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Search for Dormant Black Holes in Ellipsoidal Variables I. Revisiting
the Expected Amplitudes of the Photometric Modulation: Ellipsoidal variables present light-curve modulations caused by stellar
distortion, induced by tidal interaction with their companions. An analytical
approximated model of the ellipsoidal modulation is given as a discrete Fourier
series by Morris and Naftilan 1993 (MN93). Based on numerical simulations using
the PHOEBE code we present here updated amplitudes of the first three harmonics
of the model. The expected amplitudes are given as a function of the mass ratio
and inclination of the binary system and the fillout factor of the
primary---the ratio between the stellar radius and that of its Roche lobe. The
corrections can get up to 30% relative to the MN93 model for fillout factors
close to unity. The updated model can be instrumental in searching for
short-period binaries with compact-object secondaries in large data sets of
photometric light curves. As shown in one OGLE light-curve example, the minimum
mass ratio can be obtained by using only the amplitudes of the three harmonics
and an estimation of the stellar temperature. High enough amplitudes can help
to identify binaries with mass ratios larger than unity, some of which might
have compact companions. | astro-ph_SR |
Gas inside the 97 au cavity around the transition disk Sz\,91: We present ALMA (Cycle 0) band-6 and band-3 observations of the transition
disk Sz\,91. The disk inclination and position angle are determined to be
$i=49.5\degr\pm3.5\degr$ and $\mathrm{PA}=18.2\degr\pm3.5\degr$ and the dusty
and gaseous disk are detected up to $\sim220$ au and $\sim400$ au from the
star, respectively. Most importantly, our continuum observations indicate that
the cavity size in the mm-sized dust distribution must be $\sim97$ au in
radius, the largest cavity observed around a T Tauri star. Our data clearly
confirms the presence of \co(2-1) well inside the dust cavity. Based on these
observational constrains we developed a disk model that simultaneously accounts
for the \co and continuum observations (i.e., gaseous and dusty disk).
According to our model, most of the millimeter emission comes from a ring
located between 97 and 140 au. We also find that the dust cavity is divided
into an innermost region largely depleted of dust particles ranging from the
dust sublimation radius up to 85 au, and a second, moderately dust-depleted
region, extending from 85 to 97 au. The extremely large size of the dust
cavity, the presence of gas and small dust particles within the cavity and the
accretion rate of Sz\,91 are consistent with the formation of multiple (giant)
planets. | astro-ph_SR |
Diagnosing the Clumpy Protoplanetary Disk of the UXor Type Young Star GM
Cephei: UX Orionis stars (UXors) are Herbig Ae/Be or T Tauri stars exhibiting
sporadic occultation of stellar light by circumstellar dust. GM\,Cephei is such
a UXor in the young ($\sim4$~Myr) open cluster Trumpler\,37, showing prominent
infrared excess, emission-line spectra, and flare activity. Our photometric
monitoring (2008--2018) detects (1)~an $\sim$3.43~day period, likely arising
from rotational modulation by surface starspots, (2)~sporadic brightening on
time scales of days due to accretion, (3)~irregular minor flux drops due to
circumstellar dust extinction, and (4)~major flux drops, each lasting for a
couple of months with a recurrence time, though not exactly periodic, of about
two years. The star experiences normal reddening by large grains, i.e., redder
when dimmer, but exhibits an unusual "blueing" phenomenon in that the star
turns blue near brightness minima. The maximum extinction during relatively
short (lasting $\leq 50$~days) events, is proportional to the duration, a
consequence of varying clump sizes. For longer events, the extinction is
independent of duration, suggestive of a transverse string distribution of
clumps. Polarization monitoring indicates an optical polarization varying
$\sim3\%$--8$\%$, with the level anticorrelated with the slow brightness
change. Temporal variation of the unpolarized and polarized light sets
constraints on the size and orbital distance of the circumstellar clumps in the
interplay with the young star and scattering envelope. These transiting clumps
are edge-on manifestations of the ring- or spiral-like structures found
recently in young stars with imaging in infrared of scattered light, or in
submillimeter of thermalized dust emission. | astro-ph_SR |
Determining the ionization rates of interstellar neutral species using
direct-sampling observations of their direct and indirect beams: A good understanding of the ionization rates of neutral species in the
heliosphere is important for studies of the heliosphere and planetary
atmospheres. So far, the intensities of the ionization reactions have been
studied based on observations of the contributing phenomena, such as the solar
spectral flux in the EUV band and the flux of the solar wind protons, alpha
particles, and electrons. The results strongly depend on absolute calibration
of these measurements, which, especially for the EUV measurements, is
challenging. Here, we propose a novel method of determining the ionization rate
of neutral species based on direct sampling of interstellar neutral gas from
two locations in space distant to each other. In particular, we suggest
performing observations from the vicinity of Earth's orbit and using ratios of
fluxes of ISN He for the direct and indirect orbits of interstellar atoms. We
identify the most favorable conditions and observations geometries, suitable
for implementation on the forthcoming NASA mission Interstellar Mapping and
Acceleration Probe. | astro-ph_SR |
A Volume-limited Sample of 63 M7--M9.5 Dwarfs I. Space Motion, Kinematic
Age, and Lithium: In a volume-limited sample of 63 ultracool dwarfs of spectral type M7--M9.5,
we have obtained high-resolution spectroscopy with UVES at the Very Large
Telescope and HIRES at Keck Observatory. In this first paper we introduce our
volume-complete sample from DENIS and 2MASS targets, and we derive radial
velocities and space motion. Kinematics of our sample are consistent with the
stars being predominantly members of the young disk. The kinematic age of the
sample is 3.1 Gyr. We find that six of our targets show strong Li lines
implying that they are brown dwarfs younger than several hundred million years.
Five of the young brown dwarfs were unrecognized before. Comparing the fraction
of Li detections to later spectral types, we see a hint of an unexpected local
maximum of this fraction at spectral type M9. It is not yet clear whether this
maximum is due to insufficient statistics, or to a combination of physical
effects including spectral appearance of young brown dwarfs, Li line formation,
and the star formation rate at low masses. | astro-ph_SR |
NSCC -- A New Scheme of Classification of C-rich Stars Devised from
Optical and Infrared Observations: A new classification system for carbon-rich stars is presented based on an
analysis of 51 AGB carbon stars through the most relevant classifying indices
available. The extension incorporated, that also represents the major advantage
of this new system, is the combination of the usual optical indices that
describe the photospheres of the objects, with new infrared ones, which allow
an interpretation of the circumstellar environment of the carbon-rich stars.
This new system is presented with the usual spectral subclasses and $C_2$-, j-,
MS- and temperature indices, and also with the new SiC- (SiC/C.A. abundance
estimation) and $\tau$- (opacity) indices. The values for the infrared indices
were carried out through a Monte Carlo simulation of the radiative transfer in
the circumstellar envelopes of the stars. The full set of indices, when applied
to our sample, resulted in a more efficient system of classification, since an
examination in a wide spectral range allows us to obtain a complete scenario
for carbon stars. | astro-ph_SR |
Evidence for a physically bound third component in HD 150136: Context. HD150136 is one of the nearest systems harbouring an O3 star.
Although this system was for a long time considered as binary, more recent
investigations have suggested the possible existence of a third component.
Aims. We present a detailed analysis of HD 150136 to confirm the triple nature
of this system. In addition, we investigate the physical properties of the
individual components of this system. Methods. We analysed high-resolution,
high signal-to-noise data collected through multi-epoch runs spread over ten
years. We applied a disentangling program to refine the radial velocities and
to obtain the individual spectra of each star. With the radial velocities, we
computed the orbital solution of the inner system, and we describe the main
properties of the orbit of the outer star such as the preliminary mass ratio,
the eccentricity, and the orbital-period range. With the individual spectra, we
determined the stellar parameters of each star by means of the CMFGEN
atmosphere code. Results. We offer clear evidence that HD 150136 is a triple
system composed of an O3V((f\ast))-3.5V((f+)), an O5.5-6V((f)), and an
O6.5-7V((f)) star. The three stars are between 0-3 Myr old. We derive dynamical
masses of about 64, 40, and 35 Msun for the primary, the secondary and the
third components by assuming an inclination of 49{\deg}. It currently
corresponds to one of the most massive systems in our galaxy. The third star
moves with a period in the range of 2950 to 5500 d on an outer orbit with an
eccentricity of at least 0.3. This discovery makes HD 150136 the first
confirmed triple system with an O3 primary star. However, because of the long
orbital period, our dataset is not sufficient to constrain the orbital solution
of the tertiary component with high accuracy. | astro-ph_SR |
When flux standards go wild: white dwarfs in the age of Kepler: White dwarf stars have been used as flux standards for decades, thanks to
their staid simplicity. We have empirically tested their photometric stability
by analyzing the light curves of 398 high-probability candidates and
spectroscopically confirmed white dwarfs observed during the original Kepler
mission and later with K2 Campaigns 0-8. We find that the vast majority (>97
per cent) of non-pulsating and apparently isolated white dwarfs are stable to
better than 1 per cent in the Kepler bandpass on 1-hr to 10-d timescales,
confirming that these stellar remnants are useful flux standards. From the
cases that do exhibit significant variability, we caution that binarity,
magnetism, and pulsations are three important attributes to rule out when
establishing white dwarfs as flux standards, especially those hotter than
30,000 K. | astro-ph_SR |
Properties and applications of a predicted population of runaway
He-sdO/B stars ejected from single degenerate He-donor SNe: This study builds on previous works, producing the most extensive prediction
of the properties of such a hypothetical population to date, taking into
account both Chandrasekhar and non-Chandrasekhar mass events. These results are
then used to define criteria for membership of this population and characterise
putative subpopulations
This study contains 6x10^6 individual ejection trajectories out of the
Galactic plane calculated with the stellar kinematics framework SHyRT, which
are analysed with regard to their bulk observational properties. These are then
put into context with the only previously identified population member US\,708
and applied to a number of other possible candidate objects.
We find that two additional previously observed objects possess properties to
warrant a designation as candidate objects. Characterisation of these object
with respect to the predicted population finds all of them to be extreme in at
least one astrometric observable. We find that current observations support a
Galactic SN rate on the order of ~3x10^-7/yr to ~2x10^-6/yr, three orders of
magnitude below the inferred Galactic SN Ia rate and two orders of magnitude
below the formation rate of predicted He-donor progenitors. The number of
currently observed population members suggests that the He-donor scenariois not
a dominant contributor to the number of observed SNe Ia. However, we find that,
even at the low event rate suggested, the majority of possibly detectable
population members is still undetected. The extreme nature of current
population members suggests that a still larger number of objects has simply
evaded detection up to this point, hinting at a higher contribution than is
currently supported by observation. - abridged - | astro-ph_SR |
The age-metallicity relation in the solar neighbourhood from a pilot
sample of white dwarf-main sequence binaries: The age-metallicity relation (AMR) is a fundamental observational constraint
for understanding how the Galactic disc formed and evolved chemically in time.
However, there is not yet an agreement on the observational properties of the
AMR for the solar neighbourhood, primarily due to the difficulty in obtaining
accurate stellar ages for individual field stars. We have started an
observational campaign for providing the much needed observational input by
using wide white dwarf-main sequence (WDMS) binaries. White dwarfs are natural
clocks and can be used to derive accurate ages. Metallicities can be obtained
from the main sequence companions. Since the progenitors of white dwarfs and
the main sequence stars were born at the same time, WDMS binaries provide a
unique opportunity to observationally constrain in a robust way the properties
of the AMR. In this work we present the AMR derived from analysing a pilot
sample of 23 WDMS binaries and provide clear observational evidence for the
lack of correlation between age and metallicity at young and intermediate ages
(0-7 Gyrs). | astro-ph_SR |
Investigation of the Progenitors of Nova Explosions: Recurrent novae (RNe) play an important role as one of the suspected
progenitor systems of Type Ia supernovae (SNe) which are used as primary
distance indicators in cosmology. Thus, it is important to investigate the
nature of their central binary systems to determine the relation between the
parameters of the central system and outburst type, and finally ascertain the
population of novae that might be available to give rise to the progenitors of
Type Ia SNe. The details of the preliminary investigation looking for
characteristics that may distinguish recurrent and classical novae (CNe)
systems, the selection of initial targets for detailed study, and preliminary
results are presented. We identified 10 suspected RNe among the Galactic CNe
for investigation with the Liverpool Telescope population and our initial
photometric observations of their quiescent systems suggest 2 may belong to the
RS Oph type and 2 to the U Sco type RNe. Spectroscopic follow-up is now
underway. | astro-ph_SR |
Release note: Massive peak bagging of red giants in the Kepler field: The NASA satellite Kepler has gathered about 1420 days-long photometric time
series for more than 20000 red giant stars. For about 6600 of them also APOGEE
spectroscopic parameters are available, making the sample of high interest for
various astrophysical investigations. To optimally exploit the full wealth of
the seismic information, extraction of mode parameters of all significant
individual frequencies is necessary. However, the complex structure of the
mixed mode pattern makes it challenging to automate the peak bagging (i.e., the
extraction of the individual mode parameters from the stars power density
spectra). Even though several approaches have been successfully implemented,
the available results are still limited to a handful of stars. Here I present
frequencies, amplitudes, and lifetimes of more than a quarter of a million
oscillation modes of the spherical degree l=0 to 3, which have been observed in
6179 Kepler red giants. The sample covers evolutionary stages from the lower
red-giant branch to high up the asymptotic giant branch. The modes were
extracted with the Automated Bayesian Peak-Bagging Algorithm (ABBA) and are
publicly available at https://github.com/tkallinger/KeplerRGpeakbagging | astro-ph_SR |
On the relationship between G-band bright point dynamics and their
magnetic field strengths: G-band bright points (GBPs) are regarded as good manifestations of magnetic
flux concentrations. We aim to investigate the relationship between the dynamic
properties of GBPs and their longitudinal magnetic field strengths. High
spatial and temporal resolution observations were recorded simultaneously with
G-band filtergrams and Narrow-band Filter Imager (NFI) Stokes I and V images
with Hinode /Solar Optical Telescope. The GBPs are identified and tracked in
the G-band images automatically, and the corresponding longitudinal magnetic
field strength of each GBP is extracted from the calibrated NFI magnetograms by
a point-to-point method. After categorizing the GBPs into five groups by their
longitudinal magnetic field strengths, we analyze the dynamics of GBPs of each
group. The results suggest that with increasing longitudinal magnetic field
strengths of GBPs correspond to a decrease in their horizontal velocities and
motion ranges as well as by showing more complicated motion paths. This
suggests that magnetic elements showing weaker magnetic field strengths prefer
to move faster and farther along straighter paths, while stronger ones move
more slowly in more erratic paths within a smaller region. The dynamic
behaviors of GBPs with different longitudinal magnetic field strengths can be
explained by that the stronger flux concentrations withstand the convective
flows much better than weaker ones. | astro-ph_SR |
The Solar-Stellar Connection: We discuss how recent advances in observations, theory and numerical
simulations have allowed the stellar community to progress in its understanding
of stellar convection, rotation and magnetism and to assess the degree to which
the Sun and other stars share similar dynamical properties. Ensemble
asteroseismology has become a reality with the advent of large time domain
studies, especially from space missions. This new capability has provided
improved constraints on stellar rotation and activity, over and above that
obtained via traditional techniques such as spectropolarimetry or CaII H&K
observations. New data and surveys covering large mass and age ranges have
provided a wide parameter space to confront theories of stellar magnetism.
These new empirical databases are complemented by theoretical advances and
improved multi-D simulations of stellar dynamos. We trace these pathways
through which a lucid and more detailed picture of magnetohydrodynamics of
solar-like stars is beginning to emerge and discuss future prospects. | astro-ph_SR |
Three ways to solve the orbit of KIC11558725: a 10 day beaming sdB+WD
binary with a pulsating subdwarf: The recently discovered subdwarf B (sdB) pulsator KIC11558725 features a rich
g-mode frequency spectrum, with a few low-amplitude p-modes at short periods,
and is a promising target for a seismic study aiming to constrain the internal
structure of this star, and of sdB stars in general.
We have obtained ground-based spectroscopic Balmer-line radial-velocity
measurements of KIC11558725, spanning the 2010 and 2011 observing seasons. From
these data we have discovered that KIC11558725 is a binary with period P=10.05
d, and that the radial-velocity amplitude of the sdB star is 58 km/s.
Consequently the companion of the sdB star has a minimum mass of 0.63 M\odot,
and is therefore most likely an unseen white dwarf.
We analyse the near-continuous 2010-2011 Kepler light curve to reveal orbital
Doppler-beaming light variations at the 238 ppm level, which is consistent with
the observed spectroscopic orbital radial-velocity amplitude of the subdwarf.
We use the strongest 70 pulsation frequencies in the Kepler light curve of the
subdwarf as clocks to derive a third consistent measurement of the orbital
radial-velocity amplitude, from the orbital light-travel delay.
We use our high signal-to-noise average spectra to study the atmospheric
parameters of the sdB star, deriving Teff = 27 910K and log g = 5.41 dex, and
find that carbon, nitrogen and oxygen are underabundant relative to the solar
mixture.
Furthermore, we extract more than 160 significant frequencies from the Kepler
light curve. We investigate the pulsation frequencies for expected period
spacings and rotational splittings. We find period-spacing sequences of
spherical-harmonic degrees \ell=1 and \ell=2, and we associate a large fraction
of the g-modes in KIC11558725 with these sequences. From frequency splittings
we conclude that the subdwarf is rotating subsynchronously with respect to the
orbit. | astro-ph_SR |
The GAPS programme at TNG LII. Spot modeling of V1298 Tau using SpotCCF
tool: The intrinsic variability due to the magnetic activity of young active stars
is one of the main challenges in detecting and characterising exoplanets. We
present a method able to model the stellar photosphere and its surface
inhomogeneities (starspots) in young/active and fast-rotating stars, based on
the cross-correlation function (CCF) technique, to extract information about
the spot configuration of the star. Within the Global Architecture of Planetary
Systems (GAPS) Project at the Telescopio Nazionale Galileo, we analysed more
than 300 spectra of the young planet-hosting star V1298 Tau provided by HARPS-N
high-resolution spectrograph. By applying the SpotCCF model to the CCFs we
extracted the spot configuration (latitude, longitude and projected filling
factor) of this star, and also provided the new RVs time series of this target.
We find that the features identified in the CCF profiles of V1298 Tau are
modulated by the stellar rotation, supporting our assumption that they are
caused by starspots. The analysis suggests a differential rotation velocity of
the star with lower rotation at higher latitudes. Also, we find that SpotCCF
provides an improvement in RVs extraction with a significantly lower dispersion
with respect to the commonly used pipelines, with consequent mitigation of the
stellar activity contribution modulated with stellar rotation. A detection
sensitivity test, by the direct injection of a planetary signal into the data,
confirmed that the SpotCCF model improves the sensitivity and ability to
recover planetary signals. Our method enables the modelling of the stellar
photosphere, extracting the spot configuration of young/active and rapidly
rotating stars. It also allows for the extraction of optimised RV time series,
thereby enhancing our detection capabilities for new exoplanets and advancing
our understanding of stellar activity. | astro-ph_SR |
Detection of transit timing variations in excess of one hour in the
Kepler multi-planet candidate system KOI 806 with the GTC: We report the detection of transit timing variations (TTVs) well in excess of
one hour in the Kepler multi-planet candidate system KOI 806. This system
exhibits transits consistent with three separate planets -- a Super-Earth, a
Jupiter, and a Saturn -- lying very nearly in a 1:2:5 resonance, respectively.
We used the Kepler public data archive and observations with the Gran
Telescopio de Canarias to compile the necessary photometry. For the largest
candidate planet (KOI 806.02) in this system, we detected a large transit
timing variation of -103.5$\pm$6.9 minutes against previously published
ephemeris. We did not obtain a strong detection of a transit color signature
consistent with a planet-sized object; however, we did not detect a color
difference in transit depth, either. The large TTV is consistent with
theoretical predictions that exoplanets in resonance can produce large transit
timing variations, particularly if the orbits are eccentric. The presence of
large TTVs among the bodies in this systems indicates that KOI806 is very
likely to be a planetary system. This is supported by the lack of a strong
color dependence in the transit depth, which would suggest a blended eclipsing
binary. | astro-ph_SR |
The Evolution of Circumstellar Disks Surrounding Intermediate Mass
Stars: IC 1805: We report the results of a study of the intermediate and high mass stars in
the young, rich star-forming complex IC 1805, based on a combination of
optical, near-infrared, and mid-infrared photometry, and classification
spectra. These data provide the basis for characterizing the masses and ages
for stars more massive than ~2 Msun and enable a study of the frequency and
character of circumstellar disks associated with intermediate- and high-mass
stars. Optically thick accretion disks among stars with masses 2 < M/Msun <4
are rare (~2% of members) and absent among more massive stars. A larger
fraction (~10%) of stars with masses 2 < M/Msun < 4 appear to be surrounded by
disks that have evolved from the initial optically thick accretion phase. We
identify four classes of such disks. These classes are based on spectral energy
distributions (SEDs) of excess emsission above photospheric levels: disks that
are (1) optically thin based on the magnitude of the observed excess emission
from 2 to 24 um; (2) optically thin in their inner regions (r< 20 AU) and
optically thick in their outer regions; (3) exhibit empty inner regions (r < 10
AU) and optically thin emission in their outer regions; and (4) exhibit empty
inner regions and optically thick outer regions. We discuss, and assess the
merits and liabilities of, proposed explanations for disks exhibiting these SED
types and suggest additional observations that would test these proposals. | astro-ph_SR |
Binary interactions and UV photometry on photometric redshift: Using the Hyperz code (Bolzonella et al. 2000) we present photometric
redshift estimates for a random sample of galaxies selected from the SDSS/DR7
and GALEX/DR4, for which spectroscopic redshifts are also available.
We confirm that the inclusion of ultraviolet photometry improves the accuracy
of photo-zs for those galaxies with g*-r* < 0.7 and z_spec < 0.2. We also
address the problem of how binary interactions can affect photo-z estimates,
and find that their effect is negligible. | astro-ph_SR |
On the vertical-shear instability in astrophysical discs: We explore the linear stability of astrophysical discs exhibiting vertical
shear, which arises when there is a radial variation in the temperature or
entropy. Such discs are subject to a "vertical-shear instability", which recent
nonlinear simulations have shown to drive hydrodynamic activity in the
MRI-stable regions of protoplanetary discs. We first revisit locally isothermal
discs using the quasi-global reduced model derived by Nelson et al. (2013).
This analysis is then extended to global axisymmetric perturbations in a
cylindrical domain. We also derive and study a reduced model describing discs
with power law radial entropy profiles ("locally polytropic discs"), which are
somewhat more realistic in that they possess physical (as opposed to numerical)
surfaces. In all cases the fastest growing modes have very short wavelengths
and are localised at the disc surfaces (if present), where the vertical shear
is maximal. An additional class of modestly growing vertically global body
modes is excited, corresponding to destabilised classical inertial waves
("r-modes"). We discuss the properties of both types of modes, and stress that
those that grow fastest occur on the shortest available length scales
(determined either by the numerical grid or the physical viscous length). This
ill-posedness makes simulations of the instability difficult to interpret. We
end with some brief speculation on the nonlinear saturation and resulting
angular momentum transport. | astro-ph_SR |
Radio light curves and imaging of the helium nova V445 Puppis reveal
seven years of synchrotron emission: V445 Puppis is the only helium nova observed to date; its eruption in late
2000 showed high velocities up to 8500 km/s and a remarkable bipolar morphology
cinched by an equatorial dust disc. Here we present multi-frequency radio
observations of V445 Pup obtained with the Very Large Array (VLA) spanning 1.5
to 43.3 GHz, and between 2001 January and 2008 March (days 89 to 2700 after
eruption). The radio light curve is dominated by synchrotron emission over
these seven years, and shows four distinct radio flares. Resolved radio images
obtained in the VLA A configuration show that the synchrotron emission hugs the
equatorial disc, and comparisons to near-IR images of the nova clearly
demonstrate that it is the densest ejecta, not the fastest ejecta, that are the
sites of the synchrotron emission in V445 Pup. The data are consistent with a
model where the synchrotron emission is produced by a wind from the white dwarf
impacting the dense equatorial disc, resulting in shocks and particle
acceleration. The individual synchrotron flares may be associated with density
enhancements in the equatorial disc and/ or velocity variations in the wind
from the white dwarf. This overall scenario is similar to a common picture of
shock production in hydrogen-rich classical novae, but V445 Pup is remarkable
in that these shocks persist for almost a decade, much longer than the weeks or
months for which shocks are typically observed in classical novae. | astro-ph_SR |
The shear-Hall instability in newborn neutron stars: Aims. In the first few minutes of a newborn neutron star's life the Hall
effect and differential rotation may both be important. We demonstrate that
these two ingredients are sufficient for generating a 'shear-Hall instability'
and for studying its excitation conditions, growth rates, and characteristic
magnetic field patterns.
Methods. We numerically solve the induction equation in a spherical shell,
with a kinematically prescribed differential rotation profile {\Omega}(s),
where s is the cylindrical radius. The Hall term is linearized about an imposed
uniform axial field. The linear stability of individual azimuthal modes, both
axisymmetric and non-axisymmetric, is then investigated.
Results. For the shear-Hall instability to occur, the axial field must be
parallel to the rotation axis if {\Omega}(s) decreases outward, whereas if
{\Omega}(s) increases outward it must be anti-parallel. The instability draws
its energy from the differential rotation, and occurs on the short rotational
timescale rather than on the much longer Hall timescale. It operates most
efficiently if the Hall time is comparable to the diffusion time. Depending on
the precise field strengths B0, either axisymmetric or non-axisymmetric modes
may be the most unstable.
Conclusions. Even if the differential rotation in newborn neutron stars is
quenched within minutes, the shear-Hall instability may nevertheless amplify
any seed magnetic fields by many orders of magnitude. | astro-ph_SR |
Detection of Strong Millimeter Emission from the Circumstellar Dust Disk
Around V1094 Sco: Cold and Massive Disk around a T Tauri Star in a Quiescent
Accretion Phase?: We present the discovery of a cold massive dust disk around the T Tauri star
V1094 Sco in the Lupus molecular cloud from the 1.1 millimeter continuum
observations with AzTEC on ASTE. A compact ($r\lesssim$320 AU) continuum
emission coincides with the stellar position having a flux density of 272 mJy
which is largest among T Tauri stars in Lupus. We also present the detection of
molecular gas associated with the star in the five-point observations in
$^{12}$CO J=3--2 and $^{13}$CO J=3--2. Since our $^{12}$CO and $^{13}$CO
observations did not show any signature of a large-scale outflow or a massive
envelope, the compact dust emission is likely to come from a disk around the
star. The observed SED of V1094 Sco shows no distinct turnover from near
infrared to millimeter wavelengths, which can be well described by a flattened
disk for the dust component, and no clear dip feature around 10 $\micron$
suggestive of absence of an inner hole in the disk. We fit a simple power-law
disk model to the observed SED. The estimated disk mass ranges from 0.03 to
$\gtrsim$0.12 $M_\sun$, which is one or two orders of magnitude larger than the
median disk mass of T Tauri stars in Taurus. | astro-ph_SR |
Mapping the youngest and most massive stars in the Tarantula nebula with
MUSE-NFM: The evolution of the most massive stars is a puzzle with many missing pieces.
Statistical analyses are the key to provide anchors to calibrate theory,
however performing these studies is an arduous job. The state-of-the-art
integral field spectrograph MUSE has stirred up stellar astrophysicists who are
excited about the capability to take spectra of up to a thousand stars in a
single exposure. The excitement was even higher with the commissioning of the
MUSE narrow-field-mode (NFM) that has demonstrated angular resolutions akin to
the Hubble Space Telescope. We present the first mapping of the dense stellar
core R136 in the Tarantula nebula based on a MUSE-NFM mosaic. We aim to deliver
the first homogeneous analysis of the most massive stars in the local Universe
and to explore the impact of these peculiar objects to the interstellar medium. | astro-ph_SR |
Short-term variability and mass loss in Be stars V. Space photometry and
ground-based spectroscopy of $γ$ Cas: Context. Be stars are physically complex systems that continue to challenge
theory to understand their rapid rotation, complex variability and decretion
disks. $\gamma$ Cassiopeiae ($\gamma$ Cas) is one such star but is even more
curious because of its unexplained hard thermal X-ray emission. Aims. We aim to
examine the optical variability of $\gamma$ Cas and thereby to shed more light
on its puzzling behaviour. Methods. Three hundred twenty-one archival H$\alpha$
spectra from 2006 to 2017 are analysed to search for frequencies corresponding
to the 203.5 day orbit of the companion. Space photometry from the SMEI
satellite from 2003 to 2011 and the BRITE-Constellation of nano-satellites
between 2015 and 2019 is investigated in the period range from a couple of
hours to a few days. Results. The orbital period of the companion of 203.5 days
is confirmed with independent measurements from the structure of the H$\alpha$
line emission. A strong blue/red asymmetry in the amplitude distribution across
the H$\alpha$ emission line could hint at a spiral structure in the decretion
disk. With the space photometry, the known frequency of 0.82 d$^{-1}$ is
confirmed in data from the early 2000s. A higher frequency of 2.48 d$^{-1}$ is
present in the data from 2015 to 2019 and possibly also in the early 2000s. A
third frequency at 1.25 d$^{-1}$ is proposed to exist in both SMEI and BRITE
data. The only explanation covering all three rapid variations seems to be
nonradial pulsation. The two higher frequencies are incompatible with rotation. | astro-ph_SR |
Theory of fossil magnetic field: Theory of fossil magnetic field is based on the observations, analytical
estimations and numerical simulations of magnetic flux evolution during star
formation in the magnetized cores of molecular clouds. Basic goals, main
features of the theory and manifestations of MHD effects in young stellar
objects are discussed. | astro-ph_SR |
Silicate Extinction Profile Based on the Stellar Spectrum by Spitzer/IRS: The 9.7$\mu m$ and 18$\mu m$ interstellar spectral features, arising from the
Si--O stretching and O--Si--O bending mode of amorphous silicate dust, are the
strongest extinction feature in the infrared. Here we use the "pair method" to
determine the silicate extinction profile by comparing the \emph{Spitzer}/IRS
spectra of 49 target stars with obvious extinction with that of un-reddened
star of the same spectral type. The 9.7$\mu m$ extinction profile is determined
from all the 49 stars and the 18$\mu m$ profile is determined from six stars.
It is found that the profile has the peak wavelength around $\sim$9.2- 9.8$\mu
m$ and $\sim$18-22$\mu m$ respectively. The peak wavelength of the 9.7$\mu m$
feature seems to become shorter from the stars of late spectral type, meanwhile
the FWHM seems irrelevant to the spectral type, which may be related to
circumstellar silicate emission. The silicate optical depth at 9.7$\mu m$,
$\Delta\tau_{9.7}$, mostly increases with the color excess in $J-K_S$ ($E_{\rm
JK_S}$). The mean ratio of the visual extinction to the 9.7$\mu m$ silicate
absorption optical depth is $A_{\rm V}/\Delta\tau_{9.7}\approx 17.8$, in close
agreement with that of the solar neighborhood diffuse ISM. When $E_{\rm JK_S}$
> 4, this proportionality changes. The correlation coefficient between the peak
wavelength and FWHM of the 9.7$\mu m$ feature is 0.4, which indicates a
positive correlation considering the uncertainties of the parameters. The
method is compared with replacing the reference star by an atmospheric model
SED and no significant difference is present. | astro-ph_SR |
Helium-Star Models with Optically Thick Winds: Implications for the
Internal Structures and Mass-Loss Rates of Wolf-Rayet Stars: We construct helium (He) star models with optically thick winds and compare
them with the properties of Galactic Wolf-Rayet (WR) stars. Hydrostatic He-core
solutions are connected smoothly to trans-sonic wind solutions that satisfy the
regularity conditions at the sonic point. Velocity structures in the supersonic
parts are assumed by a simple beta-type law. By constructing a
center-to-surface structure, a mass-loss rate can be obtained as an eigenvalue
of the equations. Sonic points appear at temperatures ~ 1.8e5 - 2.8e5 K below
the Fe-group opacity peak, where the radiation force becomes comparable to the
local gravity. Photospheres are located at radii 3-10 times larger than sonic
points. The obtained mass-loss rates are comparable to those of WR stars. Our
mass-loss rate - luminosity relation agrees well with the relation recently
obtained by Graefener et al. (2017). Photospheric temperatures of WR stars tend
to be cooler than our predictions. We discuss the effects of stellar evolution,
detailed radiation transfer, and wind clumping, which are ignored in this
paper. | astro-ph_SR |
The IACOB project VIII. Searching for empirical signatures of binarity
in fast-rotating O-type stars: The empirical distribution of projected rotational velocities (vsini) in
massive O-type stars is characterized by a dominant slow velocity component and
a tail of fast rotators. Binary interaction has been proposed to play a
dominant role in the formation of this tail. We perform a complete and
homogeneous search for empirical signatures of binarity in a sample of 54
fast-rotating stars with the aim of evaluating this hypothesis. This working
sample has been extracted from a larger sample of 415 Galactic O-type stars
which covers the full range of vsini values. We use new and archival
multi-epoch spectra in order to detect spectroscopic binary systems. We
complement this information with Gaia proper motions and TESS photometric data
to aid in the identification of runaway stars and eclipsing binaries,
respectively. The identified fraction of single-lined spectroscopic binary
(SB1) systems and apparently single stars among the fast-rotating sample is
$\sim$18% and $\sim$70%, respectively. When comparing these percentages with
those corresponding to the slow-rotating sample we find that our sample of fast
rotators is characterized by a slightly larger percentage of SB1 systems
($\sim$18% vs. $\sim$13%) and a considerably smaller fraction of clearly
detected SB2 systems (8% vs. 33%). Overall, there seems to be a clear deficit
of spectroscopic binaries (SB1+SB2) among fast-rotating O-type stars ($\sim$26%
vs. $\sim$46%). On the contrary, the fraction of runaway stars is significantly
higher in the fast-rotating domain ($\sim$33-50%) than among those stars with
vsini < 200 km/s. Lastly, almost 65% of the apparently single fast-rotating
stars are runaways. Our empirical results seem to be in good agreement with the
idea that the tail of fast-rotating O-type stars (with vsini > 200 km/s) is
mostly populated by post-interaction binary products. | astro-ph_SR |
The Color and Brightness of the F-Corona Inferred from the 2019 July 2
Total Solar Eclipse: Total solar eclipses (TSEs) provide a unique opportunity to quantify the
properties of the K-corona (electrons), F-corona (dust) and E-corona (ions)
continuously from the solar surface out to a few solar radii. We apply a novel
inversion method to separate emission from the K- and F-corona continua using
unpolarized total brightness (tB) observations from five 0.5 nm bandpasses
acquired during the 2019 July 2 TSE between 529.5 nm and 788.4 nm. The
wavelength dependence relative to the photosphere (i.e., color) of the F-corona
itself is used to infer the tB of the K- and F-corona for each line-of-sight.
We compare our K-corona emission results with the Mauna Loa Solar Observatory
(MLSO) K-Cor polarized brightness (pB) observations from the day of the
eclipse, and the forward modeled K-corona intensity from the Predictive Science
Inc. (PSI) Magnetohydrodynamic (MHD) model prediction. Our results are
generally consistent with previous work and match both the MLSO data and
PSI-MHD predictions quite well, supporting the validity of our approach and of
the PSI-MHD model. However, we find that the tB of the F-corona is higher than
expected in the low corona, perhaps indicating that the F-corona is slightly
polarized -- challenging the common assumption that the F-corona is entirely
unpolarized. | 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 |
Recycled Pulsars: Spins, Masses and Ages: Recycled pulsars are mainly characterized by their spin periods, B-fields and
masses. All these quantities are affected by previous interactions with a
companion star in a binary system. Therefore, we can use these quantities as
fossil records and learn about binary evolution. Here, I briefly review the
distribution of these observed quantities and summarize our current
understanding of the pulsar recycling process. | astro-ph_SR |
Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV
and X-ray emission lines: Context. The accretion process in Classical T Tauri Stars (CTTSs) can be
studied through the analysis of some UV and X-ray emission lines which trace
hot gas flows and act as diagnostics of the post-shock downfalling plasma. In
the UV band, where higher spectral resolution is available, these lines are
characterized by rather complex profiles whose origin is still not clear.
Aims. We investigate the origin of UV and X-ray emission at impact regions of
density structured (fragmented) accretion streams.We study if and how the
stream fragmentation and the resulting structure of the post-shock region
determine the observed profiles of UV and X-ray emission lines.
Methods. We model the impact of an accretion stream consisting of a series of
dense blobs onto the chromosphere of a CTTS through 2D MHD simulations. We
explore different levels of stream fragmentation and accretion rates. From the
model results, we synthesize C IV (1550 {\AA}) and OVIII (18.97 {\AA}) line
profiles.
Results. The impacts of accreting blobs onto the stellar chromosphere produce
reverse shocks propagating through the blobs and shocked upflows. These
upflows, in turn, hit and shock the subsequent downfalling fragments. As a
result, several plasma components differing for the downfalling velocity,
density, and temperature are present altoghether. The profiles of C IV doublet
are characterized by two main components: one narrow and redshifted to speed
$\approx$ 50 km s$^{-1}$ and the other broader and consisting of subcomponents
with redshift to speed in the range 200 $\approx$ 400 km s$^{-1}$. The profiles
of OVIII lines appear more symmetric than C IV and are redshifted to speed
$\approx$ 150 km s$^{-1}$.
Conclusions. Our model predicts profiles of C IV line remarkably similar to
those observed and explains their origin in a natural way as due to stream
fragmentation. | astro-ph_SR |
Coronal Magnetic Field Topology From Total Solar Eclipse Observations: Measuring the global magnetic field of the solar corona remains exceptionally
challenging. The fine-scale density structures observed in white light images
taken during Total Solar Eclipses (TSEs) are currently the best proxy for
inferring the magnetic field direction in the corona from the solar limb out to
several solar radii (Rs). We present, for the first time, the topology of the
coronal magnetic field continuously between 1 and 6 Rs, as quantitatively
inferred with the Rolling Hough Transform (RHT) for 14 unique eclipse coronae
that span almost two complete solar cycles. We find that the direction of the
coronal magnetic field does not become radial until at least 3 Rs, with a high
variance between 1.5 and 3 Rs at different latitudes and phases of the solar
cycle. We find that the most non-radial coronal field topologies occur above
regions with weaker magnetic field strengths in the photosphere, while stronger
photospheric fields are associated with highly radial field lines in the
corona. In addition, we find an abundance of field lines which extend
continuously from the solar surface out to several solar radii at all
latitudes, regardless of the presence of coronal holes. These results have
implications for testing and constraining coronal magnetic field models, and
for linking in situ solar wind measurements to their sources at the Sun. | astro-ph_SR |
A method to identify and characterise binary candidates - a study of
CoRoT data: The analysis of the CoRoT space mission data was performed aiming to test a
method that selects, among the several light curves observed, the transiting
systems that likely host a low-mass star orbiting the main target. The method
identifies stellar companions by fitting a model to the observed transits.
Applying this model, that uses equations like Kepler's third law and an
empirical mass-radius relation, it is possible to estimate the mass and radius
of the primary and secondary objects as well as the semimajor axis and
inclination angle of the orbit. We focus on how the method can be used in the
characterisation of transiting systems having a low-mass stellar companion with
no need to be monitored with radial-velocity measurements or ground-based
photometric observations. The model, which provides a good estimate of the
system parameters, is also useful as a complementary approach to select
possible planetary candidates. A list of confirmed binaries together with our
estimate of their parameters are presented. The characterisation of the first
twelve detected CoRoT exoplanetary systems was also performed and agrees very
well with the results of their respective announcement papers. The comparison
with confirmed systems validates our method, specially when the radius of the
secondary companion is smaller than 1.5 Rjup, in the case of planets, or larger
than 2 Rjup, in the case of low-mass stars. Intermediate situations are not
conclusive. | astro-ph_SR |
The PASTEL catalogue: 2016 version: The bibliographical compilation of stellar atmospheric parameters (Teff,
logg, [Fe/H]) relying on high-resolution, high signal-to-noise spectroscopy
started in the eighties with the so-called [Fe/H] catalogue, and was continued
in 2010 with the PASTEL catalogue, which also includes determinations of Teff
alone, based on various methods. Here we present an update of the PASTEL
catalogue. The main journals and the CDS database have been surveyed to find
relevant publications presenting new determinations of atmospheric parameters.
As of February 2016, PASTEL includes 64082 determinations of either Teff or
(Teff, logg, [Fe/H]) for 31401 stars, corresponding to 1142 bibliographical
references. Some 11197 stars have a determination of the three parameters
(Teff, logg, [Fe/H]) with a high-quality spectroscopic metallicity. | astro-ph_SR |
Budget equations and astrophysical nonlinear mean-field dynamos: Solar, stellar and galactic large-scale magnetic fields are originated due to
a combined action of non-uniform (differential) rotation and helical motions of
plasma via mean-field dynamos. Usually, nonlinear mean-field dynamo theories
take into account algebraic and dynamic quenching of alpha effect and algebraic
quenching of turbulent magnetic diffusivity. However, the theories of the
algebraic quenching do not take into account the effect of modification of the
source of turbulence by the growing large-scale magnetic field. This phenomenon
is due to the dissipation of the strong large-scale magnetic field resulting in
an increase of the total turbulent energy. This effect has been studied using
the budget equation for the total turbulent energy (which takes into account
the feedback of the generated large-scale magnetic field on the background
turbulence) for (i) a forced turbulence, (ii) a shear-produced turbulence and
(iii) a convective turbulence. As the result of this effect, a nonlinear dynamo
number decreases with increase of the large-scale magnetic field, so that that
the mean-field $\alpha\Omega$, $\alpha^2$ and $\alpha^2\Omega$ dynamo
instabilities are always saturated by the strong large-scale magnetic field. | astro-ph_SR |
Surface distribution of small-scale magnetic field on the active cool
star LQ Hydrae: It is well known that small-scale magnetism dominates the surface magnetic
field topologies of active late-type stars. However, little information is
available on the spatial distribution of this key magnetic field component.
Here, we take advantage of the recently developed magnetic field diagnostic
procedure relying on the magnetic intensification of iron atomic lines in the
optical. We extend this methodology from measuring a single average field
strength value to simultaneous Doppler imaging reconstruction of the
two-dimensional maps of temperature and magnetic field strength. We applied
this novel surface mapping approach to two spectroscopic data sets of the young
active Sun-like star LQ Hya. For both epochs, we found a fairly uniform field
strength distribution, apart from a latitudinal trend of the field strength
increasing from 1.5-2.0 kG at low latitudes to 3.0-3.5 kG, close to the
rotational poles. This distribution of the small-scale field does not display a
clear correlation with the locations of temperature spots or the global
magnetic field structure reconstructed for the same epochs. | astro-ph_SR |
Mid-IR observations of circumstellar disks -- Part III: A mixed sample
of PMS stars and Vega-type objects: We present new mid-infrared spectra for a sample of 15 targets (1 FU Orionis
object, 4 Herbig Ae stars, 5 T Tauri stars and 5 Vega type stars), obtained
with the TIMMI2 camera at La Silla Observatory (ESO). Three targets are members
of the beta Pic moving group (HD 155555, HD 181296 and HD 319139). PAH bands
are observed towards the T Tauri star HD 34700 and the Herbig Ae star PDS 144
N. For HD 34700, the band profiles indicate processed PAHs. The spectrum of the
Vega-type object eta Corvi (HD 109085), for which a resolved disk at sub-mm
wavelengths is known, is entirely stellar between 8--13 micron. Similarly, no
indication for circumstellar matter at mid-infrared wavelengths is found
towards the Vega-like stars HD 3003, HD 80951, HD 181296 and, surprisingly, the
T Tauri system HD 155555.
The silicate emission features of the remaining eight sources are modelled
with a mixture of silicates of different grain sizes and composition.
Unprocessed dust dominates FU Ori, HD 143006 and CD-43 344. Large amorphous
grains are the main dust component around HD 190073, HD 319139, KK Oph and PDS
144 S. Both small grains and crystalline dust is found for the Vega-type HD
123356, with a dominance of small amorphous grains. We show that the infrared
emission of the binary HD 123356 is dominated by its late-type secondary, but
optical spectroscopy is still required to confirm the age of the system and the
spectral class of the companion. For most targets this is their first
mid-infrared spectroscopic observation. We investigate trends between stellar,
disk and silicate properties and confirm correlations of previous studies.
Several objects present an exciting potential for follow-up high-resolution
disk studies. | astro-ph_SR |
Metallicity estimation of MW, SMC and LMC classical Cepheids from the
shape of the $V$- and $I$-band light curves: Estimating metallicity of classical Cepheids is of prime importance for
studying metallicity effect on stellar evolution, chemical evolution of
galaxies, and ultimately its impact on period-luminosity relation used in the
extragalactic distance scale. We aim at establishing new empirical relations
for estimating the iron content of classical Cepheids for short and
long-periods based on Fourier parameters from the $V$-band light curves. We
calibrate new interrelations of Fourier parameters to convert $V$-band
empirical relations into the $I$-band. Then we apply these relation in $V$ and
$I$-bands to Cepheids from Milky Way (MW), Small and Large Magellanic Clouds
(SMC and LMC) available in the literature. Last, we map the metallicity
distribution in these galaxies for investigating potential application in
galactic archeology. These empirical relations in $V$ and $I$ bands are able to
derive the mean metallicity of a sample of MW, SMC and LMC Cepheids in
agreement with literature values within 1$\sigma$. We also show that these
relations are precise enough to reconstruct the radial metallicity gradients
within the MW from OGLE data. The empirical relations in the $V$ and $I$ bands
calibrated in this paper for short and long-period Cepheids provide a new
useful tool to estimate the metallicity of Cepheids which are not accessible by
spectroscopy. The calibration can be improved with further high-resolution
spectroscopic observations of metal-poor Cepheids and homogeneous photometries
in $V$ and $I$ bands. | astro-ph_SR |
On the Correlation between L Dwarf Optical and Infrared Variability and
Radio Aurorae: Photometric variability attributed to cloud phenomena is common in L/T
transition brown dwarfs. Recent studies show that such variability may also
trace aurorae, suggesting that localized magnetic heating may contribute to
observed brown dwarf photometric variability. We assess this potential
correlation with a survey of 17 photometrically variable brown dwarfs using the
Karl G. Jansky Very Large Array (VLA) at 4 -- 8 GHz. We detect quiescent and
highly circularly polarized flaring emission from one source, 2MASS
J17502484-0016151, which we attribute to auroral electron cyclotron maser
emission. The detected auroral emission extends throughout the frequency band
at $\sim$5 -- 25$\sigma$, and we do not detect evidence of a cutoff. Our
detection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength
of $\geq$2.9 kG, similar to those of other radio-bright ultracool dwarfs. We
show that H$\alpha$ emission continues to be an accurate tracer of auroral
activity in brown dwarfs. Supplementing our study with data from the
literature, we calculate the occurrence rates of quiescent emission in L dwarfs
with low- and high-amplitude variability and conclude that high amplitude O/IR
variability does not trace radio magnetic activity in L dwarfs. | astro-ph_SR |
Solar-like stars observed by Kepler: an incredible adventure: The NASA Kepler mission -in flight since March 2009- is producing an enormous
number of high-quality continuous light curves. Now, and for the first time
ever, we are able to do ensemble asteroseismology, i.e., to do an asteroseismic
analysis with a statistically significant sub-sample of solar-like stars
covering a wide range of stellar characteristics. In the present work, I
highlight some of the most recent studies carried out using these data. | astro-ph_SR |
Interacting winds in massive binaries: Massive stars feature highly energetic stellar winds that interact whenever
two such stars are bound in a binary system. The signatures of these
interactions are nowadays found over a wide range of wavelengths, including the
radio domain, the optical band, as well as X-rays and even gamma-rays. A proper
understanding of these effects is thus important to derive the fundamental
parameters of the components of massive binaries from spectroscopic and
photometric observations. | astro-ph_SR |
Observations of microglitches in HartRAO radio pulsars: A detailed observation of microglitch phenomenon in relatively slow radio
pulsars is presented. Our analyses for these small amplitude jumps in pulse
rotation frequency ($\nu$) and/or spin down rate ($\dot{\nu}$) combine the
traditional manual detection method (which hinges on careful visual inspections
of the residuals of pulse phase residuals) and a new, and perhaps more
objective, automated search technique (which exploits the power of the
computer, rather than the eyes, for resolving discrete events in pulsar spin
parameters). The results of the analyses of a sample of 26 radio pulsars reveal
that: (i) only 20 pulsars exhibit significant fluctuations in their arrival
times to be considered suitable for meaningful microglitch analyses; (ii) a
phenomenal 299 microglitch events were identified in $\nu$ and/or $\dot{\nu}$:
266 of these events were found to be simultaneously significant in $\nu$ and
$\dot{\nu}$, while 19 and 14 were noticeable only in $\nu$ and $\dot{\nu}$,
respectively; (iii) irrespective of sign, the microglitches have fractional
sizes which cover about 3 orders of magnitude in $\nu$ and $\dot{\nu}$
($10^{-11} < |\Delta{\nu}/\nu| < 2.0\times10^{-8}$ and $5.0\times10^{-5} <
|\Delta{\dot{\nu}}/\dot{\nu}| < 2.0\times10^{-2}$) with median values as
$0.78\times10^{-9}$ and $0.36\times10^{-3}$, respectively. | astro-ph_SR |
Water around IRAS15398-3359 observed with ALMA: How protostars accrete mass is one of the fundamental problems of star
formation. High column densities and complex kinematical structures make direct
observations challenging and they only provide a snapshot. Chemical tracers
provide an interesting alternative to characterise the infall histories of
protostars. Previous observations of H13CO+ towards the low-mass protostar
IRAS15398-3359 showed a depression in the abundance. This is a sign of
destruction of HCO+ by an enhanced presence of gaseous water in an extended
region, possibly related to a recent burst in the accretion. Direct
observations of water vapour can determine the exact extent of the emission and
confirm the hypothesis that HCO+ is indeed a good tracer of the water
snow-line. IRAS15398 was observed using ALMA at 0.5" resolution. Maps of
HDO(101-000) and H218O(414-321) were taken simultaneously with observations of
the CS(8-7) and N2H+(5-4) lines and continuum at 0.65 and 0.75 mm. The maps
were interpreted using dust radiative transfer calculations of the protostellar
infalling envelope with an outflow cavity. HDO is clearly detected and extended
over the scales of the H13CO+ depression, although it is displaced by ~500 AU
in the direction of the outflow. H218O is tentatively detected towards the
red-shifted outflow lobe, but otherwise it is absent from the mapped region,
which suggests that temperatures are low. Based on the temperature structure
obtained from dust radiative transfer models, we conclude that the water was
most likely released from the grains in an extended hour-glass configuration
during a recent accretion burst. HDO is only detected in the region closest to
the protostar, at distances of up to 500 AU. These signatures can only be
explained if the luminosity has recently been increased by orders of
magnitudes. Additionally, the densities in the outflow cones must be
sufficiently low. | astro-ph_SR |
A survey of nulling pulsars using the Giant Meterwave Radio Telescope: Several pulsars show sudden cessation of pulsed emission, which is known as
pulsar nulling. In this paper, the nulling behaviour of 15 pulsars is
presented. The nulling fractions of these pulsars, along with the degree of
reduction in the pulse energy during the null phase, are reported for these
pulsars. A quasi-periodic null-burst pattern is reported for PSR J1738-2330.
The distributions of lengths of the null and the burst phases as well as the
typical nulling time scales are estimated for eight strong pulsars. The nulling
pattern of four pulsars with similar nulling fraction are found to be different
from each other, suggesting that the fraction of null pulses does not quantify
the nulling behaviour of a pulsar in full detail. Analysis of these
distributions also indicate that while the null and the burst pulses occur in
groups, the underlying distribution of the interval between a transition from
the null to the burst phase and vice verse appears to be similar to that of a
stochastic Poisson point process. | astro-ph_SR |
White-light superflare and long-term activity of the nearby M7 type
binary EI~Cnc observed with GWAC system: Stellar white-light flares are believed to play an essential role on the
physical and chemical properties of the atmosphere of the surrounding
exoplanets. Here we report an optical monitoring campaign on the nearby flaring
system EI~Cnc carried out by the Ground-based Wide Angle Cameras (GWAC) and its
dedicated follow-up telescope. A superflare, coming from the brighter component
EI~CncA, was detected and observed, in which four components are required to
properly model the complex decay light curve. The lower limit of flare energy
in the $R-$band is estimated to be $3.3\times10^{32}$ ergs. 27 flares are
additionally detected from the GWAC archive data with a total duration of 290
hours. The inferred cumulative flare frequency distribution follows a quite
shallow power-law function with a slope of $\beta=-0.50\pm 0.03$ over the
energy range between $10^{30}$ and $10^{33}$ erg, which reinforces the trend
that stars cooler than M4 show enhanced superflare activity. The flares
identified in EI~Cnc enable us to extend the $\tau-E$ relationship previously
established in the white-light superflares of solar-type stars down to an
energy as low as $\sim10^{30}$erg (i.e., by three orders): $\tau\propto
E^{0.42\pm0.02}$, which suggests a common flare mechanism for stars with a type
from M to solar-like, and implies an invariant of $B^{1/3}\upsilon_{\rm A}$ in
the white-light flares. | astro-ph_SR |
Analytical maximum likelihood estimation of stellar magnetic fields: The polarised spectrum of stellar radiation encodes valuable information on
the conditions of stellar atmospheres and the magnetic fields that permeate
them. In this paper, we give explicit expressions to estimate the magnetic
field vector and its associated error from the observed Stokes parameters. We
study the solar case where specific intensities are observed and then the
stellar case, where we receive the polarised flux. In this second case, we
concentrate on the explicit expression for the case of a slow rotator with a
dipolar magnetic field geometry. Moreover, we also give explicit formulae to
retrieve the magnetic field vector from the LSD profiles without assuming mean
values for the LSD artificial spectral line. The formulae have been obtained
assuming that the spectral lines can be described in the weak field regime and
using a maximum likelihood approach. The errors are recovered by means of the
hermitian matrix. The bias of the estimators are analysed in depth. | astro-ph_SR |
Bayesian Characterization of Main Sequence Binaries in the Old Open
Cluster NGC 188: The binary fractions of open and globular clusters yield powerful constraints
on their dynamical state and evolutionary history. We apply publicly available
Bayesian analysis code to a $UBVRIJHK_{S}$ photometric catalog of the old open
cluster NGC 188 to detect and characterize photometric binaries along the
cluster main sequence. This technique has the advantage that it
self-consistently handles photometric errors, missing data in various
bandpasses, and star-by-star prior constraints on cluster membership.
Simulations are used to verify uncertainties and quantify selection biases in
our analysis, illustrating that among binaries with mass ratios >0.5, we
recover the binary fraction to better than 7% in the mean, with no significant
dependence on binary fraction and a mild dependence on assumed mass ratio
distribution. Using our photometric catalog, we recover the majority
(65%$\pm$11%) of spectroscopically identified main sequence binaries, including
8 of the 9 with spectroscopically measured mass ratios. Accounting for
incompleteness and systematics, we derive a mass ratio distribution that rises
toward lower mass ratios (within our $q >$0.5 analysis domain). We observe a
raw binary fraction for solar-type main sequence stars with mass ratios $q
>$0.5 of 42%$\pm$4%, independent of the assumed mass ratio distribution to
within its uncertainties, consistent with literature values for old open
clusters but significantly higher than the field solar-type binary fraction. We
confirm that the binaries identified by our method are more concentrated than
single stars, in agreement with previous studies, and we demonstrate that the
binary nature of those candidates which remain unidentified spectroscopically
is strongly supported by photometry from Gaia DR2. | astro-ph_SR |
A key factor to the spin parameter of uniformly rotating compact stars:
crust structure: We study the key factor to determine the dimensionless spin parameter
$j\equiv cJ/(GM^2)$ of different kinds of uniformly rotating compact stars,
including the traditional neutron stars, hyperonic neutron stars, and hybrid
stars, and check the reliability of the results on various types of equations
of state of dense matter. The equations of state from the relativistic mean
field theory and the MIT bag model are adopted to simulate compact stars.
Numerical calculations of rigidly rotating neutron stars are performed using
the RNS code in the framework of general relativity by solving the Einstein
equations for stationary axis-symmetric spacetime. The crust structure of
compact stars is found to be a key factor to determine the maximum value of the
spin parameter $j_{\rm max}$. For the stars with inclusion of the crust,
$j_{\rm max}\sim 0.7$ is sustained for various kinds of compact stars with
$M>0.5 M_{\odot}$, and is found to be insensitive to the mass of star and
selected equations of state. For the traditional neutron stars, hyperonic
neutron stars and hybrid stars without crust structure, the value $j_{\rm max}$
lies in the range of $[0.7, 1.0]$. Thus, not $j>0.7$ but $j>1$ could be treated
as the candidate criterion to distinguish the strange quark stars from the
other kinds of compact stars. Furthermore, a universal formula
$j=0.48(f/f_K)^3-0.42(f/f_K)^2+0.63(f/f_K)$ is suggested to calculate the spin
parameter at any rotational frequency for all kinds of compact stars with crust
structure and $M>0.5M_{\odot}$. | astro-ph_SR |
Massive Outflows Driven by Magnetic Effects II: Comparison with
Observations: The driving mechanism of massive outflows observed in high-mass star-forming
regions is investigated using three-dimensional magnetohydrodynamics (MHD) and
protostellar evolution calculations. In our previous paper, we showed that the
mass outflow rate depends strongly on the mass accretion rate onto the
circumstellar disk around a high-mass protostar, and massive outflows may be
driven by the magnetic effect in high-mass star-forming cores. In the present
study, in order to verify that the MHD disk wind is the primary driving
mechanism of massive outflows, we quantitatively compare outflow properties
obtained through simulations and observations. Since the outflows obtained
through simulations are slightly younger than those obtained through
observations, the time-integrated quantities of outflow mass, momentum, and
kinetic energy are slightly smaller than those obtained through observations.
On the other hand, time-derivative quantities of mass ejection rate, outflow
momentum flux, and kinetic luminosity obtained through simulations are in very
good agreement with those obtained through observations. This indicates that
the MHD disk wind greatly contributes to the massive outflow driving from
high-mass protostars, and the magnetic field might significantly control the
high-mass star formation process. | astro-ph_SR |
Lithium in the Hyades L5 brown dwarf 2MASSJ04183483+2131275: From the luminosity, effective temperature, and age of the Hyades brown dwarf
2MASSJ04183483+2131275 (2M0418), sub-stellar evolutionary models predict a mass
in the range 39-55 Jupiter masses (M_Jup) which is insufficient to produce any
substantial lithium burning except for the very upper range >53 M_Jup. Our goal
is to measure the abundance of lithium in this object, test the consistency
between models and observations and refine constraints on the mass and age of
the object.
We used the 10.4-m Gran Telescopio Canarias (GTC) with its low-dispersion
optical spectrograph to obtain ten spectra of 2277s each covering the range
6300-10300 Angstroms with a resolving power of R~500.
In the individual spectra, which span several months, we detect persistent
unresolved H_alpha in emission with pseudo equivalent widths (pEW) in the range
45-150 Angstroms and absorption lines of various alkalis with the typical
strengths found in objects of L5 spectral type. The lithium resonance line at
6707.8 Angstroms is detected with pEW of 18+/-4 Angstroms in 2M0418 (L5).
We determine a lithium abundance of log N(Li) = 3.0+/-0.4 dex consistent with
a minimum preservation of 90% of this element which confirms 2M0418 as a brown
dwarf with a maximum mass of 52 M_Jup. We infer a maximum age for the Hyades of
775 Myr from a comparison with the BHAC15 models. Combining recent results from
the literature with our study, we constrain the mass of 2M0418 to 45-52 M_Jup
and the age of the cluster to 580-775 Myr (1 sigma) based on the lithium
depletion boundary method. | astro-ph_SR |
Stability of the coronal magnetic field around large confined and
eruptive solar flares: In order to improve our understanding on the pre-requisites of eruptive solar
flares, we study and compare different measures that characterize the eruptive
potential of solar active regions - the critical height for torus instability
as a local measure and the helicity ratio as a global measure - with the
structural properties of the underlying magnetic field, namely the altitude of
the center of the current-carrying magnetic structure. Using time series of 3D
optimization-based nonlinear force-free magnetic field models for 10 different
active regions (ARs) around the time of large solar flares, we determine the
altitudes of the current-weighted centers of the non-potential model
structures. Based on the potential magnetic field, we inspect the decay index,
$n$, in multiple vertical planes oriented along of or perpendicular to the
flare-relevant polarity inversion line, and estimate the critical height
($h_{\mathrm{crit}}$) for torus instability (TI) using different thresholds of
$n$. The critical heights are interpreted with respect to the altitudes of the
current-weighted centers of the associated non-potential structures, as well as
the eruptive character of the associated flares, and the eruptive potential of
the host AR, as characterized by the helicity ratio. Our most important
findings are that (i) $h_{\mathrm{crit}}$ is more segregated in terms of flare
type than the helicity ratio, and that (ii) coronal field configurations with a
higher eruptive potential (in terms of the helicity ratio) also appear to be
more prone to TI. Furthermore, we find no pronounced differences in the
altitudes of the non-potential structures prior to confined and eruptive
flares. | astro-ph_SR |
The past and future evolution of a star like Betelgeuse: We discuss the physics and the evolution of a typical massive star passing
through an evolutionary stage similar to that of Betelgeuse. After a brief
introduction recalling various observed parameters of Betelgeuse, we discuss
the Pre-Main-Sequence phase (PMS), the Main-Sequence (MS) phase, the physics
governing the duration of the first crossing of the HR diagram, the red
supergiant stage (RSG), the post-red supergiant phases and the final fate of
solar metallicity stars with masses between 9 and 25 M$_\odot$. We examine the
impact of different initial rotation and of various prescriptions for the mass
loss rates during the red supergiant phase. We show that, whatever the initial
rotation rate (chosen between 0 and 0.7$\times\upsilon_{\rm crit}$,
$\upsilon_{\rm crit}$ being the surface equatorial velocity producing a
centrifugal acceleration balancing exactly the gravity) and the mass loss rates
during the RSG stage (varied between a standard value and 25 times that value),
a 15 M$_\odot$ star always ends its lifetime as a RSG and explodes as a type
II-P or II-L supernova. | astro-ph_SR |
Radial Speed Evolution of Interplanetary Coronal Mass Ejections During
Solar Cycle 23: We report radial speed evolution of interplanetary coronal mass ejections
(ICMEs) detected by the SOHO/LASCO coronagraph, interplanetary scintillation
(IPS) at 327 MHz, and in-situ observations. In this study, we analyze solar
wind disturbance factor (g-value) data derived from IPS observations during
1997-2009 covering nearly the whole period of Solar Cycle 23. By comparing
observations from the SOHO/LASCO, IPS, and in situ, we then identify 39 ICMEs
that could be analyzed carefully. Here, we define two speeds V_SOHO and V_bg,
which are initial speed of ICME and the speed of the background solar wind,
respectively. Examinations of these speeds yield the following results; i) Fast
ICMEs (with V_SOHO - V_bg > 500 km/s) rapidly decelerate, moderate ICMEs (with
0 km/s < V_SOHO - V_bg < 500 km/s) show either gradually decelerating or
uniform motion, and slow ICMEs (with V_SOHO - V_bg < 0 km/s) accelerate. The
radial speeds converge on the speed of background solar wind during their
outward propagation. We subsequently find; ii) both the acceleration and
deceleration are nearly complete by 0.79 (+/- 0.04) AU, and those are ended
when the ICME speed reaches a given speed. We find the value of that to be 480
(+/- 21) km/s. iii) For the fast and moderate ICMEs, a linear equation with a
constant coefficient is more appropriate than a quadratic equation to describe
their kinematics, because the chi-square for the linear equation satisfies the
statistical significance level of 0.05, while the quadratic one is not. These
results support the hypothesis that the radial motion of ICMEs is governed by a
drag force due to interaction with the background solar wind. These findings
also suggest that ICMEs propagating faster than background solar wind are
controlled mainly by the hydrodynamic Stokes drag. | astro-ph_SR |
Discovery of three self-lensing binaries from Kepler: We report the discovery of three edge-on binaries with white dwarf companions
that gravitationally magnify (instead of eclipsing) the light of their stellar
primaries, as revealed by a systematic search for pulses with long periods in
the Kepler photometry. We jointly model the self-lensing light curves and
radial-velocity orbits to derive the white dwarf masses, all of which are close
to 0.6 Solar masses. The orbital periods are long, ranging from 419 to 728
days, and the eccentricities are low, all less than 0.2. These characteristics
are reminiscent of the orbits found for many blue stragglers in open clusters
and the field, for which stable mass transfer due to Roche-lobe overflow from
an evolving primary (now a white dwarf) has been proposed as the formation
mechanism. Because the actual masses for our three white dwarf companions have
been accurately determined, these self-lensing systems would provide excellent
tests for models of interacting binaries. | astro-ph_SR |
Astrometric detection of exoplanets from the ground: Astrometry is a powerful technique to study the populations of extrasolar
planets around nearby stars. It gives access to a unique parameter space and is
therefore required for obtaining a comprehensive picture of the properties,
abundances, and architectures of exoplanetary systems. In this review, we
discuss the scientific potential, present the available techniques and
instruments, and highlight a few results of astrometric planet searches, with
an emphasis on observations from the ground. In particular, we discuss
astrometric observations with the Very Large Telescope (VLT) Interferometer and
a programme employing optical imaging with a VLT camera, both aimed at the
astrometric detection of exoplanets. Finally, we set these efforts into the
context of Gaia, ESA's astrometry mission scheduled for launch in 2013, and
present an outlook on the future of astrometric exoplanet detection from the
ground. | astro-ph_SR |
Exploring the spectral \textit{information content} in the LAMOST
medium-resolution survey (MRS): Low-resolution spectra are proved competitive to high-resolution spectra in
determining many stellar labels at comparable precision. It is useful to
consider the spectral information content when assessing the capability of a
stellar spectrum in deriving precise stellar labels. In this work, we quantify
the information content brought by the LAMOST-II medium-resolution
spectroscopic survey (MRS) using the gradient spectra and the
coefficients-of-dependence (CODs). In general, the wavelength coverage of the
MRS well constrains the stellar labels but the sensitivities of different
stellar labels vary with spectral types and metallicity of the stars of
interest and, therefore, affect the performance of the stellar label
determination from the MRS spectra. Applying the SLAM to the synthetic spectra
which mimic the MRS data, we find the precision of the fundamental stellar
parameters Teff, logg and [M/H] are better when combining both the blue and red
bands of the MRS. This is especially important for warm stars since the
H$\alpha$ line located in the red part plays a more important role in
determining the effective temperature for warm stars. With blue and red parts
together, we are able to reach similar performance to the low-resolution
spectra except for warm stars. However, at [M/H]$\sim-2.0$ dex, the
uncertainties of fundamental stellar labels estimated from MRS are
substantially larger than those from low-resolution spectra. We also tested the
uncertainties of Teff, logg and [M/H] of from MRS data induced from the radial
velocity mismatch and find that a mismatch of about 1 km s$^{-1}$, which is
typical for LAMOST MRS data, would not significantly affect the stellar label
estimates. At last, reference precision limits are calculated using synthetic
gradient spectra, according to which we expect abundances of at least 17
elements to be measured precisely from MRS spectra. | astro-ph_SR |
Signatures of ubiquitous magnetic reconnection in the deep atmosphere of
sunspot penumbrae: Ellerman bombs are regions with enhanced Balmer line wing emission and mark
magnetic reconnection in the deep solar atmosphere in active regions and quiet
Sun. They are often found in regions where opposite magnetic polarities are in
close proximity. Recent high resolution observations suggest that Ellerman
bombs are more prevalent than thought before. We aim to determine the
occurrence of Ellerman bombs in the penumbra of sunspots. We analyze high
spatial resolution observations of sunspots in the Balmer H-alpha and H-beta
lines as well as auxiliary continuum channels obtained with the Swedish 1-m
Solar Telescope and apply the k-means clustering technique to systematically
detect and characterize Ellerman Bombs. Features with all the defining
characteristics of Ellerman bombs are found in large numbers over the entire
penumbra. The true prevalence of these events is only fully appreciated in the
H-beta line due to highest spatial resolution and lower chromospheric opacity.
We find that the penumbra hosts some of the highest Ellerman bomb densities,
only surpassed by the moat in the immediate surroundings of the sunspot. Some
penumbral Ellerman bombs show flame morphology and rapid dynamical evolution.
Many penumbral Ellerman bombs are fast moving with typical speed of 3.7 km/s
and sometimes more than 10 km/s. Many penumbral Ellerman bombs migrate from the
inner to the outer penumbra over hundreds of km and some continue moving beyond
the outer penumbral boundary into the moat. Many penumbral Ellerman bombs are
found in the vicinity of regions with opposite magnetic polarity. We conclude
that reconnection is a near continuous process in the low atmosphere of the
penumbra of sunspots as manifest in the form of penumbral Ellerman bombs. These
are so prevalent that they may be a major sink of sunspot magnetic energy. | astro-ph_SR |
The process of data formation for the Spectrometer/Telescope for Imaging
X-rays (STIX) in Solar Orbiter: The Spectrometer/Telescope for Imaging X-rays (STIX) is a hard X-ray imaging
spectroscopy device to be mounted in the Solar Orbiter cluster with the aim of
providing images and spectra of solar flaring regions at different photon
energies in the range from a few keV to around 150 keV. The imaging modality of
this telescope is based on the Moire pattern concept and utilizes 30
sub-collimators, each one containing a pair of co-axial grids. This paper
applies Fourier analysis to provide the first rigorous description of the data
formation process in STIX. Specifically, we show that, under first harmonic
approximation, the integrated counts measured by STIX sub-collimators can be
interpreted as specific spatial Fourier components of the incoming photon flux,
named visibilities. Fourier analysis also allows the quantitative assessment of
the reliability of such interpretation. The description of STIX data in terms
of visibilities has a notable impact on the image reconstruction process, since
it fosters the application of Fourier-based imaging algorithms. | astro-ph_SR |
Dust-temperature of an isolated star-forming cloud: Herschel
observations of the Bok globule CB244: We present Herschel observations of the isolated, low-mass star-forming Bok
globule CB244. It contains two cold sources, a low-mass Class 0 protostar and a
starless core, which is likely to be prestellar in nature, separated by 90
arcsec (~ 18000 AU). The Herschel data sample the peak of the Planck spectrum
for these sources, and are therefore ideal for dust-temperature and column
density modeling. With these data and a near-IR extinction map, the MIPS 70
micron mosaic, the SCUBA 850 micron map, and the IRAM 1.3 mm map, we model the
dust-temperature and column density of CB244 and present the first measured
dust-temperature map of an entire star-forming molecular cloud. We find that
the column-averaged dust-temperature near the protostar is ~ 17.7 K, while for
the starless core it is ~ 10.6K, and that the effect of external heating causes
the cloud dust-temperature to rise to ~ 17 K where the hydrogen column density
drops below 10^21 cm^-2. The total hydrogen mass of CB244 (assuming a distance
of 200 pc) is 15 +/- 5 M_sun. The mass of the protostellar core is 1.6 +/- 0.1
M_sun and the mass of the starless core is 5 +/- 2 M_sun, indicating that ~ 45%
of the mass in the globule is participating in the star-formation process. | astro-ph_SR |
A new extremely low-mass white dwarf in the NLTT catalogue: We report on the discovery of the extremely low-mass, hydrogen-rich white
dwarf, NLTT 11748. Based on measurements of the effective temperature
(8540+/-50 K) and surface gravity (log g = 6.20+/-0.15) obtained by fitting the
observed Balmer line profiles with synthetic spectra, we derive a mass of
0.167+/-0.005 M_solar. This object is one of only a handful of white dwarfs
with masses below 0.2 M_solar that are believed to be the product of close
binary evolution with an episode of Roche lobe overflow onto a degenerate
companion (neutron star or white dwarf). Assuming membership in the halo
population, as suggested by the kinematics and adopting a cooling age of 4.0 -
6.3 Gyrs for the white dwarf, we infer a progenitor mass of 0.87 - 0.93
M_solar. The likely companion has yet to be identified, but a search for radial
velocity variations may help constrain its nature. | astro-ph_SR |
Formation of Extremely Low-mass White Dwarfs Binaries: Motivated by the discovery of a handful of pulsating, extremely low mass
white dwarfs (ELM WDs, mass $M \lesssim 0.17\, M_\odot$) which likely have WD
companions, this paper discusses binary formation models for these systems.
Formation of an ELM WD by unstable mass transfer (MT) or a common envelope (CE)
event is unlikely. Stable Roche-lobe overflow with conservative MT produces
only $M \gtrsim 0.2\, M_\odot$. This paper discusses the formation of ELM WD
using angular momentum losses by magnetic braking. The initially more massive
star becomes the companion WD through a CE event. The initially less massive
star becomes the ELM WD as an evolved donor in a Cataclysmic Variable binary.
Evolutionary models are constructed using the Modules for Experiments in
Stellar Astrophysics (MESA), with ELM WD progenitors in the range
$M_d=1.0-1.5\, M_\odot$ and WD companions in the range $M_{\rm a} \simeq
0.4-0.9\, M_\odot$. A prescription to reduce magnetic braking for thin surface
convection zones is included. Upon the thinning of the evolved donor's
envelope, the donor star shrinks out of contact and MT ceases, revealing the
ELM WD. Systems with small masses have previously been suggested as possible AM
CVN's. Systems with large masses, up to the limit $M \simeq 0.18\, M_\odot$ at
which shell flashes occur on the WD cooling track, will tend to expand out to
orbital periods $P_{\rm orb} \gtrsim 15\, {\rm hr}$. In between this range, ELM
WD may become pulsators both as pre-WD and on the WD cooling track. Brickhill's
criterion for convective mode driving is used to estimate the location of the
blue edge of the g-mode instability strip. | astro-ph_SR |
Neutrino Quantum Kinetic Equations: Neutrinos propagate in astrophysical and cosmological environments modifying
their flavor in intriguing ways. The study of neutrino propagation in media is
based on the mean-field, extended mean-field and Boltzmann equations. We
summarise salient features of these evolution equations and the methods
employed so far to derive them. We emphasize applications to situations of
observational interest. | astro-ph_SR |
The nature of Elsässer variables in compressible MHD: The Els\"{a}sser variables are often used in studies of plasma turbulence, in
helping differentiate between MHD waves propagating parallel or anti-parallel
to the main magnetic field. While for pure Alfv\'en waves in a homogeneous
plasma the method is strictly valid, we show that compressible, magnetoacoustic
waves are in general described by both Els\"{a}sser variables. Furthermore, in
a compressible and inhomogeneous plasma, the pure MHD waves (Alfv\'en, fast and
slow) are no longer normal modes, but waves become linearly coupled or display
mixed properties of Alfv\'en and magnetoacoustic nature. These waves are
necessarily described by both Els\"{a}sser variables and therefore the
Els\"{a}sser formalism cannot be used to strictly separate parallel and
anti-parallel propagating waves. Nevertheless, even in an inhomogeneous plasma,
for a highly Alfv\'enic wave the Els\"{a}sser variable corresponding to the
propagation direction appears still dominating. We suggest that for Alfv\'enic
waves, the relative amplitude of Els\"{a}sser variables depends on the local
degree of inhomogeneity and other plasma and wave properties. This finding has
implications for turbulence studies in inhomogeneous and compressible plasmas,
such as the solar corona and solar wind. | astro-ph_SR |
Algebraic and machine learning approach to hierarchical triple-star
stability: We present two approaches to determine the dynamical stability of a
hierarchical triple-star system. The first is an improvement on the
Mardling-Aarseth stability formula from 2001, where we introduce a dependence
on inner orbital eccentricity and improve the dependence on mutual orbital
inclination. The second involves a machine learning approach, where we use a
multilayer perceptron (MLP) to classify triple-star systems as `stable' and
`unstable'. To achieve this, we generate a large training data set of 10^6
hierarchical triples using the N-body code MSTAR. Both our approaches perform
better than previous stability criteria, with the MLP model performing the
best. The improved stability formula and the machine learning model have
overall classification accuracies of 93 % and 95 % respectively. Our MLP model,
which accurately predicts the stability of any hierarchical triple-star system
within the parameter ranges studied with almost no computation required, is
publicly available on Github in the form of an easy-to-use Python script. | astro-ph_SR |
Colors of c-type RR Lyrae Stars and Interstellar Reddening: RR Lyrae stars pulsating in the fundamental mode have long been used to
measure interstellar reddening, based on their observed uniformity of $B-V$
color at minimum light after small corrections for metallicity and period are
applied. However, little attention has been paid to the first overtone
pulsators (RRc or RR1). We present new $V-I$ observations of field RRc stars,
supplemented with published data from uncrowded RRc in globular clusters.
Preliminary results indicate the RRc colors are correlated with period, but
appear to be independent of the stars' metallicity. The scatter around the
period-color relation is slightly larger than a comparable relation for RRab.
Thus, RRc can be useful indicators of line of sight reddening toward old
stellar systems, particularly when multiple stars are available as in
Oosterhoff II globular clusters and metal-poor galaxies. | astro-ph_SR |
Chromospheric magnetic field: A comparison of He I 10830 A observations
with nonlinear force-free field extrapolation: The nonlinear force-free field (NLFFF) modeling has been extensively used to
infer the three-dimensional (3D) magnetic field in the solar corona. One of the
assumptions in the NLFFF extrapolation is that the plasma beta is low, but this
condition is considered to be incorrect in the photosphere. We examine direct
measurements of the chromospheric magnetic field in two active regions through
spectropolarimetric observations at He I 10830 A, which are compared with the
potential fields and NLFFFs extrapolated from the photosphere. The comparisons
allow quantitative estimation of the uncertainty in the NLFFF extrapolation
from the photosphere. Our analysis shows that observed chromospheric magnetic
field may have larger non-potentiality compared to the photospheric magnetic
field. Moreover, the large non-potentiality in the chromospheric height may not
be reproduced by the NLFFF extrapolation from the photospheric magnetic field.
The magnitude of the underestimation of the non-potentiality at chromospheric
heights may reach 30-40 degree in shear signed angle in some locations. This
deviation may be caused by the non-force-freeness in the photosphere. Our study
suggests the importance of the inclusion of measured chromospheric magnetic
fields in the NLFFF modeling for the improvement of the coronal extrapolation. | astro-ph_SR |
The geometry of the close environment of SV Psc as probed by VLTI/MIDI: Context. SV Psc is an asymptotic giant branch (AGB) star surrounded by an
oxygen-rich dust envelope. The mm-CO line profile of the object's outflow shows
a clear double-component structure. Because of the high angular resolution,
mid-IR interferometry may give strong constraints on the origin of this
composite profile.
Aims. The aim of this work is to investigate the morphology of the
environment around SV Psc using high-angular resolution interferometry
observations in the mid-IR with the Very Large Telescope MID-infrared
Interferometric instrument (VLTI/MIDI).
Methods. Interferometric data in the N-band taken at different baseline
lengths (ranging from 32-64 m) and position angles (73- 142{\deg}) allow a
study of the morphology of the circumstellar environment close to the star. The
data are interpreted on the basis of 2-dimensional, chromatic geometrical
models using the fitting software tool GEM-FIND developed for this purpose.
Results. The results favor two scenarios: (i) the presence of a highly
inclined, optically thin, dusty disk surrounding the central star; (ii) the
presence of an unresolved binary companion at a separation of 13.7 AU and a
position angle of 121.8{\deg} NE. The derived orbital period of the binary is
38.1 yr. This detection is in good agreement with hydrodynamic simulations
showing that a close companion could be responsible for the entrainment of the
gas and dust into a circumbinary structure. | astro-ph_SR |
Asteroseismic stellar activity relations: In asteroseismology an important diagnostic of the evolutionary status of a
star is the small frequency separation which is sensitive to the gradient of
the mean molecular weight in the stellar interior. It is thus interesting to
discuss the classical age-activity relations in terms of this quantity.
Moreover, as the photospheric magnetic field tends to suppress the amplitudes
of acoustic oscillations, it is important to quantify the importance of this
effect by considering various activity indicators. We propose a new class of
age-activity relations that connects the Mt. Wilson $S$ index and the average
scatter in the light curve with the small frequency separation and the
amplitude of the p-mode oscillations. We used a Bayesian inference to compute
the posterior probability of various empirical laws for a sample of 19
solar-like active stars observed by the Kepler telescope. We demonstrate the
presence of a clear correlation between the Mt. Wilson $S$ index and the
relative age of the stars as indicated by the small frequency separation, as
well as an anti-correlation between the $S$ index and the oscillation
amplitudes. We argue that the average activity level of the stars shows a
stronger correlation with the small frequency separation than with the absolute
age that is often considered in the literature. The phenomenological laws
discovered in this paper have the potential to become new important diagnostics
to link stellar evolution theory with the dynamics of global magnetic fields.
In particular we argue that the relation between the Mt. Wilson $S$ index and
the oscillation amplitudes is in good agreement with the findings of direct
numerical simulations of magneto-convection. | astro-ph_SR |
The rapid dispersal of low-mass virialised clusters: Infant mortality brought about by the expulsion of a star cluster's natal gas
is widely invoked to explain cluster statistics at different ages. While a well
studied problem, most recent studies of gas expulsion's effect on a cluster
have focused on massive clusters, with stellar counts of order $10^4$. Here we
argue that the evolutionary timescales associated with the compact low-mass
clusters typical of the median cluster in the Solar neighborhood are short
enough that significant dynamical evolution can take place over the ages
usually associated with gas expulsion. To test this we perform {\it N}-body
simulations of the dynamics of a very young star forming region, with initial
conditions drawn from a large-scale hydrodynamic simulation of gravitational
collapse and fragmentation. The subclusters we analyse, with populations of a
few hundred stars, have high local star formation efficiencies and are roughly
virialised even after the gas is removed. Over 10 Myr they expand to a similar
degree as would be expected from gas expulsion if they were initially gas-rich,
but the expansion is purely due to the internal stellar dynamics of the young
clusters. The expansion is such that the stellar densities at 2 Myr match those
of YSOs in the Solar neighborhood. We argue that at the low-mass end of the
cluster mass spectrum, a deficit of clusters at 10s of Myr does not necessarily
imply gas expulsion as a disruption mechanism. | astro-ph_SR |
Variability in Hot Carbon-Dominated Atmosphere (hot DQ) White Dwarfs:
Rapid Rotation?: Hot white dwarfs with carbon-dominated atmospheres (hot DQs) are a cryptic
class of white dwarfs. In addition to their deficiency of hydrogen and helium,
most of these stars are highly magnetic, and a large fraction vary in
luminosity. This variability has been ascribed to nonradial pulsations, but
increasing data call this explanation into question. We present studies of
short-term variability in seven hot DQ white dwarfs. Three (SDSS J1426+5752,
SDSS J2200-0741, and SDSS J2348-0942) were known to be variable. Their
photometric modulations are coherent over at least two years, and we find no
evidence for variability at frequencies that are not harmonics. We present the
first time-series photometry for three additional hot DQs (SDSS J0236-0734,
SDSS J1402+3818, and SDSS J1615+4543); none are observed to vary, but the
signal-to-noise is low. Finally, we present high speed photometry for SDSS
J0005-1002, known to exhibit a 2.1 d photometric variation; we do not observe
any short-term variability. Monoperiodicity is rare among pulsating white
dwarfs, so we contemplate whether the photometric variability is due to
rotation rather than pulsations; similar hypotheses have been raised by other
researchers. If the variability is due to rotation, then hot DQ white dwarfs as
a class contain many rapid rotators. Given the lack of companions to these
stars, the origin of any fast rotation is unclear -- both massive progenitor
stars and double degenerate merger remnants are possibilities. We end with
suggestions on future work that would best clarify the nature of these rare,
intriguing objects. | astro-ph_SR |
The gravitational redshift of solar-type stars from Gaia DR3 wide
binaries: Light escaping from a gravitational potential suffers a redshift with
magnitude proportional to the depth of the potential. This "gravitational
redshift" is easily measurable in dense stars such as white dwarfs, but is much
weaker and has evaded unambiguous detection in main-sequence stars. I show that
the effect is directly measurable in the Gaia DR3 radial velocities (RVs) of
the components of wide binary stars. In a sample of $\sim$500 wide binaries
containing a solar-type main-sequence star and a red giant or red clump
companion, the apparent RV of the giant is on average $0.49 \pm 0.02 \,\, \rm
km\,s^{-1}$ lower than that of the main-sequence star. This owes primarily to
the giants' weaker gravitational fields and is in reasonably good agreement
with the value expected from general relativity. | astro-ph_SR |
Consecutive Narrow and Broad Quasi-periodic Fast-propagating Wave Trains
Associated with a Flare: The excitation mechanism of coronal quasi-period fast-propagating (QFP) wave
trains remains unresolved. Using Atmospheric Imaging Assembly onboard the Solar
Dynamics Observatory observations, we study a narrow and a broad QFP wave train
excited one after another during the successive eruptions of filaments hosted
within a fan-spine magnetic system on 2013 October 20. The consecutive
occurrence of these two types of QFP wave trains in the same event provides an
excellent opportunity to explore their excitation mechanisms and compare their
physical parameters. Our observational results reveal that narrow and broad QFP
wave trains exhibit distinct speeds, periods, energy fluxes, and relative
intensity amplitudes, although originating from the same active region and
being associated with the same {\em GOES} C2.9 flare. Using wavelet analysis,
we find that the narrow QFP wave train shares a similar period with the flare
itself, suggesting its possible excitation through the pulsed energy release in
the magnetic reconnection process that generated the accompanying flare. On the
other hand, the broad QFP wave train appears to be associated with the energy
pulses released by the successive expansion and unwinding of filament threads.
Additionally, it is plausible that the broad QFP wave train was also excited by
the sequential stretching of closed magnetic field lines driven by the erupting
filament. These findings shed light on the different excitation mechanisms and
origins of the QFP wave trains. | astro-ph_SR |
Panchromatic evolution of three luminous red novae: Forbidden hugs in
pandemic times -- IV: We present photometric and spectroscopic data on three extragalactic luminous
red novae (LRNe): AT2018bwo, AT2021afy, and AT2021blu. AT2018bwo was discovered
in NGC45 (at 6.8 Mpc) a few weeks after the outburst onset. During the
monitoring period, the transient reached a peak luminosity of 10^40 erg/s.
AT2021afy, hosted by UGC10043 (49.2 Mpc), showed a double-peaked light curve,
with the two peaks reaching a similar luminosity of 2.1(+-0.6)x10^41 erg/s. For
AT2021blu in UGC5829, (8.6 Mpc), the pre-outburst phase was well-monitored by
several photometric surveys, and the object showed a slow luminosity rise
before the outburst. The light curve of AT2021blu was sampled with an
unprecedented cadence until the object disappeared behind the Sun, and it was
then recovered at late phases. The light curve of AT2021blu shows a double
peak, with a prominent early maximum reaching a luminosity of 6.5x10^40 erg/s,
which is half of that of AT2021afy. The spectra of AT2021afy and AT2021blu
display the expected evolution for LRNe: a blue continuum dominated by
prominent Balmer lines in emission during the first peak, and a redder
continuum consistent with that of a K-type star with narrow absorption metal
lines during the second, broad maximum. The spectra of AT2018bwo are markedly
different, with a very red continuum dominated by broad molecular features in
absorption. As these spectra closely resemble those of LRNe after the second
peak, AT2018bwo was probably discovered at the very late evolutionary stages.
This would explain its fast evolution and the spectral properties compatible
with that of an M-type star. From the analysis of deep frames of the LRN sites
years before the outburst, and considerations of the light curves, the
quiescent progenitor systems of the three LRNe were likely massive, with
primaries ranging from 13Mo for AT2018bwo, to 13-18Mo for AT2021blu, and over
40Mo for AT2021afy. | astro-ph_SR |
EPIC 216747137: a new HW Vir eclipsing binary with a massive sdOB
primary and a low-mass M-dwarf companion: EPIC 216747137 is a new HW~Virginis system discovered by the Kepler
spacecraft during its K2 "second life". Like the other HW Vir systems, EPIC
216747137 is a post-common-envelope eclipsing binary consisting of a hot
subluminous star and a cool low-mass companion. The short orbital period of
3.87 hours produces a strong reflection effect from the secondary (~9% in the R
band). Together with AA Dor and V1828 Aql, EPIC 216747137 belongs to a small
subgroup of HW Vir systems with a hot evolved sdOB primary. We find the
following atmospheric parameters for the hot component: Teff=40400$\pm$1000 K,
logg=5.56$\pm$0.06, log(N(He)/N(H))=$-$2.59$\pm$0.05. The sdOB rotational
velocity vsini=51$\pm$10 km/s implies that the stellar rotation is slower than
the orbital revolution and the system is not synchronized. When we combine
photometric and spectroscopic results with the Gaia parallax, the best solution
for the system corresponds to a primary with a mass of about 0.62 Msun close
to, and likely beyond, the central helium exhaustion, while the cool M-dwarf
companion has a mass of about 0.11 Msun. | astro-ph_SR |
Damping of filament thread oscillations: effect of the slow continuum: Transverse oscillations of small amplitude are commonly seen in
high-resolution observations of filament threads, i.e. the fine-structures of
solar filaments/prominences, and are typically damped in a few periods. Kink
wave modes supported by the thread body offer a consistent explanation of these
observed oscillations. Among the proposed mechanisms to explain the kink mode
damping, resonant absorption in the Alfven continuum seems to be the most
efficient as it produces damping times of about 3 periods. However, for a
nonzero-beta plasma and typical prominence conditions, the kink mode is also
resonantly coupled to slow (or cusp) continuum modes, which could further
reduce the damping time. In this Letter, we explore for the first time both
analytically and numerically the effect of the slow continuum on the damping of
transverse thread oscillations. The thread model is composed of a homogeneous
and straight cylindrical plasma, an inhomogeneous transitional layer, and the
homogeneous coronal plasma. We find that the damping of the kink mode due to
the slow resonance is much less efficient than that due to the Alfven
resonance. | astro-ph_SR |
Radial velocity photon limits for the dwarf stars of spectral classes
F--M: The determination of extrasolar planet masses with the radial velocity (RV)
technique requires spectroscopic Doppler information from the planet's host
star, which varies with stellar brightness and temperature. We analyze Doppler
information in spectra of F--M dwarfs utilizing empirical information from
HARPS and CARMENES, and from model spectra. We come to the conclusions that an
optical setup ($BVR$-bands) is more efficient that a near-infrared one ($YJHK$)
in dwarf stars hotter than 3200\,K. We publish a catalogue of 46,480
well-studied F--M dwarfs in the solar neighborhood and compare their
distribution to more than one million stars from Gaia DR2. For all stars, we
estimate the RV photon noise achievable in typical observations assuming no
activity jitter and slow rotation. We find that with an ESPRESSO-like
instrument at an 8m-telescope, a photon noise limit of 10\,cm\,s$^{-1}$ or
lower can be reached in more than 280 stars in a 5\,min observation. At
4m-telescopes, a photon noise limit of 1\,m\,s$^{-1}$ can be reached in a
10\,min exposure in approx.\ 10,000 predominantly sun-like stars with a
HARPS-like (optical) instrument. The same applies to $\sim$3000 stars for a
red-optical setup covering the $RIz$-bands, and to $\sim$700 stars for a
near-infrared instrument. For the latter two, many of the targets are nearby M
dwarfs. Finally, we identify targets in which Earth-mass planets within the
liquid water habitable zone can cause RV amplitudes comparable to the RV photon
noise. Assuming the same exposure times, we find that an ESPRESSO-like
instrument can reach this limit for 1\,M$_\Earth$ planets in more than 1000
stars. The optical, red-optical, and near-infrared configurations reach the
limit for 2\,M$_\Earth$ planets in approximately 500, 700, and 200 stars,
respectively. | astro-ph_SR |
CSI 2264: Characterizing Young Stars in NGC 2264 with Stochastically
Varying Light Curves: We provide CoRoT and Spitzer light curves, as well as broad-band
multi-wavelength photometry and high resolution, multi- and single-epoch
spectroscopy for 17 classical T Tauris in NGC 2264 whose CoRoT light curves
(LCs) exemplify the "stochastic" LC class as defined in Cody et al. (2014). The
most probable physical mechanism to explain the optical variability in this LC
class is time-dependent mass accretion onto the stellar photosphere, producing
transient hot spots. As evidence in favor of this hypothesis, multi-epoch high
resolution spectra for a subset of these stars shows that their veiling levels
also vary in time and that this veiling variability is consistent in both
amplitude and timescale with the optical LC morphology. Furthermore, the
veiling variability is well-correlated with the strength of the HeI 6678A
emission line, a feature predicted by models to arise in accretion shocks on or
near the stellar photosphere. Stars with accretion burst LC morphology
(Stauffer et al. 2014) are also attributed to variable mass accretion. Both the
stochastic and accretion burst LCs can be explained by a simple model of
randomly occurring flux bursts, with the stochastic LC class having a higher
frequency of lower amplitude events. Based on their UV excesses, veiling, and
mean Ha equivalent widths, members of the stochastic LC class have only
moderate time-averaged mass accretion rates. The most common feature of their
Ha profiles is for them to exhibit blue-shifted absorption features, most
likely originating in a disk wind. The lack of periodic signatures in the LCs
suggests that little of the variability is due to long-lived hot spots rotating
into or out of our line of sight; instead, the primary driver of the observed
photometric variability is likely to be instabilities in the inner disk that
lead to variable mass accretion. | astro-ph_SR |
Classical Cepheids Require Enhanced Mass Loss: Measurements of rates of period change of Classical Cepheids probe stellar
physics and evolution. Additionally, better understanding of Cepheid structure
and evolution provides greater insight into their use as standard candles and
tools for measuring the Hubble constant. Our recent study of the period change
of the nearest Cepheid, Polaris, suggested that it is undergoing enhanced mass
loss when compared to canonical stellar evolution model predictions. In this
work, we expand the analysis to rates of period change measured for about 200
Galactic Cepheids and compare them to population synthesis models of Cepheids
including convective core overshooting and enhanced mass loss. Rates of period
change predicted from stellar evolution models without mass loss do not agree
with observed rates whereas including enhanced mass loss yields predicted rates
in better agreement with observations. This is the first evidence that enhanced
mass loss as suggested previously for Polaris and delta Cephei must be a
ubiquitous property of Classical Cepheids. | astro-ph_SR |
The quasi-biennial periodicity as a window on the solar magnetic dynamo
configuration: Manifestations of the solar magnetic activity through periodicities of about
11 and 2 years are now clearly seen in all solar activity indices.In this
paper, we add information about the mechanism driving the 2 year period by
studying the time and latitudinal properties of acoustic modes that are
sensitive probes of the subsurface layers. We use almost 17 years of high
quality resolved data provided by the Global Oscillation Network Group (GONG)
to investigate the solar cycle changes in p-mode frequencies for spherical
degrees l from 0 to 120 and 1.6 mHz < nu < 3.5 mHz. For both periodic
components of solar activity, we locate the origin of the frequency shift in
the subsurface layers and put in evidence for a sudden enhancement in amplitude
just in the last few hundred kilometers. We also show that, in both cases, the
size of the shift increases towards equatorial latitudes and from minimum to
maximum of solar activity, but, in agreement with previous findings,the
quasi-biennial periodicity (QBP) causes a weaker shift in mode frequencies and
a slower enhancement than the one caused by the 11 year cycle. We compare our
observational findings with the features predicted by different models that try
to explain the origin of this QBP and conclude that the observed
propertiescould result from the beating between a dipole and quadrupole
magnetic configuration of the dynamo. | astro-ph_SR |
Nature of grand minima and maxima from fully non-linear Flux-Transport
Dynamos: We aim to investigate the nature and occurrence characteristics of grand
solar minimum and maximum periods, which are observed in the solar proxy
records such as 10Be and 14C, using a fully non-linear Babcock-Leighton type
flux-transport dynamo including momentum and entropy equations. The
differential rotation and meridional circulation are generated from the effect
of turbulent Reynolds stress and are subjected to back-reaction from the
magnetic field. To generate grand minimum and maximum-like periods in our
simulations, we used random fluctuations in the angular momentum transport
process, namely the Lambda-mechanism, and in the Babcock-Leighton mechanism. To
characterise the nature and occurrences of the identified grand minima and
maxima in our simulations, we used the waiting time distribution analyses,
which reflects whether the underlying distribution arises from a random or a
memory-bearing process. The results show that, in majority of the cases, the
distributions of grand minima and maxima reveal that the nature of these events
originates from memoryless processes. We also found that in our simulations the
meridional circulation speed tends to be smaller during grand maximum, while it
is faster during grand minimum periods. The radial differential rotation tend
to be larger during grand maxima, while it is smaller during grand minima. The
latitudinal differential rotation on the other hand is found to be larger
during grand minima. | astro-ph_SR |
The Envelope Kinematics and a Possible Disk Around the Class 0 Protostar
within BHR7: We present a characterization of the protostar embedded within the BHR7 dark
cloud, based on both photometric measurements from the near-infrared to
millimeter and interferometric continuum and molecular line observations at
millimeter wavelengths. We find that this protostar is a Class 0 system, the
youngest class of protostars, measuring its bolometric temperature to be
50.5~K, with a bolometric luminosity of 9.3~L$_{\odot}$. The near-infrared and
\textit{Spitzer} imaging show a prominent dark lane from dust extinction
separating clear bipolar outflow cavities. Observations of $^{13}$CO
($J=2\rightarrow1$), C$^{18}$O ($J=2\rightarrow1$), and other molecular lines
with the Submillimeter Array (SMA) exhibit a clear rotation signature on scales
$<$1300~AU. The rotation can be traced to an inner radius of $\sim$170~AU and
the rotation curve is consistent with an R$^{-1}$ profile, implying that
angular momentum is being conserved. Observations of the 1.3~mm dust continuum
with the SMA reveal a resolved continuum source, extended in the direction of
the dark lane, orthogonal to the outflow. The deconvolved size of the continuum
indicates a radius of $\sim$100~AU for the continuum source at the assumed
distance of 400~pc. The visibility amplitude profile of the continuum emission
cannot be reproduced by an envelope alone and needs a compact component. Thus,
we posit that the resolved continuum source could be tracing a Keplerian disk
in this very young system. If we assume that the continuum radius traces a
Keplerian disk (R$\sim$120~AU) the observed rotation profile is consistent with
a protostar mass of 1.0~$M_{\odot}$. | astro-ph_SR |
Tests of sunspot number sequences: 4. Discontinuities around 1946 in
various sunspot number and sunspot group number reconstructions: We use 5 test data series to quantify putative discontinuities around 1946 in
5 annual-mean sunspot number or group number sequences. The series tested are:
the original and new versions of the Wolf/Zurich/International sunspot number
composite [$R_{ISNv1}$ and $R_{ISNv2}$] ; the corrected version of $R_{ISNv1}$
[$R_C$]; the backbone group number [$R_{BB}$]; and the group number composite
[$R_{UEA}$]. The test data are: the group number $N_G$ and total sunspot area
$A_G$ from the RGO photoheliographic data; the CaK index from re-analysis of
MWO CaII K spectroheliograms; the group number from the MWO sunspot drawings,
$N_{MWO}$; and ionospheric critical frequencies at Slough [$foF2$]. The test
data all vary with sunspot numbers, in some cases non-linearly. Tests use both
before-and-after fit-residual comparison and correlation methods, applied to
intervals iterated to minimise errors and eliminate the effect of the
discontinuity date. It is not assumed that the correction required is by a
constant factor, nor even linear in sunspot number. A non-linear correction is
required by $R_C$, $R_{BB}$ and $R_{ISNv1}$, but not by $R_{ISNv2}$ or
$R_{UEA}$. The test datasets give very similar results in all cases. By
multiplying the probability distribution functions together we obtain the
optimum correction for each data series that must be applied to
pre-discontinuity data to make them consistent with the post-discontinuity
data. It is shown that, on average, values for 1932-1943 are too small
(relative to later values) by about 12.3% for $R_{ISNv1}$ but are too large for
$R_{ISNv2}$ and $R_{BB}$ by 3.8% and 5.2%. The correction applied to generate
$R_C$ from $R_{ISNv1}$ reduces this average factor to 0.5% but does not remove
the non-linear variation, and other errors remain uncorrected. A test is
provided by $R_{UEA}$, which is identical to the RGO $N_G$ values over the
interval used. | astro-ph_SR |
A Theory about Electric Current and Heating in Plasma: The traditional generalized Ohm's law in MHD do not explicitly present the
relation of electric currents and electric fields in fully ionized plasma, and
lead to some unexpected concepts, such as "the magnetic frozen-in plasma",
magnetic reconnection etc. In the present paper, we solve the balance equation
can give exact solution of the velocities of electrons and ions, and then
derived the electric current in fully ionized plasma. In the case ignoring
boundary condition, there is no electric current in the plane perpendicular to
the magnetic field when external forces are ignored. The electric field in the
plane perpendicular to magnetic field do not contribute to the electric
currents, so do the induced electric field from the motion of the plasma across
magnetic field. The lack of induced electric current will keep magnetic field
in space unaffected. The velocity of the bulk velocity of the plasma
perpendicular to magnetic field is not free, it is decided by electromagnetic
field and the external forces. We conclude that the bulk velocity of the fully
ionized plasma is not coupled with the magnetic field. The motion of the plasma
do not change the magnetic field in space, but the plasma will be confined by
magnetic field.
Due to the confinement of magnetic field, the plasma kinetic energy will be
transformed into plasma thermal energy by the Lamor motion and collisions
between the same species of particles inside plasma. Because the electric field
perpendicular to magnetic field do not contribute electric current, the
variation of magnetic field will transfer energy directly into the plasma
thermal energy by induced electric field. The heating of plasma could be from
the kinetic energy and the variation of magnetic field. | astro-ph_SR |
A blueprint of state-of-the-art techniques for detecting quasi-periodic
pulsations in solar and stellar flares: Quasi-periodic pulsations (QPPs) appear to be a common feature observed in
the light curves of both solar and stellar flares. However, their
quasi-periodic nature, along with the fact that they can be small in amplitude
and short-lived, makes QPPs difficult to unequivocally detect. In this paper,
we test the strengths and limitations of state-of-the-art methods for detecting
QPPs using a series of hare-and-hounds exercises. The hare simulated a set of
flares, both with and without QPPs of a variety of forms, while the hounds
attempted to detect QPPs in blind tests. We use the results of these exercises
to create a blueprint for anyone who wishes to detect QPPs in real solar and
stellar data. We present eight clear recommendations to be kept in mind for
future QPP detections, with the plethora of solar and stellar flare data from
new and future satellites. These recommendations address the key pitfalls in
QPP detection, including detrending, trimming data, accounting for colored
noise, detecting stationary-period QPPs, detecting QPPs with nonstationary
periods, and ensuring that detections are robust and false detections are
minimized. We find that QPPs can be detected reliably and robustly by a variety
of methods, which are clearly identified and described, if the appropriate care
and due diligence are taken. | astro-ph_SR |
The Lyman-alpha Emission in Solar Flares. I. a Statistical Study on Its
Relationship with the 1--8 Å Soft X-ray Emission: We statistically study the relationship between the Lyman-alpha (\lya) and
1--8 \AA\ soft X-ray (SXR) emissions from 658 M- and X-class solar flares
observed by the {\em Geostationary Operational Environmental Satellite} during
2006--2016. Based on the peak times of the two waveband emissions, we divide
the flares into three types. Type I (III) has an earlier (a later) peak time in
the \lya\ emission than that in the SXR emission, while type II has nearly a
same peak time (within the time resolution of 10 s) between the \lya\ and SXR
emissions. In these 658 flares, we find that there are 505 (76.8\%) type I
flares, 10 (1.5\%) type II flares, and 143 (21.7\%) type III flares, and that
the three types appear to have no dependence on the flare duration, flare
location, or solar cycle. Besides the main peak, the \lya\ emission of the
three type flares also shows sub-peaks which can appear in the impulsive or
gradual phase of the flare. It is found that the main-peak (for type I) and
sub-peak (for type III) emissions of \lya\ that appear in the impulsive phase
follow the Neupert effect in general. This indicates that such \lya\ emissions
are related to the nonthermal electron beam heating. While the main-peak (for
type III) and sub-peak (for type I) emissions of \lya\ that appear in the
gradual phase are supposed to be primarily contributed by the thermal plasma
that cools down. | astro-ph_SR |
Coronal Hole and Solar Global Magnetic Field Evolution in 1976-2012: Coronal hole spatial-temporal evolution is studied and comparison made with
that of the solar global magnetic field in cycles 21-23 (1976-2012). The
latitude-longitude distribution dynamics of coronal holes and the regularities
in the global magnetic field associated with the solar polar field reversal are
analyzed. Polar and non-polar coronal hole populations are considered. The
investigation reveals some temporal and spatial regularities in coronal hole
distributions that match well the global magnetic-field cycle evolution. The
results show that the non-polar coronal hole longitudinal distribution follows
all configuration changes in the global magnetic-field structure.
Reorganizations of the global magnetic-field and coronal hole distributions
occur simultaneously during a time interval of a few solar rotations. The cycle
evolution of the non-polar coronal holes reflects the transition of the solar
global magnetic field from the zonal structure to sectorial andvice versa. Two
different type waves of non-polar coronal holes are revealed from their
latitudinal distribution. The first one is short poleward waves. They trace the
poleward motion of the unipolar photospheric magnetic fields from approximately
35 deg to the associated pole in each hemisphere and the redevelopment of a
new-polarity polar CH. Although they start the poleward movement before the
change of the polar magnetic field in the associated hemisphere, they reach the
pole after the polar reversal. The other type of non-polar CH wave forms two
sinusoidal branches associated with the positive- and negative-polarity
magnetic fields. The complete period of the wave was equal to approximately 268
CRs (22 years). These wave CHs arrive at high latitudes during declining phases
when the new polarity polar CHs are already completely formed. | astro-ph_SR |
Absolute dimensions of detached eclipsing binaries. II. The
metallic-lined system XY Ceti: We present phase-resolved spectroscopy and extensive survey photometry of the
detached eclipsing binary system XY Cet, which is composed of two
metallic-lined stars. We measure their masses to be 1.773 +/- 0.016 and 1.615
+/- 0.014 Msun and their radii to be 1.873 +/- 0.035 and 1.773 +/- 0.029 Rsun,
resulting in logarithmic surface gravities of 4.142 +/- 0.016 and 4.149 +/-
0.014 (cgs). We determine effective temperatures of 7870 +/- 115 and 7620 +/-
125 K. The projected rotational velocities are 34.4 +/- 0.4 and 34.1 +/- 0.4
km/s, which are close to synchronous. Theoretical models cannot match all of
these properties, but come closest for a solar helium and metal abundance and
an age in the region of 850 Myr. We obtain the individual spectra of the two
stars by the spectral disentangling method, and compare them to synthetic
spectra calculated for the measured effective temperatures and a solar chemical
composition. Both stars show enhanced abundances of iron-group elements and
clear deficiencies of Ca I and Sc II, confirming their classification as Am
stars. We also find strong overabundances of Zr II and the rare-earth species
La II, Ce II and Nd II, a hallmark of chemically peculiar A-stars. XY Cet is a
prime candidate for detailed spectroscopic analyses of metallic-lined stars
whose masses and radii are known to accuracies of 1-2%. | astro-ph_SR |
Disk Evolution in OB Associations - Deep Spitzer/IRAC Observations of IC
1795: We present a deep Spitzer/IRAC survey of the OB association IC 1795 carried
out to investigate the evolution of protoplanetary disks in regions of massive
star formation. Combining Spitzer/IRAC data with Chandra/ACIS observations, we
find 289 cluster members. An additional 340 sources with an infrared excess,
but without X-ray counterpart, are classified as cluster member candidates.
Both surveys are complete down to stellar masses of about 1 Msun. We present
pre-main sequence isochrones computed for the first time in the Spitzer/IRAC
colors. The age of the cluster, determined via the location of the Class III
sources in the [3.6]-[4.5]/[3.6] color-magnitude diagram, is in the range of 3
- 5 Myr. As theoretically expected, we do not find any systematic variation in
the spatial distribution of disks within 0.6 pc of either O-type star in the
association. However, the disk fraction in IC 1795 does depend on the stellar
mass: sources with masses >2 Msun have a disk fraction of ~20%, while lower
mass objects (2-0.8 Msun) have a disk fraction of ~50%. This implies that disks
around massive stars have a shorter dissipation timescale. | astro-ph_SR |
The Intricate Structure of HH 508, the Brightest Microjet in the Orion
Nebula: We present Magellan adaptive optics H$\alpha$ imaging of HH 508, which has
the highest surface brightness among protostellar jets in the Orion Nebula. We
find that HH 508 actually has a shorter component to the west, and a longer and
knotty component to the east. The east component has a kink at 0.3" from the
jet-driving star $\theta^1$ Ori B2, so it may have been deflected by the
wind/radiation from the nearby $\theta^1$ Ori B1B5. The origin of both
components is unclear, but if each of them is a separate jet, then $\theta^1$
Ori B2 may be a tight binary. Alternatively, HH 508 may be a slow-moving
outflow, and each component represents an illuminated cavity wall. The
ionization front surrounding $\theta^1$ Ori B2B3 does not directly face
$\theta^1$ Ori B1B5, suggesting that the EUV radiation from $\theta^1$ Ori C
plays a dominant role in affecting the morphology of proplyds even in the
vicinity of $\theta^1$ Ori B1B5. Finally, we report an H$\alpha$ blob that
might be ejected by the binary proplyd LV 1. | astro-ph_SR |
Dynamics of fast and slow magnetoacoustic waves in plasma slabs with
thermal misbalance: Non-uniformity of the solar atmosphere along with the presence of
non-adiabatic processes such as radiation cooling and unspecified heating can
significantly affect the dynamics and properties of magnetoacoustic (MA) waves.
To address the co-influence of these factors on the dispersion properties of MA
waves, we considered a single magnetic slab composed of the thermally active
plasma. Using the perturbation theory, we obtained a differential equation that
determines the dynamics of the two-dimensional perturbations. Applying the
assumption of strong magnetic structuring, we derived the dispersion relations
for the sausage and kink MA modes. The numerical solution of the dispersion
relations for the coronal conditions was performed to investigate the interplay
between the non-uniformity and the thermal misbalance. For the heating scenario
considered, it was obtained that the phase speed of both the sausage and kink
slow MA waves is highly affected by the thermal misbalance in the long
wavelength limit. The obtained characteristic timescales of the slow waves
dissipation coincide with the periods of waves observed in the corona.
Simultaneously, the phase speed of the fast waves is not affected by the
thermal misbalance. The geometry of the magnetic structure still remains the
main dispersion mechanism for the fast waves. Our estimation reveals that
dissipation of the fast waves is weaker than dissipation of the slow waves in
the coronal conditions. The obtained results are of importance for using the
magnetoacoustic waves not only as a tool for estimating plasma parameters, but
also as a tool for estimating the non-adiabatic processes. | astro-ph_SR |
Forward seismic modelling of the pulsating magnetic B-type star HD 43317: The large-scale magnetic fields detected at the surface of about 10% of hot
stars extend into the stellar interior, where they may alter the structure.
Deep inner regions of stars are only observable using asteroseismology. Here,
we investigated the pulsating magnetic B3.5V star HD43317, inferred its
interior properties and assessed whether the dipolar magnetic field with a
surface strength of $B_p = 1312 \pm 332$G caused different properties compared
to those of non-magnetic stars. We analysed the latest version of the star's
150d CoRoT light curve and extracted 35 significant frequencies, 28 of which
were determined to be independent and not related to the known surface rotation
period of $P_{\rm rot} = 0.897673$d. We performed forward seismic modelling
based on non-magnetic, non-rotating 1D MESA models and the adiabatic module of
the pulsation code GYRE, utilizing a grid-based approach. Our aim was to
estimate the stellar mass, age, and convective core overshooting. The GYRE
calculations were done for uniform rotation with $P_{\rm rot}$. This modelling
was able to explain 16 of the 28 frequencies as gravity modes belonging to
retrograde modes with $(\ell, m) = (1, -1)$ and $(2, -1)$ period spacing
patterns and one distinct prograde $(2,2)$ mode. The modelling resulted in a
stellar mass $M_{\star} = 5.8^{+0.1}_{-0.2}$$\mathrm{M_{\odot}}$, a central
hydrogen mass fraction $X_c = 0.54^{+0.01}_{-0.02}$, and exponential convective
core overshooting parameter $f_{\rm ov} = 0.004^{+0.014}_{-0.002}$. The low
value for $f_{\rm ov}$ is compatible with the suppression of near-core mixing
due to a magnetic field but the uncertainties are too large to pinpoint such
suppression as the sole physical interpretation. $[...]$ | astro-ph_SR |
The magnetic field of the double-lined spectroscopic binary system HD
5550: (Abridged) In the framework of the BinaMicS project, we have begun a study of
the magnetic properties of a sample of intermediate-mass and massive
short-period binary systems, as a function of binarity properties. We report in
this paper the characterisation of the magnetic field of HD 5550, a
double-lined spectroscopic binary system of intermediate-mass, using
high-resolution spectropolarimetric Narval observations of HD 5550. We first
fit the intensity spectra using Zeeman/ATLAS9 LTE synthetic spectra to estimate
the effective temperatures, microturbulent velocities, and the abundances of
some elements of both components, as well as the light-ratio of the system. We
then fit the least-square deconvolved $I$ profiles to determine the radial and
projected rotational velocities of both stars. We then analysed the shape and
evolution of the LSD $V$ profiles using the oblique rotator model to
characterise the magnetic fields of both stars.
We confirm the Ap nature of the primary, previously reported in the
literature, and find that the secondary displays spectral characteristics
typical of an Am star. While a magnetic field is clearly detected in the lines
of the primary, no magnetic field is detected in the secondary, in any of our
observation. If a dipolar field were present at the surface of the Am star, its
polar strength must be below 40 G. The faint variability observed in the Stokes
$V$ profiles of the Ap star allowed us to propose a rotation period of
$6.84_{-0.39}^{+0.61}$ d, close to the orbital period ($\sim$6.82 d),
suggesting that the star is synchronised with its orbit. By fitting the
variability of the $V$ profiles, we propose that the Ap component hosts a
dipolar field inclined with the rotation axis at an angle $\beta=156\pm17$
$^{\circ}$ and a polar strength $B_{\rm d}=65 \pm 20$ G. The field strength is
the weakest known for an Ap star. | astro-ph_SR |
Evolving waves and turbulence in the outer corona and inner heliosphere:
the accelerating expanding box: Alfv\'enic fluctuations in the solar wind display many properties reflecting
an ongoing nonlinear cascade, e.g. a well-defined spectrum in frequency,
together with some characteristics more commonly associated with the linear
propagation of waves from the Sun, such as the variation of fluctuation
amplitude with distance, dominated by solar wind expansion effects. Therefore
both nonlinearities and expansion must be included simultaneously in any
successful model of solar wind turbulence evolution. Because of the disparate
spatial scales involved, direct numerical simulations of turbulence in the
solar wind represent an arduous task, especially if one wants to go beyond the
incompressible approximation. Indeed, most simulations neglect solar wind
expansion effects entirely. Here we develop a numerical model to simulate
turbulent fluctuations from the outer corona to 1 AU and beyond, including the
sub-Alfv\'enic corona. The accelerating expanding box (AEB) extends the
validity of previous expanding box models by taking into account both the
acceleration of the solar wind and the inhomogeneity of background density and
magnetic field. Our method incorporates a background accelerating wind within a
magnetic field that naturally follows the Parker spiral evolution using a
two-scale analysis in which the macroscopic spatial effect coupling
fluctuations with background gradients becomes a time-dependent coupling term
in a homogeneous box. In this paper we describe the AEB model in detail and
discuss its main properties, illustrating its validity by studying Alfv\'en
wave propagation across the Alfv\'en critical point. | astro-ph_SR |
Time delay between outer heliosheath crossing and observation of
interstellar neutral atoms: In situ measurements of the heliospheric particle populations by the Voyager
spacecraft can only be put in an appropriate context with remote-sensing
observations of energetic and interstellar neutral atoms (ENAs and ISN,
respectively) at 1 au when the time delay between the production and the
observation times is taken into account. ENA times of flight from the
production regions in the heliosheath are relatively easy to estimate because
these atoms follow almost constant speed, force-free trajectories. For the ISN
populations, dynamical and ballistic selection effects are important, and times
of flight are much longer. We estimate these times for ISN He and H atoms
observed by IBEX and in the future by IMAP using the WTPM model with synthesis
method. We show that for the primary population atoms, the times of flight are
on the order of three solar cycle periods, with a spread equivalent to one
solar cycle. For the secondary populations, the times of flight are on the
order of ten solar cycle periods, and during the past ten years of
observations, IBEX has been collecting secondary He atoms produced in the OHS
during almost entire 19th century. ISN atoms penetrating the heliopause at the
time of Voyager crossing will become gradually visible about 2027, during the
planned IMAP observations. Hypothetical variations in the ISN flow in the Local
Interstellar Medium are currently not detectable. Nevertheless, we expect
steady-state heliosphere models used with appropriately averaged solar wind
parameters to be suitable for understanding the ISN observations. | astro-ph_SR |
Explaining the Early Exit of Eta Carinae from its 2009 X-Ray Minimum
with the Accretion Model: We use the accretion model to explain the early exit of Eta Carinae from its
2009 X-ray minimum. In the accretion model the secondary star accretes mass
from the primary wind near periastron passage, a process that suppresses the
secondary wind. As the shocked secondary wind is responsible for most of the
X-ray emission, the accretion process accounts for the X-ray minimum. The early
exit from the 2009 X-ray minimum after four weeks, instead of ten weeks as in
the two previous minima, is attributed to the primary wind that during the last
minimum was somewhat faster and of lower mass loss rate than during the two
previous X-ray minima. This results in a much lower mass accretion rate during
the X-ray minimum. We show that using fluctuations in these quantities that are
within the range deduced from fluctuations in the X-ray flux outside the
minimum, can account for the short duration of the last X-ray minimum. The
shorter X-ray minimum may have further implications on the recovery of the
system from the spectroscopic event. | astro-ph_SR |
The VMC Survey - XIV. First results on the look-back time star-formation
rate tomography of the Small Magellanic Cloud: We analyse deep images from the VISTA survey of the Magellanic Clouds in the
YJKs filters, covering 14 sqrdeg (10 tiles), split into 120 subregions, and
comprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply
a colour--magnitude diagram reconstruction method that returns their
best-fitting star formation rate SFR(t), age-metallicity relation (AMR),
distance and mean reddening, together with 68% confidence intervals. The
distance data can be approximated by a plane tilted in the East-West direction
with a mean inclination of 39 deg, although deviations of up to 3 kpc suggest a
distorted and warped disk. After assigning to every observed star a probability
of belonging to a given age-metallicity interval, we build high-resolution
population maps. These dramatically reveal the flocculent nature of the young
star-forming regions and the nearly smooth features traced by older stellar
generations. They document the formation of the SMC Wing at ages <0.2 Gyr and
the peak of star formation in the SMC Bar at 40 Myr. We clearly detect periods
of enhanced star formation at 1.5 Gyr and 5 Gyr. The former is possibly related
to a new feature found in the AMR, which suggests ingestion of metal-poor gas
at ages slightly larger than 1 Gyr. The latter constitutes a major period of
stellar mass formation. We confirm that the SFR(t) was moderately low at even
older ages. | astro-ph_SR |
Destination exoplanet: Habitability conditions influenced bystellar
winds properties: The cumulative effect of the magnetized stellar winds on exoplanets dominates
over other forms of star-planet interactions. When combined with
photoevaporation, these winds will lead to atmospheric erosion. This is
directly connected with the concept of Habitable Zone (HZ) planets around
late-type stars. Our knowledge of these magnetized winds is limited, making
numerical models useful tools to explore them. In this preliminary study, we
focus on solar-like stars exploring how different stellar wind properties scale
with one another. We used one of the most detailed physics-based models, the 3D
Alfv\'en Wave Solar Model part of the Space Weather ModelingFramework, and
applied it to the stellar winds domain. Our simulations showed that the
magnetic field topology on the star surface plays a fundamental role in shaping
the different stellar wind properties (wind speed, mass loss rate, angular
momentum loss rate). We conclude that a characterization of the Alfv\'en
surface is crucial when studying star-planet interaction as it can serve as an
inner-boundary of the HZ | astro-ph_SR |
Convective scale and subadiabatic layers in simulations of rotating
compressible convection: (abridged) Context: Rotation is thought to influence the size of convective
eddies and the efficiency of convective energy transport in the deep convection
zones of stars. Rotationally constrained convection has been invoked to explain
the lack of large-scale power in observations of solar flows. Aims: The main
aims are to quantify the effects of rotation on the scale of convective eddies
and velocity, the depths of convective overshoot, and the subadiabatic
Deardorff layers. Methods: Three-dimensional hydrodynamic simulations of
rotating convection in Cartesian domains were run. The results were compared
with theoretical scaling results that assume a balance between Coriolis,
inertial, and buoyancy (Archemedean) forces (CIA balance). Results: The scale
of convective eddies decreases as rotation increases, and ultimately reaches a
rotationally constrained regime consistent with the CIA balance. Using a new
measure of the rotational influence on the system, it is shown that even the
deep parts of the solar convection zone are not in the rotationally constrained
regime. The simulations capture the slowly and rapidly rotating scaling laws
predicted by theory, and the Sun appears to be in between these two regimes.
Both, the overshooting depth and the extent of the Deardorff layer, decrease as
rotation becomes more rapid. For sufficiently rapid rotation the Deardorff
layer is absent. Conclusions: Relating the simulations with the Sun suggests
that the convective scale even in the deep parts of the Sun is only mildly
affected by rotation and that some other mechanism is needed to explain the
lack of strong large-scale flows in the Sun. Taking the current results at face
value, the overshoot and Deardorff layers are estimated to span roughly five
per cent of the pressure scale height at the base of the convection zone in the
Sun. | astro-ph_SR |
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