduce the rotational periods obtained from CoRoT light curves. These new
measurements of rotation periods and stellar models probe the non-rigid
rotation of subgiant stars. | astro-ph_SR |
On the nature of Nova 1670 (CK Vulpeculae): a merger of a red giant with
a helium white dwarf: Nova 1670 is a historical transient bearing strong similarities to a
recently-recognized type of stellar eruptions known as red novae, which are
thought to be powered by stellar mergers. The remnant of the transient, CK Vul,
is observable today mainly through cool circumstellar gas and dust, and
recombining plasma, but we have no direct view on the stellar object. Within
the merger hypothesis, we aim to infer the most likely makeup of the progenitor
system that resulted in Nova 1670. We collect and summarize the literature data
on the physical properties of the outburst and the remnant, and on the chemical
composition of the circumstellar material which resulted from optical and
submillimeter observations of the circumstellar gas of CK Vul. Simple
simulations yield the form and level of mixing of material associated with the
merger. Products of nuclear burning are identified, among them ashes of H
burning in the CNO cycles and the MgAl chain, as well as of partial He burning.
Based on the luminosity and chemical composition of the remnant, we find that
the progenitor primary had to be an evolutionarily advanced red-giant branch
star of a mass of 1-2 M$_{\odot}$. The secondary was either a very similar
giant, or a He white dwarf. While the eruption event was mainly powered by
accretion, we estimate that about 12% of total energy might have come from He
burning activated during the merger. The coalescence of a first-ascent giant
with a He white dwarf created a star with a rather unique internal structure
and composition, which resemble these of early R-type carbon stars. Nova 1670
was the result of a merger between a He white dwarf and a first-ascent red
giant and is likely now evolving to become an early R-type carbon star. | astro-ph_SR |
An Infrared Census Of Dust In Nearby Galaxies With Spitzer (DUSTiNGS):
V. The Period-luminosity Relation For Dusty Metal-Poor AGB Stars: The survey for DUST In Nearby Galaxies with Spitzer (DUSTiNGS) has identified
hundreds of candidate dust-producing Asymptotic Giant Branch (AGB) stars in
several nearby metal-poor galaxies. We have obtained multi-epoch follow-up
observations for these candidates with the Spitzer Space Telescope and measured
their infrared (IR) lightcurves. This has allowed us to confirm their AGB
nature and investigate pulsation behavior at very low metallicity. We have
obtained high-confidence pulsation periods for 88 sources in seven galaxies. We
have confirmed DUSTiNGS variable star candidates with a 20% success rate, and
determined the pulsation properties of 19 sources already identified as
Thermally-Pulsing AGB (TP-AGB) stars. We find that the AGB pulsation properties
are similar in all galaxies surveyed here, with no discernible difference
between the DUSTiNGS galaxies (down to 1.4% solar metallicity; [Fe/H]=-1.85)
and the far more metal-rich Magellanic Clouds (up to 50% solar metallicity;
[Fe/H]=-0.38). These results strengthen the link between dust production and
pulsation in AGB stars and establish the IR Period-Luminosity (P - L) relation
as a reliable tool (+/- 4%) for determining distances to galaxies, regardless
of metallicity. | astro-ph_SR |
Large ion-neutral drift velocities and plasma heating in partially
ionized coronal rain blobs: In this paper we present a numerical study of the dynamics of partially
ionized coronal rain blobs. We use a two-fluid model to perform a
high-resolution 2D simulation that takes into account the collisional
interaction between the charged and neutral particles contained in the plasma.
We follow the evolution of a cold plasma condensation as it falls through an
isothermal vertically stratified atmosphere that represents the much hotter and
lighter solar corona. We study the consequences of the different degrees of
collisional coupling that are present in the system. On the one hand, we find
that at the dense core of the blob there is a very strong coupling and the
charged and neutral components of the plasma behave as a single fluid, with
negligible drift velocities (of a few cm s^-1). On the other hand, at the edges
of the blob the coupling is much weaker and larger drift velocities (of the
order of 1 km s^-1) appear. In addition, frictional heating causes large
increases of temperature at the transition layers between the blob and the
corona. For the first time we show that such large drift velocities and
temperature enhancements can develop as a consequence of ion-neutral decoupling
associated to coronal rain dynamics. This can lead to enhanced emission coming
from the plasma at the coronal rain-corona boundary, which possesses transition
region temperature. | astro-ph_SR |
Mid-infrared observations of O-type stars: spectral morphology: We present mid-infrared observations for a sample of 16 O-type stars. The
data were acquired with the NASA Spitzer Space Telescope, using the IRS
instrument at moderate resolution (R $\sim$ 600), covering the range $\sim
10-37$ microns. Our sample includes early, mid and late O supergiants and
dwarfs. We explore for the first time their mid-IR spectral morphology in a
quantitative way. We use NLTE expanding atmosphere models to help with line
identifications, analyze profile contributions and line-formation regions. The
O supergiants present a rich emission line spectra. The most intense features
are from hydrogen - $6\alpha$, 7$\alpha$, and $8\alpha$ - which have
non-negligible contributions of HeI or HeII lines, depending on the spectral
type. The spectrum of early O supergiants is a composite of HI and HeII lines,
HeI lines being absent. On the other hand, late O supergiants present features
composed mainly by HI and HeI lines. All emission lines are formed throughout
the stellar wind. We found that O dwarfs exhibit a featureless mid-IR spectrum.
Two stars of our sample exhibit very similar mid-IR features, despite having a
very different optical spectral classification. The analysis of O-type stars
based on mid-IR spectra alone to infer spectral classes or to estimate physical
parameters may thus be prone to substantial errors. Our results may therefore
inform spectroscopic observations of massive stars located in heavily obscured
regions and help establish an initial framework for observations of massive
stars using the Mid-Infrared Instrument on the James Webb Space Telescope. | astro-ph_SR |
An Incipient Debris Disk in the Chamaeleon I Cloud: The point at which a protoplanetary disk becomes a debris disk is difficult
to identify. To better understand this, here we study the $\sim$40~AU
separation binary T~54 in the Chamaeleon I cloud. We derive a K5 spectral type
for T~54~A (which dominates the emission of the system) and an age of
$\sim$2~Myr. However, the dust disk properties of T~54 are consistent with
those of debris disks seen around older and earlier-type stars. At the same
time, T~54 has evidence of gas remaining in the disk as indicated by [Ne II],
[Ne III], and [O I] line detections. We model the spectral energy distribution
of T~54 and estimate that $\sim$3$\times$10$^{-3}$ Earth-masses of small dust
grains ($<$0.25~$\mu$m) are present in an optically thin circumbinary disk
along with at least $\sim$3$\times$10$^{-7}$ Earth-masses of larger
($>$10~$\mu$m) grains within a circumprimary disk. Assuming a solar-like
mixture, we use Ne line luminosities to place a minimum limit on the gas mass
of the disk ($\sim$3$\times$10$^{-4}$ Earth-masses) and derive a gas-to-dust
mass ratio of $\sim$0.1. We do not detect substantial accretion, but we do see
H$\alpha$ in emission in one epoch, suggestive that there may be intermittent
dumping of small amounts of matter onto the star. Considering the low dust
mass, the presence of gas, and young age of T~54, we conclude that this system
is on the bridge between the protoplanetary and debris disk stages. | astro-ph_SR |
The LAMOST spectroscopic survey of star clusters in M31. II.
Metallicities, ages and masses: We select from Paper I a sample of 306 massive star clusters observed with
the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) in the
vicinity fields of M31 and M33 and determine their metallicities, ages and
masses. Metallicities and ages are estimated by fitting the observed integrated
spectra with stellar synthesis population (SSP) models with a pixel-to-pixel
spectral fitting technique. Ages for most young clusters are also derived by
fitting the multi-band photometric measurements with model spectral energy
distributions (SEDs). The estimated cluster ages span a wide range, from
several million years to the age of the universe. The numbers of clusters
younger and older than 1 Gyr are respectively 46 and 260. With ages and
metallicities determined, cluster masses are then estimated by comparing the
multi-band photometric measurements with SSP model SEDs. The derived masses
range from $\sim 10^{3}$ to $\sim 10^7$ $M_{\odot}$, peaking at $\sim 10^{4.3}$
and $\sim 10^{5.7}$ $M_{\odot}$ for young ($< 1$ Gyr) and old ($>1$ Gyr)
clusters, respectively. Our estimated metallicities, ages and masses are in
good agreement with available literature values. Old clusters richer than
[Fe/H] $\sim -0.7$ dex have a wide range of ages. Those poorer than [Fe/H]
$\sim -0.7$ dex seem to be composed of two groups, as previously found for
Galactic GCs -- one of the oldest ages with all values of metallicity down to
$\sim -2$ dex and another with metallicity increasing with decreasing age. The
old clusters in the inner disk of M\,31 (0 -- 30 kpc) show a clear metallicity
gradient measured at $-0.038\pm0.023$ dex/kpc. | astro-ph_SR |
HD 156324: a tidally locked magnetic triple spectroscopic binary with a
disrupted magnetosphere: HD 156324 is an SB3 (B2V/B5V/B5V) system in the Sco OB4 association. The
He-strong primary possesses both a strong magnetic field, and H$\alpha$
emission believed to originate in its Centrifugal Magnetosphere (CM). We
analyse a large spectroscopic and high-resolution spectropolarimetric dataset.
The radial velocities (RVs) indicate that the system is composed of two
sub-systems, which we designate A and B. Period analysis of the RVs of the
three components yields orbital periods $P_{\rm orb} = 1.5806(1)$~d for the Aa
and Ab components, and 6.67(2)~d for the B component, a PGa star. Period
analysis of the longitudinal magnetic field \bz~and H$\alpha$ equivalent
widths, which should both be sensitive to the rotational period $P_{\rm rot}$
of the magnetic Aa component, both yield $\sim$1.58~d. Since $P_{\rm orb} =
P_{\rm rot}$ Aa and Ab must be tidally locked. Consistent with this, the orbit
is circularized, and the rotational and orbital inclinations are identical
within uncertainty, as are the semi-major axis and the Kepler corotation
radius. The star's H$\alpha$ emission morphology differs markedly from both
theoretical and observational expectations in that there is only one, rather
than two, emission peaks. We propose that this unusual morphology may be a
consequence of modification of the gravitocentrifugal potential by the presence
of the close stellar companion. We also obtain upper limits on the magnetic
dipole strength $B_{\rm d}$ for the Ab and B components, respectively finding
$B_{\rm d} < 2.6$~kG and $<0.7$~kG. | astro-ph_SR |
He white dwarfs with large H contamination: Convective mixing or
accretion?: White dwarfs are compact objects with atmospheres containing mainly light
elements, hydrogen or helium. Because of their surface high gravitational
field, heavy elements diffuse downwards in a very short timescale compared to
the evolutionary timescale, leaving the lightest ones on the top of the
envelope. This results in the main classification of white dwarfs as hydrogen
rich or helium rich. But many helium rich white dwarfs show also the presence
of hydrogen traces in their atmosphere, whose origin is still unsettled. Here
we study, by means of full evolutionary calculations, the case for a
representative model of the "He-H-Z" white dwarfs, a sub-group of helium rich
white dwarfs showing both heavy elements and a large amount of hydrogen in
their atmosphere. We find it impossible to explain its hydrogen atmospheric
content by the convective mixing of a primordial hydrogen present in the star.
We conclude that the most likely explanation is the accretion of hydrogen rich
material, presumably water-bearing, coming from a debris disk. | astro-ph_SR |
Coronal Fourier power spectra: implications for coronal seismology and
coronal heating: The dynamics of regions of the solar corona are investigated using
Atmospheric Imaging Assembly (AIA) 171\AA\ and 193\AA\ data. The coronal
emission from the quiet Sun, coronal loop footprints, coronal moss, and from
above a sunspot is studied. It is shown that the mean Fourier power spectra in
these regions can be described by a power law at lower frequencies that tails
to flat spectrum at higher frequencies, plus a Gaussian-shaped contribution
that varies depending on the region studied. This Fourier spectral shape is in
contrast to the commonly-held assumption that coronal time-series are well
described by the sum of a long time-scale background trend plus
Gaussian-distributed noise, with some specific locations also showing an
oscillatory signal. The implications of this discovery to the field of coronal
seismology and the automated detections of oscillations are discussed. The
power law contribution to the shape of the Fourier power spectrum is
interpreted as being due to the summation of a distribution of exponentially
decaying emission events along the line of sight. This is consistent with the
idea that the solar atmosphere is heated everywhere by small energy deposition
events. | astro-ph_SR |
Observations of the forward scattering Hanle effect in the Ca i 4227
Å line: Chromospheric magnetic fields are notoriously diffcult to measure. The
chromospheric lines are broad, while the fields are producing a minuscule
Zeeman-effect polarization. A promising diagnostic alternative is provided by
the forward-scattering Hanle effect, which can be recorded in chromospheric
lines such as the He i 10830 {\AA} and the Ca i 4227 {\AA} lines. We present a
set of spectropolarimetric observations of the full Stokes vector obtained near
the center of the solar disk in the Ca i 4227 {\AA} line with the ZIMPOL
polarimeter at the IRSOL observatory.We detect a number of interesting
forward-scattering Hanle effect signatures, which we model successfully using
polarized radiative transfer. Here we focus on the observational aspects, while
a separate companion paper deals with the theoretical modeling. | astro-ph_SR |
A Universal Law for Solar-Wind Turbulence at Electron Scales: The interplanetary magnetic fluctuation spectrum obeys a Kolmogorovian power
law at scales above the proton inertial length and gyroradius which is well
regarded as an inertial range. Below these scales a power law index around
$-2.5$ is often measured and associated to nonlinear dispersive processes.
Recent observations reveal a third region at scales below the electron inertial
length. This region is characterized by a steeper spectrum that some refer to
it as the dissipation range. We investigate this range of scales in the
electron magnetohydrodynamic approximation and derive an exact and universal
law for a third-order structure function. This law can predict a magnetic
fluctuation spectrum with an index of $-11/3$ which is in agreement with the
observed spectrum at the smallest scales. We conclude on the possible existence
of a third turbulence regime in the solar wind instead of a dissipation range
as recently postulated. | astro-ph_SR |
Evolution towards and beyond accretion-induced collapse of massive white
dwarfs and formation of millisecond pulsars: Millisecond pulsars (MSPs) are generally believed to be old neutron stars
(NSs), formed via type Ib/c core-collapse supernovae (SNe), which have been
spun up to high rotation rates via accretion from a companion star in a
low-mass X-ray binary (LMXB). In an alternative formation channel, NSs are
produced via the accretion-induced collapse (AIC) of a massive white dwarf (WD)
in a close binary. Here we investigate binary evolution leading to AIC and
examine if NSs formed in this way can subsequently be recycled to form MSPs
and, if so, how they can observationally be distinguished from pulsars formed
via the standard core-collapse SN channel in terms of their masses, spins,
orbital periods and space velocities. Numerical calculations with a detailed
stellar evolution code were used for the first time to study the combined pre-
and post-AIC evolution of close binaries. We investigated the mass transfer
onto a massive WD in 240 systems with three different types of non-degenerate
donor stars: main-sequence stars, red giants, and helium stars. When the WD is
able to accrete sufficient mass (depending on the mass-transfer rate and the
duration of the accretion phase) we assumed it collapses to form a NS and we
studied the dynamical effects of this implosion on the binary orbit.
Subsequently, we followed the mass-transfer epoch which resumes once the donor
star refills its Roche lobe and calculated the continued LMXB evolution until
the end. We demonstrate that the final properties of these MSPs are, in
general, remarkably similar to those of MSPs formed via the standard
core-collapse SN channel. However, the resultant MSPs created via the AIC
channel preferentially form in certain orbital period intervals. Finally, we
discuss the link between AIC and young NSs in globular clusters. Our
calculations are also applicable to progenitor binaries of SNe Ia under certain
conditions. [Abridged] | astro-ph_SR |
First solar butterfly diagram from Schwabe's observations in 1825-1867: The original sunspot observations by Heinrich Samuel Schwabe of 1825-1867
were digitized and a first subset of spots was measured. In this initial
project, we determined more than 14 000 sunspot positions and areas comprising
about 11% of the total amount of spots available from that period. The
resulting butterfly diagram has a typical appearance, but with evident
north-south asymmetries. | astro-ph_SR |
Is it possible to detect coronal mass ejections on solar-type stars
through extreme-ultraviolet spectral observations?: Stellar coronal mass ejections (CMEs) from host stars are an important factor
that affects the habitability of exoplanets. Although their solar counterparts
have been well observed for decades, it is still very difficult to find solid
evidence of stellar CMEs. Using the spectral line profile asymmetry caused by
the Doppler shift of erupting plasma, several stellar CME candidates have been
identified from spectral lines formed at chromospheric or transition region
temperatures of the stars. However, a successful detection of stellar CME
signals based on the profile asymmetries of coronal lines is still lacking. It
is unclear whether we can detect such signals. Here we construct an analytical
model for CMEs on solar-type stars, and derive an expression of stellar
extreme-ultraviolet (EUV) line profiles during CME eruptions. For different
instrumental parameters, exposure times, CME propagation directions and stellar
activity levels, we synthesized the corresponding line profiles of Fe IX 171.07
\AA\ and Fe XV 284.16 \AA. Further investigations provide constraints on the
instrumental requirements for successful detection and characterization of
stellar CMEs. Our results show that it is possible to detect stellar CME
signals and infer their velocities based on spectral profile asymmetries using
an EUV spectrometer with a moderate spectral resolution and signal-to-noise
ratio. Our work provides important references for the design of future EUV
spectrometers for stellar CME detection and the development of observation
strategies. | astro-ph_SR |
The Stability of the Electron Strahl against the Oblique
Fast-magnetosonic/Whistler Instability in the Inner Heliosphere: We analyze the micro-kinetic stability of the electron strahl in the solar
wind depending on heliocentric distance. The oblique fast-magnetosonic/whistler
(FM/W) instability has emerged in the literature as a key candidate mechanism
for the effective scattering of the electron strahl into the electron halo
population. Using data from Parker Solar Probe (PSP) and Helios, we compare the
measured strahl properties with the analytical thresholds for the oblique FM/W
instability in the low- and high-$\beta_{\parallel c}$ regimes, where
$\beta_{\parallel c}$ is the ratio of the core parallel thermal pressure to the
magnetic pressure. Our PSP and Helios data show that the electron strahl is on
average stable against the oblique FM/W instability in the inner heliosphere.
Our analysis suggests that the instability, if at all, can only be excited
sporadically and on short timescales. We discuss the caveats of our analysis
and potential alternative explanations for the observed scattering of the
electron strahl in the solar wind. Furthermore, we recommend the numerical
evaluation of the stability of individual distributions in the future to
account for any uncertainties in the validity of the analytical expressions for
the instability thresholds. | astro-ph_SR |
Characteristics of transverse waves in chromospheric mottles: Using data obtained by the high temporal and spatial resolution Rapid
Oscillations in the Solar Atmosphere (ROSA) instrument on the Dunn Solar
Telescope, we investigate at an unprecedented level of detail transverse
oscillations in chromospheric fine structures near the solar disk center. The
oscillations are interpreted in terms of propagating and standing
magnetohydrodynamic kink waves. Wave characteristics including the maximum
transverse velocity amplitude and the phase speed are measured as a function of
distance along the structure's length. Solar magneto-seismology is applied to
these measured parameters to obtain diagnostic information on key plasma
parameters (e.g., magnetic field, density, temperature, flow speed) of these
localised waveguides. The magnetic field strength of the mottle along the
$\sim$2 Mm length is found to decrease by a factor of 12, while the local
plasma density scale height is $\sim280\pm$80 km. | astro-ph_SR |
Physical properties and astrometry of radio-emitting brown dwarf
TVLM513-46546 revisited: We present multi-epoch astrometric observations of the M9 ultra-cool dwarf
TVLM513-46546 that is placed at the brown dwarf boundary. The new observations
have been performed with the European VLBI Network (EVN) at 6cm band. The
target has been detected at 7~epochs spanning three years, with measured
quiescent emission flux in the range 180-300 muJy. We identified four
short-duration flaring events (0.5-2 mJy) with very high circular polarization
(~75%-100%). Properties of the observed radio flares support the physical model
of the source that is characterized by the electron cyclotron maser instability
responsible for outbursts of radio emission. Combined with Very Long Baseline
Array (VLBA) earlier data, our detections make it possible to refine the
absolute parallax pi=93.27(+0.18,-0.17) mas. Our measurements rule out
TVLM513-46546 companions more massive than Jupiter in orbits with periods
longer than ~1 yr. | astro-ph_SR |
The CARMENES search for exoplanets around M dwarfs: Wing asymmetries of
H$α$, Na I D, and He I lines: Stellar activity is ubiquitously encountered in M dwarfs and often
characterised by the H$\alpha$ line. In the most active M dwarfs, H$\alpha$ is
found in emission, sometimes with a complex line profile. Previous studies have
reported extended wings and asymmetries in the H$\alpha$ line during flares. We
used a total of 473 high-resolution spectra of 28 active M dwarfs obtained by
the CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths
with Near-infrared and optical Echelle Spectrographs) spectrograph to study the
occurrence of broadened and asymmetric H$\alpha$ line profiles and their
association with flares, and examine possible physical explanations. We
detected a total of 41 flares and 67 broad, potentially asymmetric, wings in
H$\alpha$. The broadened H$\alpha$ lines display a variety of profiles with
symmetric cases and both red and blue asymmetries. Although some of these line
profiles are found during flares, the majority are at least not obviously
associated with flaring. We propose a mechanism similar to coronal rain or
chromospheric downward condensations as a cause for the observed red
asymmetries; the symmetric cases may also be caused by Stark broadening. We
suggest that blue asymmetries are associated with rising material, and our
results are consistent with a prevalence of blue asymmetries during the flare
onset. Besides the H$\alpha$ asymmetries, we find some cases of additional line
asymmetries in \ion{He}{i} D$_{3}$, \ion{Na}{i}~D lines, and the \ion{He}{i}
line at 10830\,\AA\, taken all simultaneously thanks to the large wavelength
coverage of CARMENES. Our study shows that asymmetric H$\alpha$ lines are a
rather common phenomenon in M~dwarfs and need to be studied in more detail to
obtain a better understanding of the atmospheric dynamics in these objects. | astro-ph_SR |
The B Fields in OB Stars (BOB) Survey: The B fields in OB stars (BOB) survey is an ESO large programme collecting
spectropolarimetric observations for a large number of early-type stars in
order to study the occurrence rate, properties, and ultimately the origin of
magnetic fields in massive stars. As of July 2014, a total of 98 objects were
observed over 20 nights with FORS2 and HARPSpol. Our preliminary results
indicate that the fraction of magnetic OB stars with an organised, detectable
field is low. This conclusion, now independently reached by two different
surveys, has profound implications for any theoretical model attempting to
explain the field formation in these objects. We discuss in this contribution
some important issues addressed by our observations (e.g., the lower bound of
the field strength) and the discovery of some remarkable objects. | astro-ph_SR |
Note on the Power-2 Limb Darkening Law: Recently there has been a renewed interest in the power-2 limb darkening law
for modeling exoplanet transits. This law provides a better match to the
intensities generated by spherical stellar atmosphere models than other
2-parameter laws. To help facilitate a wider use of the power-2 law we correct
a minor error and, expanding on previous work, suggest a parametrization that
can improve the sampling required by some numerical methods such as MCMC. | astro-ph_SR |
Examining the Magnetic Field Strength and the Horizontal and Vertical
Motions in an Emerging Active Region: Earlier observational studies have used the time evolution of emerging
magnetic flux regions at the photosphere to infer their subsurface structures,
assuming that the flux structure does not change significantly over the
near-surface layer.In this study, we test the validity of this assumption by
comparing the horizontal and vertical motions of an emerging active region. The
two motions would be correlated if the emerging structure is rigid. The
selected active region (AR) NOAA 11645 is not embedded in detectable
preexisting magnetic field. The observed horizontal motion is quantified by the
separation of the two AR polarities and the extension of the region. The
vertical motion is derived from the magnetic buoyancy theory. Our results show
that the separation of the polarities is fastest at the beginning with a
velocity of $\approx$~4~Mm hr$^{-1}$ and decreases to $\le$~1~Mm hr$^{-1}$
after the main growing phase of flux emergence. The derived thick flux-tube
buoyant velocity is between 1 and 3~Mm hr$^{-1}$ while the thin flux-tube
approximation results in an unreasonably high buoyant velocity, consistent with
the expectation that the approximation is inappropriate at the surface layer.
The observed horizontal motion is not found to directly correlate with either
the magnetic field strength or the derived buoyant velocities. However, the
percentage of the horizontally oriented fields and the temporal derivatives of
the field strength and the buoyant velocity show some positive correlations
with the separation velocity. The results of this study imply that the
assumption that the emerging active region is the cross section of a rising
flux tube whose structure can be considered rigid as it rises through the
near-surface layer should be taken with caution. | astro-ph_SR |
The Gray Needle: Large Grains in the HD 15115 Debris Disk from
LBT/PISCES/Ks and LBTI/LMIRcam/L' Adaptive Optics Imaging: We present diffraction-limited \ks band and \lprime adaptive optics images of
the edge-on debris disk around the nearby F2 star HD 15115, obtained with a
single 8.4 m primary mirror at the Large Binocular Telescope. At \ks band the
disk is detected at signal-to-noise per resolution element (SNRE) \about 3-8
from \about 1-2\fasec 5 (45-113 AU) on the western side, and from \about
1.2-2\fasec 1 (63-90 AU) on the east. At \lprime the disk is detected at SNRE
\about 2.5 from \about 1-1\fasec 45 (45-90 AU) on both sides, implying more
symmetric disk structure at 3.8 \microns . At both wavelengths the disk has a
bow-like shape and is offset from the star to the north by a few AU. A surface
brightness asymmetry exists between the two sides of the disk at \ks band, but
not at \lprime . The surface brightness at \ks band declines inside 1\asec
(\about 45 AU), which may be indicative of a gap in the disk near 1\asec. The
\ks - \lprime disk color, after removal of the stellar color, is mostly grey
for both sides of the disk. This suggests that scattered light is coming from
large dust grains, with 3-10 \microns -sized grains on the east side and 1-10
\microns dust grains on the west. This may suggest that the west side is
composed of smaller dust grains than the east side, which would support the
interpretation that the disk is being dynamically affected by interactions with
the local interstellar medium. | astro-ph_SR |
Flares and rotation of M dwarfs with habitable zones accessible to TESS
planet detections: We characterize the magnetic activity of M dwarfs to provide the planet
community with information on the energy input from the star; in particular, in
addition to the frequency of optical flares directly observed with TESS, we aim
at estimating the corresponding X-ray flare frequencies, making use of the
small pool of known events observed simultaneously in both wavebands. We
identified 112 M dwarfs with a TESS magnitude <= 11.5 for which TESS can probe
the full habitable zone for transits. These 112 stars have 1276 two-minute
cadence TESS light curves from the primary mission, which we searched for
rotational modulation and flares. We study the link between rotation and flares
and between flare properties, for example the flare amplitude-duration relation
and cumulative flare energy frequency distributions (FFDs). Assuming that each
optical flare is associated with a flare in the X-ray band, and making use of
published simultaneous Kepler/K2 and XMM-Newton flare studies, we estimate the
X-ray energy released by our detected TESS flare events. Our calibration also
involves the relation between flare energies in the TESS and K2 bands. We
detected more than 2500 optical flare events on a fraction of about 32% of our
targets and found reliable rotation periods only for 12 stars, which is a
fraction of about 11%. For these 12 targets, we present cumulative FFDs and FFD
power law fits. We construct FFDs in the X-ray band by calibrating optical
flare energies to the X-rays. In the absence of directly observed X-ray FFDs
for main-sequence stars, our predictions can serve for estimates of the
high-energy input to the planet of a typical fast-rotating early- or mid-M
dwarf. | astro-ph_SR |
An Interface Region Imaging Spectrograph first view on Solar Spicules: Solar spicules have eluded modelers and observers for decades. Since the
discovery of the more energetic type II, spicules have become a heated topic
but their contribution to the energy balance of the low solar atmosphere
remains unknown. Here we give a first glimpse of what quiet Sun spicules look
like when observed with NASA's recently launched Interface Region Imaging
Spectrograph (IRIS). Using IRIS spectra and filtergrams that sample the
chromosphere and transition region we compare the properties and evolution of
spicules as observed in a coordinated campaign with Hinode and the Atmospheric
Imaging Assembly. Our IRIS observations allow us to follow the thermal
evolution of type II spicules and finally confirm that the fading of Ca II H
spicules appears to be caused by rapid heating to higher temperatures. The IRIS
spicules do not fade but continue evolving, reaching higher and falling back
down after 500-800 s. Ca II H type II spicules are thus the initial stages of
violent and hotter events that mostly remain invisible in Ca II H filtergrams.
These events have very different properties from type I spicules, which show
lower velocities and no fading from chromospheric passbands. The IRIS spectra
of spicules show the same signature as their proposed disk counterparts,
reinforcing earlier work. Spectroheliograms from spectral rasters also confirm
that quiet Sun spicules originate in bushes from the magnetic network. Our
results suggest that type II spicules are indeed the site of vigorous heating
(to at least transition region temperatures) along extensive parts of the
upward moving spicular plasma. | 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 |
Star-spot distributions and chromospheric activity on the RS CVn type
eclipsing binary SV Cam: Using a time series of high-resolution spectra and high-quality multi-colour
photometry, we reconstruct surface maps of the primary component of the RS CVn
type rapidly rotating eclipsing binary, SV Cam (F9V + K4V). We measure a mass
ratio, q, of 0.641(2) using our highest quality spectra and obtain surface
brightness maps of the primary component, which exhibit predominantly
high-latitude spots located between 60-70-degree latitudes with a mean filling
factor of about 35%. This is also indicated by the R-band light curve
inversion, subjected to rigourous numerical tests. The spectral subtraction of
the H-alpha line reveals strong activity of the secondary component. The excess
H-alpha absorption detected near the secondary minimum hints to the presence of
cool material partially obscuring the primary star. The flux ratios of Ca II
IRT excess emission indicate that the contribution of chromospheric plage
regions associated with star-spots is dominant, even during the passage of the
filament-like absorption feature. | astro-ph_SR |
Can Kilonova Light curves be Standardized?: Binary neutron star mergers have been recently confirmed to be the
progenitors of the optical transients kilonovae (KNe). KNe are powered by the
radioactive decay of neutron-rich elements (r-process elements) which are
believed to be the product of disruption of neutron stars during their merger.
KNe exhibit interesting parallels with type Ia supernovae (SNe), whose light
curves show specific correlations which allow them to be used as standardizable
candles. In this paper, we investigate the possibility of the KN light curves
exhibiting similar correlations. While a satisfactory answer to this question
can only be provided by future KN observations, employing theoretical models we
explore whether there is any ground for harboring such expectations. Using
semi-analytic models of KN light curves in conjunction with results from
numerical relativity simulations of binary neutron star mergers, we obtain the
maximum bolometric luminosity ($L_{\mathrm{Bol}}^{\mathrm{max}}$) and decline
in luminosity ($\Delta L_{\mathrm{Bol}}$) for a simulated population of
mergers. We find that theoretical light curves of KNe show remarkable
correlations despite the complex physics governing their behavior. This
presents a possibility of future observations to uncover such correlations in
the observed light curves, eventually allowing observers to standardize these
light curves and to use them for local distance measurements. | astro-ph_SR |
ASASSN-15jd: WZ Sge-type star with intermediate superoutburst between
single and double ones: We present optical photometry of a WZ Sge-type dwarf nova (DN), ASASSN-15jd.
Its light curve showed a small dip in the middle of the superoutburst in 2015
for the first time among WZ Sge-type DNe. The unusual light curve implies a
delay in the growth of the 3:1 resonance tidal instability. Also, the light
curve is similar to those of other two WZ Sge-type stars, SSS
J122221.7$-$311523 and OT J184228.1$+$483742, which are believed to be the best
candidates for a period bouncer on the basis of their small values of the mass
ratio ($q \equiv M_{2}/M_{1}$). Additionally, the small mean superhump
amplitude ($<$ 0.1 mag) and the long duration of no ordinary superhumps at the
early stage of the superoutburst are common to the best candidates for a period
bouncer. The average superhump period was $P_{\rm sh}$ = 0.0649810(78) d and no
early superhumps were detected. Although we could not estimate the mass ratio
of ASASSN-15jd with high accuracy, this object is expected to be a candidate
for a period bouncer, a binary accounting for the missing population of
post-period minimum cataclysmic variables, based on the above characteristics. | astro-ph_SR |
Two-stage evolution of an extended C-class eruptive flaring activity
from sigmoid active region NOAA 12734: SDO and Udaipur-CALLISTO observations: We present a multi-wavelength investigation of a C-class flaring activity
that occurred in the active region NOAA 12734 on 8 March 2019. The
investigation utilises data from AIA and HMI on board the SDO and the
Udaipur-CALLISTO solar radio spectrograph of the Physical Research Laboratory.
This low intensity C1.3 event is characterised by typical features of a long
duration event (LDE), viz. extended flare arcade, large-scale two-ribbon
structures and twin coronal dimmings. The eruptive event occurred in a coronal
sigmoid and displayed two distinct stages of energy release, manifested in
terms of temporal and spatial evolution. The formation of twin dimming regions
are consistent with the eruption of a large flux rope with footpoints lying in
the western and eastern edges of the coronal sigmoid. The metric radio
observations obtained from Udaipur-CALLISTO reveals a broad-band
($\approx$50-180 MHz), stationary plasma emission for $\approx$7 min during the
second stage of the flaring activity that resemble a type IV radio burst. A
type III decametre-hectometre radio bursts with starting frequency of
$\approx$2.5 MHz precedes the stationary type IV burst observed by
Udaipur-CALLISTO by $\approx$5 min. The synthesis of multi-wavelength
observations and Non-Linear Force Free Field (NLFFF) coronal modelling together
with magnetic decay index analysis suggests that the sigmoid flux rope
underwent a zipping-like uprooting from its western to eastern footpoints in
response to the overlying asymmetric magnetic field confinement. The
asymmetrical eruption of the flux rope also accounts for the observed
large-scale structures viz. apparent eastward shift of flare ribbons and post
flare loops along the polarity inversion line (PIL), and provides an evidence
for lateral progression of magnetic reconnection site as the eruption proceeds. | astro-ph_SR |
A multi-wavelength analysis of BL Her stars: Models versus Observations: We present new theoretical period--luminosity (PL) and period--radius (PR)
relations at multiple wavelengths (Johnson--Cousins--Glass and {\sl Gaia}
passbands) for a fine grid of BL~Herculis models computed using {\sc mesa-rsp}.
The non-linear models were computed for periods typical of BL~Her stars, i.e.
$1\leq P ({\rm days}) \leq4$, covering a wide range of input parameters:
metallicity ($-$2.0 dex $\leq$ [Fe/H] $\leq$ 0.0 dex), stellar mass (0.5--0.8
M$_{\odot}$), luminosity (50--300 L$_{\odot}$) and effective temperature (full
extent of the instability strip; in steps of 50K). We investigate the impact of
four sets of convection parameters on multi-wavelength properties. Most
empirical relations match well with theoretical relations from the BL~Her
models computed using the four sets of convection parameters. No significant
metallicity effects are seen in the PR relations. Another important result from
our grid of BL~Her models is that it supports combining PL relations of RR
Lyrae and Type~II Cepheids together as an alternative to classical Cepheids for
the extragalactic distance scale calibration. | astro-ph_SR |
Uncovering a 260 pc wide, 35 Myr old filamentary relic of star formation: Several recent studies have shown that the Vela OB2 region hosts a complex
constellation of sub-populations with ages in the range 10 to 50 Myr. Such
populations might represent the best example of the outcome of clustered star
formation in Giant Molecular clouds (GMC). We use Gaia DR2 data over an area of
40 deg radius around the open cluster Collinder 135 to extend the study of the
stellar populations of the Vela OB2 region over an area of several hundreds of
parsecs on sky. Detailed clustering algorithms combined with the exquisite
astrometric quality of the GAIA catalogue allow us to detect a new cluster
named BBJ 1 that shows the same age as NGC 2547 (30 to 35 Myr), but located at
a distance of 260 pc from it. Deeper investigation of the region via clustering
in 5D parameter space and in the colour-magnitude diagram allows us to detect a
filamentary structure of stars that bridges the two clusters. Given the extent
in space of such structure (260 pc) and the young age (~35 Myr), we exclude
that such population originates by the same mechanism responsible to create
tidal streams around older clusters. Even if we miss a complete picture of the
3D motion of the studied stellar structure because of the lack of accurate
radial velocity measurements, we propose that such structure represent the
detection of a 35 Myr old outcome of a mechanism of filamentary star formation
in a GMC. | astro-ph_SR |
The timing behaviour of radio pulsars: The purpose of this review paper is to summarise the pulsar timing method, to
provide an overview of recent research into the spin-down of pulsars over
decadal timescales and to highlight the science that can be achieved using
high-precision timing of millisecond pulsars. | astro-ph_SR |
The Luminous Blue Variable RMC127 as seen with ALMA and ATCA: We present ALMA and ATCA observations of the luminous blue variable \rmc. The
radio maps show for the first time the core of the nebula and evidence that the
nebula is strongly asymmetric with a Z-pattern shape. Hints of this morphology
are also visible in the archival \emph{HST} $\rm H\alpha$ image, which overall
resembles the radio emission. The emission mechanism in the outer nebula is
optically thin free-free in the radio. At high frequencies, a component of
point-source emission appears at the position of the star, up to the ALMA
frequencies. The rising flux density distribution ($S_{\nu}\sim
\nu^{0.78\pm0.05}$) of this object suggests thermal emission from the ionized
stellar wind and indicates a departure from spherical symmetry with
$n_{e}(r)\propto r^{-2}$. We examine different scenarios to explain this excess
of thermal emission from the wind and show that this can arise from a bipolar
outflow, supporting the suggestion by other authors that the stellar wind of
\rmc is aspherical. We fit the data with two collimated ionized wind models and
we find that the mass-loss rate can be a factor of two or more smaller than in
the spherical case. We also fit the photometry obtained by IR space telescopes
and deduce that the mid- to far-IR emission must arise from extended, cool
($\sim80\,\rm K$) dust within the outer ionized nebula. Finally we discuss two
possible scenarios for the nebular morphology: the canonical single star
expanding shell geometry, and a precessing jet model assuming presence of a
companion star. | astro-ph_SR |
Dynamic kink instability and transverse motions of solar spicules: Hydrodynamic jets are unstable to the kink instability (m=1 mode in
cylindrical geometry) owing to the centripetal force, which increases the
transverse displacement of the jet. When the jet moves along a magnetic field,
then the Lorentz force tries to decrease the displacement and stabilises the
instability of sub-Alfvenic flows. The threshold of the instability depends on
the Alfven Mach number (the ratio of Alfven and jet speeds). We suggest that
the dynamic kink instability may be of importance to explain observed
transverse motions of type II spicules in the solar atmosphere. We show that
the instability may start for spicules which rise up at the peripheries of
vertically expanding magnetic flux tubes owing to the decrease of the Alfven
speed in both, the vertical and the radial directions. Therefore, inclined
spicules may be more unstable and have more higher transverse speeds. Periods
and growth times of unstable modes in the conditions of type II spicules have
the values of 30 s and 25-100 s, respectively, which are comparable to the life
time of the structures. This may indicate to the interconnection between high
speed flow and rapid disappearance of type II spicules in chromospheric
spectral lines. | astro-ph_SR |
Photospheric magnetic structure of coronal holes: In this study, we investigate in detail the photospheric magnetic structure
of 98 coronal holes using line-of-sight magnetograms of SDO/HMI, and for a
subset of 42 coronal holes using HINODE/SOT G-band filtergrams. We divided the
magnetic field maps into magnetic elements and quiet coronal hole regions by
applying a threshold at $\pm 25$ G. We find that the number of magnetic bright
points in magnetic elements is well correlated with the area of the magnetic
elements (cc=$0.83\pm 0.01$). Further, the magnetic flux of the individual
magnetic elements inside coronal holes is related to their area by a power law
with an exponent of $1.261\pm 0.004$ (cc=$0.984\pm 0.001$). Relating the
magnetic elements to the overall structure of coronal holes, we find that on
average ($69\pm 8$) % of the overall unbalanced magnetic flux of the coronal
holes arises from long-lived magnetic elements with lifetimes > 40 hours. About
($22\pm 4$) % of the unbalanced magnetic flux arises from a very weak
background magnetic field in the quiet coronal hole regions with a mean
magnetic field density of about 0.2 to 1.2 G. This background magnetic field is
correlated to the flux of the magnetic elements with lifetimes of > 40 h
(cc=$0.88\pm 0.02$). The remaining flux arises from magnetic elements with
lifetimes < 40 hours. By relating the properties of the magnetic elements to
the overall properties of the coronal holes, we find that the unbalanced
magnetic flux of the coronal holes is completely determined by the total area
that the long-lived magnetic elements cover (cc=$0.994\pm 0.001$). | astro-ph_SR |
Spatially resolving the inhomogeneous structure of the dynamical
atmosphere of Betelgeuse with VLTI/AMBER: We present spatially resolved high-spectral resolution K-band observations of
the red supergiant Betelgeuse (alpha Ori) using AMBER at the Very Large
Telescope Interferometer (VLTI). Betelgeuse was observed between 2.28 and 2.31
micron using baselines of 16, 32, and 48m with spectral resolutions of 4800 --
12000. Spectrally dispersed interferograms have been obtained in the 2nd, 3rd,
and 5th lobes, which represents the highest spatial resolution (9 mas) achieved
for Betelgeuse, corresponding to 5 resolution elements over its stellar disk.
The AMBER data in the continuum can be reasonably fitted by a uniform disk with
a diameter of 43.19+/-0.03 mas or a limb-darkening disk with 43.56+/-0.06 mas.
The K-band interferometric data taken at various epochs suggest that Betelgeuse
seen in the continuum shows much smaller deviations from the above
uniform/limb-darkened disk than predicted by 3-D convection simulations. On the
other hand, our AMBER data in the CO lines reveal that the blue and red wings
of the CO lines originate in spatially distinct regions over the stellar disk,
indicating an inhomogeneous velocity field. Our AMBER data in the CO lines can
be roughly explained by a simple model, in which a patch of CO gas is moving
outward or inward at velocities of 10--15 km s^-1, while the CO gas in the
remaining region in the atmosphere is moving in the opposite direction at the
same velocities. The AMBER data are also consistent with the presence of warm
molecular layers at ~1.4--1.5 Rstar with a CO column density of ~1 x 10^20
cm^-2. Our AMBER observations of Betelgeuse are the first spatially resolved
study of the so-called macroturbulence in a stellar atmosphere other than the
Sun. The spatially resolved CO gas motion is likely to be related to convective
motion or intermittent mass ejections in clumps or arcs. | astro-ph_SR |
The intermediate neutron capture process. V. The i-process in AGB stars
with overshoot: The intermediate neutron capture process (i-process) can develop during
proton ingestion events (PIE), potentially during the early stages of low-mass
low-metallicity asymptotic giant branch (AGB) stars. We examine the impact of
overshoot mixing on the triggering and development of i-process nucleosynthesis
in AGB stars of various initial masses and metallicities. We computed AGB
stellar models, with initial masses of 1, 2, 3, and 4 M$_{\odot}$ and
metallicities in the $-2.5 \le $ [Fe/H] $\le 0$ range, using the stellar
evolution code STAREVOL with a network of 1160 nuclei coupled to the transport
equations. We considered different overshooting profiles below and above the
thermal pulses, and below the convective envelope. The occurrence of PIEs is
found to be primarily governed by the amount of overshooting at the top of
pulse ($f_{\rm top}$) and to increase with rising $f_{\rm top}$. For $f_{\rm
top} =$ 0, 0.02, 0.04, and 0.1, we find that 0 %, 6 %, 24 %, and 86 % of our 21
AGB models with $-2<$ [Fe/H] $<0$ experience a PIE, respectively. We also find
that PIEs leave a $^{13}$C-pocket at the bottom of the pulse that can give rise
to an additional radiative s-process nucleosynthesis, and ultimately produce a
noticeable mixed i+s chemical signature at the surface. Finally, the chemical
abundance patterns of 22 observed r/s-stars candidates with $-2<$ [Fe/H] $<-1$
are found to be in reasonable agreement with our AGB model predictions. The
binary status of the dwarfs/giants being unclear, we suggest that these stars
have acquired their chemical pattern either from the mass transfer of a
now-extinct AGB companion or from an early generation AGB star that polluted
the natal cloud. Stricter constraints from multi-dimensional hydrodynamical
models on overshoot coefficients could deliver new insights into the
contribution of AGB stars to heavy elements in the Universe. | astro-ph_SR |
A new class of Roche lobe-filling hot subdwarf binaries: We present the discovery of the second binary with a Roche lobe-filling hot
subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute
binary was discovered using data from the Zwicky Transient Facility.
Spectroscopic observations reveal an He-sdOB star with an effective temperature
of $T_{\rm eff}=33,700\pm1000$ K and a surface gravity of $log(g)=5.54\pm0.11$.
The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the
He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well
as a weak eclipse of the WD. We infer a He-sdOB mass of $M_{\rm
sdOB}=0.41\pm0.04$ M$_\odot$ and a WD mass of $M_{\rm WD}=0.68\pm0.05$
M$_\odot$. The weak eclipses imply a WD black-body temperature of
$63,000\pm10,000$ K and a radius $R_{\rm WD}=0.0148\pm0.0020$ M$_\odot$ as
expected for a WD of such high temperature.
The He-sdOB star is likely undergoing hydrogen shell burning and will
continue transferring mass for $\approx1$ Myrs at a rate of $10^{-9} M_\odot
{\rm yr}^{-1}$ which is consistent with the high WD temperature. The hot
subdwarf will then turn into a WD and the system will merge in $\approx30$
Myrs. We suggest that Galactic reddening could bias discoveries towards
preferentially finding Roche lobe-filling systems during the short-lived shell
burning phase. Studies using reddening corrected samples should reveal a large
population of helium core-burning hot subdwarfs with $T_{\rm eff}\approx25,000$
K in binaries of 60-90 minutes with WDs. Though not yet in contact, these
binaries would eventually come into contact through gravitational wave emission
and explode as a sub-luminous thermonuclear supernova or evolve into a massive
single WD. | astro-ph_SR |
IAU 2015 Resolution B2 on Recommended Zero Points for the Absolute and
Apparent Bolometric Magnitude Scales: The XXIXth IAU General Assembly in Honolulu adopted IAU 2015 Resolution B2 on
recommended zero points for the absolute and apparent bolometric magnitude
scales. The resolution was proposed by the IAU Inter-Division A-G Working Group
on Nominal Units for Stellar and Planetary Astronomy after consulting with a
broad spectrum of researchers from the astronomical community. Resolution B2
resolves the long-standing absence of an internationally-adopted zero point for
the absolute and apparent bolometric magnitude scales. Resolution B2 defines
the zero point of the absolute bolometric magnitude scale such that a radiation
source with $M_{\rm Bol}$ = 0 has luminosity L$_{\circ}$ = 3.0128e28 W. The
zero point of the apparent bolometric magnitude scale ($m_{\rm Bol}$ = 0)
corresponds to irradiance $f_{\circ}$ = 2.518021002e-8 W/m$^2$. The zero points
were chosen so that the nominal solar luminosity (3.828e26 W) adopted by IAU
2015 Resolution B3 corresponds approximately to $M_{\rm Bol}$(Sun) = 4.74, the
value most commonly adopted in recent literature. The nominal total solar
irradiance (1361 W/m$^2$) adopted in IAU 2015 Resolution B3 corresponds
approximately to apparent bolometric magnitude $m_{\rm bol}$(Sun) = -26.832.
Implicit in the IAU 2015 Resolution B2 definition of the apparent bolometric
magnitude scale is an exact definition for the parsec (648000/$\pi$ au) based
on the IAU 2012 Resolution B2 definition of the astronomical unit. | astro-ph_SR |
Water deuterium fractionation in the high-mass star-forming region
G34.26+0.15 based on Herschel/HIFI data: Understanding water deuterium fractionation is important for constraining the
mechanisms of water formation in interstellar clouds. Observations of HDO and
H$_2^{18}$O transitions were carried out towards the high-mass star-forming
region G34.26+0.15 with the HIFI instrument onboard the Herschel Space
Observatory, as well as with ground-based single-dish telescopes. Ten HDO lines
and three H$_2^{18}$O lines covering a broad range of upper energy levels
(22-204 K) were detected. We used a non-LTE 1D analysis to determine the
HDO/H$_2$O ratio as a function of radius in the envelope. Models with different
water abundance distributions were considered in order to reproduce the
observed line profiles. The HDO/H$_2$O ratio is found to be lower in the hot
core ($\sim$3.5 $\times$ 10$^{-4}$ - 7.5 $\times$ 10$^{-4}$) than in the colder
envelope ($\sim$1.0 $\times$ 10$^{-3}$ - 2.2 $\times$ 10$^{-3}$). This is the
first time that a radial variation of the HDO/H$_2$O ratio has been found to
occur in a high-mass source. The chemical evolution of this source was modeled
as a function of its radius and the observations are relatively well
reproduced. The comparison between the chemical model and the observations
leads to an age of $\sim$10$^5$ years after the infrared dark cloud stage. | astro-ph_SR |
Using stellar rotation to identify tidally stripped members of the
Praesepe open cluster: As an open cluster orbits the Milky Way, gravitational fields distort it,
stripping stars from the core and forming tidal tails. Recent work has
identified tidal tails of the Praesepe cluster; we explore rotation periods as
a way to confirm these candidate members. In open clusters, the rotation period
distribution evolves over time due to magnetic braking. Since tidally stripped
stars originally formed within the cluster, they should follow the same period
distribution as in the cluster core. We analyze 96 candidate members observed
by NASA's Transiting Exoplanet Survey Satellite (TESS) mission. We measure
reliable rotation periods for 32 stars, while 64 light curves are
noise-dominated. The 32 newly identified rotators are consistent with the
period distribution in the core, and with past membership in Praesepe. We
therefore suggest that for nearby open clusters, stellar rotation offers a
quick and inexpensive method for confirming past members dispersed into tidal
tails. | astro-ph_SR |
Forward Modelling of Standing Kink Modes in Coronal Loops I. Synthetic
Views: Kink magnetohydrodynamic (MHD) waves are frequently observed in various
magnetic structures of the solar atmosphere. They may contribute significantly
to coronal heating and could be used as a tool to diagnose the solar plasma. In
this study, we synthesise the \ion{Fe}{9} $\lambda171.073$ emission of a
coronal loop supporting a standing kink MHD mode. The kink MHD wave solution of
a plasma cylinder is mapped into a semi-torus structure to simulate a curved
coronal loop. We decompose the solution into a quasi-rigid kink motion and a
quadrupole term, which dominate the plasma inside and outside the flux tube,
respectively. At the loop edges, the line-of-sight integrates relatively more
ambient plasma, and the background emission becomes significant. The plasma
motion associated with the quadrupole term causes spectral line broadening and
emission suppression. The periodic intensity suppression will modulate the
integrated intensity and the effective loop width, which both exhibit
oscillatory variations at half of the kink period. The quadrupole term can be
directly observed as a pendular motion at front view. | astro-ph_SR |
High Fidelity Imaging of the Inner AU Mic Debris Disk: Evidence of
Differential Wind Sculpting?: We present new high fidelity optical coronagraphic imagery of the inner
$\sim$50 au of AU Mic's edge-on debris disk using the BAR5 occulter of the
Hubble Space Telescope Imaging Spectrograph (HST/STIS) obtained on 26-27 July
2018. This new imagery reveals that "feature A", residing at a projected
stellocentric separation of 14.2 au on SE-side of the disk, exhibits an
apparent "loop-like" morphology at the time of our observations. The loop has a
projected width of 1.5 au and rises 2.3 au above the disk midplane. We also
explored TESS photometric observations of AU Mic that are consistent with
evidence of two starspot complexes in the system. The likely co-alignment of
the stellar and disk rotational axes breaks degeneracies in detailed spot
modeling, indicating that AU Mic's projected magnetic field axis is offset from
its rotational axis. We speculate that small grains in AU Mic's disk could be
sculpted by a time-dependent wind that is influenced by this offset magnetic
field axis, analogous to co-rotating Solar interaction regions that sculpt and
influence the inner and outer regions of our own Heliosphere. Alternatively, if
the observed spot modulation is indicative of a significant mis-alignment of
the stellar and disk rotational axes, we suggest the disk could still be
sculpted by the differential equatorial versus polar wind that it sees with
every stellar rotation. | astro-ph_SR |
Revealing the structure of the outer disks of Be stars: Context. The structure of the inner parts of Be star disks (20 stellar radii)
is well explained by the viscous decretion disk (VDD) model, which is able to
reproduce the observable properties of most of the objects studied so far. The
outer parts, on the ther hand, are not observationally well-explored, as they
are observable only at radio wavelengths. A steepening of the spectral slope
somewhere between infrared and radio wavelengths was reported for several Be
stars that were previously detected in the radio, but a convincing physical
explanation for this trend has not yet been provided. Aims. We test the VDD
model predictions for the extended parts of a sample of six Be disks that have
been observed in the radio to address the question of whether the observed
turndown in the spectral energy distribution (SED) can be explained in the
framework of the VDD model, including recent theoretical development for
truncated Be disks in binary systems. Methods. We combine new multi-wavelength
radio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama
Pathfinder Experiment (APEX) with previously published radio data and archival
SED measurements at ultraviolet, visual, and infrared wavelengths. The density
structure of the disks, including their outer parts, is constrained by
radiative transfer modeling of the observed spectrum using VDD model
predictions. In the VDD model we include the presumed effects of possible tidal
influence from faint binary companions. Results. For 5 out of 6 studied stars,
the observed SED shows strong signs of SED turndown between far-IR and radio
wavelengths. A VDD model that extends to large distances closely reproduces the
observed SEDs up to far IR wavelengths, but fails to reproduce the radio SED.
... (abstract continues but did not fit here) | astro-ph_SR |
Discovery of A Binary System in IRAM 04191+1522: We present high angular resolution observations of the Class 0 protostar
IRAM04191+1522, using the Submillimeter Array (SMA). The SMA 1.3 mm continuum
images reveal within IRAM04191+1522 two distinct sources with an angular
separation of 7.8\,$\pm$\,0.2$"$. The two continuum sources are located in the
southeast-northwest direction, with total gas masses of about 0.011 M_sun and
about 0.005 M_sun, respectively. The southeastern source, associated with an
infrared source seen in the Spitzer images, is the well-known Class 0 protostar
with a bolometric luminosity of about 0.08 L_sun. The newly-discovered
northwestern continuum source is not visible in the Spitzer images at
wavelengths from 3.6 to 70 micron, and has an extremely low bolometric
luminosity (< 0.03 L_sun). Complementary IRAM N2H+(1-0) data that probe the
dense gas in the common envelope suggest that the two sources were formed
through the rotational fragmentation of an elongated dense core. Furthermore,
comparisons between IRAM04191+1522 and other protostars suggest that most cores
with binary systems formed therein have ratios of rotational energy to
gravitational energy $\beta_{\rm rot}$ > 1%. This is consistent with
theoretical simulations and indicates that the level of rotational energy in a
dense core plays an important role in the fragmentation process. | astro-ph_SR |
Doppler Probe of Accretion onto a T Tauri star: The YY Ori stars are T Tauri stars with prominent time-variable redshifted
absorption components that flank certain emission lines. One of the brightest
in this class is S CrA, a visual double star. We have obtained a series of
high-resolution spectra of the two components during four nights with the UVES
spectrograph at the Very Large Telescope. We followed the spectral changes
occurring in S CrA to derive the physical structure of the accreting gas.
We found that both stars are very similar with regard to surface temperature,
radius, and mass. Variable redshifted absorption components are particularly
prominent in the SE component. During one night, this star developed a spectrum
unique among the T Tauri stars: extremely strong and broad redshifted
absorption components appeared in many lines of neutral and ionized metals, in
addition to those of hydrogen and helium. The absorption depths of cooler, low
ionization lines peak at low velocities - while more highly ionized lines have
peak absorption depths at high velocities. The different line profiles indicate
that the temperature and density of the accretion stream increase as material
approaches the star. We derive the physical conditions of the flow at several
points along the accretion funnel directly from the spectrum of the infalling
gas. We estimated mass accretion rates of about 10^(-7) solar masses per year,
which is similar to that derived from the relation based on the strength of H
alpha emission line.
This is the first time the density and temperature distributions in accretion
flows around a T Tauri star have been inferred from observations. Compared with
predictions from standard models of accretion in T Tauri stars, which assume a
dipole stellar magnetic field, we obtained higher densities and a steeper
temperature rise toward the star. | astro-ph_SR |
Stability of thermal modes in cool prominence plasmas: Context: Magnetohydrodynamic thermal modes may play an important role in the
formation, plasma condensation, and evolution of solar prominences. Unstable
thermal modes due to unbalance between radiative losses and heating can lead to
rapid plasma cooling and condensation. An accurate description of the radiative
loss function is therefore crucial for this process. Aims: We study the
stability of thermal modes in unbounded and uniform plasmas with properties
akin to those in solar prominences. Effects due to partial ionization are taken
into account. Three different parametrizations of the radiative loss function
are used. Methods: By means of a normal mode analysis, we investigate linear
nonadiabatic perturbations superimposed on the equilibrium state. We find an
approximate instability criterion for thermal modes, while the exact linear
growth rate is obtained by numerically solving the general dispersion relation.
The stability of thermal disturbances is compared for the three different loss
functions considered. Results: Using up-to-date computations of radiative
losses derived from the CHIANTI atomic database, we find that thermal modes may
be unstable in prominences for lower temperatures than those predicted with
previously existing loss functions. Thermal instability can take place for
temperatures as low as 15,000 K, approximately. The obtained linear growth
rates indicate that this instability might have an important impact on the
dynamics and evolution of cool prominence condensations. | astro-ph_SR |
Extended atmospheres of AGB stars: modeling and measurement: Encoded in the time- and wavelength dependent properties of pulsating AGB
stars are the underlying fundamental parameters of mass, composition and
evolutionary state. However, the standard technique of placing stars on a HR
diagram, even with the aid of pulsation periods, can not be done easily for
extended AGB stars, because of the difficulty of defining a radius or
temperature. The atmospheres of Mira variables are so extended that the optical
depth unity radius can vary by a factor of ~3 over the energetically important
region of the spectrum. Many important constituents in the radiative transfer
are far from local thermodynamic equilibrium, and for the coolest stars, the
process of dust formation and destruction requires a time-dependent model of
grain growth. I will describe the challenges and some of the solutions to
modeling these atmospheres, and describe the utility of different kinds of
observations in helping understand both fundamental parameters and chaotic
processes in complex AGB atmospheres. | astro-ph_SR |
The binary nature of the Galactic Centre X-ray source CXOGC
J174536.1-285638: X-ray and near-IR surveys of the central 2x0.8 degrees of the Galactic Centre
have revealed a population of X-ray bright massive stars. However, the nature
of the emission, originating in a wind collision zone or via accretion onto a
compact object, is uncertain. In order to address this we investigated the
nature of the luminous X-ray source CXOGC J174536.1-285638. An analysis of the
near-IR spectrum with a non-LTE model atmosphere code demonstrated that it was
an highly luminous WN9h star, for which comparison to evolutionary tracks
suggests an age of 2-2.5Myr and an initial mass of ~110Msun. The X-ray
properties of CXOGC J174536.1-285638 resemble those of 3 of the WN9h stars
within the Arches cluster and in turn other very massive WNLh colliding wind
binaries. Simple analytical arguments demonstrate consistency between the X-ray
emission and a putative WN9h+mid O V-III binary. However, we may not exclude a
high mass X-ray binary interpretation, which, if correct, would provide a
unique insight into the (post-SN) evolution of extremely massive stars.
Irrespective of the nature of the secondary, CXOGC J174536.1-285638 adds to the
growing list of known and candidate WNLh binaries. Of the subset of WNLh stars
subject to a radial velocity survey, we find a lower limit to the binary
fraction of ~45%; of interest for studies of massive stellar formation, given
that they currently possess the highest dynamically determined masses of any
type of star. [ABRIDGED] | astro-ph_SR |
New Double Periodic Variable Stars in the ASAS-SN Catalog: We report the discovery of 3 new Double Periodic Variables based on the
analysis of ASAS-SN light curves: GSD J11630570-510306, V593 Sco and TYC
6939-678-1. These systems have orbital periods between 10 and 20 days and long
cycles between 300 and 600 days. | astro-ph_SR |
Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar
Diffusion: II. Comparison with Observation, Clump Properties, and Scaling to
Physical Units: Ambipolar diffusion is important in redistributing magnetic flux and in
damping Alfven waves in molecular clouds. The importance of ambipolar diffusion
on a length scale $\ell$ is governed by the ambipolar diffusion Reynolds
number, $\rad=\ell/\lad$, where $\lad$ is the characteristic length scale for
ambipolar diffusion. The logarithmic mean of the AD Reynolds number in a sample
of 15 molecular clumps with measured magnetic fields (Crutcher 1999) is 17,
comparable to the theoretically expected value. We identify several regimes of
ambipolar diffusion in a turbulent medium, depending on the ratio of the flow
time to collision times between ions and neutrals; the clumps observed by
Crutcher (1999) are all in the standard regime of ambipolar diffusion, in which
the neutrals and ions are coupled over a flow time. We have carried out
two-fluid simulations of ambipolar diffusion in isothermal, turbulent boxes for
a range of values of $\rad$. The mean Mach numbers were fixed at $\calm=3$ and
$\ma=0.67$; self-gravity was not included. We study the properties of
overdensities--i.e., clumps--in the simulation and show that the slope of the
higher-mass portion of the clump mass spectrum increases as $\rad$ decreases,
which is qualitatively consistent with Padoan et al. (2007)'s finding that the
mass spectrum in hydrodynamic turbulence is significantly steeper than in ideal
MHD turbulence. For a value of $\rad$ similar to the observed value, we find a
slope that is consistent with that of the high-mass end of the Initial Mass
Function for stars. However, the value we find for the spectral index in our
ideal MHD simulation differs from theirs, presumably because our simulations
have different initial conditions. This suggests that the mass spectrum of the
clumps in the Padoan et al. (2007) turbulent fragmentation model for the IMF
depends on the environment, which would conflict with evidence ... | astro-ph_SR |
Annihilation of Magnetic Islands at the Top of Solar Flare Loops: The dynamics of magnetic reconnection in the solar current sheet (CS) is
studied by high-resolution 2.5-dimensional MHD simulation. With the commence of
magnetic reconnection, a number of magnetic islands are formed intermittently
and move quickly upward and downward along the CS. When colliding with the
semi-closed flux of flare loops, the downflow islands cause a second
reconnection with a rate even comparable with that in the main CS. Though the
time-integrated magnetic energy release is still dominated by the reconnection
in main CS, the second reconnection can release substantial magnetic energy,
annihilating the main islands and generating secondary islands with various
scales at the flare loop top. The distribution function of the flux of the
second islands is found to follow a power-law varying from
$f\left(\psi\right)\sim\psi^{-1}$ (small scale) to $\psi^{-2}$ (large scale),
which seems to be independent with background plasma $\beta$ and if including
thermal conduction. However, the spatial scale and the strength of the
termination shocks driven by main reconnection outflows or islands decrease if
$\beta$ increases or thermal conduction is included. We suggest that the
annihilation of magnetic islands at the flare loop top, which is not included
in the standard flare model, plays a non-negligible role in releasing magnetic
energy to heat flare plasma and accelerate particles. | astro-ph_SR |
First evidence of non-Gaussian solar flare EUV spectral line profiles
and accelerated non-thermal ion motion: The properties of solar flare plasma can be determined from the observation
of optically thin lines. The emitting ion distribution determines the shape of
the line, with an isothermal Maxwellian ion distribution producing a Gaussian
profile. Non-Gaussian lines may indicate more complex ion distributions. We
investigate the possibility of determining flare-accelerated non-thermal ion or
plasma velocity distributions. We study EUV spectral lines during a flare
SOL2013-05-15T01:45 using the Hinode EUV Imaging Spectrometer (EIS). The flare
is located close to the eastern solar limb with an extended loop structure,
allowing the different flare features: ribbons, hard X-ray (HXR) footpoints and
the loop-top source to be clearly observed in UV, EUV and X-rays. EUV line
spectroscopy is performed in seven different regions. We study the line
profiles of isolated and unblended Fe XVI lines (262.976 A) formed at
temperatures of 2-4 MK. Fe XVI line profiles at one time close to the peak soft
X-ray emission and free of directed mass motions are examined using: 1. a
higher moments analysis, 2. Gaussian fitting, and 3. by fitting a kappa
distribution profile convolved with a Gaussian to account for the EIS
instrumental profile. Fe XVI line profiles in the loop-top, HXR footpoint and
ribbon regions can be confidently fitted with a kappa line profile, giving low,
non-thermal kappa values between 2-3.3. A higher moments analysis also finds
that many of the line kurtosis values are higher than the Gaussian value of 3,
even with the presence of a broad Gaussian instrumental profile. A
flare-accelerated non-thermal ion population could account for both the
observed non-Gaussian line profiles, and for the Fe XVI excess broadening found
from Gaussian fitting, if the emitting ions are interacting with a thermalised
4 MK electron population, and the instrumental profile is well-approximated by
a Gaussian. | astro-ph_SR |
Three-dimensional kinetic-MHD model of the global heliosphere with the
heliopause-surface fitting: This paper provides a detailed description of the latest version of our model
of the solar wind (SW) interaction with the local interstellar medium (LISM).
This model has already been applied to the analysis of Lyman-alpha absorption
spectra toward nearby stars and for analyses of Solar and Heliospheric
Observatory/SWAN data. Katushkina et al. (this issue) used the model results to
analyze IBEX-Lo data. At the same time, the details of this model have not yet
been published. This is a three-dimensional (3D) kinetic-magnetohydrodynamical
(MHD) model that takes into account SW and interstellar plasmas (including
$\alpha$ particles in SW and helium ions in LISM), the solar and interstellar
magnetic fields, and the interstellar hydrogen atoms. The latitudinal
dependence of SW and the actual flow direction of the interstellar gas with
respect to the Sun are also taken into account in the model. It was very
essential that our numerical code had been developed in such a way that any
numerical diffusion or reconnection across the heliopause had not been allowed
in the model. The heliospheric current sheet is a rotational discontinuity in
the ideal MHD and can be treated kinematically. In the paper, we focus in
particular on the effects of the heliospheric magnetic field and on the
heliolatitudinal dependence of SW. | astro-ph_SR |
Correlation between the solar magnetic field strength and the millimeter
brightness temperature: Images of the Sun at millimeter wavelengths obtained by ALMA show a
significant correspondence with the magnetograms. In this paper, we investigate
this correspondence by comparing ALMA full-disk solar image taken at 1.2 mm
with a SDO/HMI magnetogram and analyze their correlation. It is found that
chromospheric network and active regions show a positive correlation where
brightness temperature is increasing with the line-of-sight magnetic field
strength, while sunspots have a negative correlation. Quiet Sun regions do not
show any dependence of the brightness temperature with the magnetic field.
Thermal bremsstrahlung is given as the best explanation for the observed
correlations. | astro-ph_SR |
Exploring wind-driving dust species in cool luminous giants III. Wind
models for M-type AGB stars: dynamic and photometric properties: Stellar winds observed in asymptotic giant branch (AGB) stars are usually
attributed to a combination of stellar pulsations and radiation pressure on
dust. Shock waves triggered by pulsations propagate through the atmosphere,
compressing the gas and lifting it to cooler regions, which create favourable
conditions for grain growth. If sufficient radiative acceleration is exerted on
the newly formed grains through absorption or scattering of stellar photons, an
outflow can be triggered. Strong candidates for wind-driving dust species in
M-type AGB stars are magnesium silicates (Mg$_2$SiO$_4$ and MgSiO$_3$). Such
grains can form close to the stellar surface, they consist of abundant
materials and, if they grow to sizes comparable to the wavelength of the
stellar flux maximum, they experience strong acceleration by photon scattering.
We use a frequency-dependent radiation-hydrodynamics code with a detailed
description for the growth of Mg$_2$SiO$_4$ grains to calculate the first
extensive set of time-dependent wind models for M-type AGB stars. The resulting
wind properties, visual and near-IR photometry and mid-IR spectra are compared
with observations.We show that the models can produce outflows for a wide range
of stellar parameters. We also demonstrate that they reproduce observed
mass-loss rates and wind velocities, as well as visual and near-IR photometry.
However, the current models do not show the characteristic silicate features at
10 and 18 $\mu$m as a result of the cool temperature of Mg$_2$SiO$_4$ grains in
the wind. Including a small amount of Fe in the grains further out in the
circumstellar envelope will increase the grain temperature and result in
pronounced silicate features, without significantly affecting the photometry in
the visual and near-IR wavelength regions. | astro-ph_SR |
Mass and Angular Momentum Transport in a Gravitationally Unstable
Protoplanetary Disk with Improved 3D Radiative Hydrodynamics: During early phases of a protoplanetary disks's life, gravitational
instabilities can produce significant mass transport, can dramatically alter
disk structure, can mix and shock-process gas and solids, and may be
instrumental in planet formation. We present a 3D grid-based radiative
hydrodynamics study with varied resolutions of a 0.07 M$_\odot$ disk orbiting a
0.5 M$_\odot$ star as it settles over most of its radial extent into a
quasi-steady asymptotic state that maintains approximate balance between
heating produced by GIs and radiative cooling governed by realistic dust
opacities. We assess disk stability criteria, thermodynamic properties,
strengths of GIs, characteristics of density waves and torques produced by GIs,
radial mass transport arising from these torques, and the level to which
transport can be represented as local or nonlocal processes. Physical and
thermal processes display distinct differences between inner optically thick
and outer optically thin regions of the disk. In the inner region,
gravitational torques are dominated by low-order Fourier components of the
azimuthal mass distribution. These torques are strongly variable on the local
dynamical time and are subject to rapid flaring presumably driven by recurrent
swing amplification. In the outer region, m=1 torques dominate. Ring-like
structures exhibiting strong noncircular motions, and vortices develop near the
inner edge between 8 and 14 au. We find that GI-induced spiral modes erupt in a
chaotic manner over the whole low-Q part of the disk, with many spiral modes
appearing and disappearing, producing gravitoturbulence, but dominated by
fluctuating large-scale modes, very different from a simple $\alpha$-disk. | astro-ph_SR |
Origin of Quasi-Periodic Pulsation at the Base of Kink Unstable Jet: We study a blowout jet that occurs at the west limb of the Sun on August
29$^{th}$, 2014 using high-resolution imaging/spectroscopic observations
provided by SDO/AIA and IRIS. An inverse $\gamma$-shape flux-rope appears
before the jet{--} morphological indication of the onset of kink instability.
The twisted field lines of kink-unstable flux-rope reconnect at its bright knot
and launch the blowout jet at $\approx$06:30:43 UT with an average speed of 234
km s$^{-1}$. Just after the launch, the northern leg of the flux rope erupts
completely. The time-distance diagrams show multiple spikes or bright dots,
which is the result of periodic fluctuations, i.e., quasi-periodic fluctuations
(QPPs). The wavelet analysis confirms that QPPs have a dominant period of
$\approx$ 03 minutes. IRIS spectra (Si~{\sc iv}, C~{\sc ii}, and Mg~{\sc ii})
may also indicate the occurrence of magnetic reconnection through existence of
broad $\&$ complex profiles and bi-directional flows in the jet. Further, we
have found that line broadening is periodic with a period of $\approx$ 03
minutes, and plasma upflow is always occurs when the line width is high, i.e.,
multiple reconnection may produce periodic line broadening. The EM curves also
show the same period of $\approx$ 03 minutes in different temperature bins. The
images and EM show that this jets spire is mainly cool
(chromospheric/transition region) rather than hot (coronal) material. Further,
line broadening, intensity, and EM curves have a period of $\approx$03 minutes,
which strongly supports that multiple magnetic reconnection triggers QPPs in
the blowout jet. | astro-ph_SR |
Evolution and Consequences of Interacting CMEs of 2012 November 9-10
using STEREO/SECCHI and In Situ Observations: Understanding of the kinematic evolution of Coronal Mass Ejections (CMEs) in
the heliosphere is important to estimate their arrival time at the Earth. It is
found that kinematics of CMEs can change when they interact or collide with
each other as they propagate in the heliosphere. In this paper, we analyze the
collision and post-interaction characteristics of two Earth-directed CMEs,
launched successively on 2012 November 9 and 10, using white light imaging
observations from STEREO/SECCHI and in situ observations taken from WIND
spacecraft. We tracked two density enhanced features associated with leading
and trailing edge of November 9 CME and one density enhanced feature associated
with leading edge of November 10 CME by constructing J-maps. We found that the
leading edge of November 10 CME interacted with the trailing edge of November 9
CME. We also estimated the kinematics of these features of the CMEs and found a
significant change in their dynamics after interaction. In in situ
observations, we identified distinct structures associated with interacted CMEs
and also noticed their heating and compression as signatures of CME-CME
interaction. Our analysis shows an improvement in arrival time prediction of
CMEs using their post-collision dynamics than using pre-collision dynamics.
Estimating the true masses and speeds of these colliding CMEs, we investigated
the nature of observed collision which is found to be close to perfectly
inelastic. The investigation also places in perspective the geomagnetic
consequences of the two CMEs and their interaction in terms of occurrence of
geomagnetic storm and triggering of magnetospheric substorms. | astro-ph_SR |
The Mass Budget of Planet Forming Discs: Isolating the Epoch of
Planetesimal Formation: The high rate of planet detection among solar-type stars argues that planet
formation is common. It is also generally assumed that planets form in
protoplanetary discs like those observed in nearby star forming regions. On
what timescale does the transformation from discs to planets occur? Here we
show that current inventories of planets and protoplanetary discs are sensitive
enough to place basic constraints on the timescale and efficiency of the planet
formation process. A comparison of planet detection statistics and the measured
solid reservoirs in T Tauri discs suggests that planet formation is likely
already underway at the few Myr age of the discs in Taurus-Auriga, with a large
fraction of solids having been converted into large objects with low millimeter
opacity and/or sequestered at small disc radii where they are difficult to
detect at millimeter wavelengths. | astro-ph_SR |
Horizontal flow fields observed in Hinode G-band images III. The decay
of a satellite sunspot and the role of magnetic flux removal in flaring: The flare-prolific active region NOAA 10930 offered both a developing
delta-spot and a decaying satellite sunspot of opposite polarity. The objective
of this study is to characterize the photometric decay of the satellite sunspot
and the evolution of photospheric and chromospheric horizontal proper motions
in its surroundings. We apply the local correlation tracking technique to a
16-hour time-series of Hinode G-band and CaIIH images and study the horizontal
proper motions in the vicinity of the satellite sunspot on 2006 December 7.
Decorrelation times were computed to measure the lifetime of solar features in
intensity and flow maps. We observed shear flows in the dominant umbral cores
of the satellite sunspot. These flows vanished once the penumbra had
disappeared. This slow penumbral decay had an average rate of 152Mm2/day over
an 11-hour period. Typical lifetimes of intensity features derived from an
autocorrelation analysis are 3-5min for granulation, 25-35min for G-band bright
points, and up to 200-235min for penumbrae, umbrae, and pores. Long-lived
intensity features (i.e., the dominant umbral cores) are not related to
long-lived flow features in the northern part of the sunspot, where flux
removal, slowly decaying penumbrae, and persistent horizontal flows of up to 1
km/s contribute to the erosion of the sunspot. Finally, the restructuring of
magnetic field topology was responsible for a homologous M2.0 flare, which
shared many characteristics with an X6.5 flare on the previous day.
Notwithstanding the prominent role of delta-spots in flaring, we conclude based
on the decomposition of the satellite sunspot, the evolution of the surrounding
flow fields, and the timing of the M2.0 flare that the vanishing magnetic flux
in the decaying satellite sunspot played an instrumental role in triggering the
homologous M2.0 flare and the eruption of a small Halpha filament. | astro-ph_SR |
Asteroseismic sensitivity to internal rotation along the red-giant
branch: Transport of angular momentum in stellar interiors is currently not well
understood. Asteroseismology can provide us with estimates of internal rotation
of stars and thereby advances our understanding of angular momentum transport.
We can measure core-rotation rates in red-giant stars and we can place upper
bounds on surface-rotation rates using measurements of dipole ($l=1$) modes.
Here, we aim to determine the theoretical sensitivity of modes of different
spherical degree towards the surface rotation. Additionally, we aim to identify
modes that can potentially add sensitivity at intermediate radii. We used
asteroseismic rotational inversions to probe the internal stellar rotation
profiles in red-giant models from the base of the red-giant branch up to the
luminosity bump. We used the inversion method of multiplicative optimally
localised averages (MOLA) to assess how well internal and surface rotation
rates can be recovered from different mode sets and different synthetic
rotation profiles. We confirm that dipole mixed modes are sufficient to set
constraints on the average core-rotation rates in red giants. However,
surface-rotation rates estimated with only dipole mixed modes are contaminated
by the core rotation. We show that the sensitivity to the surface rotation
decreases from the base of the red-giant branch until it reaches a minimum at
0.6-0.8$L_\text{bump}$ due to a glitch in the buoyancy frequency. Thereafter a
narrow range of increased surface sensitivity just below the bump luminosity
exists. Quadrupole and octopole modes have more sensitivity in the outer parts
of the star. If observed, quadrupole and octopole modes enable us to
distinguish between differential and solid body rotation in the convection
zone. To obtain accurate estimates of rotation rates at intermediate radii,
acoustic oscillation modes with a spherical degree of $l\approx10$ are needed. | astro-ph_SR |
New Horizons Solar Wind Around Pluto (SWAP) Observations of the Solar
Wind From 11-33 AU: The Solar Wind Around Pluto (SWAP) instrument on NASA's New Horizon Pluto
mission has collected solar wind observations en route from Earth to Pluto, and
these observations continue beyond Pluto. Few missions have explored the solar
wind in the outer heliosphere making this dataset a critical addition to the
field. We created a forward model of SWAP count rates, which includes a
comprehensive instrument response function based on laboratory and flight
calibrations. By fitting the count rates with this model, the proton density
(n), speed (V), and temperature (T) parameters are determined. Comparisons
between SWAP parameters and both propagated 1 AU observations and prior Voyager
2 observations indicate consistency in both the range and mean wind values.
These comparisons as well as our additional findings confirm that small and
midsized solar wind structures are worn down with increasing distance due to
dynamic interaction of parcels of wind with different speed. For instance, the
T-V relationship steepens, as the range in V is limited more than the range in
T with distance. At times the T-V correlation clearly breaks down beyond 20 AU,
which may indicate wind currently expanding and cooling may have an elevated T
reflecting prior heating and compression in the inner heliosphere. The power of
wind parameters at shorter periodicities decreases with distance as the longer
periodicities strengthen. The solar rotation periodicity is present in
temperature beyond 20 AU indicating the observed parcel temperature may reflect
not only current heating or cooling, but also heating occurring closer to the
Sun. | astro-ph_SR |
Ionization: a possible explanation for the difference of mean disk sizes
in star-forming regions: Surveys of protoplanetary disks in star-forming regions of similar age
revealed significant variations in average disk mass between some regions. For
instance, disks in the Orion Nebular Cluster (ONC) and Corona Australis (CrA)
are on average smaller than disks observed in Lupus, Taurus, Chamaeleon I or
Ophiuchus. In contrast to previous models that study truncation of disks at a
late stage of their evolution, we investigate whether disks may already be born
with systematically smaller disk sizes in more massive star-forming regions as
a consequence of enhanced ionization rates. Assuming various cosmic-ray
ionization rates, we compute the resistivities for ambipolar diffusion and
Ohmic dissipation with a chemical network, and perform 2D non-ideal
magnetohydrodynamical protostellar collapse simulations. A higher ionization
rate leads to stronger magnetic braking, and hence to the formation of smaller
disks. Accounting for recent findings that protostars act as forges of cosmic
rays and considering only mild attenuation during the collapse phase, we show
that a high average cosmic-ray ionization rate in star-forming regions like the
ONC or CrA can explain the detection of smaller disks in these regions. Our
results show that on average a higher ionization rate leads to the formation of
smaller disks. Therefore, smaller disks in regions of similar age can be the
consequence of different levels of ionization, and may not exclusively be
caused by disk truncation via external photoevaporation. We strongly encourage
observations that allow measuring the cosmic-ray ionization degrees in
different star-forming regions to test our hypothesis. | astro-ph_SR |
Stellar disk destruction by dynamical interactions in the Orion
Trapezium star cluster: We compare the observed size distribution of circum stellar disks in the
Orion Trapezium cluster with the results of $N$-body simulations in which we
incorporated an heuristic prescription for the evolution of these disks. In our
simulations, the sizes of stellar disks are affected by close encounters with
other stars (with disks). We find that the observed distribution of disk sizes
in the Orion Trapezium cluster is excellently reproduced by truncation due to
dynamical encounters alone. The observed distribution appears to be a sensitive
measure of the past dynamical history of the cluster, and therewith on the
conditions of the cluster at birth. The best comparison between the observed
disk size distribution and the simulated distribution is realized with a
cluster of $N = 2500\pm500$ stars with a half-mass radius of about 0.5\,pc in
virial equilibrium (with a virial ratio of $Q = 0.5$, or somewhat colder $Q
\simeq 0.3$), and with a density structure according to a fractal dimension of
$F \simeq 1.6$. Simulations with these parameters reproduce the observed
distribution of circum stellar disks in about 0.2--0.5\,Myr. | astro-ph_SR |
Direct diagnostics of forming massive stars: stellar pulsation and
periodic variability of maser sources: The 6.7 GHz methanol maser emission, a tracer of forming massive stars,
sometimes shows enigmatic periodic flux variations over several 10-100 days. In
this Letter, we propose that this periodic variations could be explained by the
pulsation of massive protostars growing under rapid mass accretion with rates
of Mdot > 10^-3 Msun/yr. Our stellar evolution calculations predict that the
massive protostars have very large radius exceeding 100 Rsun at maximum, and we
here study the pulsational stability of such the bloated protostars by way of
the linear stability analysis. We show that the protostar becomes pulsationally
unstable with various periods of several 10-100 days, depending on different
accretion rates. With the fact that the stellar luminosity when the star is
pulsationally unstable also depends on the accretion rate, we derive the
period-luminosity relation log (L/Lsun) = 4.62 + 0.98log(P/100 day), which is
testable with future observations. Our models further show that the radius and
mass of the pulsating massive protostar should also depend on the period. It
would be possible to infer such protostellar properties and the accretion rate
with the observed period. Measuring the maser periods enables a direct
diagnosis of the structure of accreting massive protostars, which are deeply
embedded in dense gas and inaccessible with other observations. | astro-ph_SR |
Modelling depletion by re-accretion of gas from a dusty disc in post-AGB
stars: Many disc-type post-asymptotic giant branch (post-AGB) stars are chemically
peculiar, showing underabundances of refractory elements in their photospheres
that correlate with condensation temperature. The aim of this paper is to
investigate how accretion from a circumbinary disc can cause this phenomenon of
depletion and how this impacts the evolution of post-AGB stars. We used the
\texttt{MESA} code to evolve stars in the post-AGB phase, while including
accretion of metal-poor gas. We compared the models to a sample of 58 observed
disc-type post-AGB stars with chemical abundance data. For each of these stars,
we estimated the luminosity and the mass using the Gaia distance. We modelled
the accretion rate onto the binary from a viscously evolving disc for a range
of initial accretion rates and disc masses. We find that large initial
accretion rates ($\gtrsim 3\times10^{-7}$ $M_\odot$/yr) and large initial disc
masses ($\sim10^{-2}$ $M_\odot$) are needed to reproduce the observed depleted
post-AGB stars. Based on these high accretion rates, the evolution timescale of
post-AGB stars can be significantly extended by a factor between two and five.
We distinguish depletion patterns that are unsaturated (plateau profile) from
those that are saturated, and we expect that post-red giant branch (post-RGB)
stars are much more likely to show an unsaturated abundance pattern compared to
post-AGB stars. Finally, because of the slower evolution of the low-mass
post-RGB stars, we find that these systems can become depleted at lower
effective temperatures ($< 5000$ K). We conclude that accretion from a
circumbinary disc successfully accounts for the chemical peculiarity of
post-AGB stars. | astro-ph_SR |
A solar blowout jet caused by the eruption of a magnetic flux rope: We investigate the three-dimensional (3D) magnetic structure of a blowout jet
originated in the west edge of NOAA Active Region (AR) 11513 on 02 July 2012 by
means of recently developed forced field extrapolation (FFE) model. The results
show that the blowout jet was caused by the eruption of the magnetic flux rope
(MFR) consisting of twisted field lines. We further calculate the twist number
$\mathcal{T}_{w}$ and squashing factor Q of the reconstructed magnetic field
and find that (1) the MFR corresponds well to the high $\mathcal{T}_{w}$ region
(2) the MFR outer boundary corresponds well to the high Q region, probably
interpreting the bright structure at the base of the jet. The twist number of
the MFR is estimated to be $\mathcal{T}_{w}=-1.54\pm 0.67$. Thus, the kink
instability is regarded as the initiation mechanism of the blowout jet as
$\mathcal{T}_{w}$ reaching or even exceeding the threshold value of the kink
instability. Our results also indicate that the bright point at the decaying
phase is actually comprised of some small loops that are heated by the
reconnection occurred above. In summary, the blowout jet is mostly consistent
with the scenario proposed by \citet{mcs10} except that the kink instability is
found to be a possible trigger. | astro-ph_SR |
Investigating the IBEX Ribbon Structure a Solar Cycle Apart: A Ribbon of enhanced energetic neutral atom (ENA) emissions was discovered by
the Interstellar Boundary Explorer (IBEX) in 2009, redefining our understanding
of the heliosphere boundaries and the physical processes occurring at the
interstellar interface. The Ribbon signal is intertwined with that of a
globally distributed flux (GDF) that spans the entire sky. To a certain extent,
Ribbon separation methods enabled examining its evolution independent of the
underlying GDF. Observations over a full solar cycle revealed the Ribbon's
evolving nature, with intensity variations closely tracking those of the solar
wind (SW) structure after a few years delay accounting for the SW-ENA recycling
process. In this work, we examine the Ribbon structure, namely, its ENA fluxes,
angular extent, width, and circularity properties for two years, 2009 and 2019,
representative of the declining phases of two adjacent solar cycles. We find
that, (i) the Ribbon ENA fluxes have recovered in the nose direction and south
of it down to ~ 25{\deg} (for energies below 1.7 keV) and not at mid and high
ecliptic latitudes; (ii) The Ribbon width exhibits significant variability as a
function of azimuthal angle; (iii) Circularity analysis suggests that the 2019
Ribbon exhibits a statistically consistent radius with that in 2009. The
Ribbon's partial recovery is aligned with the consensus of a heliosphere with
its closest point being southward of the nose region. The large variability of
the Ribbon width as a function of Azimuth in 2019 compared to 2009 is likely
indicative of small-scale processes within the Ribbon. | astro-ph_SR |
Photometric search for variable stars in young open cluster Berkeley 59: We present time-series photometry of stars located in the extremely young
open cluster Berkeley 59. Using the 1.04 m telescope at ARIES, Nainital, we
have identified 42 variables in a field of 13'x13' around the cluster. The
probable members of the cluster are identified using (V, V-I) colour-magnitude
diagram and (J-H, H-K) colour-colour diagram. Thirty one variables are found to
be pre-main sequence stars associated with the cluster. The ages and masses of
pre-main sequence stars are derived from colour-magnitude diagram by fitting
theoretical models to the observed data points. The ages of the majority of the
probable pre-main sequence variable candidates range from 1 to 5 Myrs. The
masses of these pre-main sequence variable stars are found to be in the range
of ~0.3 to ~3.5 Msun and these could be T Tauri stars. The present statistics
reveal that about 90% T Tauri stars have periods < 15 days. The classical T
Tauri stars are found to have larger amplitude in comparison to the weak line T
Tauri stars. There is an indication that the amplitude decreases with increase
of the mass, which could be due to the dispersal of disk of relatively massive
stars. | astro-ph_SR |
An estimate of the magnetic field strength associated with a solar
coronal mass ejection from low frequency radio observations: We report ground based, low frequency heliograph (80 MHz), spectral (85-35
MHz) and polarimeter (80 and 40 MHz) observations of drifting, non-thermal
radio continuum associated with the `halo' coronal mass ejection (CME) that
occurred in the solar atmosphere on 2013 March 15. The magnetic field strengths
($B$) near the radio source were estimated to be $B \approx 2.2 \pm 0.4$ G at
80 MHz and $B \approx 1.4 \pm 0.2$ G at 40 MHz. The corresponding radial
distances ($r$) are $r \approx 1.9~R_{\odot}$ (80 MHz) and $r \approx
2.2~R_{\odot}$ (40 MHz). | astro-ph_SR |
On the incidence of episodic accretion in Class I YSOs from VVV: Episodic accretion is one of the competing models to explain the observed
luminosity spread in young stellar clusters. These short-lived high accretion
events could also have a strong impact on planet formation. Observations of
high-amplitude variability in young stellar objects (YSOs) due to large changes
in the accretion rate provide direct observational evidence for episodic
accretion. However, there are still uncertainties in the frequency of these
events and if episodic accretion is universal among YSOs. To determine the
frequency of outbursts in Class I YSOs, we built a large and robust sample of
objects at this evolutionary stage, and searched for high-amplitude
near-infrared ($\Delta K_{\rm S}>2$~mag) variability in the VIRAC2 database of
the Vista Variables in the Via Lactea (VVV) survey. By complementing with
near-IR (2MASS and DENIS) and mid-IR (WISE/Neo-WISE) data, we find that from
$\sim$ 7000 Class I YSOs, 97 objects can be classified as eruptive variable
YSOs. The duration of the outbursts vary from a few months to longer than 9
years, and cover a similar range of amplitudes. Values of $\Delta K_{\rm
S}>5$~mag, however, are only observed in outbursts with duration longer than 9
years. When considering different effects of completeness and contamination we
estimate that the incidence of episodic accretion in Class I YSOs is between
2\% and 3\%. Finally, we determine a recurrence timescale of long-term
outbursts (a.k.a FUors) of $\tau=1.75^{+1.12}_{-0.87}$~kyr. The latter value
agrees with previous estimates and is in line with the expectations of higher
frequency of FUor outbursts during younger stages of evolution. | astro-ph_SR |
Amplitude of solar gravity modes generated by penetrative plumes: The detection of gravity modes is expected to give us unprecedented insights
into the inner dynamics of the Sun. Within this framework, predicting their
amplitudes is essential to guide future observational strategies and seismic
studies. In this work, we predict the amplitude of low-frequency asymptotic
gravity modes generated by penetrative convection at the top of the radiative
zone. The result is found to depend critically on the time evolution of the
plumes inside the generation region. Using a solar model, we compute the GOLF
apparent surface radial velocity of low-degree gravity modes in the frequency
range $10~\mu H_z\le \nu \le 100~\mu H_z$. In case of a Gaussian plume time
evolution, gravity modes turn out to be undetectable because of too small
surface amplitudes. This holds true despite a wide range of values considered
for the parameters of the model. In the other limiting case of an exponential
time evolution, plumes are expected to drive gravity modes in a much more
efficient way because of a much higher temporal coupling between the plumes and
the modes than in the Gaussian case. Using reasonable values for the plume
parameters based on semi-analytical models, the apparent surface velocities in
this case turn out to be one order of magnitude smaller than the 22-years GOLF
detection threshold and than the previous estimates considering turbulent
pressure as the driving mechanism, with a maximum value of $0.05$ cm s${}^{-1}$
for $\ell =1$ and $\nu\approx 100~\mu H_z$. When accounting for uncertainties
on the plume parameters, the apparent surface velocities in the most favorable
plausible case become comparable to those predicted with turbulent pressure,
and the GOLF observation time required for a detection at $ \nu \approx100~\mu
H_z$ and $\ell=1$ is reduced to about 50 yrs. | astro-ph_SR |
Low-Resolution Spectroscopy of the Recurrent Nova T Pyxidis at its Early
Stage of 2011 Outburst: We present our observational results of the recurrent nova T Pyxidis at its
early stage of 2011 outburst, using a low-resolution spectrograph
($R\approx400$) attached to a 28cm telescope. Total nights of our observation
are 11, among which 9 nights are during the pre-maximum stage. As a result we
have obtained a detailed evolutional feature of this recurrent nova on the way
to its maximum light. At first, on the earliest three nights ($-25 \sim -21$
days before maximum), broad and prominent emission lines such as Balmer series,
He I, He II, N II, N III and O I together with P Cygni profile are seen on the
spectra. The blueshifted absorption minima of H$\alpha$ yields a maximum
expansion velocity of approximately 2200 km s$^{-1}$, and the velocity
gradually decreases. Then, Helium and Nitrogen lines are weakened day by day.
After that (18 days before maximum light), Fe II (multiplets) lines emerge on
the spectra. These lines are then strengthened day by day, and the P Cygni
profiles also become more prominent. Accordingly, the expansion velocities
turns to be gradual increase. In addition, during the pre-maximum stage, nova
spectral type of T Pyx is thought to evolve from He/N type to Fe II one. | astro-ph_SR |
The Solar Minimum Eclipse of 2019 July 2. III. Inferring the Coronal
$T_e$ with a Radiative Differential Emission Measure Inversion: Differential Emission Measure (DEM) inversion methods use the brightness of a
set of emission lines to infer the line-of-sight (LOS) distribution of the
electron temperature ($T_e$) in the corona. DEM inversions have been
traditionally performed with collisionally excited lines at wavelengths in the
Extreme Ultraviolet (EUV) and X-ray. However, such emission is difficult to
observe beyond the inner corona (1.5 R$_\odot$), particularly in coronal holes.
Given the importance of the $T_e$ distribution in the corona for exploring the
viability of different heating processes, we introduce an analog of the DEM
specifically for radiatively excited coronal emission lines, such as those
observed during total solar eclipses (TSEs) and with coronagraphs. This
Radiative DEM (R-DEM) inversion utilizes visible and infrared emission lines
which are excited by photospheric radiation out to at least 3 R$_\odot$.
Specifically, we use the Fe X (637 nm), Fe XI (789 nm), and Fe XIV (530 nm)
coronal emission lines observed during the 2019 July 2 TSE near solar minimum.
We find that despite a large $T_e$ spread in the inner corona, the distribution
converges to an almost isothermal yet bimodal distribution beyond 1.4
R$_\odot$, with $T_e$ ranging from 1.1 to 1.4 in coronal holes, and from 1.4 to
1.65 MK in quiescent streamers. Application of the R-DEM inversion to the
Predictive Science Inc. magnetohydrodynamic (MHD) simulation for the 2019
eclipse validates the R-DEM method and yields a similar LOS Te distribution to
the eclipse data. | astro-ph_SR |
Gas phase SiO in the circumstellar environment of the recurrent nova T
Coronae Borealis: We report the discovery of the diatomic molecule SiO in the gas phase in the
environment of the recurrent nova T Coronae Borealis. While some of the SiO is
photospheric, a substantial portion must arise in the wind from the red giant
component of T CrB. A simple fit to the SiO feature, assuming local
thermodynamic equilibrium, suggests a SiO column density of 2.8x10^17 /cm2 and
temperature ~1000K; the SiO column density is similar to that present in the
winds of field red giants. A search for SiO maser emission is encouraged both
before and after the next anticipated eruption. We find that the 12C/13C ratio
in the red giant is <9, with a best fit value of ~5, a factor ~18 times lower
than the solar value of 89. We find no convincing evidence for the presence of
dust in the environment of T CrB, which we attribute to the destructive effects
on nucleation sites of hard X-ray emission. When the next eruption of T CrB
occurs, the ejected material will shock the wind, producing X-ray and coronal
line emission, as is the case for the recurrent nova RS Oph. T CrB is also a
good candidate for very high energy gamma-ray emission, as first observed
during the 2010 outburst of V407 Cyg. We include in the paper a wide variety of
infrared spectroscopic and photometric data. | astro-ph_SR |
Cycling Dynamo in a Young Sun: Grand Minima and Equatorward Propagation: We assess the global-scale dynamo action achieved in a simulation of a
sun-like star rotating at three times the solar rate. The 3-D MHD Anelastic
Spherical Harmonic code, using slope-limited diffusion, is employed to capture
convection and dynamo processes in such a young sun. The simulation is carried
out in a spherical shell that encompasses 3.8 density scale heights of the
solar convection zone. We find that dynamo action with a high degree of time
variation occurs, with many periodic polarity reversals every 6.2 years. The
magnetic energy also rises and falls with a regular period, with two magnetic
energy cycles required to complete a polarity cycle. These magnetic energy
cycles arise from a Lorentz-force feedback on the differential rotation,
whereas the polarity reversals are present due to the spatial separation of the
equatorial and polar dynamos. Moreover, an equatorial migration of toroidal
field is found, which is linked to the changing differential rotation and to a
near-surface shear layer. This simulation also enters a grand minimum lasting
roughly 20 years, after which the dynamo recovers its regular polarity cycles. | astro-ph_SR |
HIFISTARS Herschel/HIFI observations of VY Canis Majoris. Molecular-line
inventory of the envelope around the largest known star: To gain insight into the physical conditions and kinematics of the warm
(100-1000 K) gas around the red hyper-giant VY CMa, we performed sensitive high
spectral resolution observations of molecular lines in the sub-mm/FIR using the
HIFI instrument of the Herschel Space Observatory. We observed CO, H2O, and
other molecular species, sampling excitation energies from a few tens to a few
thousand K. These observations are part of the Herschel Guaranteed Time Key
Program HIFISTARS. We detected the J=6-5, J=10-9, and J=16-15 lines of 12CO and
13CO at about 100, 300, and 750K above the ground state (and the 13CO J=9-8
line). These lines are crucial for improving the modelling of the internal
layers of the envelope around VY CMa. We also detected 27 lines of H2O and its
isotopomers, and 96 lines of species such as NH3, SiO, SO, SO2 HCN, OH and
others, some of them originating from vibrationally excited levels. Three lines
were not unambiguously assigned. Our observations confirm that VY CMa's
envelope must consist of two or more detached components. The molecular
excitation in the outer layers is significantly lower than in the inner ones,
resulting in strong self-absorbed profiles in molecular lines that are
optically thick in this outer envelope, for instance, low-lying lines of H2O.
Except for the most abundant species, CO and H2O, most of the molecular
emission detected at these sub-mm/FIR wavelengths arise from the central parts
of the envelope. The spectrum of VY CMa is very prominent in vibrationally
excited lines, which are caused by the strong IR pumping present in the central
regions. Compared with envelopes of other massive evolved stars, VY CMa's
emission is particularly strong in these vibrationally excited lines, as well
as in the emission from less abundant species such as H13CN, SO, and NH3. | astro-ph_SR |
Photometric light curve solutions of three ultra-short period eclipsing
binaries: We present the results of our study of the eclipsing binary systems CSS
J112237.1+395219, LINEAR 1286561 and LINEAR 2602707 based on new CCD $B$, $V$,
$R_c$ and $I_c$ complete light curves. The ultra-short period nature of the
stars \citep{Drake2014} is confirmed and the system's periods are revised. The
light curves were modelled using the 2005 version of the Wilson-Devinney code.
When necessary, cool spots on the surface of the primary component were
introduced to account for asymmetries in the light curves. As a result, we
found that CSS J112237.1+395219 is a W UMa type contact binary system belonging
to W subclass with a mass ratio of $q=1.61$ and a shallow degree of contact of
14.8\% where the primary component is hotter than the secondary one by $500K$.
LINEAR 1286561 and LINEAR 2602707 are detached binary systems with mass ratios
$q=3.467$ and $q=0.987$ respectively. These detached systems are low-mass
M-type eclipsing binaries of similar temperatures. The marginal contact, the
fill-out factor and the temperature difference between components of CSS
J112237.1+395219 suggest that this system may be at a key evolutionary state
predicted by the Thermal Relaxation Oscillation theory (TRO). From the
estimated absolute parameters we conclude that our systems share common
properties with others ultra-short period binaries. | astro-ph_SR |
Discovery of deep eclipses in the cataclysmic variable ZTF17aaaeefu
(2MASS J00594349+6454419): I performed photometric observations of the cataclysmic variable candidate
ZTF17aaaeefu and discovered very deep eclipses. The observations were obtained
over 8 nights covering 7 months. The eclipse profile is similar to the eclipse
profiles observed in other cataclysmic variables. During the observations,
ZTF17aaaeefu showed brightness changes resembling dwarf nova outbursts and
quiescent states. When ZTF17aaaeefu was bright (16.0-16.2 mag), the average
eclipse depth was 2.50+/-0.18 mag. When ZTF17aaaeefu was faint (17.2-17.4 mag),
the average eclipse depth was 1.45+/-0.06 mag. However, these differences in
the eclipse depth may be mainly caused by the contaminating effect of three
faint stars around ZTF17aaaeefu. In both cases, the average width of the
prominent parts of the eclipses was the same and was about 30 min. Due to the
large coverage of observations, I measured the orbital period with high
precision, Porb=0.18821155+/-0.00000014 d. I derived the eclipse ephemeris, the
validity time of which is 700 years in accordance with the precision of the
orbital period. This ephemeris can be used for future studies of the orbital
period changes. Because ZTF17aaaeefu has a long orbital period, this
cataclysmic variable is of interest for determining the masses of its stellar
components. In future radial velocity measurements, my precise eclipse
ephemeris may be useful for determining the orbital phases. | astro-ph_SR |
Chemistry in a forming protoplanetary disk: main accretion phase: We investigate the chemistry in a radiation-hydrodynamics model of
star-forming core which evolves from a cold ($\sim 10$ K) prestellar core to
the main accretion phase in $\sim 10^5$ yr. A rotationally-supported
gravitationally unstable disk is formed around a protostar. We extract the
temporal variation of physical parameters in $\sim 1.5 \times 10^3$ SPH
particles which end up in the disk, and perform post-processing calculations of
the gas-grain chemistry adopting a three-phase model. Inside the disk, the SPH
particles migrate both inward and outward. Since a significant fraction of
volatiles such as CO can be trapped in the water-dominant ice in the
three-phase model, the ice mantle composition depends not only on the current
position in the disk but also on whether the dust grain has ever experienced
higher temperatures than the water sublimation temperature. Stable molecules
such as H$_2$O, CH$_4$, NH$_3$ and CH$_3$OH are already abundant at the onset
of gravitational collapse and simply sublimated as the fluid parcels migrate
inside the water snow line. On the other hand, various molecules such as carbon
chains and complex organic molecules (COMs) are formed in the disk. COMs
abundance sensitively depends on the outcomes of photodissociation and
diffusion rates of photofragments in bulk ice mantle. As for S-bearing species,
H$_2$S ice is abundant in the collapse phase. In the warm regions in the disk,
H$_2$S is sublimated to be destroyed, while SO, H$_2$CS, OCS and SO$_2$ become
abundant. | astro-ph_SR |
A $ΔR\sim 9.5$ mag Super Flare of An Ultracool Star Detected by
$\text{SVOM/GWAC}$ System: In this paper, we report the detection and follow-ups of a super stellar
flare GWAC\,181229A with an amplitude of $\Delta R\sim$9.5 mag on a M9 type
star by $\text{SVOM/GWAC}$ and the dedicated follow-up telescopes. The
estimated bolometric energy $E_{bol}$ is $(5.56-9.25)\times10^{34}$ ergs, which
places the event to be one of the most powerful flares on ultracool stars. The
magnetic strength is inferred to be (3.6-4.7) kG. Thanks to the sampling with a
cadence of 15 seconds, a new component near the peak time with a very steep
decay is detected in the $R$-band light curve, followed by the two-component
flare template given by Davenport et al. (2014). An effective temperature of
$5340\pm40$ K is measured by a blackbody shape fitting to the spectrum at the
shallower phase during the flare. The filling factors of the flare are
estimated to be $\sim$30\% and 19\% at the peak time and at 54 min after the
first detection. The detection of the particular event with large amplitude,
huge-emitted energy and a new component demonstrates that a high cadence sky
monitoring cooperating with fast follow-up observations is very essential for
understanding the violent magnetic activity. | astro-ph_SR |
A Very Small and Super Strong Zebra Pattern Burst at the Beginning of a
Solar Flare: Microwave emission with spectral zebra pattern structures (ZPs) is observed
frequently in solar flares and the Crab pulsar. The previous observations show
that ZP is only a structure overlapped on the underlying broadband continuum
with slight increments and decrements. This work reports an extremely unusual
strong ZP burst occurring just at the beginning of a solar flare observed
simultaneously by two radio telescopes located in China and Czech Republic and
by the extreme ultraviolet (EUV) telescope on board NASA's satellite Solar
Dynamics Observatory on 2013 April 11. It is a very short and super strong
explosion whose intensity exceeds several times that of the underlying flaring
broadband continuum emission, lasting for just 18 s. EUV images show that the
flare starts from several small flare bursting points (FBPs). There is a sudden
EUV flash with extra enhancement in one of these FBPs during the ZP burst.
Analysis indicates that the ZP burst accompanying EUV flash is an unusual
explosion revealing a strong coherent process with rapid particle acceleration,
violent energy release, and fast plasma heating simultaneously in a small
region with short duration just at the beginning of the flare. | astro-ph_SR |
The Frequency and Mass-Ratio Distribution of Binaries in Clusters I:
Description of the method and application to M67: We present a new method for probabilistic generative modelling of stellar
colour-magnitude diagrams (CMDs) to infer the frequency of binary stars and
their mass-ratio distribution. The method invokes a mixture model to account
for overlapping populations of single stars, binaries and outliers in the CMD.
We apply the model to Gaia observations of the old open cluster, M67, and find
a frequency $f_B(q > 0.5) = 0.258 \pm 0.019$ for binary stars with mass ratio
greater than 0.5. The form of the mass-ratio distribution function rises
towards higher mass ratios for $q > 0.3$. | astro-ph_SR |
Signatures of ubiquitous magnetic reconnection in the lower solar
atmosphere: Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the
quiet Sun (QSEBs) are a signature of the fundamental process of magnetic
reconnection at the smallest observable scale in the solar lower atmosphere. We
analyze high spatial resolution observations (0.1 arcsec) obtained with the
Swedish 1-m Solar Telescope to explore signatures of QSEBs in the H$\beta$
line. We find that QSEBs are ubiquitous and uniformly distributed throughout
the quiet Sun, predominantly occurring in intergranular lanes. We find up to
120 QSEBs in the FOV for a single moment in time; this is more than an order of
magnitude higher than the number of QSEBs found in earlier H$\alpha$
observations. This suggests that about half a million QSEBs could be present in
the lower solar atmosphere at any given time. The QSEB brightening found in the
H$\beta$ line wings also persist in the line core with a temporal delay and
spatial offset towards the nearest solar limb. Our results suggest that QSEBs
emanate through magnetic reconnection along vertically extended current sheets
in the solar lower atmosphere. The apparent omnipresence of small-scale
magnetic reconnection may play an important role in the energy balance of the
solar chromosphere. | astro-ph_SR |
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