text
stringlengths 89
2.49k
| category
stringclasses 19
values |
|---|---|
Discovery of a Highly Eccentric, Chromospherically Active Binary:
ASASSN-V J192114.84+624950.8: As part of an All-Sky Automated Survey for SuperNovae (ASAS-SN) search for
sources with large flux decrements, we discovered a transient where the
quiescent, stellar source, ASASSN-V J192114.84+624950.8, rapidly decreased in
flux by $\sim55\%$ ($\sim0.9$ mag) in the g-band. The \textit{TESS} light curve
revealed that the source is a highly eccentric, eclipsing binary. Fits to the
light curve using \textsc{phoebe} find the binary orbit to have $e=0.79$,
$P_{\rm orb}=18.462~\text{days}$, and $i=88.6^{\circ}$ and the ratios of the
stellar radii and temperatures to be $R_2/R_1 = 0.71$ and $T_{e,2}/T_{e,1} =
0.82$. Both stars are chromospherically active, allowing us to determine their
rotational periods of $P_1=1.52$ days and $P_2=1.79$ days, respectively. A
LBT/MODS spectrum shows that the primary is a late-G or early-K type dwarf.
Fits to the SED show that the luminosities and temperatures of the two stars
are $L_1 = 0.48~L_{\sun}$, $T_1= 5050~K$, $L_2 = 0.12~L_{\sun}$, and $T_{2} =
4190~K$. We conclude that ASASSN-V J192114.84+624950.8 consists of two
chromospherically active, rotational variable stars in a highly elliptical
eclipsing orbit. | astro-ph_SR |
CNO abundances in the globular clusters NGC 1851 and NGC 6752: We measure the C+N+O abundance sum in red giant stars in two Galactic
globular clusters, NGC 1851 and NGC 6752. NGC 1851 has a split subgiant branch
which could be due to different ages or C+N+O content while NGC 6752 is
representative of the least complex globular clusters. For NGC 1851 and NGC
6752, we obtain average values of A(C+N+O) = 8.16 +/- 0.10 (\sigma = 0.34) and
7.62 +/- 0.02 (\sigma = 0.06), respectively. When taking into account the
measurement errors, we find a constant C+N+O abundance sum in NGC 6752. The
C+N+O abundance dispersion is only 0.06 dex, and such a result requires that
the source of the light element abundance variations does not increase the
C+N+O sum in this cluster. For NGC 1851, we confirm a large spread in C+N+O. In
this cluster, the anomalous RGB has a higher C+N+O content than the canonical
RGB by a factor of four (~0.6 dex). This result lends further support to the
idea that the two subgiant branches in NGC 1851 are roughly coeval, but with
different CNO abundances. | astro-ph_SR |
Populations of rotating stars. - I. Models from 1.7 to 15 Msun at Z =
0.014, 0.006, and 0.002 with Ω/Ωcrit between 0 and 1: B-type stars are known to rotate at various velocities, including very fast
rotators near the critical velocity as the Be stars. In this paper, we provide
stellar models covering the mass range between 1.7 to 15 Msun, which includes
the typical mass of known Be stars, at Z = 0.014, 0.006, and 0.002 and for an
extended range of initial velocities on the zero-age main sequence. We used the
Geneva stellar-evolution code, including the effects of shellular rotation,
with a numerical treatment that has been improved so the code can precisely
track the variation in the angular momentum content of the star as it changes
under the influence of radiative winds and/or mechanical mass loss. We discuss
the impact of the initial rotation rate on the tracks in the
Hertzsprung-Russell diagram, the main-sequence (MS) lifetimes, the evolution of
the surface rotation and abundances, as well as on the ejected masses of
various isotopes. Among the new results obtained from the present grid we find
that 1) fast-rotating stars with initial masses around 1.7 Msun present at the
beginning of the core hydrogen-burning phase quite small convective cores with
respect to their slowly rotating counterparts. This fact may be interesting to
keep in mind in the framework of the asteroseismic studies of such stars. 2)
The contrast between the core and surface angular velocity is higher in slower
rotating stars. The values presently obtained are in agreement with the very
few values obtained for B-type stars from asteroseismology. 3) At Z = 0.002,
the stars in the mass range of 1.7 to 3 Msun with a mean velocity on the MS of
the order of 150 km/s show N/H enhancement superior to 0.2 dex at mid-MS, and
superior to 0.4 dex at the end of the MS phase. At solar metallicity the
corresponding values are below 0.2 dex at any time in the MS. | astro-ph_SR |
SDSS/SEGUE Spectral Feature Analysis For Stellar Atmospheric Parameter
Estimation: Large-scale and deep sky survey missions are rapidly collecting a large
amount of stellar spectra, which necessitate the estimation of atmospheric
parameters directly from spectra and makes it feasible to statistically
investigate latent principles in a large dataset. We present a technique for
estimating parameters $T_{eff}$, log$~g$ and [Fe/H] from stellar spectra. With
this technique, we first extract features from stellar spectra using the LASSO
algorithm; then, the parameters are estimated from the extracted features using
the SVR. On a subsample of 20~000 stellar spectra from SDSS with reference
parameters provided by SDSS/SEGUE Pipeline SSPP, estimation consistency are
0.007458 dex for log$~T_{eff}$ (101.609921 K for $T_{eff}$), 0.189557 dex for
log$~g$ and 0.182060 for [Fe/H], where the consistency is evaluated by mean
absolute error. Prominent characteristics of the proposed scheme are
sparseness, locality, and physical interpretability. In this work, every
spectrum consists of 3821 fluxes, and 10, 19, and 14 typical wavelength
positions are detected respectively for estimating $T_{eff}$, log$~g$ and
[Fe/H]. It is shown that the positions are related to typical lines of stellar
spectra. This characteristic is important in investigating physical indications
from analysis results. Then, stellar spectra can be described by the individual
fluxes on the detected positions (PD) or local integration of fluxes near them
(LI). The abovementioned consistency is the result based on features described
by LI. If features are described by PD, consistency are 0.009092 dex for
log$~T_{eff}$ (124.545075 K for $T_{eff}$), 0.198928 dex for log$~g$, and
0.206814 dex for [Fe/H]. | astro-ph_SR |
PolarBase: a data base of high resolution spectropolarimetric stellar
observations: PolarBase is an evolving data base that contains all stellar data collected
with the ESPaDOnS and NARVAL high-resolution spectropolarimeters, in their
reduced form, as soon as they become public. As of early 2014, observations of
2,000 stellar objects throughout the Hertzsprung-Russell diagram are available.
Intensity spectra are available for all targets, and the majority of the
observations also include simultaneous spectra in circular or linear
polarization, with the majority of the polarimetric measurements being
performed only in circularly polarized light (Stokes V). Observations are
associated with a cross-correlation pseudo-line profile in all available Stokes
parameters, greatly increasing the detectability of weak polarized signatures.
Stokes V signatures are detected for more than 300 stars of all masses and
evolutionary stages, and linear polarization is detected in 35 targets. The
detection rate in Stokes V is found to be anti-correlated with the stellar
effective temperature. This unique set of Zeeman detections offers the first
opportunity to run homogeneous magnetometry studies throughout the H-R diagram.
The web interface of PolarBase is available at http://polarbase.irap.omp.eu. | astro-ph_SR |
Precise mass measurements for the double neutron star system J1829+2456: PSR J1829+2456 is a radio pulsar in a relativistic binary system with another
neutron star. It has a rotational period of 41 ms and a mildly eccentric ($e =
0.14$) 28-hr orbit. We have continued its observations with the Arecibo radio
telescope and have now measured the individual neutron star masses of this
system. The pulsar and companion masses are $1.306\,\pm\,0.007\,M_{\odot}$ and
$1.299\,\pm\,0.007\,M_{\odot}$ (2$\sigma$ - 95% confidence, unless stated
otherwise), respectively. We have also measured the proper motion for this
system and used it to estimate a space velocity of 49$^{+77}_{-30}$ km s$^{-1}$
with respect to the local standard of rest. The relatively low values for
companion mass, space velocity and orbital eccentricity in this system make it
similar to other double neutron star systems in which the second-formed neutron
star is thought to have formed in a low-kick, low mass-loss, symmetric
supernova. | astro-ph_SR |
The Effects of Space Weather on Flight Delays: Although the sun is really far away from us, some solar activities could
still influence the performance and reliability of space-borne and ground-based
technological systems on Earth. Those time-varying conditions in space caused
by the sun are also called space weather, as the atmospheric conditions that
can affect weather on the ground. It is known that aviation activities can be
affected during space weather events, but the exact effects of space weather on
aviation are still unclear. Especially how the flight delays, the top topic
concerned by most people, will be affected by space weather has never been
thoroughly researched. By analyzing huge amount of flight data (~5X106
records), for the first time, we demonstrate that space weather events could
have systematically modulating effects on flight delays. The average arrival
delay time and 30-minute delay rate during space weather events are
significantly increased by 81.34% and 21.45% respectively compared to those
during quiet periods. The evident negative correlation between the yearly
flight regularity rate and the yearly mean total sunspot number during 22 years
also confirms such delay effects. Further studies indicate that the
interference in communication and navigation caused by geomagnetic field
fluctuations and ionospheric disturbances associated with the space weather
events will increase the flight delay time and delay rate. These results expand
the traditional field of space weather research and could also provide us with
brand new views for improving the flight delay predications. | astro-ph_SR |
Extensive photometry of the intermediate polar V1033 Cas (IGR
J00234+6141): To measure the spin period of the white dwarf in V1033 Cas with high
precision, we performed extensive photometry. Observations were obtained over
34 nights in 2017. The total duration of the observations was 143 h. We found
that the spin period of the white dwarf is equal to 563.11633+/-0.00010 s.
Using this period, we derived the oscillation ephemeris with a long validity of
100 years. The spin oscillation semi-amplitude was stable and was equal to
95.5+/-1.3 mmag. This is a very large semi-amplitude of the spin oscillation
among intermediate polars, which have similar and lesser spin periods. This
large semi-amplitude suggests that the system is noticeably inclined. The spin
pulse profile was sinusoidal with high accuracy. This may mean that the spin
oscillation is produced by a single accretion curtain whereas the second
accretion curtain may be obscured by the accretion disc. Despite the large
amount of our observations, we did not detect sidebands. The semi-amplitudes of
the undetected sideband oscillations do not exceed 10 mmag. The absence of
sideband oscillations seems puzzling. We detected the orbital variability of
V1033 Cas with a period of 4.0243+/-0.0028 h and with a semi-amplitude of
55+/-4 mmag. The orbital variability semi-amplitude seems large and also
suggests that the system is noticeably inclined. Using our oscillation
ephemeris and the times of spin pulse maximum obtained in the past, we found
that the spin period is very stable. dP/dt is most probably less than -4 X
10^(-12). This contradicts the assumption that the white dwarf in V1033 Cas is
not spinning at equilibrium. Our spin period and our oscillation ephemeris can
be used for further investigations of the stability of the spin period in V1033
Cas. | astro-ph_SR |
Discovery of a ~23 Mjup Brown Dwarf Orbiting ~700 AU from the Massive
Star HIP 78530 in Upper Scorpius: We present the discovery of a substellar companion on a wide orbit around the
~2.5 Msun star HIP 78530, which is a member of the 5 Myr-old Upper Scorpius
association. We have obtained follow-up imaging over two years and show that
the companion and primary share common proper motion. We have also obtained JHK
spectroscopy of the companion and confirm its low surface gravity, in
accordance with the young age of the system. A comparison with Drift-Phoenix
synthetic spectra indicates an effective temperature of 2800+/-200 K and a
comparison with template spectra of young and old dwarfs indicates a spectral
type of M8+/-1. The mass of the companion is estimated to be 19-26 Mjup based
on its bolometric luminosity and the predictions of evolutionary models. The
angular separation of the companion is 4.5", which at the distance of the
primary star, 156.7 pc, corresponds to a projected separation of ~710 AU. This
companion features one of the lowest mass ratios (~0.009) of any known
companion at separations greater than 100 AU. | astro-ph_SR |
Statistical Study of Emerging Flux Regions and the Upper Atmosphere
Response: We statistically study the property of emerging flux regions (EFRs) and the
upper solar atmosphere response to the flux emergence by using data from the
Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly
(AIA) on board the Solar Dynamics Observatory (SDO). Parameters including the
total emerged flux, the flux growth rate, the maximum area, the duration of the
emergence and the separation speed of the opposite polarities are adopted to
delineate the property of the EFRs. The response of the upper atmosphere is
addressed by the response of the atmosphere at different wavelengths (and thus
at different temperatures). According to our results, the total emerged fluxes
are in the range of (0.44 -- 11.2)$\times10^{19}$ Mx while the maximum area
ranges from 17 to 182 arcsec$^2$. The durations of the emergence are between 1
and 12 hours, which are positively correlated to both the total emerged flux
and the maximum area. The maximum distances between the opposite polarities are
7 -- 25 arcsec and are also correlated to the duration positively. The
separation speeds are from 0.05 to 1.08 km s$^{-1}$, negatively correlated to
the duration. The derived flux growth rates are (0.1 -- 1.3)$\times10^{19}$ Mx
hr$^{-1}$, which are positively correlated to the total emerging flux. The
upper atmosphere responds to the flux emergence in the 1600\AA\ chromospheric
line first, and then tens and hundreds of seconds later, in coronal lines, such
as the 171\AA\ (T=10$^{5.8}$ K) and 211\AA\ (T=10$^{6.3}$ K) lines almost
simultaneously, suggesting the successively heating of atmosphere from the
chromosphere to the corona. | astro-ph_SR |
The quiet Sun at mm Wavelengths as Seen by ALMA: Solar observations at sub-mm, mm and cm wavelengths offer a straightforward
diagnostic of physical conditions in the solar atmosphere because they yield
measurement of brightness temperature which, for optically thick features,
equals intrinsic temperature - much unlike solar diagnostics in other spectral
ranges. The Atacama Large Millimeter and sub-millimeter Array (ALMA) has
therefore opened a new, hitherto underexplored, spectral window for studying
the enigmatic solar chromosphere. In this review we discuss initial ALMA
studies of the quiet chromosphere that used both single-dish and compact-array
interferometric observing modes. We present results on the temperature
structure of the chromosphere, comparison with classic empirical models of the
chromosphere, and observations of the chromospheric network and spicules.
Furthermore, we discuss what may be expected in the future, since the ALMA
capabilities continuously expand and improve towards higher angular resolution,
wavelength coverage, and polarization measurement for magnetometry. | astro-ph_SR |
Gravitational fragmentation and formation of giant protoplanets on
tens-of-au orbits: Migration of dense gaseous clumps that form in young protostellar disks via
gravitational fragmentation is investigated to determine the likelihood of
giant planet formation. High-resolution numerical hydrodynamics simulations in
the thin-disk limit are employed to compute the formation and long-term
evolution of a gravitationally unstable protostellar disk around a solar-mass
star. We show that gaseous clumps that form in the outer regions of the disk
(>100 AU) through disk fragmentation are often perturbed by other clumps or
disk structures, such as spiral arms, and migrate toward the central star on
timescales from a few 10^3 to few 10^4 yr. The migration timescale is slowest
when stellar motion in response to the disk gravity is considered. When
approaching the star, the clumps first gain mass (up to several tens of M_Jup),
but then quickly lose most of their diffuse envelopes through tidal torques.
Part of the clump envelope can be accreted on the central star causing an
FU-Ori-type accretion and luminosity outburst. The tidal mass loss helps the
clumps to significantly slow down or even halt their inward migration at a
distance of a few tens of AU from the protostar. The resulting clumps are
heavily truncated both in mass and size compared to their wider-orbit
counterparts, keeping only a dense and hot nucleus. During the inward
migration, the temperature in the clump interiors may exceed the molecular
hydrogen dissociation limit (2000 K) and the central region of the clump can
collapse into a gas giant protoplanet. We argue that FU-Orionis-type luminosity
outbursts may be the end product of disk fragmentation and clump inward
migration, ushering the formation of giant protoplanets on tens-of-au orbits in
systems such as HR~8799. | astro-ph_SR |
Detection of the 69 μm band of crystalline forsterite in the
Herschel MESS-program: In this article we present the detection of the 69 {\mu}m band of the
crystalline olivine forsterite within the MESS key program of Herschel. We
determine the temperature of the forsterite grains by fitting the 69 {\mu}m
band. | astro-ph_SR |
WIRED for EC: New White Dwarfs with $\textit{WISE}$ Infrared Excesses
and New Classification Schemes from the Edinburgh-Cape Blue Object Survey: We present a simple method for identifying candidate white dwarf systems with
dusty exoplanetary debris based on a single temperature blackbody model fit to
the infrared excess. We apply this technique to a sample of Southern Hemisphere
white dwarfs from the recently completed Edinburgh-Cape Blue Object Survey and
identify four new promising dusty debris disk candidates. We demonstrate the
efficacy of our selection method by recovering three of the four
$\textit{Spitzer}$ confirmed dusty debris disk systems in our sample. Further
investigation using archival high resolution imaging shows $\textit{Spitzer}$
data of the un-recovered fourth object is likely contaminated by a
line-of-sight object that either led to a mis-classification as a dusty disk in
the literature or is confounding our method. Finally, in our diagnostic plot we
show that dusty white dwarfs which also host gaseous debris lie along a
boundary of our dusty debris disk region, providing clues to the origin and
evolution of these especially interesting systems. | astro-ph_SR |
Radii of Rapidly-Rotating Stars, with Application to Transiting-Planet
Hosts: The currently favored method for estimating radii and other parameters of
transiting-planet host stars is to match theoretical models to observations of
the stellar mean density rho_*, the effective temperature T_eff, and the
composition parameter [Z]. This explicitly model-dependent approach is based on
readily-available observations, and results in small formal errors. Here I use
two calibration samples of stars (eclipsing binaries and stars for which
asteroseismic analyses are available) having well-determined masses and radii
to estimate the accuracy and systematic errors inherent in the rho_* method.
When matching to the Yonsei-Yale stellar evolution models, I find the most
important systematic error results from selection bias favoring
rapidly-rotating (hence probably magnetically active) stars among the eclipsing
binary sample. If unaccounted for, this bias leads to a mass-dependent
underestimate of stellar radii by as much as 4% for stars of 0.4 M_sun,
decreasing to zero for masses above about 1.4 M_sun. The asteroseismic sample
suggests (albeit with significant uncertainty) that systematic errors are small
for slowly-rotating, inactive stars. Systematic errors arising from failings of
the Yonsei-Yale models of inactive stars probably exist, but are difficult to
assess because of the small number of well-characterized comparison stars
having low mass and slow rotation. Poor information about [Z] is an important
source of random error, and may be a minor source of systematic error as well.
With suitable corrections for rotation, it is likely that systematic errors in
the rho_* method can be comparable to or smaller than the random errors,
yielding radii that are accurate to about 2% for most stars. | astro-ph_SR |
Long-term forcing of Sun's coronal field, open flux and cosmic ray
modulation potential during grand minima, maxima and regular activity phases
by the solar dynamo mechanism: Magnetic fields generated in the Sun's interior by the solar dynamo mechanism
drive solar activity over a range of time-scales. While space-based
observations of the Sun's corona exist only for few decades, direct sunspot
observations exist for a few centuries, solar open flux and cosmic ray flux
variations can be reconstructed through studies of cosmogenic isotopes over
thousands of years. While such reconstructions indicate the presence of extreme
solar activity fluctuations in the past, causal links between millennia scale
dynamo activity, consequent coronal field, solar open flux and cosmic ray
modulation remain elusive. By utilizing a stochastically forced solar dynamo
model we perform long-term simulations to illuminate how the dynamo generated
magnetic fields govern the structure of the solar corona and the state of the
heliosphere -- as indicated by variations in the open flux and cosmic ray
modulation potential. We establish differences in the nature of the large-scale
structuring of the solar corona during grand maximum, minimum, and regular
solar activity phases and simulate how the open flux and cosmic ray modulation
potential varies over time scales encompassing these different phases of solar
activity. We demonstrate that the power spectrum of simulated and reconstructed
solar open flux are consistent with each other. Our study provides the
theoretical basis for interpreting long-term solar cycle variability based on
reconstructions relying on cosmogenic isotopes and connects solar internal
variations to the forcing of the state of the heliosphere. | astro-ph_SR |
Interactions of the magnetospheres of stars and close-in giant planets: Since the first discovery of an extrasolar planetary system more than a
decade ago, hundreds more have been discovered. Surprisingly, many of these
systems harbor Jupiter-class gas giants located close to the central star, at
distances of 0.1 AU or less. Observations of chromospheric 'hot spots' that
rotate in phase with the planetary orbit, and elevated stellar X-ray
luminosities,suggest that these close-in planets significantly affect the
structure of the outer atmosphere of the star through interactions between the
stellar magnetic field and the planetary magnetosphere. Here we carry out the
first detailed three-dimensional MagnetoHydroHynamics (MHD) simulation
containing the two magnetic bodies and explore the consequences of such
interactions on the steady-state coronal structure. The simulations reproduce
the observable features of 1) increase in the total X-ray luminosity, 2)
appearance of coronal hot spots, and 3) phase shift of these spots with respect
to the direction of the planet. The proximate cause of these is an increase in
the density of coronal plasma in the direction of the planet, which prevents
the corona from expanding and leaking away this plasma via a stellar wind. The
simulations produce significant low temperature heating. By including dynamical
effects, such as the planetary orbital motion, the simulation should better
reproduce the observed coronal heating. | astro-ph_SR |
Helioseismology with Solar Orbiter: The Solar Orbiter mission, to be launched in July 2017, will carry a suite of
remote sensing and in-situ instruments, including the Polarimetric and
Helioseismic Imager (PHI). PHI will deliver high-cadence images of the Sun in
intensity and Doppler velocity suitable for carrying out novel helioseismic
studies. The orbit of the Solar Orbiter spacecraft will reach a solar latitude
of up to 21 deg (up to 34 deg by the end of the extended mission) and thus will
enable the first local helioseismology studies of the polar regions. Here we
consider an array of science objectives to be addressed by helioseismology
within the baseline telemetry allocation (51 Gbit per orbit, current baseline)
and within the science observing windows (baseline 3 x 10 days per orbit). A
particularly important objective is the measurement of large-scale flows at
high latitudes (rotation and meridional flow), which are largely unknown but
play an important role in flux transport dynamos. The full range of
Earth-Sun-spacecraft angles provided by the orbit will enable helioseismology
from two vantage points by combining PHI with another instrument: stereoscopic
helioseismology will allow the study of the deep solar interior and a better
understanding of the physics of solar oscillations in both quiet Sun and
sunspots. We have used a model of the PHI instrument to study its performance
for helioseismology applications. As input we used a 6 hr time-series of
realistic solar magneto-convection simulation (Stagger code) and the SPINOR
radiative transfer code to synthesize the observables. The simulated power
spectra of solar oscillations show that the instrument is suitable for
helioseismology. In particular, the specified point spread function, image
jitter, and photon noise are no obstacle to a successful mission. | astro-ph_SR |
The Of?p stars of the Magellanic Clouds: Are they strongly magnetic?: All known Galactic Of?p stars have been shown to host strong, organized,
magnetic fields. Recently, five Of?p stars have been discovered in the
Magellanic Clouds. They posses photometric \citep{Naze} and spectroscopic
\citep{Walborn} variability compatible with the Oblique Rotator Model (ORM).
However, their magnetic fields have yet to be directly detected. We have
developed an algorithm allowing for the synthesis of photometric observables
based on the Analytic Dynamical Magnetosphere (ADM) model of \citet{Owocki}. We
apply our model to OGLE photometry in order to constrain their magnetic
geometries and surface dipole strengths. We predict that the field strengths
for some of these candidate extra-Galactic magnetic stars may be within the
detection limits of the FORS2 instrument. | astro-ph_SR |
COBRaS: The e-MERLIN 21 cm Legacy survey of Cygnus OB2: The role of massive stars is central to an understanding of galactic ecology.
It is important to establish the details of how massive stars provide
radiative, chemical, and mechanical feedback in galaxies. Central to these
issues is an understanding of the evolution of massive stars, and the critical
role of mass loss via strongly structured winds and stellar binarity.
Ultimately, massive stellar clusters shape the structure and energetics of
galaxies. We aim to conduct high-resolution, deep field mapping at 21cm of the
core of the massive Cygnus OB2 association and to characterise the properties
of the massive stars and colliding winds at this waveband. We used seven
stations of the e-MERLIN radio facility, with its upgraded bandwidth and
enhanced sensitivity to conduct a 21cm census of Cygnus OB2. Based on 42 hours
of observations, seven overlapping pointings were employed over multiple epochs
during 2014 resulting in 1 sigma sensitivities down to ~21microJy and a
resolution of ~180mas. A total of 61 sources are detected at 21cm over a
~0.48deg x 0.48deg region centred on the heart of the Cyg OB2 association. Of
these 61 sources, 33 are detected for the first time. We detect a number of
previously identified sources including four massive stellar binary systems,
two YSOs, and several known X-ray and radio sources. We also detect the LBV
candidate (possible binary system) and blue hypergiant (BHG) star of Cyg OB2
#12. The 21cm observations secured in the COBRaS Legacy project provide data to
constrain conditions in the outer wind regions of massive stars; determine the
non-thermal properties of massive interacting binaries; examine evidence for
transient sources, including those associated with young stellar objects; and
provide unidentified sources that merit follow-up observations. The 21cm data
are of lasting value and will serve in combination with other key surveys of
Cyg OB2. | astro-ph_SR |
The characteristics of flare- and CME-productive solar active regions: Solar flares and coronal mass ejections (CMEs) cause immediate and adverse
effects on the interplanetary space and geospace. The deeper understanding of
the mechanisms that produce them and the construction of efficient prediction
schemes are of paramount importance. The source regions of flares and CMEs
exhibit some common morphological characteristics associated with strongly
sheared magnetic polarity inversion lines, indicative of the complex magnetic
configurations that store huge amounts of free magnetic energy and helicity.
This knowledge is transformed into parameters that can help us distinguish
efficiently between quiet, flare-, and CME-productive active regions.
Nonetheless, flare and CME prediction still faces a number of challenges. The
magnetic field information is constrained at the photosphere and accessed only
from one vantage point of observation; the dynamic behavior of active regions
is still not fully incorporated into predictions; the stochasticity of flares
and CMEs renders their prediction probabilistic. To meet these challenges, new
properties have been put forward to describe different aspects of magnetic
energy storage mechanisms in active regions and offer the opportunity of
parametric studies for over an entire solar cycle. This inventory of predictors
now includes information from flow fields, transition region/coronal
spectroscopy, data-driven modeling of the coronal magnetic field, as well as
parameterizations of dynamic effects from time series. Further work towards
these directions may help alleviate the current limitations in observing the
magnetic field of higher atmospheric layers. This paper reviews these efforts
as well as the importance of transforming new knowledge into more efficient
predictors and including new types of data. | astro-ph_SR |
CCD photometric study of the W UMa-type binary II CMa in the field of
Berkeley 33: The CCD photometric data of the EW-type binary, II CMa, which is a contact
star in the field of the middle-aged open cluster Berkeley 33, are presented.
The complete R light curve was obtained. In the present paper, using the five
CCD epochs of light minimum (three of them are calculated from Mazur et al.
(1993)'s data and two from our new data), the orbital period P was revised to
0.22919704 days. The complete R light curve was analyzed by using the 2003
version of W-D (Wilson-Devinney) program. It is found that this is a contact
system with a mass ratio $q=0.9$ and a contact factor $f=4.1%$. The high mass
ratio ($q=0.9$) and the low contact factor ($f=4.1%$) indicate that the system
just evolved into the marginal contact stage. | astro-ph_SR |
Transition region contribution to AIA observations in the context of
coronal heating: We investigate the relative contributions from the transition region and
corona of coronal loops observed by the Atmospheric Imaging Assembly (AIA) on
the Solar Dynamics Observatory (SDO). Using EBTEL (Enthalpy-Based Thermal
Evolution of Loops) hydrodynamic simulations, we model loops with multiple
lengths and energy fluxes heated randomly by events drawn from power-law
distributions with different slopes and minimum delays between events to
investigate how each of these parameters influences observable loop properties.
We generate AIA intensities from the corona and transition region for each
realization. The variations within and between models generated with these
different parameters illustrate the sensitivity of narrowband imaging to the
details of coronal heating. We then analyze the transition region and coronal
emission from a number of observed active regions and find broad agreement with
the trends in the models. In both models and observations, the transition
region brightness is significant, often greater than the coronal brightness in
all six "coronal" AIA channels. We also identify an inverse relationship,
consistent with heating theories, between the slope of the differential
emission measure (DEM) coolward of the peak temperature and the observed ratio
of coronal to transition region intensity. These results highlight the use of
narrowband observations and the importance of properly considering the
transition region in investigations of coronal heating. | astro-ph_SR |
Transition region contribution to AIA observations in the context of
coronal heating: We investigate the relative contributions from the transition region and
corona of coronal loops observed by the Atmospheric Imaging Assembly (AIA) on
the Solar Dynamics Observatory (SDO). Using EBTEL (Enthalpy-Based Thermal
Evolution of Loops) hydrodynamic simulations, we model loops with multiple
lengths and energy fluxes heated randomly by events drawn from power-law
distributions with different slopes and minimum delays between events to
investigate how each of these parameters influences observable loop properties.
We generate AIA intensities from the corona and transition region for each
realization. The variations within and between models generated with these
different parameters illustrate the sensitivity of narrowband imaging to the
details of coronal heating. We then analyze the transition region and coronal
emission from a number of observed active regions and find broad agreement with
the trends in the models. In both models and observations, the transition
region brightness is significant, often greater than the coronal brightness in
all six "coronal" AIA channels. We also identify an inverse relationship,
consistent with heating theories, between the slope of the differential
emission measure (DEM) coolward of the peak temperature and the observed ratio
of coronal to transition region intensity. These results highlight the use of
narrowband observations and the importance of properly considering the
transition region in investigations of coronal heating. | astro-ph_SR |
Rapid Changes of Photospheric Magnetic Field after Tether-Cutting
Reconnection and Magnetic Implosion: The rapid, irreversible change of the photospheric magnetic field has been
recognized as an important element of the solar flare process. This Letter
reports such a rapid change of magnetic fields during the 2011 February 13 M6.6
flare in NOAA AR 11158 that we found from the vector magnetograms of the
Helioseismic and Magnetic Imager with 12-min cadence. High-resolution
magnetograms of Hinode that are available at ~-5.5, -1.5, 1.5, and 4 hrs
relative to the flare maximum are used to reconstruct three-dimensional coronal
magnetic field under the nonlinear force-free field (NLFFF) assumption. UV and
hard X-ray images are also used to illuminate the magnetic field evolution and
energy release. The rapid change is mainly detected by HMI in a compact region
lying in the center of the magnetic sigmoid, where the mean horizontal field
strength exhibited a significant increase by 28%. The region lies between the
initial strong UV and hard X-ray sources in the chromosphere, which are
cospatial with the central feet of the sigmoid according to the NLFFF model.
The NLFFF model further shows that strong coronal currents are concentrated
immediately above the region, and that more intriguingly, the coronal current
system underwent an apparent downward collapse after the sigmoid eruption.
These results are discussed in favor of both the tether-cutting reconnection
producing the flare and the ensuing implosion of the coronal field resulting
from the energy release. | astro-ph_SR |
Red giants seismology: The space-borne missions CoRoT and Kepler are indiscreet. With their
asteroseismic programs, they tell us what is hidden deep inside the stars.
Waves excited just below the stellar surface travel throughout the stellar
interior and unveil many secrets: how old is the star, how big, how massive,
how fast (or slow) its core is dancing. This paper intends to \emph{paparazze}
the red giants according to the seismic pictures we have from their interiors. | astro-ph_SR |
Characterization of mid-infrared polarization due to scattering in
protoplanetary disks: It is generally assumed that magnetic fields play an important role in the
formation and evolution of protoplanetary disks. One way of observationally
constraining magnetic fields is to measure polarized emission and absorption
produced by magnetically aligned elongated dust grains. The fact that radiation
also becomes linearly polarized by light scattering at optical to millimeter
wavelengths complicates magnetic field studies. We characterize the linear
polarization of mid-infrared radiation due to scattering of the stellar
radiation and dust thermal re-emission radiation (self-scattering). We find
that the thermal re-emission radiation is stronger than the scattered stellar
radiation for disks with inner holes smaller than 10 au within the considered
parameter range. The mid-infrared polarization due to scattering shows several
clear trends: For scattered stellar radiation only, the linear polarization
degree decreases slightly with increasing radial distance, while it increases
with radial distance for thermal re-emission radiation only and for a
combination of scattered stellar radiation and thermal re-emission radiation.
The linear polarization degree decreases with increasing disk flaring and
luminosity of the central star. An increasing inner radius shifts the increase
of the linear polarization degree further outside, while a larger scale height
increases the linear polarization degree for small radial distances and
decreases this degree further outside. For longer wavelengths, the linear
polarization degree converges more slowly. | astro-ph_SR |
Mass loss from inhomogeneous hot star winds I. Resonance line formation
in 2D models: Small-scale clumping in the winds of hot, massive stars is conventionally
included in spectral analyses by assuming optically thin clumps, a void
inter-clump medium, and a smooth velocity field. To reconcile investigations of
different diagnostics within such models, a highly clumped wind with very low
mass-loss rates needs to be invoked. Particularly, unsaturated UV resonance
lines seem to indicate rates an order of magnitude (or even more) lower than
previously accepted values. We investigate resonance line formation in
inhomogeneous hot star winds with non-monotonic velocity fields by means of 2D
stochastic and pseudo-2D radiation-hydrodynamic wind models. A Monte-Carlo
radiative transfer code is presented and used to produce synthetic line
spectra. Results: The optically thin clumping limit is only valid for very weak
lines. For intermediate strong lines, the velocity spans of the clumps are of
central importance. Current hydrodynamical models predict spans that are too
large to reproduce observed profiles unless a very low mass-loss rate is
invoked. By simulating lower spans in 2D stochastic models, the profile
strengths become drastically reduced, and are consistent with higher mass-loss
rates. To simultaneously meet the constraints from strong lines, the
inter-clump medium must be non-void. A first comparison to the observed PV
doublet in the O6 supergiant lam Cep confirms that a stochastic 2D model
reproduces observations with a mass-loss rate roughly ten times higher than
that derived from the same lines but assuming optically thin clumping.
Tentatively this may resolve discrepancies between theoretical predictions,
evolutionary constraints, and recent derived mass-loss rates, and suggests a
re-investigation of the structure predicted by current hydrodynamical models. | astro-ph_SR |
Intermediate Resolution Near-Infrared Spectroscopy of 36 late-M Dwarfs: We present observations of 36 late-M dwarfs obtained with the KeckII/NIRSPEC
in the J-band at a resolution of \sim20,000. We have measured projected
rotational velocities, absolute radial velocities, and pseudo-equivalent widths
of atomic lines. 12 of our targets did not have previous measurements in the
literature.
For the other 24 targets, we confirm previously reported measurements. We
find that 13 stars from our sample have vsini below our measurement threshold
(12 km/s) whereas four of our targets are fast rotators (vsini > 30 km/s). As
fast rotation causes spectral features to be washed out, stars with low
projected rotational velocities are sought for radial velocity surveys.
At our intermediate spectral resolution we have confirmed the identification
of neutral atomic lines reported in Mclean et al. 2007. We also calculated
pseudo-equivalent widths (p-EW) of 12 atomic lines. Our results confirm that
the p-EW of K I lines are strongly dependent on spectral types. We observe that
the p-EW of Fe I and Mn I lines remain fairly constant with later spectral
type. We suggest that those lines are particularly suitable for deriving
metallicities for late-M dwarfs. | astro-ph_SR |
Rotational studies in the Orion Nebula Cluster: from solar mass stars to
brown dwarfs: Rotational studies at a variety of ages and masses are important for
constraining the angular momentum evolution of young stellar objects (YSO). Of
particular interest are the very low mass (VLM) stars and brown dwarfs (BDs),
because of the significant lack of known rotational periods in that mass range.
We provide for the first time information on rotational periods for a large
sample of young VLM stars and BDs. This extensive rotational period study in
the 1 Myr old Orion Nebula Cluster (ONC) is based on a deep photometric
monitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG
2.2m telescope on La Silla, Chile. Accurate I-band photometry of 2908 stars was
obtained, extending three magnitudes deeper than previous studies in the ONC.
We found 487 periodic variables with estimated masses between 0.5 Msun and
0.015 Msun, 124 of which are BD candidates. This is by far the most extensive
and complete rotational period data set for young VLM stars and BDs. In
addition, 808 objects show non-periodic brightness variations. We study the
dependence of the period distribution on mass and variability level and compare
this with known higher mass objects in the ONC (Herbst et al. 2002) and with
the 2 Myr old cluster NGC 2264 (Lamm et al., 2004). We find that substellar
objects rotate on average faster than the VLM stars. Our rotational data also
suggest a dependence of the rotational periods on position within the field,
which can be explained by a possible age spread in the ONC. In addition,
periodic variables with larger peak-to-peak amplitudes rotate on average slower
than those with small peak-to-peak amplitude variations, which can possibly be
explained by different magnetic field topologies. | astro-ph_SR |
Empirical Estimate of the Shape of the Upstream Heliopause from IBEX-Lo
Helium Measurements: Preliminary Results: We present a simplified model of the outer heliosheath to help interpret the
observations of interstellar neutral helium by the IBEX-Lo instruments. We
assume that the measured particles are composed of the superposition of a
primary beam population, with the properties of the local interstellar medium,
and a secondary population, created by charge exchange between the primary beam
neutrals and the ions that have been deflected as they approach the heliopause.
We extract information on the large-scale shape of the heliopause by comparing
the helium flux measured at IBEX along four different look directions with
simple models of deflected plasma flow around hypothetical obstacles of
different aspect ratios to the flow. As a first step in this paper, we model
the deflected plasma flow with the analytical solutions for compressible gas
flow around a series of oblate ellipsoidal obstacles. Our comparisons between
the model results and the observations indicate that the heliopause is very
blunt in the vicinity of the heliospheric nose, especially compared to a
Rankine half-body or cometary shape. The upstream heliopause seems to be highly
elongated in the directions parallel to the interstellar magnetic field, and
relatively more compact and symmetric in the directions transverse to that
field. The IBEX-Lo helium observations are not consistent with a heliopause
elongated in directions parallel to the solar rotation axis. | astro-ph_SR |
Intensity contrast from MHD simulations and from HINODE observations: Changes in the solar surface area covered by small-scale magnetic elements
are thought to cause long-term changes in the solar spectral irradiance, which
are important for determining the impact on Earth's climate. To study the
effect of small-scale magnetic elements on total and spectral irradiance, we
derive their contrasts from 3-D MHD simulations of the solar atmosphere. Such
calculations are necessary since measurements of small-scale flux tube
contrasts are confined to a few wavelengths and suffer from scattered light and
instrument defocus, even for space observations. To test the contrast
calculations, we compare rms contrasts from simulations with those obtained
with the broad-band filter imager mounted on the Solar Optical Telescope (SOT)
onboard the Hinode satellite and also analyse centre-to-limb variations (CLV).
The 3-D MHD simulations include the interaction between convection and magnetic
flux tubes. They have been run with non-grey radiative transfer using the MURaM
code. Simulations have an average vertical magnetic field of 0G, 50G, and 200G.
Emergent intensities are calculated with the spectral synthesis code ATLAS9 and
are convolved with a theoretical point-spread function to account for the
properties of the observations' optical system. We find reasonable agreement
for simulated and observed intensity distributions in the visible continuum
bands. Agreement is poorer for the CN and G-Bands. The analysis of the
simulations exhibits a potentially more realistic centre-to-limb behaviour than
calculations based on 1-D model atmospheres. We conclude that starting from 3-D
MHD simulations represents a powerful approach to obtaining intensity contrasts
for a wide wavelength coverage and for different positions on the solar disk.
This also paves the way for future calculations of facular and network contrast
as a function of magnetic fluxes. | astro-ph_SR |
Spectral Classification; Old and Contemporary: Beginning with a historical account of the spectral classification, its
refinement through additional criteria is presented. The line strengths and
ratios used in two dimensional classifications of each spectral class are
described. A parallel classification scheme for metal-poor stars and the
standards used for classification are presented. The extension of spectral
classification beyond M to L and T and spectroscopic classification criteria
relevant to these classes are described. Contemporary methods of
classifications based upon different automated approaches are introduced. | astro-ph_SR |
The dynamical importance of binary systems in young massive star
clusters: Characterization of the binary fractions in star clusters is of fundamental
importance for many fields in astrophysics. Observations indicate that the
majority of stars are found in binary systems, while most stars with masses
greater than $0.5 M_\odot$ are formed in star clusters. In addition, since
binaries are on average more massive than single stars, in resolved star
clusters these systems are thought to be good tracers of (dynamical) mass
segregation. Over time, dynamical evolution through two-body relaxation will
cause the most massive objects to migrate to the cluster center, while the
relatively lower-mass objects remain in or migrate to orbits at greater radii.
This process will globally dominate a cluster's stellar distribution. However,
close encounters involving binary systems may disrupt `soft' binaries. This
process will occur more frequently in a cluster's central, dense region than in
its periphery, which may mask the effects of mass segregation. Using high
resolution Hubble Space Telescope observations, combined with sophisticated
$N$-body simulations, we investigate the radial distributions of the
main-sequence binary fractions in massive young Large Magellanic Cloud star
clusters. We show that binary disruption may play an important role on very
short timescales, depending on the environmental conditions in the cluster
cores. This may lead to radial binary fractions that initially decline in the
cluster centers, which is contrary to the effects expected from dynamical mass
segregation. | astro-ph_SR |
The Mouse that Squeaked: A small flare from Proxima Cen observed in the
millimeter, optical, and soft X-ray with Chandra and ALMA: We present millimeter, optical, and soft X-ray observations of a stellar
flare with an energy squarely in the regime of typical X1 solar flares. The
flare was observed from Proxima Cen on 2019 May 6 as part of a larger
multi-wavelength flare monitoring campaign and was captured by Chandra, LCOGT,
du Pont, and ALMA. Millimeter emission appears to be a common occurrence in
small stellar flares that had gone undetected until recently, making it
difficult to interpret these events within the current multi-wavelength picture
of the flaring process. The May 6 event is the smallest stellar millimeter
flare detected to date. We compare the relationship between the soft X-ray and
millimeter emission to that observed in solar flares. The X-ray and optical
flare energies of 10$^{30.3\pm0.2}$ and 10$^{28.9\pm0.1}$ erg, respectively,
the coronal temperature of T=11.0$\pm$2.1 MK, and the emission measure of
9.5$\pm$2.2 X 10$^{49}$ cm$^{-3}$ are consistent with M-X class solar flares.
We find the soft X-ray and millimeter emission during quiescence are consistent
with the Gudel-Benz Relation, but not during the flare. The millimeter
luminosity is >100X higher than that of an equivalent X1 solar flare and lasts
only seconds instead of minutes as seen for solar flares. | astro-ph_SR |
The evolutionary state of the chemically peculiar members of the open
cluster NGC2516: We aim at establishing safe membership and evolutionary status of 11
chemically peculiar (CP) stars that are residing in the domain of the open
cluster NGC2516 and are frequently referred to as cluster members. We queried
the Gaia EDR3 catalogue in an area with a radius of 1deg and selected 37508
stars brighter than G=19mag. The cluster membership was determined in
parallax-proper motion-space and 719 probable and 764 possible members were
found. The obtained average astrometric and photometric parameters of the
cluster are in good agreement with the most recent literature data. The
evolutionary status of the target stars was determined with respect to Padova
isochrones. After minor adjustments including the metallicity, the reddening,
and the transformation scale variation, a perfect fit of the model to the
observations over the whole observed magnitude range was achieved. Only 5 of
the 11 considered CP stars could be classified as highly probable cluster
members. Among the Ap/Bp stars with previously detected magnetic fields HD65987
and HD65712 have a high membership probability and the magnetic star CPD-60
944B is a possible cluster member. Further we discuss the blue straggler nature
of HD66194 and the magnetic star HD65987. To our knowledge, HD65987 is
currently the only known blue straggler, with a field of the order of a few
hundred Gauss. The most striking result of our study is that the strongly
magnetic A0p star HD66318 with previously reported very low fractional age does
not belong to the NGC2516 cluster at a high level of confidence. | astro-ph_SR |
KIC 4142768: An Evolved Gamma Doradus/Delta Scuti Hybrid Pulsating
Eclipsing Binary with Tidally Excited Oscillations: We present the characterization of KIC 4142768, an eclipsing binary with two
evolved A-type stars in an eccentric orbit with a period of 14 days. We measure
the fundamental parameters of the two components ($M_1=2.05M_{\odot},
R_1=2.96R_{\odot}$ and $M_2=2.05M_{\odot}, R_2=2.51R_{\odot}$) by combining
{\it Kepler} photometry and spectra from {\it Keck} HIRES. The measured surface
rotation rates are only one-fifth of the pseudo-synchronous rate of the
eccentric orbit. Fourier spectrum of the light curve reveals hybrid pulsations
of $\delta$ Scuti and $\gamma$ Doradus type, with pulsation frequencies at
about $15-18$ day$^{-1}$ for p modes and about $0.2-1.2$ day$^{-1}$ for
low-frequency g modes. Some of the g modes are exact orbital harmonics and are
likely tidally excited. Their pulsation amplitudes and phases both agree with
predictions from the linear tidal theory for $l=2, m=2$ prograde modes. We
examine the period spacing patterns in the free oscillating g modes and
identify them mostly as prograde sectoral dipole modes. The unstable frequency
range and frequency spacing of p modes and the inferred asymptotic g-mode
period spacings both agree with the stellar model for the primary star evolved
to a late stage of the main sequence. The inferred rotation rate of the
convective core boundary is very slow, similar to the small surface rotation
rate inferred from the spectroscopy. The measured surface and near-core
rotation rates provide constraints for testing the mechanism of angular
momentum transfer and tidal synchronization in evolved eccentric binary star
systems. | astro-ph_SR |
Amplitude and lifetime of radial modes in red giant star spectra
observed by Kepler: Context: the space-borne missions CoRoT and Kepler have provided photometric
observations of unprecedented quality. The study of solar-like oscillations
observed in red giant stars by these satellites allows a better understanding
of the different physical processes occurring in their interiors. In
particular, the study of the mode excitation and damping is a promising way to
improve our understanding of stellar physics that has, so far, been performed
only on a limited number of targets. Aims: the recent asteroseismic
characterization of the evolutionary status for a large number of red giants
allows us to study the physical processes acting in the interior of red giants
and how they are modify during stellar evolution. In this work, we aim to
obtain information on the excitation and damping of pressure modes through the
measurement of the stars' pressure mode widths and amplitudes and to analyze
how they are modified with stellar evolution. The objective is to bring
observational constraints on the modeling of the physical processes behind mode
excitation and damping. Methods: we fit the frequency spectra of red giants
with well defined evolutionary status using Lorentzians functions to derive the
pressure mode widths and amplitudes. To strengthen our conclusions, we used two
different fitting techniques. Results: pressure mode widths and amplitudes were
determined for more than 5000 red giants. With a stellar sample two orders of
magnitude larger than previous results, we confirmed that the mode width
depends on stellar evolution and varies with stellar effective temperature. In
addition, we discovered that the mode width depends on stellar mass. We also
confirmed observationally the influence of the stellar metallicity on the mode
amplitudes, as predicted by models. | astro-ph_SR |
Solar Simulations for the Atacama Large Millimeter Observatory Network: The Atacama Large Millimeter/submillimeter Array (ALMA) will be a valuable
tool for observing the chromosphere of our Sun at (sub-)millimeter wavelengths
at high spatial, temporal and spectral resolution and as such has great
potential to address long-standing scientific questions in solar physics. In
order to make the best use of this scientific opportunity, the Solar
Simulations for the Atacama Large Millimeter Observatory Network has been
initiated. A key goal of this international collaboration is to support the
preparation and interpretation of future observations of the Sun with ALMA. | astro-ph_SR |
The FERRUM project: laboratory-measured transition probabilities for Cr
II: Aims: We measure transition probabilities for Cr II transitions from the z
^4H_J, z ^2D_J, y ^4F_J, and y ^4G_J levels in the energy range 63000 to 68000
cm^{-1}. Methods: Radiative lifetimes were measured using time-resolved
laser-induced fluorescence from a laser-produced plasma. In addition, branching
fractions were determined from intensity-calibrated spectra recorded with a UV
Fourier transform spectrometer. The branching fractions and radiative lifetimes
were combined to yield accurate transition probabilities and oscillator
strengths. Results: We present laboratory measured transition probabilities for
145 Cr II lines and radiative lifetimes for 14 Cr II levels. The
laboratory-measured transition probabilities are compared to the values from
semi-empirical calculations and laboratory measurements in the literature. | astro-ph_SR |
The polluted atmosphere of the white dwarf NLTT 25792 and the diversity
of circumstellar environments: We present an analysis of X-Shooter spectra of the polluted, hydrogen-rich
white dwarf NLTT 25792. The spectra show strong lines of calcium (Ca H&K,
near-infrared calcium triplet, and Ca I 4226 and numerous lines of iron along
with magnesium and aluminum lines from which we draw the abundance pattern.
Moreover, the photospheric Ca H&K lines are possibly blended with a
circumstellar component shifted by -20 km/s relative to the photosphere. A
comparison with a sample of four white dwarfs with similar parameters show
considerable variations in their abundance patterns, particularly in the
calcium to magnesium abundance ratio that varies by a factor of five within
this sample. The observed variations, even after accounting for diffusion
effects, imply similar variations in the putative accretion source. Also, we
find that silicon and sodium are significantly underabundant in the atmosphere
of NLTT 25792, a fact that may offer some clues on the nature of the accretion
source. | astro-ph_SR |
Evolution and chemical yields of AGB stars: effects of low-temperature
opacities: The studies focused on the Thermally-Pulsing Asymptotic Giant Branch phase
experienced by low- and intermediate-mass stars are extremely important in many
astrophysical contexts. In particular, a detailed computation of their chemical
yields is essential for several issues, ranging from the chemical evolution of
galaxies, to the mechanisms behind the formation of globular clusters. Among
all the uncertainties affecting the theoretical modelling of this phase, and
described in the literature, it remains to be fully clarified which results are
severely affected by the use of inadequate low-temperature opacities, that are
in most cases calculated on the basis of the original chemical composition of
the stars, and do not consider the changes in the surface chemistry due to the
occurrence of the third dredge-up and hot-bottom burning. Our investigation is
aimed at investigating this point. By means of full evolutionary models
including new set of molecular opacities computed specifically with the AESOPUS
tool, we highlight which stellar models, among those present in the literature,
need a substantial revision, mainly in relation to the predicted chemical
yields. The interplay among convection, hot bottom burning and the
low-temperature opacity treatment is also discussed | astro-ph_SR |
Pulsations, eruptions, and evolution of four yellow hypergiants: We aim to explore the variable photometric and stellar properties of four
yellow hypergiants (YHGs), HR8752, HR 5171A, $\rho$ Cas, and HD 179821, and
their pulsations of hundreds of days, and long-term variations (LTVs) of years.
We tackled multi-colour and visual photometric data sets, looked for
photometric indications betraying eruptions or enhanced mass-loss episodes,
calculated stellar properties mainly using a published temperature calibration,
and investigated the nature of LTVs and their influence on quasi-periods and
stellar properties. The $BV$ photometry revealed a high-opacity layer in the
atmospheres. When the temperature rises the mass loss increases as well,
consequently, as the density of the high-opacity layer. As a result, the
absorption in $B$ and $V$ grow. The absorption in $B$, presumably of the order
of one to a few 0\fm1, is always higher than in $V$. This difference renders
redder and variable $(B-V)$ colour indexes, but the absorption law is unknown.
This property of YHGs is unpredictable and explains why spectroscopic
temperatures are always higher than photometric ones. We propose shorter
distances for $\rho$ Cas and HR 5171A than the accepted ones. Therefore, a
correction to decrease the blue luminescence of HR 5171A by polycyclic aromatic
hydrocarbon (PAH) molecules is necessary, and HR 5171A would no longer be a
member of the cluster Gum48d. HR 5171A is only subject to one source of light
variation, not by two as the literature suggests. Eruptive episodes of YHGs
prefer relatively cool circumstances when a red evolutionary loop (RL) has
shifted the star to the red on the HR diagram. After the eruption, a blue loop
evolution (BL) is triggered lasting one to a few decades. The reddening episode
of HR 5171A between 1960 and 1974 was most likely due to a red loop evolution,
and the reddening after the 1975 eruption was likely due to a shell ejection. | astro-ph_SR |
The potential of combining MATISSE and ALMA observations: Constraining
the structure of the innermost region in protoplanetary discs: In order to study the initial conditions of planet formation, it is crucial
to obtain spatially resolved multi-wavelength observations of the innermost
region of protoplanetary discs. We evaluate the advantage of combining
observations with MATISSE/VLTI and ALMA to constrain the radial and vertical
structure of the dust in the innermost region of circumstellar discs in nearby
star-forming regions. Based on a disc model with a parameterized dust density
distribution, we apply 3D radiative-transfer simulations to obtain ideal
intensity maps. These are used to derive the corresponding wavelength-dependent
visibilities we would obtain with MATISSE as well as ALMA maps simulated with
CASA. Within the considered parameter space, we find that constraining the dust
density structure in the innermost $5\,$au around the central star is
challenging with MATISSE alone, whereas ALMA observations with reasonable
integration times allow us to derive significant constraints on the disc
surface density. However, we find that the estimation of the different disc
parameters can be considerably improved by combining MATISSE and ALMA
observations. For example, combining a 30-minute ALMA observation (at 310 GHz
with an angular resolution of 0.03$^{\prime\prime}$) for MATISSE observations
in the L and M bands (with visibility accuracies of about $3\,\%$) allows the
radial density slope and the dust surface density profile to be constrained to
within $\Delta \alpha=0.3$ and $\Delta (\alpha-\beta)=0.15$, respectively. For
an accuracy of ${\sim 1\%}$ even the disc flaring can be constrained to within
$\Delta \beta=0.1$. To constrain the scale height to within $5\,$au, M band
accuracies of $0.8\,\%$ are required. While ALMA is sensitive to the number of
large dust grains settled to the disc midplane we find that the impact of the
surface density distribution of the large grains on the observed quantities is
small. | astro-ph_SR |
Statistical Study of Coronal Mass Ejection Source Locations: II. Role of
Active Regions in CME Production: This is the second paper of the statistical study of coronal mass ejection
(CME) source locations, in which the relationship between CMEs and active
regions (ARs) is statistically studied on the basis of the information of CME
source locations and the ARs automatically extracted from magnetic synoptic
charts of Michelson Doppler Imager (MDI) during 1997 -- 1998. It is found that
about 63% of the CMEs are related with ARs, at least about 53% of the ARs
produced one or more CMEs, and particularly about 14% of ARs are CME-rich (3 or
more CMEs were generated) during one transit across the visible disk. Several
issues are then tried to clarify: whether or not the CMEs originating from ARs
are distinct from others, whether or not the CME kinematics depend on AR
properties, and whether or not the CME productivity depends on AR properties.
The statistical results suggest that (1) there is no evident difference between
AR-related and non-AR-related CMEs in terms of CME speed, acceleration and
width, (2) the size, strength and complexity of ARs do little with the
kinematic properties of CMEs, but have significant effects on the CME
productivity, and (3) the sunspots in all the most productive ARs at least
belong to $\beta\gamma$ type, whereas 90% of those in CME-less ARs are $\alpha$
or $\beta$ type only. A detailed analysis on CME-rich ARs further reveals that
(1) the distribution of the waiting time of same-AR CMEs, consists of two parts
with a separation at about 15 hours, which implies that the CMEs with a waiting
time shorter than 15 hours are probably truly physical related, and (2) an AR
tends to produce such related same-AR CMEs at a pace of 8 hours, but cannot
produce two or more fast CMEs (>800 km/s) within a time interval of 15 hours.
This interesting phenomenon is particularly discussed. | astro-ph_SR |
A critical analysis of three near-infrared photometric methods of
estimating distances to cataclysmic variables: A critical analysis of three methods of estimating distances to cataclysmic
variables (CVs) is performed. These methods, by Ak et al., Beuermann, and
Knigge, all use near-infrared (JHK or Ks) magnitudes and the Barnes-Evans
relation. We compare all three methods to distances measured by trigonometric
parallax by Thorstensen, with Hubble Space Telescope, and with the HIPPARCOS
spacecraft.
We find that the method of Ak et al. works best overall for all CVs,
predicting distances on the average 4% less than those measured by
trigonometric parallaxes. The method of Beuermann overestimates distances by
52%. The method of Knigge underestimates distances by 26%, although this was
only ever meant as a lower limit, since it assumes all light comes from the
secondary star. | astro-ph_SR |
The $UBV$ Color Evolution of Classical Novae. III. Time-Stretched
Color-Magnitude Diagram of Novae in Outburst: We propose a modified color-magnitude diagram for novae in outburst, i.e.,
$(B-V)_0$ versus $(M_V-2.5 \log f_{\rm s})$, where $f_{\rm s}$ is the
timescaling factor of a (target) nova against a comparison (template) nova,
$(B-V)_0$ is the intrinsic $B-V$ color, and $M_V$ is the absolute $V$
magnitude. We dub it the time-stretched color-magnitude diagram. We carefully
reanalyzed 20 novae based on the time-stretching method and revised their
extinctions $E(B-V)$, distance moduli in the $V$ band $(m-M)_V$, distances $d$,
and timescaling factors $f_{\rm s}$ against the template nova LV Vul. We have
found that these 20 nova outburst tracks broadly follow one of the two template
tracks, LV Vul/V1668 Cyg or V1500 Cyg/V1974 Cyg group, in the time-stretched
color-magnitude diagram. In addition, we estimate the white dwarf masses and
$(m-M)_V$ of the novae by directly fitting the absolute $V$ model light curves
($M_V$) with observational apparent $V$ magnitudes ($m_V$). A good agreement in
the two estimates of $(m-M)_V$ confirms the consistency of the time-stretched
color-magnitude diagram. Our distance estimates are in good agreement with the
results of Gaia Data Release 2. | astro-ph_SR |
Fundamental Parameters of Main-Sequence Stars in an Instant with Machine
Learning: Owing to the remarkable photometric precision of space observatories like
Kepler, stellar and planetary systems beyond our own are now being
characterized en masse for the first time. These characterizations are pivotal
for endeavors such as searching for Earth-like planets and solar twins,
understanding the mechanisms that govern stellar evolution, and tracing the
dynamics of our Galaxy. The volume of data that is becoming available, however,
brings with it the need to process this information accurately and rapidly.
While existing methods can constrain fundamental stellar parameters such as
ages, masses, and radii from these observations, they require substantial
computational efforts to do so.
We develop a method based on machine learning for rapidly estimating
fundamental parameters of main-sequence solar-like stars from classical and
asteroseismic observations. We first demonstrate this method on a
hare-and-hound exercise and then apply it to the Sun, 16 Cyg A & B, and 34
planet-hosting candidates that have been observed by the Kepler spacecraft. We
find that our estimates and their associated uncertainties are comparable to
the results of other methods, but with the additional benefit of being able to
explore many more stellar parameters while using much less computation time. We
furthermore use this method to present evidence for an empirical diffusion-mass
relation. Our method is open source and freely available for the community to
use.
The source code for all analyses and for all figures appearing in this
manuscript can be found electronically at
https://github.com/earlbellinger/asteroseismology | astro-ph_SR |
CoRoT-TESS eclipsing binaries with light-travel-time effect: Identifying long-period eclipsing binaries with space-based photometry is
still a challenge even in the century of space telescopes due to the relatively
short observation sequences and short lifetime of these missions. The
Transiting Exoplanet Survey Satellite (TESS) space telescope is an appropriate
tool to supplement previous space-based observations. In this paper we report
the first results of the eclipse timing variation (ETV) analyses of eclipsing
binaries (EBs) measured by CoRoT and TESS space telescopes. Among the 1428 EB
candidates we found 4 new potential triple candidates, for which ETV was
analysed and fitted by the well-known light-travel-time effect (LTTE). One of
them shows significant phase shift in its folded light curve which required
extra care. In this paper we also present some other systems showing
significant ETV signals that could be explained by mass transfer or apsidal
motion. | astro-ph_SR |
Numerical Investigation of a Coronal Mass Ejection from an Anemone
Active Region: Reconnection and Deflection of the 2005 August 22 Eruption: We present a numerical investigation of the coronal evolution of a coronal
mass ejection (CME) on 2005 August 22 using a 3-D thermodynamics
magnetohydrodynamic model, the SWMF. The source region of the eruption was
anemone active region (AR) 10798, which emerged inside a coronal hole. We
validate our modeled corona by producing synthetic extreme ultraviolet (EUV)
images, which we compare to EIT images. We initiate the CME with an
out-of-equilibrium flux rope with an orientation and chirality chosen in
agreement with observations of a H-alpha filament. During the eruption, one
footpoint of the flux rope reconnects with streamer magnetic field lines and
with open field lines from the adjacent coronal hole. It yields an eruption
which has a mix of closed and open twisted field lines due to interchange
reconnection and only one footpoint line-tied to the source region. Even with
the large-scale reconnection, we find no evidence of strong rotation of the CME
as it propagates. We study the CME deflection and find that the effect of the
Lorentz force is a deflection of the CME by about 3 deg/Rsun towards the East
during the first 30 minutes of the propagation. We also produce coronagraphic
and EUV images of the CME, which we compare with real images, identifying a
dimming region associated with the reconnection process. We discuss the
implication of our results for the arrival at Earth of CMEs originating from
the limb and for models to explain the presence of open field lines in magnetic
clouds. | astro-ph_SR |
Acceleration and propagation of Solar Energetic Particles: Solar Energetic Particles (SEPs) are an important component of Space Weather,
including radiation hazard to humans and electronic equipment, and the
ionisation of the Earth's atmosphere. We review the key observations of SEPs,
our current understanding of their acceleration and transport, and discuss how
this knowledge is incorporated within Space Weather forecasting tools.
Mechanisms for acceleration during solar flares and at shocks driven by Coronal
Mass Ejections are discussed, as well as the timing relationships between
signatures of solar eruptive events and the detection of SEPs in interplanetary
space. Evidence on how the parameters of SEP events are related to those of the
parent solar activity is reviewed and transport effects influencing SEP
propagation to near-Earth locations are examined. Finally, the approaches to
forecasting Space Weather SEP effects are discussed. We conclude that both
flare and CME shock acceleration contribute to Space Weather relevant SEP
populations and need to be considered within forecasting tools. | astro-ph_SR |
Metric Radio Bursts and Fine Structures Observed on January 20, 2005: A major radio event, associated with an X7.1/2B flare in AR720 and a fast CME
was observed on January 20, 2005 with the radio-spectrograph ARTEMIS-IV; it was
particularly intense and with a complex radio signature with rich fine
structure which was recorded in the 270-420 MHz range at high resolution (100
samples/sec). The fine structure is compared with similar results in the
decimetric and microwave frequency range. It was found to approximately match
the comprehensive Ondrejov Classification in the spectral range 0.8-2 GHz. | astro-ph_SR |
Lorentz Force Evolution Reveals the Energy Buildup Processes during
Recurrent Eruptive Solar Flares: The energy release and build-up processes in the solar corona have
significant implications in particular for the case of large recurrent flares,
which pose challenging questions about the conditions that lead to the episodic
energy release processes. It is not yet clear whether these events occur due to
the continuous supply of free magnetic energy to the solar corona or because
not all of the available free magnetic energy is released during a single major
flaring event. In order to address this question, we report on the evolution of
photospheric magnetic field and the associated net Lorentz force changes in ARs
11261 and 11283, each of which gave rise to recurrent eruptive M- and X-class
flares. Our study reveals that after the abrupt downward changes during each
flare, the net Lorentz force increases by (2-5)x10^22 dyne in between the
successive flares. This distinct rebuild-up of net Lorentz forces is the first
observational evidence found in the evolution of any non-potential parameter of
solar active regions (ARs), which suggests that new energy was supplied to the
ARs in order to produce the recurrent large flares. The rebuild-up of magnetic
free energy of the ARs is further confirmed by the observations of continuous
shearing motion of moving magnetic features of opposite polarities near the
polarity inversion line. The evolutionary pattern of the net Lorentz force
changes reported in this study has significant implications, in particular, for
the forecasting of recurrent large eruptive flares from the same AR and hence
the chances of interaction between the associated CMEs. | astro-ph_SR |
A New Transition Wolf-Rayet WN/C Star in the Milky Way: We report the discovery of a new transition type Wolf-Rayet (WR) WN/C star in
the Galaxy. According to its coordinates (R.A., Dec)J2000 = 18h51m39.7s,
-05d34m51.1s, and the distance (7.11 kpc away from Earth) inferred from the
second Gaia, data release, it's found that WR 121-16 is located in the Far 3
kpc Arm, and it is 3.75 kpc away from the Galactic Center. The optical spectra
obtained by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope
(LAMOST) and the 2.16 m telescope, both located at the Xinglong Observatory in
China, indicate that this is a WR star of the transitional WN7o/WC subtype. A
current stellar mass of about 7.1 M_solar, a mass-loss rate of M_dot =
10^(-4.97) M_solar/yr, a bolometric luminosity of log L/L_solar = 4.88, and a
stellar temperature of T_* = 47 kK are derived, by fitting the observed
spectrum with a specific Potsdam Wolf-Rayet (PoWR) model. The magnitude in
V-band varies between 13.95 and 14.14 mag, while no period is found. Based on
the optical spectra, the time domain data, and the indices of the astrometric
solution of the Gaia data, WR 121-16 is likely a transitional WN/C single star
rather than a WN+WC binary. | astro-ph_SR |
The RR Lyrae Variable Population in the Phoenix Dwarf Galaxy: We present the first detailed study of the RR Lyrae variable population in
the Local Group dSph/dIrr transition galaxy, Phoenix, using previously obtained
HST/WFPC2 observations of the galaxy. We utilize template light curve fitting
routines to obtain best fit light curves for RR Lyrae variables in Phoenix. Our
technique has identified 78 highly probable RR Lyrae stars (54 ab-type; 24
c-type) with about 40 additional candidates. We find mean periods for the two
populations of $\langle P_{ab}\rangle = 0.60 \pm 0.03$ days and $\langle
P_{c}\rangle = 0.353 \pm 0.002$ days. We use the properties of these light
curves to extract, among other things, a metallicity distribution function for
ab-type RR Lyrae. Our analysis yields a mean metallicity of $\langle
[Fe/H]\rangle = -1.68 \pm 0.06$ dex for the RRab stars. From the mean period
and metallicity calculated from the ab-type RR Lyrae, we conclude that Phoenix
is more likely of intermediate Oosterhoff type; however the morphology of the
Bailey diagram for Phoenix RR Lyraes appears similar to that of an Oosterhoff
type I system. Using the RRab stars, we also study the chemical enrichment law
for Phoenix. We find that our metallicity distribution is reasonably well
fitted by a closed-box model. The parameters of this model are compatible with
the findings of Hidalgo et al. (2009) further supporting the idea that Phoenix
appears to have been chemically enriched as a closed-box-like system during the
early stage of its formation and evolution. | astro-ph_SR |
Constraining general massive-star physics by exploring the unique
properties of magnetic O-stars: Rotation, macroturbulence, and sub-surface
convection: A quite remarkable aspect of non-interacting O-stars with detected surface
magnetic fields is that they all are very slow rotators. This paper uses this
unique property to first demonstrate that the projected rotational speeds of
massive, hot stars, as derived using current standard spectroscopic techniques,
can be severely overestimated when significant "macroturbulent" line-broadening
is present. This may, for example, have consequences for deriving the
statistical distribution of rotation rates in massive-star populations, and for
the use of these rates in stellar evolution models. It is next shown how such
macroturbulence (seemingly a universal feature of hot, massive stars) is
present in all but one of the magnetic O-stars, namely NGC 1624-2. Assuming
then a simple model in which NGC 1624-2's exceptionally strong, large-scale
magnetic field suppresses atmospheric motions down to layers where the magnetic
and gas pressures are comparable, first empirical constraints on the formation
depth of this enigmatic hot-star macroturbulence are derived. The results
suggest an origin in the thin sub-surface convection zone of massive stars,
consistent with a physical origin due to, e.g., stellar pulsations excited by
the convective motions. | astro-ph_SR |
Coadded Spectroscopic Stellar Parameters and Abundances from the LAMOST
Low Resolution Survey: I combine duplicate spectroscopic stellar parameter estimates in the Large
Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 6 Low
Resolution Spectral Survey A, F, G, and K Type stellar parameter catalog.
Combining repeat measurements results in a factor of two improvement in the
precision of the spectroscopic stellar parameter estimates. Moreover, this
trivializes the process of performing coordinate-based cross-matching with
other catalogs. Similarly, I combine duplicate stellar abundance estimates for
the Xiang et al. catalog which was produced using LAMOST Data Release 5 Low
Resolution Spectral Survey data. These data have numerous applications in
stellar, galactic, and exoplanet astronomy. The catalogs I produce are
available as machine-readable tables at https://doi.org/10.7281/T1/QISGRU . | astro-ph_SR |
Correlation of the sunspot number and the waiting time distribution of
solar flares, coronal mass ejections, and solar wind switchback events
observed with the Parker Solar Probe: Waiting time distributions of solar flares and {\sl coronal mass ejections
(CMEs)} exhibit power law-like distribution functions with slopes in the range
of $\alpha_{\tau} \approx 1.4-3.2$, as observed in annual data sets during 4
solar cycles (1974-2012). We find a close correlation between the waiting time
power law slope $\alpha_\tau$ and the {\sl sunspot number (SN)}, i.e.,
$\alpha_\tau$ = 1.38 + 0.01 $\times$ SN. The waiting time distribution can be
fitted with a Pareto-type function of the form $N(\tau) = N_0$ $(\tau_0 +
\tau)^{-\alpha_{\tau}}$, where the offset $\tau_0$ depends on the instrumental
sensitivity, the detection threshold of events, and pulse pile-up effects. The
time-dependent power law slope $\alpha_{\tau}(t)$ of waiting time distributions
depends only on the global solar magnetic flux (quantified by the sunspot
number) or flaring rate, independent of other physical parameters of {\sl
self-organized criticality (SOC)} or {\sl magneto-hydrodynamic (MHD)}
turbulence models. Power law slopes of $\alpha_{\tau}\approx 1.2-1.6$ were also
found in solar wind switchback events, as observed with the {\sl Parker Solar
Probe (PSP)}. We conclude that the annual variability of switchback events in
the heliospheric solar wind is modulated by flare and CME rates originating in
the photosphere and lower corona. | astro-ph_SR |
Near Infrared studies of the carbon-monoxide and dust forming nova V5668
Sgr: We present near-infrared (NIR) observations of Nova V5668 Sgr, discovered in
outburst on 2015 March 15.634 UT, between 2d to 107d after outburst. NIR
spectral features are used to classify it as a FeII class of nova. The spectra
follow the evolution of the spectral lines from a P Cygni stage to a pure
emission phase where the shape of the profiles suggests the presence of a
bipolar flow. A notable feature is the presence of carbon monoxide first
overtone bands which are seen in emission. The CO emission is modeled to make
estimates of the mass, temperature and column density to be (0.5--2.0)$\times$
10$^{-8}$ M$_\odot$, 4000 $\pm$ 300K and (0.36--1.94)$\times$ 10$^{19}$
cm$^{-2}$ respectively. The $^{12}$C/$^{13}$C ratio is estimated to be $\sim$
1.5. V5668 Sgr was a strong dust producer exhibiting the classical deep dip in
its optical light curve during dust formation. Analysis of the dust SED yields
a dust mass of 2.7 $\times$ 10${^{\rm -7}}$ $M_\odot $, a blackbody angular
diameter of the dust shell of 42 mas and a distance estimate to the nova of
1.54 kpc which agrees with estimates made from MMRD relations. | astro-ph_SR |
Investigating 2MASS J06593158-0405277: a FUor burst in a triple system?: FUor outbursts in young stellar objects (YSOs) are the most dramatic events
among episodic accretion phenomena. The origin of these bursts is not clear:
disk instabilities and/or disk perturbations by an external body being the most
viable hypotheses. Here, we report our VLT/SINFONI high angular resolution
AO-assisted observations of 2MASS J06593158-0405277, which is undergoing a
recently discovered FUor outburst. Our observations reveal the presence of an
extended disc-like structure around the FUor, a very low-mass companion (2MASS
J06593158-0405277B) at ~100 au in projection, and, possibly, a third closer
companion at ~11 au. These sources appear to be young, displaying accretion
signatures. Assuming the components are physically linked, 2MASS
J06593158-0405277 would then be one of the very few triple systems observed in
FUors. | astro-ph_SR |
Ultrashort-period MS eclipsing systems. New observations and light curve
solutions of six NSVS binaries: We carried out photometric and low-resolution spectral observations of six
eclipsing ultrashort-period binaries with MS components. The light curve
solutions of the Rozhen observations show that all targets are overcontact
systems. We found well-defined empirical relation "period -- semi-major axis"
for the short-period binaries and used it for estimation of the global
parameters of the targets. Our results revealed that NSVS 925605 is quite
interesting target: (a) it is one of a few contact binaries with M components;
(b) it exhibits high activity (emission in H$\alpha$ line, X-ray emission,
large cool spots, non-Planck energy distribution); (c) its components differ in
temperature by 700 K. All appearances of high magnetic activity and huge
fillout factor (0.7) of NSVS 925605 might be assumed as a precursor of the
predicted merging of close magnetic binaries. Another unusual binary is NSVS
2700153 which reveals considerable long-term variability. | astro-ph_SR |
First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP: The
Anatomy of an Umbral Flash: The Visible Spectro-Polarimeter (ViSP) of the NSF Daniel K. Inouye Solar
Telescope (DKIST) collected its Science Verification data on May 7-8, 2021. The
instrument observed multiple layers of a sunspot atmosphere simultaneously, in
passbands of Ca-II 397 nm (H-line), Fe-I 630 nm, and Ca-II 854 nm, scanning the
region with a spatial sampling of 0.041" and average temporal cadence of 7.76
seconds, for a 38.8 minute duration. The slit moves southward across the
plane-of-the-sky at 3.83 km/s. The spectropolarimetric scans exhibit prominent
oscillatory 'ridge' structures which lie nearly perpendicular to the direction
of slit motion (north to south). These ridges are visible in maps of line
intensity, central wavelength, line width, and both linear and circular
polarizations. Contemporaneous Atmospheric Imaging Assembly observations
indicate these ridges are purely temporal in character and likely attributed to
the familiar chromospheric 3-minute umbral oscillations. We observe in detail a
steady umbral flash near the center of the sunspot umbra. Although bad seeing
limited the spatial resolution, the unique high signal-to-noise enable us to
estimate the shock Mach numbers (= 2), propagation speeds (= 9 km/s), and their
impact on longitudinal magnetic field (delta B = 50 G), gas pressure, and
temperature (delta T/T = 0.1) of the subshocks over 30 seconds. We also find
evidence for rarefaction waves situated between neighboring wave-train shocks.
The Ca-II 854 nm line width is steady throughout the umbral flash except for a
sharp 1.5 km/s dip immediately before, and comparable spike immediately after,
the passage of the shock front. This zig-zag in line width is centered on the
subshock and extends over 0.4". | astro-ph_SR |
Validating Forward Modeling and Inversions of Helioseismic Holography
Measurements: Here we use synthetic data to explore the performance of forward models and
inverse methods for helioseismic holography. Specifically, this work presents
the first comprehensive test of inverse modeling for flows using
lateral-vantage (deep-focus) holography. We derive sensitivity functions in the
Born approximation. We then use these sensitivity functions in a series of
forward models and inversions of flows from a publicly available
magnetohydrodynamic quiet-Sun simulation. The forward travel times computed
using the kernels generally compare favorably with measurements obtained by
applying holography, in a lateral-vantage configuration, on a 15-hour time
series of artificial Dopplergrams extracted from the simulation. Inversions for
the horizontal flow components are able to reproduce the flows in the upper 3Mm
of the domain, but are compromised by noise at greater depths. | astro-ph_SR |
New Parallaxes for the Upper Scorpius OB Association: Upper Scorpius is a subgroup of the nearest OB association,
Scorpius--Centaurus. Its young age makes it an important association to study
star and planet formation. We present parallaxes to 52 low mass stars in Upper
Scorpius, 28 of which have full kinematics. We measure ages of the individual
stars by combining our measured parallaxes with pre-main sequence evolutionary
tracks. We find there is a significant difference in the ages of stars with and
without circumstellar disks. The stars without disks have a mean age of
4.9+/-0.8 Myr and those with disks have an older mean age of 8.2+/-0.9 Myr.
This somewhat counterintuitive result suggests that evolutionary effects in
young stars can dominate their apparent ages. We also attempt to use the 28
stars with full kinematics (i.e.\ proper motion, radial velocity, and parallax)
to trace the stars back in time to their original birthplace to obtain a
trackback age. We find, as expected given large measurement uncertainties on
available radial velocity measurements, that measurement uncertainties alone
cause the group to diverge after a few Myr. | astro-ph_SR |
Distances of CVs and related objects derived from Gaia Data Release 1: We consider the parallaxes of sixteen cataclysmic variables and related
objects that are included in the TGAS catalogue, which is part of the Gaia
first data release, and compare these with previous parallax measurements. The
parallax of the dwarf nova SS Cyg is consistent with the parallax determination
made using the VLBI, but with only one of the analyses of the HST Fine Guidance
Sensor (FGS) observations of this system. In contrast, the Gaia parallaxes of
V603 Aql and RR Pic are broadly consistent, but less precise than the HST/FGS
measurements. The Gaia parallaxes of IX Vel, V3885 Sgr, and AE Aqr are
consistent with, but much more accurate than the Hipparcos measurements. We
take the derived Gaia distances and find that absolute magnitudes of
outbursting systems show a weak correlation with orbital period. For systems
with measured X-ray fluxes we find that the X-ray luminosity is a clear
indicator of whether the accretion disc is in the hot and ionised or cool and
neutral state. We also find evidence for the X-ray emission of both low and
high state discs correlating with orbital period, and hence the long-term
average accretion rate. The inferred mass accretion rates for the nova-like
variables and dwarf novae are compared with the critical mass accretion rate
predicted by the Disk Instability Model. While we find agreement to be good for
most systems there appears to be some uncertainty in the system parameters of
SS Cyg. Our results illustrate how future Gaia data releases will be an
extremely valuable resource in mapping the evolution of cataclysmic variables. | astro-ph_SR |
The Photometric Investigation of V921 Her using the Lunar-based
Ultraviolet Telescope of Chang'e-3 mission: The light curve of V921 Her in ultraviolet band observed by the Lunar-based
Ultraviolet Telescope (LUT) is analyzed by the Wilson-Devinney code. Our
solutions conclude that V921 Her is an early type marginal contact binary
system with an additional close-in component. The binary system is under poor
thermal contact with a temperature difference of nearly $700K$ between the two
components. The close-in component contributes about $19\,\%$ of the total
luminosity in the triple system. Combining the radial velocity study together
with our photometric solutions, the mass of the primary star and secondary one
are calculated to be $M_1 = 1.784(\pm0.055)M_\odot$, $M_2 =
0.403(\pm0.012)M_\odot$. The evolutionary scenario of V921 Her is discussed.
All times of light minimum of V921 Her available in the bibliography are taken
into account and the $O - C$ curve is analyzed for the first time. The most
probable fitting results are discussed in the paper, which also confirm the
existence of a third component ($P_3=10.2$ year) around the binary system. The
period of V921 Her is also undergoing a continuously rapid increase at a rate
of $dP/dt=+2.79\times{10^{-7}}day\cdot year^{-1}$, which may due to mass
transfer from the less massive component to the more massive one. | astro-ph_SR |
The circumbinary rings of GG Carinae: indications of disc eccentricity
growth in the B[e] supergiant's atomic emission lines: B[e] supergiants have unusual circumstellar environments which may include
thin, concentric rings displaying atomic line emission. GG Carinae is a B[e]
supergiant binary which exhibits such a geometry in its circumbinary
environment. We study atomic emission lines arising from GG Carinae's
circumbinary disc in FEROS spectra collected between 1998 and 2015. We find
that semi-forbidden Fe\,II] and permitted Ca\,II emission are formed in the
same thin circumbinary ring previously reported to have forbidden [O\,I] and
[Ca\,II] emission. We find that there are two circumbinary rings orbiting with
projected velocities of $84.6\pm1.0$\,km\,s$^{-1}$ and
$27.3\pm0.6$\,km\,s$^{-1}$. Deprojecting these velocities from the
line-of-sight, and using updated binary masses presented by
\cite{Porter2021GGPhotometry}, we find that the radii of the circumbinary rings
are $2.8^{+0.9}_{-1.1}$\,AU and $27^{+9}_{-10}$\,AU for the inner ring and
outer ring respectively. We find evidence of subtle dynamical change in the
inner circumbinary ring over the 17 years spanned by the data, manifesting in
variability in the ratio of the intensity of the blueshifted peak to the
redshifted peak of its emission lines and the central velocity becoming more
blueshifted. We perform smoothed-particle hydrodynamic simulations of the
system which suggest that these observed changes are consistent with pumping of
the eccentricity of a radially thin circumbinary ring by the inner binary. We
find a systemic velocity of the GG Carinae system of $-23.2 \pm
0.4$\,km\,s$^{-1}$. | astro-ph_SR |
ASAS Light Curves of Intermediate Mass Eclipsing Binaries and the
Parameters of HI Mon: We present a catalog of 56 candidate intermediate mass eclipsing binary
systems extracted from the 3rd data release of the All Sky Automated Survey. We
gather pertinent observational data and derive orbital properties, including
ephemerides, for these systems as a prelude to anticipated spectroscopic
observations. We find that 37 of the 56, or ~66% of the systems are not
identified in the Simbad Astronomical Database as known binaries. As a specific
example, we show spectroscopic data obtained for the system HI Mon (B0 V + B0.5
V) observed at key orbital phases based on the computed ephemeris, and we
present a combined spectroscopic and photometric solution for the system and
give stellar parameters for each component. | astro-ph_SR |
Particle acceleration by circularly and elliptically polarised
dispersive Alfven waves in a transversely inhomogeneous plasma in the
inertial and kinetic regimes: Dispersive Alfven waves (DAWs) offer, an alternative to magnetic
reconnection, opportunity to accelerate solar flare particles. We study the
effect of DAW polarisation, L-, R-, circular and elliptical, in different
regimes inertial and kinetic on the efficiency of particle acceleration. We use
2.5D PIC simulations to study how particles are accelerated when DAW, triggered
by a solar flare, propagates in transversely inhomogeneous plasma that mimics
solar coronal loop. (i) In inertial regime, fraction of accelerated electrons
(along the magnetic field), in density gradient regions is ~20% by the time
when DAW develops 3 wavelengths and is increasing to ~30% by the time DAW
develops 13 wavelengths. In all considered cases ions are heated in transverse
to the magnetic field direction and fraction of the heated particles is ~35%.
(ii) The case of R-circular, L- and R- elliptical polarisation DAWs, with the
electric field in the non-ignorable transverse direction exceeding several
times that of in the ignorable direction, produce more pronounced parallel
electron beams and transverse ion beams in the ignorable direction. In the
inertial regime such polarisations yield the fraction of accelerated electrons
~20%. In the kinetic regime this increases to ~35%. (iii) The parallel electric
field that is generated in the density inhomogeneity regions is independent of
m_i/m_e and exceeds the Dreicer value by 8 orders of magnitude. (iv) Electron
beam velocity has the phase velocity of the DAW. Thus electron acceleration is
via Landau damping of DAWs. For the Alfven speeds of 0.3c the considered
mechanism can accelerate electrons to energies circa 20 keV. (v) The increase
of mass ratio from m_i/m_e=16 to 73.44 increases the fraction of accelerated
electrons from 20% to 30-35% (depending on DAW polarisation). For the mass
ratio m_i/m_e=1836 the fraction of accelerated electrons would be >35%. | astro-ph_SR |
A CCD Search for Variable Stars of Spectral Type B in the Northern
Hemisphere Open Clusters. VII. NGC 1502: We present results of variability search in the field of the young open
cluster NGC 1502. Eight variable stars were discovered. Of six other stars in
the observed field that were suspected for variability, we confirm variability
of two, including one beta Cep star, NGC 1502-26. The remaining four suspects
were found to be constant in our photometry. In addition, UBVI photometry of
the well-known massive eclipsing binary SZ Cam was obtained. The new variable
stars include: two eclipsing binaries of which one is a relatively bright
detached system with an EA-type light curve, an alpha2 CVn-type variable, an
SPB candidate, a field RR Lyrae star and three other variables showing
variability of unknown origin. The variability of two of them is probably
related to their emission in Halpha, which has been measured by means of the
alpha index obtained for 57 stars brighter than V=16 mag in the central part of
the observed field. Four other non-variable stars with emission in Halpha were
also found. Additionally, we provide VI photometry for stars down to V=17 mag
and UB photometry for about 50 brightest stars in the observed field. We also
show that the 10-Myr isochrone fits very well the observed color-magnitude
diagram if a distance of 1 kpc and mean reddening, E(V-I)=0.9 mag, are adopted. | astro-ph_SR |
Long-Term Measurements of Sunspot Magnetic Tilt Angles: Tilt angles of close to 30,600 sunspots are determined using Mount Wilson
daily averaged magnetograms taken from 1974 to 2012, and MDI/SoHO magnetograms
taken from 1996 to 2010. Within a cycle, more than 90% of sunspots have a
normal polarity alignment along the east-west direction following Hale's law.
The median tilts increase with increasing latitude (Joy's law) at a rate of
~0.5 degree per degree of latitude. Tilt angles of spots appear largely
invariant with respect to time at a given latitude, but they decrease by
~0.9degree per year on average, a trend which largely reflects Joy's law
following the butterfly diagram. We find an asymmetry between the hemispheres
in the mean tilt angles. On average, the tilts are greater in the southern than
in the northern hemisphere for all latitude zones, and the differences increase
with increasing latitude. | astro-ph_SR |
Iron abundance in the prototype PG1159 star, GW Vir pulsator PG1159-035,
and related objects: We performed an iron abundance determination of the hot, hydrogen deficient
post-AGB star PG1159-035, which is the prototype of the PG1159 spectral class
and the GW Vir pulsators, and of two related objects (PG1520+525, PG1144+005),
based on the first detection of Fe VIII lines in stellar photospheres. In
another PG1159 star, PG1424+535, we detect Fe VII lines. In all four stars,
each within Teff = 110,000 - 150,000 K, we find a solar iron abundance. This
result agrees with our recent abundance analysis of the hottest PG1159 stars
(Teff = 150,000 - 200,000 K) that exhibit Fe X lines. On the whole, we find
that the PG1159 stars are not significantly iron deficient, in contrast to
previous notions. | astro-ph_SR |
Torsional oscillations of nonbare strange stars: Strange stars are one of the possible compact stellar objects that can be
formed after a supernova collapse. We consider a model of strange star having
an inner core in the color-flavor locked phase surmounted by a crystalline
color superconducting layer. These two phases constitute the {\it quarksphere},
which we assume to be the largest and heaviest part of the strange star. The
next layer consists of standard nuclear matter forming a ionic crust, hovering
on the top of the quarksphere and prevented from falling by a strong dipolar
electric field. The dipolar electric field arises because quark matter is
confined in the quarksphere by the strong interaction, but electrons can leak
outside forming a few hundreds Fermi thick electron layer separating the ionic
crust from the underlying quark matter. The ionic matter and the crystalline
color superconducting matter constitute two electromagnetically coupled crust
layers. We study the torsional oscillations of these two layers. Remarkably, we
find that if a fraction larger than $10^{-4}$ of the energy of a Vela-like
glitch is conveyed to a torsional oscillation, the ionic crust will likely
break. The reason is that the very rigid and heavy crystalline color
superconducting crust layer will absorb only a small fraction of the glitch
energy, leading to a large amplitude torsional oscillation of the ionic crust. | astro-ph_SR |
ASASSN-18di: discovery of a $ΔV \sim 10$ flare on a mid-M dwarf: We report and characterize a white-light superflare on a previously
undiscovered M dwarf detected by the ASAS-SN survey. Employing various
color-magnitude and color-spectral type relationships, we estimate several
stellar parameters, including the quiescent V-band magnitude, from which we
derive a flare amplitude of $\Delta V \sim 10$. We determine an r-band absolute
magnitude of $M_{r} = 11.4$, consistent with a mid-M dwarf, and an approximate
distance to the source of $2.2$ kpc. Using classical-flare models, we infer a
flare energy of $E_{V} \simeq (4.1\pm 2.2)\times 10^{36}$ ergs, making this one
of the strongest flares documented on an M dwarf. | astro-ph_SR |
Vector Magnetic Fields and Current Helicities in Coronal Holes and Quiet
Regions: In the solar photosphere, many properties of coronal holes (CHs) are not
known, especially vector magnetic fields. Using observations from
\emph{Hinode}, we investigate vector magnetic fields, current densities and
current helicities in two CHs and compare them with two normal quiet regions
(QRs) for the first time. We find that, in the CHs and QRs, the areas where
large current helicities are located are mainly co-spatial with strong vertical
and horizontal field elements both in shape and location. In the CHs,
horizontal magnetic fields, inclination angles, current densities and current
helicities are larger than those in the QRs. The mean vertical current density
and current helicity, averaged over all the observed areas including the CHs
and QRs, are approximately 0.008 A m$^{-2}$ and 0.005 G$^{2}$ m$^{-1}$,
respectively. The mean current density in magnetic flux concentrations where
the vertical fields are stronger than 100 G is as large as 0.012 $\pm$ 0.001 A
m$^{-2}$, consistent with that in the flare productive active regions. Our
results imply that the magnetic fields, especially the strong fields, both in
the CHs and QRs are nonpotential. | astro-ph_SR |
The First TESS Self-Lensing Pulses: Revisiting KIC 12254688: We report the observations of two self-lensing pulses from KIC 12254688 in
Transiting Exoplanet Survey Satellite (TESS) light curves. This system,
containing a F2V star and white-dwarf companion, was amongst the first
self-lensing binary systems discovered by the Kepler Space Telescope over the
past decade. Each observed pulse occurs when the white dwarf transits in front
of its companion star, gravitationally lensing the star's surface, thus making
it appear brighter to a distant observer. These two pulses are the very first
self-lensing events discovered in TESS observations. We describe the methods by
which the data were acquired and detrended, as well as the best-fit binary
parameters deduced from our self-lensing+radial velocity model. We highlight
the difficulties of finding new self-lensing systems with TESS, and we discuss
the types of self-lensing systems that TESS may be more likely to discover in
the future. | astro-ph_SR |
From Predicting Solar Activity to Forecasting Space Weather: Practical
Examples of Research-to-Operations and Operations-to-Research: The successful transition of research to operations (R2O) and operations to
research (O2R) requires, above all, interaction between the two communities. We
explore the role that close interaction and ongoing communication played in the
successful fielding of three separate developments: an observation platform, a
numerical model, and a visualization and specification tool. Additionally, we
will examine how these three pieces came together to revolutionize
interplanetary coronal mass ejection (ICME) arrival forecasts. A discussion of
the importance of education and training in ensuring a positive outcome from
R2O activity follows. We describe efforts by the meteorological community to
make research results more accessible to forecasters and the applicability of
these efforts to the transfer of space-weather research.We end with a
forecaster "wish list" for R2O transitions. Ongoing, two-way communication
between the research and operations communities is the thread connecting it
all. | astro-ph_SR |
Solar Electron Beam -- Langmuir Wave Interactions and How They Modify
Solar Electron Beam Spectra: Solar Orbiter Observations of a Match Made in
the Heliosphere: Solar Orbiter's four in-situ instruments have recorded numerous energetic
electron events at heliocentric distances between 0.5 and 1au. We analyse
energetic electron fluxes, spectra, pitch angle distributions, associated
Langmuir waves, and type III solar radio bursts for 3 events to understand what
causes modifications in the electron flux and identify the origin and
characteristics of features observed in the electron spectrum. We investigate
what electron beam properties and solar wind conditions are associated with
Langmuir wave growth and spectral breaks in the electron peak flux as a
function of energy. We observe velocity dispersion and quasilinear relaxation
in the electron flux caused by the resonant wave-particle interactions in the
deca-keV range, at the energies at which we observe breaks in the electron
spectrum, co-temporal with the local generation of Langmuir waves. We show, via
the evolution of the electron flux at the time of the event, that these
interactions are responsible for the spectral signatures observed around 10 and
50keV, confirming the results of simulations by Kontar & Reid (2009). These
signatures are independent of pitch angle scattering. Our findings highlight
the importance of using overlapping FOVs when working with data from different
sensors. In this work, we exploit observations from all in-situ instruments to
address, for the first time, how the energetic electron flux is modified by the
beam-plasma interactions, and results into specific features to appear in the
local spectrum. Our results, corroborated with numerical simulations, can be
extended to a wider range of heliocentric distances. | astro-ph_SR |
Wobbling jets in common envelope evolution: We find that the convective motion in the envelopes of red supergiant (RSG)
stars supplies a non-negligible stochastic angular momentum to the mass that a
secondary star accretes in a common envelope evolution (CEE), such that jets
that the secondary star launches wobble. The orbital motion of the secondary
star in a CEE and the density gradient in the envelope impose a non-zero
angular momentum to the accreted mass with a constant direction parallel to the
orbital angular momentum. From one-dimensional stellar evolution simulations
with the numerical code \textsc{mesa} we find that the stochastic convection
motion in the envelope of RSG stars adds a stochastic angular momentum
component with an amplitude that is about 0.1-1 times that of the constant
component due to the orbital motion. We mimic a CEE of the RSG star by removing
envelope mass at a high rate and by depositing energy into its envelope. The
stochastic angular momentum implies that the accretion disk around the
secondary star (which we do not simulate), and therefore the jets that it
launches, wobble with angles of up to tens of degrees with respect to the
orbital angular momentum axis. This wobbling makes it harder for jets to break
out from the envelope and can shape small bubbles in the ejecta that compress
filaments that appear as arcs in the ejected nebula, i.e., in planetary nebulae
when the giant is an asymptotic giant branch star. | astro-ph_SR |
The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic
Cloud: spectroscopy, orbital analysis, formation, and evolution: Massive Wolf-Rayet (WR) stars dominate the radiative and mechanical energy
budget of galaxies and probe a critical phase in the evolution of massive stars
prior to core-collapse. It is not known whether core He-burning WR stars
(classical WR, cWR) form predominantly through wind-stripping (w-WR) or binary
stripping (b-WR). With spectroscopy of WR binaries so-far largely avoided due
to its complexity, our study focuses on the 44 WR binaries / binary candidates
of the Large Magellanic Cloud (LMC, metallicity Z~0.5 Zsun), identified on the
basis of radial velocity variations, composite spectra, or high X-ray
luminosities. Relying on a diverse spectroscopic database, we aim to derive the
physical and orbital parameters of our targets, confronting evolution models of
evolved massive stars at sub-solar metallicity, and constraining the impact of
binary interaction in forming them. Spectroscopy is performed using the Potsdam
Wolf-Rayet (PoWR) code and cross-correlation techniques. Disentanglement is
performed using the code Spectangular or the shift-and-add algorithm.
Evolutionary status is interpreted using the Binary Population and Spectral
Synthesis (BPASS) code, exploring binary interaction and chemically-homogeneous
evolution.
No obvious dichotomy in the locations of apparently-single and binary WN
stars on the Hertzsprung-Russell diagram is apparent. According to commonly
used stellar evolution models (BPASS, Geneva), most apparently-single WN stars
could not have formed as single stars, implying that they were stripped by an
undetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing
(e.g., during the red supergiant phase) are strongly underestimated in standard
stellar evolution models. | astro-ph_SR |
First Results from the CHEPS: Exoplanets and the Discovery of an
Eccentric Brown Dwarf in the Desert: We report the discovery of a brown dwarf on an eccentric orbit and with a
semimajor axis that places it in the brown dwarf desert region around the star
HD191760. The star has a spectral type of G3IV/V and a metallicity ([Fe/H]) of
0.29 dex. HD191760 adds to the small number of metal-rich stars with brown
dwarf companions. The brown dwarf (HD191760b) is found to have an orbital
period of 505.57+/-0.40 days and semimajor axis of 1.35+/-0.01 AU, placing it
firmly in the brown dwarf desert. The eccentricity of HD191760b is found to be
0.63+/-0.01, meaning it reaches as close as 0.5 AU from the host star.
Dynamical simulations indicate that no inner planets could reside at
separations beyond ~0.17 AU due to the disastrous gravity imposed by HD191760b.
In addition to these first results we also refine the orbits found for the
exoplanets around the stars HD48265, HD143361 and HD154672. All 1-planet
solutions are in agreement with those previously published by the Magellan
Planet Search. | astro-ph_SR |
Surface convection: from the Sun to red giant stars: We check how the change in surface conditions between the Sun and red giant
branch stars changes the characteristic surface convection length scale to be
used in models. We investigate the question in the case of the mixing length
theory and of the phenomenology of full spectrum of turbulence. For the
observational part, we rely on independent measurements of effective
temperatures and interferometric radii of nearby red giants. We find that the
local red giant branch cannot be explained taking into account the solar
calibrated convective length scale. | astro-ph_SR |
Tracing the ISM magnetic field morphology: The potential of
multi-wavelength polarization measurements: $\textit{Aims.}$ We present a case study to demonstrate the potential of
multi-wavelength polarization measurements. The aim is to investigate the
effects that dichroic polarization and thermal re-emission have on tracing the
magnetic field in the interstellar medium (ISM). Furthermore, we analyze the
crucial influence of imperfectly aligned compact dust grains on the resulting
synthetic continuum polarization maps.$\\ \textit{Methods.}$ We developed an
extended version of the well-known 3D Monte-Carlo radiation transport code MC3D
for multi-wavelength polarization simulations running on an adaptive grid.We
investigated the interplay between radiation, magnetic fields and dust grains.
Our results were produced by post-processing both ideal density distributions
and sophisticated magnetohydrodynamic (MHD) collapse simulations with radiative
transfer simulations. We derived spatially resolved maps of intensity, optical
depth, and linear and circular polarization at various inclination angles and
scales in a wavelength range from 7 $\mu m$ to 1 $mm$.$\\ \textit{Results.}$ We
predict unique patterns in linear and circular polarization maps for different
types of density distributions and magnetic field morphologies for test setups
and sophisticated MHD collapse simulations. We show that alignment processes of
interstellar dust grains can significantly influence the resulting synthetic
polarization maps. Multi-wavelength polarization measurements allow one to
predict the morphology of the magnetic field inside the ISM. The interpretation
of polarization measurements of complex structures still remains ambiguous
because of the large variety of the predominant parameters in the ISM. | astro-ph_SR |
Likely Members of the Beta Pictoris and AB Doradus Moving Groups in the
North: We present first results from follow-up of targets in the northern hemisphere
Beta Pictoris and AB Doradus moving group candidate list of Schlieder, Lepine,
and Simon (2012). We obtained high-resolution, near-infrared spectra of 27
candidate members to measure their radial velocities and confirm consistent
group kinematics. We identify 15 candidates with consistent predicted and
measured radial velocities, perform analyses of their 6-dimensional
(U,V,W,X,Y,Z) Galactic kinematics, and compare to known group member
distributions. Based on these analyses, we propose that 7 Beta Pic and 8 AB Dor
candidates are likely new group members. Four of the likely new Beta Pic stars
are binaries; one a double lined spectroscopic system. Three of the proposed AB
Dor stars are binaries. Counting all binary components, we propose 22 likely
members of these young, moving groups. The majority of the proposed members are
M2 to M5 dwarfs, the earliest being of type K2. We also present preliminary
parameters for the two new spectroscopic binaries identified in the data, the
proposed Beta Pic member and a rejected Beta Pic candidate. Our candidate
selection and follow-up has thus far identified more than 40 low-mass, likely
members of these two moving groups. These stars provide a new sample of nearby,
young targets for studies of local star formation, disks and exoplanets via
direct imaging, and astrophysics in the low-mass regime. | astro-ph_SR |
Gravitoturbulent dynamo in global simulations of gaseous disks: The turbulence driven by gravitational instabilities (GIs) can amplify
magnetic fields in massive gaseous disks. This GI dynamo may appear in young
circumstellar disks, whose weak ionization challenges other amplification
routes, as well as in active galactic nuclei. Although regarded as a
large-scale dynamo, only local simulations have so far described its kinematic
regime. We study the GI dynamo in global magnetohydrodynamic (MHD) models of
accretion disks, focusing on its kinematic phase. We perform resistive MHD
simulations with the Pluto code for different radiative cooling times and
electrical resistivities. A weak magnetic field seeds the dynamo, and we adopt
mean-field and heuristic models to capture its essence. We recover the same
induction process leading to magnetic field amplification as previously
identified in local simulations. The dynamo is, however, global in nature,
connecting distant annuli of the disk via a large-scale dynamo mode of a fixed
growth rate. This large-scale amplification can be described by a mean-field
model that does not rely on conventional alpha-Omega effects. When varying the
disk parameters we find an optimal resistivity that facilitates magnetic
amplification, whose magnetic Reynolds number, Rm < 10, is substantially
smaller than in local simulations. Unlike local simulations, we find an optimal
cooling rate and the existence of global oscillating dynamo modes. The
nonlinear saturation of the dynamo puts the disk in a strongly magnetized
turbulent state on the margins of the effective range of GI. In our
simulations, the accretion power eventually exceeds the threshold required by
local thermal balance against cooling, leaving the long-term nonlinear outcome
of the GI dynamo uncertain. | astro-ph_SR |
The rotation of field stars from CoRoT data: We present period measurements of a large sample of field stars in the solar
neighbourhood, observed by CoRoT in two different directions of the Galaxy. The
presence of a period was detected using the Scargle Lomb Normalized Periodogram
technique and the autocorrelation analysis. The assessment of the results has
been performed through a consistency verification supported by the folded light
curve analysis. The data analysis procedure has discarded a non-negligible
fraction of light curves due to instrumental artifacts, however it has allowed
us to identify pulsators and binaries among a large number of field stars. We
measure a wide range of periods, from 0.25 to 100 days, most of which are
rotation periods. The final catalogue includes 1978 periods, with 1727 of them
identified as rotational periods, 169 are classified as pulsations and 82 as
orbital periods of binary systems. Our sample suffers from selection biases not
easily corrected for, thus we do not use the distribution of rotation periods
to derive the age distribution of the main-sequence population. Nevertheless,
using rotation as a proxy for age, we can identify a sample of young stars (<
600 Myr), that will constitute a valuable sample, supported by further
spectroscopic observations, to study the recent star formation history in the
solar neighborhood. | astro-ph_SR |
Rapid Enhancement of Sheared Evershed Flow Along the Neutral Line
Associated with an X6.5 Flare Observed by Hinode: We present G-band and Ca II H observations of NOAA AR 10930 obtained by
Hinode/SOT on 2006 December 6 covering an X6.5 flare. Local Correlation
Tracking (LCT) technique was applied to the foreshortening-corrected G-band
image series to acquire horizontal proper motions in this complex
beta-gamma-delta active region. With the continuous high quality, spatial and
temporal resolution G-band data, we not only confirm the rapid decay of outer
penumbrae and darkening of the central structure near the flaring neutral line,
but also unambiguously detect for the first time the enhancement of the sheared
Evershed flow (average horizontal flow speed increased from 330+-3.1 to
403+-4.6 m/s) along the neutral line right after the eruptive white-light
flare. Post-flare Ca II H images indicate that the originally fanning out field
lines at the two sides of the neutral line get connected. Since penumbral
structure and Evershed flow are closely related to photospheric magnetic
inclination or horizontal field strength, we interpret the rapid changes of
sunspot structure and surface flow as the result of flare-induced magnetic
restructuring down to the photosphere. The magnetic fields turn from fanning
out to inward connection causing outer penumbrae decay, meanwhile those near
the flaring neutral line become more horizontal leading to stronger Evershed
flow there. The inferred enhancement of horizontal magnetic field near the
neutral line is consistent with recent magnetic observations and theoretical
predictions of flare-invoked photospheric magnetic field change. | astro-ph_SR |
High-frequency wave power observed in the solar chromosphere with IBIS
and ALMA: We present observational constraints on the solar chromospheric heating
contribution from acoustic waves with frequencies between 5 and 50 mHz. We
utilize observations from the Dunn Solar Telescope in New Mexico complemented
with observations from the Atacama Large Millimeter Array collected on 2017
April 23. The properties of the power spectra of the various quantities are
derived from the spectral lines of Ca II 854.2 nm, H I 656.3 nm, and the
millimeter continuum at 1.25 mm and 3 mm. At the observed frequencies the
diagnostics almost all show a power law behavior, whose particulars (slope,
peak and white noise floors) are correlated with the type of solar feature
(internetwork, network, plage). In order to disentangle the vertical versus
transverse plasma motions we examine two different fields of view; one near
disk center and the other close to the limb. To infer the acoustic flux in the
middle chromosphere, we compare our observations with synthetic observables
from the time-dependent radiative hydrodynamic RADYN code. Our findings show
that acoustic waves carry up to about 1 kW m$^{-2}$ of energy flux in the
middle chromosphere, which is not enough to maintain the quiet chromosphere,
contrary to previous publications. | astro-ph_SR |
The ISO Long Wavelength Spectrometer line spectrum of VY Canis Majoris
and other oxygen-rich evolved stars: The far-infrared spectra of circumstellar envelopes around various
oxygen-rich stars were observed using the ISO Long Wavelength Spectrometer
(LWS). We have examined high signal-to-noise ISO LWS observations of the
luminous supergiant star, VY CMa, with the aim of identifying all of the
spectral lines. By paying particular attention to water lines, we aim to
separate the lines due to other species, in particular, to prepare for
forthcoming observations that will cover the same spectral range using Herschel
PACS and at higher spectral resolution using Herschel HIFI and SOFIA. We have
developed a fitting method to account for blended water lines using a simple
weighting scheme to distribute the flux. We have applied this approach to
several other stars which we compare with VY CMa. We present line fluxes for
the unblended H2O and CO lines, and present detections of several possible
nu_2=1 vibrationally excited water lines. We also identify blended lines of OH,
one unblended and several blended lines of NH3, and one possible detection of
H3O+. | astro-ph_SR |
Inversion of physical parameters in solar atmospheric seismology: Magnetohydrodynamic (MHD) wave activity is ubiquitous in the solar
atmosphere. MHD seismology aims to determine difficult to measure physical
parameters in solar atmospheric magnetic and plasma structures by a combination
of observed and theoretical properties of MHD waves and oscillations. This
technique, similar to seismology or helio-seismology, demands the solution of
two problems. The direct problem involves the computation of wave properties of
given theoretical models. The inverse problem implies the calculation of
unknown physical parameters, by means of a comparison of observed and
theoretical wave properties. Solar atmospheric seismology has been successfully
applied to different structures such as coronal loops, prominence fine
structures, spicules, or jets. However, it is still in its infancy. Far more is
there to come. We present an overview of recent results, with particular
emphasis in the inversion procedure. | astro-ph_SR |
Formation pathway for lonely stripped-envelope supernova progenitors:
implications for Cassiopeia A: We explore a new scenario for producing stripped-envelope supernova
progenitors. In our scenario, the stripped-envelope supernova is the second
supernova of the binary, in which the envelope of the secondary was removed
during its red supergiant phase by the impact of the first supernova. Through
2D hydrodynamical simulations, we find that $\sim$50-90$\%$ of the envelope can
be unbound as long as the pre-supernova orbital separation is $\lesssim5$ times
the stellar radius. Recombination energy plays a significant role in the
unbinding, especially for relatively high mass systems ($\gtrsim18M_\odot$). We
predict that more than half of the unbound mass should be distributed as a
one-sided shell at about $\sim$10-100pc away from the second supernova site. We
discuss possible applications to known supernova remnants such as Cassiopeia A,
RX J1713.7-3946, G11.2-0.3, and find promising agreements. The predicted rate
is $\sim$0.35-1$\%$ of the core-collapse population. This new scenario could be
a major channel for the subclass of stripped-envelope or type IIL supernovae
that lack companion detections like Cassiopeia A. | astro-ph_SR |
Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun
at solar metallicity (Z = 0.014): [abridged] Many topical astrophysical research areas, such as the properties
of planet host stars, the nature of the progenitors of different types of
supernovae and gamma ray bursts, and the evolution of galaxies, require
complete and homogeneous sets of stellar models at different metallicities in
order to be studied during the whole of cosmic history. We present here a first
set of models for solar metallicity, where the effects of rotation are
accounted for in a homogeneous way.
We computed a grid of 48 different stellar evolutionary tracks, both rotating
and non-rotating, at Z=0.014, spanning a wide mass range from 0.8 to 120 Msun.
For each of the stellar masses considered, electronic tables provide data for
400 stages along the evolutionary track and at each stage, a set of 43 physical
data are given. These grids thus provide an extensive and detailed data basis
for comparisons with the observations. The rotating models start on the ZAMS
with a rotation rate Vini/Vcrit=0.4. The evolution is computed until the end of
the central carbon-burning phase, the early AGB phase, or the core helium-flash
for, respectively, the massive, intermediate, and both low and very low mass
stars. The initial abundances are those deduced by Asplund and collaborators,
which best fit the observed abundances of massive stars in the solar
neighbourhood. We update both the opacities and nuclear reaction rates, and
introduce new prescriptions for the mass-loss rates as stars approach the
Eddington and/or the critical velocity. We account for both atomic diffusion
and magnetic braking in our low-mass star models. [...] | astro-ph_SR |
On the identification of time interval threshold in the twin-CME
scenario: Recently it has been suggested that the "twin-CME" scenario Li.etal2012 may
be a very effective mechanism in causing extreme Solar Energetic Particle (SEP)
events and in particular Ground Level Enhancement (GLE) events. Ding.etal2013
performed a statistical examination of the twin-CME scenario with a total of
$126$ fast and wide western Coronal Mass Ejections (CMEs). They found that CMEs
having a preceding CME with a speed $>$ 300 $km/s$ within $9$ hours from the
same active region have larger probability of leading to large SEP events than
CMEs that do not have preceding CMEs. The choice of $9$ hours being the time
lag $\tau$ between the preceding CME and the main CME was based on some crude
estimates of the decay time of the turbulence downstream of the shock driven by
the preceding CME. In this work, we examine this choice. For the $126$ fast
wide CMEs examined in Ding.etal2013, we vary the time lag $\tau$ from $1$ hour
to $24$ hours with an increment of $1$ hour. By considering three quantities
whose values depend on the choice of this time lag $\tau$, we show that the
choice of $13$ hours for $\tau$ is more appropriate. Our study confirms our
earlier result that twin CMEs are more likely to lead to large SEP events than
single fast CMEs. The results shown here are of great relevance to space
weather studies. | 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 |
On the accretion process in a high-mass star forming region - A
multitransitional THz Herschel-HIFI study of ammonia toward G34.26+0.15: [Abridged] Our aim is to explore the gas dynamics and the accretion process
in the early phase of high-mass star formation. The inward motion of molecular
gas in the massive star forming region G34.26+0.15 is investigated by using
high-resolution profiles of seven transitions of ammonia at THz frequencies
observed with Herschel-HIFI. The shapes and intensities of these lines are
interpreted in terms of radiative transfer models of a spherical, collapsing
molecular envelope. An accelerated Lambda Iteration (ALI) method is used to
compute the models. The seven ammonia lines show mixed absorption and emission
with inverse P-Cygni-type profiles that suggest infall onto the central source.
A trend toward absorption at increasingly higher velocities for higher
excitation transitions is clearly seen in the line profiles. The $J =
3\leftarrow2$ lines show only very weak emission, so these absorption profiles
can be used directly to analyze the inward motion of the gas. This is the first
time a multitransitional study of spectrally resolved rotational ammonia lines
has been used for this purpose. Broad emission is, in addition, mixed with the
absorption in the $1_0-0_0$ ortho-NH$_3$ line, possibly tracing a molecular
outflow from the star forming region. The best-fitting ALI model reproduces the
continuum fluxes and line profiles, but slightly underpredicts the emission and
absorption depth in the ground-state ortho line $1_0-0_0$. The derived
ortho-to-para ratio is approximately 0.5 throughout the infalling cloud core
similar to recent findings for translucent clouds in sight lines toward W31C
and W49N. We find evidence of two gas components moving inwards toward the
central region with constant velocities: 2.7 and 5.3 km$\,$s$^{-1}$, relative
to the source systemic velocity. The inferred mass accretion rates derived are
sufficient to overcome the expected radiation pressure from G34.26+0.15. | astro-ph_SR |
The Shapes of AGB Envelopes as Probes of Binary Companions: We describe how the large scale geometry of the circumstellar envelopes of
asymptotic giant branch stars can be used to probe the presence of unseen
stellar companions. A nearby companion modifies the mass loss by
gravitationally focusing the wind towards the orbital plane, and thereby
determines the shape of the envelope at large distances from the star. Using
available simulations, we develop a prescription for the observed shapes of
envelopes in terms of the binary parameters, envelope orientation, and type of
observation. The prescription provides a tool for the analysis of envelope
images at optical, infrared, and millimetre wavelengths, which can be used to
constrain the presence of companions in well observed cases. We illustrate this
approach by examining the possible role of binary companions in triggering the
onset of axi-symmetry in planetary nebula formation. If interaction with the
primary leads to axi-symmetry, the spherical halos widely seen around newly
formed nebulae set limits on the companion mass. Only low mass objects may
orbit close to the primary without observable shaping effects: they remain
invisible until the interaction causes a sudden change in the mass loss
geometry. | astro-ph_SR |
A Survey for H-alpha Emission from Late L dwarfs and T dwarfs: Recently, studies of brown dwarfs have demonstrated that they possess strong
magnetic fields and have the potential to produce radio and optical auroral
emissions powered by magnetospheric currents. This emission provides the only
window on magnetic fields in the coolest brown dwarfs and identifying
additional benchmark objects is key to constraining dynamo theory in this
regime. To this end, we conducted a new red optical (6300 - 9700 Angstrom)
survey with the Keck telescopes looking for H-alpha emission from a sample of
late L dwarfs and T dwarfs. Our survey gathered optical spectra for 29 targets,
18 of which did not have previous optical spectra in the literature, greatly
expanding the number of moderate resolution (R~2000) spectra available at these
spectral types. Combining our sample with previous surveys, we confirm an
H-alpha detection rate of 9.2 (+3.5/-2.1) % for L and T dwarfs in the optical
spectral range of L4 - T8. This detection rate is consistent with the recently
measured detection rate for auroral radio emission from Kao et al. (2016),
suggesting that geometrical selection effects due to the beaming of the radio
emission are small or absent. We also provide the first detection of H-alpha
emission from 2MASS 0036+1821, previously notable as the only electron
cyclotron maser radio source without a confirmed detection of H-alpha emission.
Finally, we also establish optical standards for spectral types T3 and T4,
filling in the previous gap between T2 and T5. | astro-ph_SR |
Differential rotation in main-sequence solar-like stars: Qualitative
inference from asteroseismic data: Understanding differential rotation of Sun-like stars is of great importance
for insight into the angular momentum transport in these stars. One means of
gaining such information is that of asteroseismology. By a forward modeling
approach we analyze in a qualitative manner the impact of different
differential rotation profiles on the splittings of p-mode oscillation
frequencies. The optimum modes for inference on differential rotation are
identified along with the best value of the stellar inclination angle. We find
that in general it is not likely that asteroseismology can be used to make an
unambiguous distinction between a rotation profile such as, e.g., a conical
Sun-like profile and a cylindrical profile. In addition, it seems unlikely that
asteroseismology of Sun-like stars will result in inferences on the radial
profile of the differential rotation, such as can be done for, e.g., red
giants. At best one could possibly obtain the sign of the radial differential
rotation gradient. Measurements of the extent of the latitudinal differential
from frequency splitting are, however, more promising. One very interesting
aspect that could likely be tested from frequency splittings is whether the
differential rotation is solar-like or anti-solar-like in nature, in the sense
that a solar-like profile has an equator rotating faster than the poles. | astro-ph_SR |
Reconstruction of a Large-scale Pre-flare Coronal Current Sheet
Associated with an Homologous X-shaped Flare: As a fundamental magnetic structure in the solar corona, electric current
sheets (CSs) can form either prior to or during solar flare, and they are
essential for magnetic energy dissipation in the solar corona by enabling
magnetic reconnection. However static reconstruction of CS is rare, possibly
due to limitation inherent in available coronal field extrapolation codes. Here
we present the reconstruction of a large-scale pre-flare CS in solar active
region 11967 using an MHD-relaxation model constrained by SDO/HMI vector
magnetogram. The CS is found to be associated with a set of peculiar homologous
flares that exhibit unique X-shaped ribbons and loops occurring in a
quadrupolar magnetic configuration. This is evidenced by that the field lines
traced from the CS to the photosphere form an X shape which nearly precisely
reproduces the shape of the observed flare ribbons, suggesting that the flare
is a product of the dissipation of the CS through reconnection. The CS forms in
a hyperbolic flux tube, which is an intersection of two quasi-separatrix
layers. The recurrence of the X-shaped flares might be attributed to the
repetitive formation and dissipation of the CS, as driven by the photospheric
footpoint motions. These results demonstrate the power of data-constrained MHD
model in reproducing CS in the corona as well as providing insight into the
magnetic mechanism of solar flares. | astro-ph_SR |
Masses of the Hyades white dwarfs: A gravitational redshift measurement: Context. It is possible to accurately measure the masses of the white dwarfs
(WDs) in the Hyades cluster using gravitational redshift, because the radial
velocity of the stars can be obtained independently of spectroscopy from
astrometry and the cluster has a low velocity dispersion. Aims. We aim to
obtain an accurate measurement of the Hyades WD masses by determining the
mass-to-radius ratio (M/R) from the observed gravitational redshift, and to
compare them with masses derived from other methods. Methods. We analyse
archive high-resolution UVES-VLT spectra of six WDs belonging to the Hyades to
measure their Doppler shift, from which M/R is determined after subtracting the
astrometric radial velocity. We estimate the radii using Gaia photometry as
well as literature data. Results. The M/R error associated to the gravitational
redshift measurement is about 5%. The radii estimates, evaluated with different
methods, are in very good agreement, though they can differ by up to 4%
depending on the quality of the data. The masses based on gravitational
redshift are systematically smaller than those derived from other methods, by a
minimum of $\sim 0.02$ up to 0.05 solar masses. While this difference is within
our measurement uncertainty, the fact that it is systematic indicates a likely
real discrepancy between the different methods. Conclusions. We show that the
M/R derived from gravitational redshift measurements is a powerful tool to
determine the masses of the Hyades WDs and could reveal interesting properties
of their atmospheres. The technique can be improved by using dedicated
spectrographs, and can be extended to other clusters, making it unique in its
ability to accurately and empirically determine the masses of WDs in open
clusters. At the same time we prove that gravitational redshift in WDs agrees
with the predictions of stellar evolution models to within a few percent. | astro-ph_SR |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.