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Does HL Tau Disk Polarization in ALMA Band 3 Come from Radiatively
Aligned Grains?: Disk polarization in (sub)millimeter dust continuum is a rapidly growing
field in the ALMA era. It opens up the exciting possibility of detecting and
characterizing magnetic fields and grain growth in disks around young stellar
objects. However, to use polarization for probing the disk properties, its
production mechanism must be ascertained first. To date, the conventional
mechanism involving magnetically aligned grains fails to explain the
polarization patterns detected in most disks. This is especially true for the
inclined disk of HL Tau in ALMA Band 3 (wavelength $\sim 3$ mm), which has an
elliptical polarization pattern. The elliptical pattern was taken as evidence
for polarized emission by dust grains aligned with their long axes
perpendicular the direction of the radiative flux. We show that the radiatively
aligned grains produce a circular, rather than elliptical, polarization pattern
even in inclined disks such as HL Tau. An elliptical polarization pattern can
be produced if the grains are aligned aerodynamically by the difference in
rotation speed between the dust and gas through the Gold mechanism. However, a
strong azimuthal variation in polarized intensity is expected for both the
radiative and aerodynamic alignment, but not observed in the HL Tau disk in
ALMA Band 3. We conclude that neither of these two mechanisms alone can explain
the data and the origin of the 3 mm polarization remains a mystery. We
speculate that this mystery may be resolved by a combination of both direct
emission and scattering by aerodynamically aligned grains. | astro-ph_SR |
Wide-angle protostellar outflows driven by narrow jets in stratified
cores: Most simulations of outflow feedback on star formation are based on the
assumption that outflows are driven by a wide angle "X-wind," rather than a
narrow jet. However, the arguments initially raised against pure jet-driven
flows were based on steady ejection in a uniform medium, a notion that is no
longer supported based on recent observations. We aim to determine whether a
pulsed narrow jet launched in a density-stratified, self-gravitating core could
reproduce typical molecular outflow properties, without the help of a
wide-angle wind component. We performed axisymmetric hydrodynamic simulations
using the MPI-AMRVAC code with optically thin radiative cooling on timescales
up to 10000 yrs. Then we computed and compared the predicted properties with
observational data. First, the jet-driven shell expands faster and wider
through a core with steeply decreasing density than through an uniform core.
Second, when blown into the same singular flattened core, a jet-driven shell
has a similar width as a wide-angle wind-driven shell in the first few hundred
years, but a decelerating expansion on long timescales. The flow adopts a
conical shape and a base opening angle reaching up to $90\unicode{xb0}$. Third,
after $\sim$ 10000 yrs, a pulsed jet-driven shell shows fitting features and a
qualitative resemblance with recent observations of protostellar outflows with
the Atacama Large Millimeter Array (ALMA), such as HH46-47 and CARMA-7. In
particular, similarities are seen in the shell widths, opening angles,
position-velocity diagrams, and mass-velocity distribution, with some showing a
closer resemblance than in simulations based on a wide-angle "X-wind" model.
Therefore, a realistic ambient density stratification in addition to
millenia-long integration times are equally essential to reliably predict the
properties of outflows driven by a pulsed jet and to confront them with the
observations. | astro-ph_SR |
Models for velocity decrease in HH34: The conservation of the energy flux in turbulent jets that propagate in the
interstellar medium (ISM) allows us to deduce the law of motion when an inverse
power law decrease of density is considered. The back-reaction that is caused
by the radiative losses for the trajectory is evaluated. The velocity
dependence of the jet with time/space is applied to the jet of HH34, for which
the astronomical data of velocity versus time/space are available. The
introduction of precession and constant velocity for the central star allows us
to build a curved trajectory for the superjet connected with HH34. The bow
shock that is visible in the superjet is explained in the framework of the
theory of the image in the case of an optically thin layer. | astro-ph_SR |
Cool dwarfs stars from the Torino Observatory Parallax Program: We investigate and parameterise high proper motion red stars in the Torino
Observatory Parallax Program. Observations of 27 objects were made over the
period 1994 - 2001 on the 1.05m Torino telescope. The trigonometric parallaxes
and proper motions were determined using standard techniques. We determine
parallaxes and proper motions, and by comparison to models we infer masses,
ages, and {metallicities}. Of the 27 objects, 22 are within 25pc and 4 appear
to be subdwarfs. There are published parallaxes for 18 objects, and all but 4
agree to within 2$\sigma$. The discrepancies are discussed. | astro-ph_SR |
Realistic model atmosphere and revised abundances of the coolest Ap star
HD101065: Among the known Ap stars, HD101065 is probably one of the most interesting
objects, demonstrating very rich spectra of rare-earth elements (REE). Strongly
peculiar photometric parameters of this star that can not be fully reproduced
by any modern theoretical calculations, even those accounting for realistic
chemistry of its atmosphere. In this study we investigate a role of missing REE
line opacity and construct a self-consistent atmospheric model based on
accurate abundance and chemical stratification analysis. We employed the
LLmodels stellar model atmosphere code together with DDAFit and SynthMag
software packages to derive homogeneous and stratified abundances for 52
chemical elements and to construct a self-consistent model of HD101065
atmosphere. The opacity in REE lines is accounted for in details, by using
up-to-date extensive theoretical calculations. We show that REE elements play a
key role in the radiative energy balance in the atmosphere of HD101065, leading
to the strong suppression of the Balmer jump and energy redistribution very
different from that of normal stars. Introducing new line lists of REEs allowed
us to reproduce, for the first time, spectral energy distribution of HD101065
and achieve a better agreement between the unusually small observed Str\"omgren
c1 index and the model predictions. Using combined photometric and
spectroscopic approaches and based on the iterative procedure of abundance and
stratification analysis we find effective temperature of HD101065 to be
Teff=6400K. | astro-ph_SR |
Contributions of structural variations to the asymptotic mixed-mode
coupling Factor in red giant stars: The advent of ultra-precise photometry space missions enable the possibility
of investigating stellar interior with mixed modes. The structural variations
induced by the discontinuity of the chemical composition left behind during the
first dredge--up is an important feature in the stellar mid-layers located
between the hydrogen-burning shell and the base of the convective zone of red
giants, as the mixed-mode properties can be significantly affected by these
variations. In this paper, the contributing factors to variations of the
mixed-mode coupling factor, $q$, are discussed with stellar models. In general,
the structural variations give rise to a subtle displacement in the Lamb
frequency and a sharp change in the buoyancy frequency, which lead to
variations in the value of $q$ computed using the asymptotic formalisms that
assuming a smooth background free of structural variations. The impact of these
two factors can be felt in detectable mixed modes in low-luminosity red giants.
Furthermore, the different nature of variations of the two characteristic
frequencies with radius near the base of the convective zone, produces a sudden
increase in $q$ in evolved red giants. This is followed by a quick drop in $q$
as the star evolves further along the red giant branch. | astro-ph_SR |
Frequency analysis of Delta Scuti stars towards the Galactic bulge: We have performed a frequency analysis of 10,092 Delta Scuti-type stars
detected in the fourth phase of the Optical Gravitational Lensing Experiment
(OGLE) towards the Galactic bulge, which is the most numerous homogeneous
sample of Delta Scuti stars observed so far. The main goal was to search for
stars pulsating in at least two radial modes simultaneously. We have found 3083
candidates for such stars, which is the largest set obtained to date. Among
them, 2655 stars pulsate in two radial modes, 414 stars pulsate in three radial
modes, and 14 stars pulsate in four radial modes at the same time. We report
the identification of 221 Delta Scuti stars pulsating in the fundamental mode,
first overtone, and third overtone simultaneously. We show the most populated
Petersen and Bailey diagrams and discuss statistical properties of the
identified frequencies based on this numerous sample. Additionally, we present
theoretical predictions of period ratios for Delta Scuti stars pulsating in
overtones from the fourth to the seventh. | astro-ph_SR |
The M Supergiant High Mass X-Ray Binary 4U 1954+31: The X-ray binary 4U 1954+31 has been classified as a Low Mass X-ray Binary
(LMXB) containing a M giant and a neutron star (NS). It has also been included
in the rare class of X-ray symbiotic binaries (SyXB). The Gaia parallax,
infrared colors, spectral type, abundances, and orbital properties of the M
star demonstrate that the cool star in this system is not a low mass giant but
a high mass M supergiant. Thus, 4U 1954+31 is a High Mass X-ray Binary (HMXB)
containing a late-type supergiant. It is the only known binary system of this
type. The mass of the M I is 9$^{+6}_{-2}$ M$_\odot$ giving an age of this
system in the range 12 - 50 Myr with the NS no more than 43 Myr old. The spin
period of the NS is one of the longest known, 5 hours. The existence of M I
plus NS binary systems is in accord with stellar evolution theory, with this
system a more evolved member of the HMXB population. | astro-ph_SR |
The Strength and Variability of the Helium 10830 Å Triplet in Young
Stars, with Implications for Exosphere Detection: Young exoplanets trace planetary evolution, particularly the atmospheric mass
loss that is most dynamic in youth. However, the high activity level of young
stars can mask or mimic the spectroscopic signals of atmospheric mass loss.
This includes the activity-sensitive He 10830 \AA\ triplet, which is an
increasingly important exospheric probe. To characterize the He-10830 triplet
at young ages, we present time-series NIR spectra for young transiting planet
hosts taken with the Habitable-zone Planet Finder. The He-10830 absorption
strength is similar across our sample, except at the fastest and slowest
rotation, indicating that young chromospheres are dense and populate metastable
helium via collisions. Photoionization and recombination by coronal radiation
only dominates metastable helium population at the active and inactive
extremes. Volatile stellar activity, such as flares and changing surface
features, drives variability in the He-10830 triplet. Variability is largest at
the youngest ages before decreasing to $\lesssim5-10$ m\AA\ (or 3%) at ages
above 300 Myr, with 6 of 8 stars in this age range agreeing with no intrinsic
variability. He-10830 triplet variability is smallest and age-independent at
the shortest timescales. Intrinsic stellar variability should not preclude
detection of young exospheres, except at the youngest ages. We recommend
out-of-transit comparison observations taken directly surrounding transit and
observation of multiple transits to minimize activity's effect. Regardless,
caution is necessary when interpreting transit observations in the context of
stellar activity, as many scenarios can lead to enhanced stellar variability
even on timescales of an hour. | astro-ph_SR |
Constraining modified gravity from tidal phenomena in binary stars: In beyond-Horndeski theories of gravity, the Vainshtein screening mechanism
might only be partially effective inside stellar objects. This results in a
modification of the pressure balance equation inside stars, often characterized
by a single parameter ($\Upsilon$) in isotropic systems. We show how to
constrain such theories of modified gravity, using tidal effects. We study such
effects in cataclysmic variable star binaries and numerically obtain limits on
the critical masses of the donor stars, below which they are tidally disrupted,
by modeling them in beyond-Horndeski theories. This is contrasted with values
of the donor masses, obtained using existing observational data, by a Monte
Carlo error progression method. A best fit scenario of the two yields a
parametric constraint in the theories that we consider, within the
approximations used. Here, we obtain the allowed range $ 0 \le \Upsilon \le
0.47 $. | astro-ph_SR |
Eleven Exoplanet Host Star Angular Diameters from the CHARA Array: We directly measured the angular diameters for 11 exoplanet host stars using
Georgia State University's CHARA Array interferometer and calculated their
linear radii and effective temperatures. The sample tends towards evolving or
evolved stars and includes one dwarf, four subgiants, and six giants. We then
estimated masses and ages for the stars using our effective temperatures
combined with metallicity measurements from the literature. | astro-ph_SR |
Tests of Sunspot Number Sequences: 2. Using Geomagnetic and Auroral Data: We compare four sunspot-number data sequences against geomagnetic and
terrestrial auroral observations. The comparisons are made for the original
SIDC composite of Wolf-Zurich-International sunspot number [$R_{ISNv1}$], the
group sunspot number [$R_{G}$] by Hoyt and Schatten (Solar Phys., 1998), the
new "backbone" group sunspot number [$R_{BB}$] by Svalgaard and Schatten (Solar
Phys., 2016), and the "corrected" sunspot number [$R_{C}$] by Lockwood at al.
(J.G.R., 2014). Each sunspot number is fitted with terrestrial observations, or
parameters derived from terrestrial observations to be linearly proportional to
sunspot number, over a 30-year calibration interval of 1982-2012. The fits are
then used to compute test sequences, which extend further back in time and
which are compared to $R_{ISNv1}$, $R_{G}$, $R_{BB}$, and $R_{C}$. To study the
long-term trends, comparisons are made using averages over whole solar cycles
(minimum-to-minimum). The test variations are generated in four ways: i) using
the IDV(1d) and IDV geomagnetic indices (for 1845-2013) fitted over the
calibration interval using the various sunspot numbers and the phase of the
solar cycle; ii) from the open solar flux (OSF) generated for 1845 - 2013 from
four pairings of geomagnetic indices by Lockwood et al. (Ann. Geophys., 2014)
and analysed using the OSF continuity model of Solanki at al. (Nature, 2000)
which employs a constant fractional OSF loss rate; iii) the same OSF data
analysed using the OSF continuity model of Owens and Lockwood (J.G.R., 2012) in
which the fractional loss rate varies with the tilt of the heliospheric current
sheet and hence with the phase of the solar cycle; iv) the occurrence frequency
of low-latitude aurora for 1780-1980 from the survey of Legrand and Simon (Ann.
Geophys., 1987). For all cases, $R_{BB}$ exceeds the test terrestrial series by
an amount that increases as one goes back in time. | astro-ph_SR |
Population Synthesis of Black Hole Binaries with Normal-Star Companions:
I. Detached Systems: Optical observations of normal-stars in binary systems with massive unseen
objects have been proposed to search for candidate black holes (BHs) and
provide a direct measurement of their dynamical masses. In this paper, we have
performed binary population synthesis calculations to simulate the potential
population of detached binaries containing BHs and normal-star companions in
the Galaxy. We focus on the influence of the BH progenitors. In the traditional
model, BHs in binaries evolve from stars more massive than $ \sim25M_{\odot} $.
However, it is difficult for this model to produce BH low-mass X-ray binaries.
Recent investigations on massive star evolution suggest that the BH progenitors
may have masses as low as $ \sim15M_{\odot} $. Based on this model, we provide
the expected distributions of various parameters for detached BH binaries with
normal-star companions, including the component masses, the orbital parameters
of the binary systems, the radial velocity semi-amplitudes, and the astrometric
signatures of the optical companions. %the detached BH systems with low-mass
companions can be easily produced in our simulations. Our calculations show
that there are more than thousands of such detached binaries in the Galaxy, and
hundreds of them are potentially observable systems with luminous companions
brighter than 20 mag. In addition, detached BH binaries are dominated by those
with main-sequence companions and only a few percent of them are expected to
have giant companions. | astro-ph_SR |
The Gaia-ESO Survey. Mg-Al anti-correlation in iDR4 globular clusters: We use Gaia-ESO Survey iDR4 data to explore the Mg-Al anti-correlation in
globular clusters, that were observed as calibrators, as a demonstration of the
quality of Gaia-ESO Survey data and analysis. The results compare well with the
available literature, within 0.1 dex or less, after a small (compared to the
internal spreads) offset between the UVES and the GIRAFFE data of 0.10-0.15 dex
was taken into account. In particular, we present for the first time data for
NGC 5927, one of the most metal-rich globular clusters studied in the
literature so far with [Fe/H]=-0.49 dex, that was included to connect with the
open cluster regime in the Gaia-ESO Survey internal calibration. The extent and
shape of the Mg-Al anti-correlation provide strong constraints on the multiple
population phenomenon in globular clusters. In particular, we studied the
dependency of the Mg-Al anti-correlation extension with metallicity,
present-day mass, and age of the clusters, using GES data in combination with a
large set of homogenized literature measurements. We find a dependency with
both metallicity and mass, that is evident when fitting for the two parameters
simultaneously, but no significant dependency with age. We confirm that the
Mg-Al anti-correlation is not seen in all clusters, but disappears for the less
massive or most metal-rich ones. We also use our dataset to see whether a
normal anti-correlation would explain the low [Mg/$\alpha$] observed in some
extragalactic globular clusters, but find that none of the clusters in our
sample can reproduce it, and more extreme chemical compositions (like the one
of NGC 2419) would be required. We conclude that GES iDR4 data already meet the
requirements set by the main survey goals, and can be used to study in detail
globular clusters even if the analysis procedures were not specifically
designed for them. | astro-ph_SR |
SST/CRISP Observations of Convective Flows in a Sunspot Penumbra: Context. Recent discoveries of intensity correlated downflows in the interior
of a sunspot penumbra provide direct evidence for overturning convection,
adding to earlier strong indications of convection from filament dynamics
observed far from solar disk center, and supporting recent simulations of
sunspots.
Aims. Using spectropolarimetric observations obtained at a spatial resolution
approaching 0'.'1 with the Swedish 1-m Solar Telescope (SST) and its
spectropolarimeter CRISP, we investigate whether the convective downflows
recently discovered in the C i line at 538.03 nm can also be detected in the
wings of the Fe i line at 630.15 nm
Methods. We make azimuthal fits of the measured LOS velocities in the core
and wings of the 538 nm and 630 nm lines to disentangle the vertical and
horizontal flows. To investigate how these depend on the continuum intensity,
the azimuthal fits are made separately for each intensity bin. By using
spatially high-pass filtered measurements of the LOS component of the magnetic
field, the flow properties are determined separately for magnetic spines
(relatively strong and vertical field) and inter-spines (weaker and more
horizontal field).
Results. The dark convective downflows discovered recently in the 538.03 nm
line are evident also in the 630.15 nm line, and have similar strength. This
convective signature is the same in spines and inter-spines. However, the
strong radial (Evershed) outflows are found only in the inter-spines.
Conclusions. At the spatial resolution of the present SST/CRISP data, the
small-scale intensity pattern seen in continuum images is strongly related to a
convective up/down flow pattern that exists everywhere in the penumbra. Earlier
failures to detect the dark convective downflows in the interior penumbra can
be explained by inadequate spatial resolution in the observed data. | astro-ph_SR |
Presupernova neutrinos: realistic emissivities from stellar evolution: We present a new calculation of neutrino emissivities and energy spectra from
a massive star going through the advanced stages of nuclear burning
(presupernova) in the months before becoming a supernova. The contributions
from beta decay and electron capture, pair annihilation, plasmon decay, and the
photoneutrino process are modeled in detail, using updated tabulated nuclear
rates. We also use realistic conditions of temperature, density, electron
fraction and nuclear isotopic composition of the star from the state of the art
stellar evolution code MESA. Results are presented for a set of progenitor
stars with mass between 15 $M_\odot$ and 30 $M_\odot$. It is found that beta
processes contribute substantially to the neutrino emissivity above realistic
detection thresholds of few MeV, at selected positions and times in the
evolution of the star. | astro-ph_SR |
A solar flare disturbing a light wall above a sunspot light bridge: With the high-resolution data from the Interface Region Imaging Spectrograph,
we detect a light wall above a sunspot light bridge in the NOAA active region
(AR) 12403. In the 1330 A slit-jaw images, the light wall is brighter than the
ambient areas while the wall top and base are much brighter than the wall body,
and it keeps oscillating above the light bridge. A C8.0 flare caused by a
filament activation occurred in this AR with the peak at 02:52 UT on 2015
August 28, and the flare's one ribbon overlapped the light bridge which was the
observational base of the light wall. Consequently, the oscillation of the
light wall was evidently disturbed. The mean projective oscillation amplitude
of the light wall increased from 0.5 Mm to 1.6 Mm before the flare, and
decreased to 0.6 Mm after the flare. We suggest that the light wall shares a
group of magnetic field lines with the flare loops, which undergo a magnetic
reconnection process, and they constitute a coupled system. When the magnetic
field lines are pushed upwards at the pre-flare stage, the light wall turns to
the vertical direction, resulting in the increase of the light wall's
projective oscillation amplitude. After the magnetic reconnection takes place,
a group of new field lines with smaller scales are formed underneath the
reconnection site and the light wall inclines. Thus, the projective amplitude
decreases remarkably at the post-flare stage. | astro-ph_SR |
On the nature of the WO3 star DR1 in IC 1613: We present the results of a quantitative spectroscopic analysis of the
oxygen-sequence Wolf- Rayet star DR1 in the low-metallicity galaxy IC 1613. Our
models suggest that the strong oxygen emission lines are the result of the high
temperature of this WO3 star and do not necessarily reflect a more advanced
evolutionary stage than WC stars. | astro-ph_SR |
Spectro-interferometry of the Be star delta Sco: Near-Infrared Continuum
and Gas Emission Region Sizes in 2007: We present near-infrared H and K-band spectro-interferometric observations of
the gaseous disk around the primary Be star in the delta Sco binary system,
obtained in 2007 (between periastron passages in 2000 and 2011). Observations
using the CHARA/MIRC instrument at H-band resolve an elongated disk with a
Gaussian FWHM 1.18 x 0.91 mas. Using the Keck Interferometer, the source of the
K-band continuum emission is only marginally spatially resolved, and
consequently we estimate a relatively uncertain K-band continuum disk FWHM of
0.7 +/- 0.3 mas. Line emission on the other hand, He1 (2.0583 micron) and Br
gamma (2.1657 micron), is clearly detected, with about 10% lower visibilities
than those of the continuum. When taking into account the continuum/line flux
ratio this translates into much larger sizes for the line emission regions: 2.2
+/- 0.4 mas and 1.9 +/- 0.3 mas for He1 and Br gamma respectively. Our KI data
also reveal a relatively flat spectral differential phase response, ruling out
significant off-center emission. We expect these new measurements will help
constrain dynamical models being actively developed in order to explain the
disk formation process in the delta Sco system and Be stars in general. | astro-ph_SR |
Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux
emergence processes: Magnetic flux emergence has been shown to be a key mechanism for unleashing a
wide variety of solar phenomena. However, there are still open questions
concerning the rise of the magnetized plasma through the atmosphere, mainly in
the chromosphere, where the plasma departs from local thermodynamic equilibrium
(LTE) and is partially ionized. We aim to investigate the impact of the
nonequilibrium (NEQ) ionization and recombination and molecule formation of
hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of
the flux emergence process. Using the Bifrost code, we performed 2.5D numerical
experiments of magnetic flux emergence from the convection zone up to the
corona. The experiments include the NEQ ionization and recombination of atomic
hydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar
diffusion term of the Generalized Ohm's Law. Our experiments show that the LTE
assumption substantially underestimates the ionization fraction in most of the
emerged region, leading to an artificial increase in the ambipolar diffusion
and, therefore, in the heating and temperatures as compared to those found when
taking the NEQ effects on the hydrogen ion population into account. We see that
LTE also overestimates the number density of H2 molecules within the emerged
region, thus mistakenly magnifying the exothermic contribution of the H2
molecule formation to the thermal energy during the flux emergence process. We
find that the ambipolar diffusion does not significantly affect the amount of
total unsigned emerged magnetic flux, but it is important in the shocks that
cross the emerged region, heating the plasma on characteristic times ranging
from 0.1 to 100 s. We also briefly discuss the importance of including elements
heavier than hydrogen in the equation of state so as not to overestimate the
role of ambipolar diffusion in the atmosphere. | astro-ph_SR |
Corona-Australis DANCe. I. Revisiting the census of stars with Gaia-DR2
data: Corona-Australis is one of the nearest regions to the Sun with recent and
ongoing star formation, but the current picture of its stellar (and substellar)
content is not complete yet. We take advantage of the second data release of
the Gaia space mission to revisit the stellar census and search for additional
members of the young stellar association in Corona-Australis. We applied a
probabilistic method to infer membership probabilities based on a
multidimensional astrometric and photometric data set over a field of 128
deg$^{2}$ around the dark clouds of the region. We identify 313
high-probability candidate members to the Corona-Australis association, 262 of
which had never been reported as members before. Our sample of members covers
the magnitude range between $G\gtrsim5$ mag and $G\lesssim20$ mag, and it
reveals the existence of two kinematically and spatially distinct subgroups.
There is a distributed `off-cloud' population of stars located in the north of
the dark clouds that is twice as numerous as the historically known `on-cloud'
population that is concentrated around the densest cores. By comparing the
location of the stars in the HR-diagram with evolutionary models, we show that
these two populations are younger than 10 Myr. Based on their infrared excess
emission, we identify 28 Class II and 215 Class III stars among the sources
with available infrared photometry, and we conclude that the frequency of Class
II stars (i.e. `disc-bearing' stars) in the on-cloud region is twice as large
as compared to the off-cloud population. The distance derived for the
Corona-Australis region based on this updated census is $d=149.4^{+0.4}_{-0.4}$
pc, which exceeds previous estimates by about 20 pc.In this paper we provide
the most complete census of stars in Corona-Australis available to date that
can be confirmed with Gaia data. | astro-ph_SR |
Cross and magnetic helicity in the outer heliosphere from Voyager 2
observations: Plasma velocity and magnetic field measurements from the Voyager 2 mission
are used to study solar wind turbulence in the slow solar wind at two different
heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to
provide information on the radial evolution of this turbulence. The magnetic
helicity and the cross-helicity, which express the correlation between the
plasma velocity and the magnetic field, are used to characterize the
turbulence. Wave number spectra are computed by means of the Taylor hypothesis
applied to time resolved single point Voyager 2 measurements. The overall
picture we get is complex and difficult to interpret. A substantial decrease of
the cross-helicity at smaller scales (over 1-3 hours of observation) with
increasing heliocentric distance is observed. At 5 AU the only peak in the
probability density of the normalized residual energy is negative, near -0.5.
At 29 AU the probability density becomes doubly peaked, with a negative peak at
-0.5 and a smaller peak at a positive values of about 0.7. A decrease of the
cross-helicity for increasing heliocentric distance is observed, together with
a reduction of the unbalance toward the magnetic energy of the energy of the
fluctuations. For the smaller scales, we found that at 29 AU the normalized
polarization is small and positive on average (about 0.1), it is instead zero
at 5 AU. For the larger scales, the polarization is low and positive at 5 AU
(average around 0.1) while it is negative (around - 0.15) at 29 AU. | astro-ph_SR |
An `Analytic Dynamical Magnetosphere' formalism for X-ray and optical
emission from slowly rotating magnetic massive stars: Slowly rotating magnetic massive stars develop "dynamical magnetospheres"
(DM's), characterized by trapping of stellar wind outflow in closed magnetic
loops, shock heating from collision of the upflow from opposite loop
footpoints, and subsequent gravitational infall of radiatively cooled material.
In 2D and 3D magnetohydrodynamic (MHD) simulations the interplay among these
three components is spatially complex and temporally variable, making it
difficult to derive observational signatures and discern their overall scaling
trends.Within a simplified, steady-state analysis based on overall conservation
principles, we present here an "analytic dynamical magnetosphere" (ADM) model
that provides explicit formulae for density, temperature and flow speed in each
of these three components -- wind outflow, hot post-shock gas, and cooled
inflow -- as a function of colatitude and radius within the closed (presumed
dipole) field lines of the magnetosphere. We compare these scalings with
time-averaged results from MHD simulations, and provide initial examples of
application of this ADM model for deriving two key observational diagnostics,
namely hydrogen H-alpha emission line profiles from the cooled infall, and
X-ray emission from the hot post-shock gas. We conclude with a discussion of
key issues and advantages in applying this ADM formalism toward derivation of a
broader set of observational diagnostics and scaling trends for massive stars
with such dynamical magnetospheres. | astro-ph_SR |
A Possible Period for the K-band Brightening Episodes of GX 17+2: The low mass X-ray binary and Z source GX 17+2 undergoes infrared K-band
brightening episodes of at least 3.5 magnitudes. The source of these episodes
is not known. Prior published K-band magnitudes and new K-band measurements
acquired between 2006 and 2008 suggest that the episodes last at least 4 hours
and have a period of 3.01254 $\pm$ 0.00002 days. Future bright episodes can be
predicted using the ephemeris JD_{max} (n) = 2454550.79829 + (3.01254 $\pm$
0.00002)(n) days. A growing body of evidence suggests that the GX 17+2 could
have a synchrotron jet, which could cause this activity. | astro-ph_SR |
An Incipient Debris Disk in the Chamaeleon I Cloud: The point at which a protoplanetary disk becomes a debris disk is difficult
to identify. To better understand this, here we study the $\sim$40~AU
separation binary T~54 in the Chamaeleon I cloud. We derive a K5 spectral type
for T~54~A (which dominates the emission of the system) and an age of
$\sim$2~Myr. However, the dust disk properties of T~54 are consistent with
those of debris disks seen around older and earlier-type stars. At the same
time, T~54 has evidence of gas remaining in the disk as indicated by [Ne II],
[Ne III], and [O I] line detections. We model the spectral energy distribution
of T~54 and estimate that $\sim$3$\times$10$^{-3}$ Earth-masses of small dust
grains ($<$0.25~$\mu$m) are present in an optically thin circumbinary disk
along with at least $\sim$3$\times$10$^{-7}$ Earth-masses of larger
($>$10~$\mu$m) grains within a circumprimary disk. Assuming a solar-like
mixture, we use Ne line luminosities to place a minimum limit on the gas mass
of the disk ($\sim$3$\times$10$^{-4}$ Earth-masses) and derive a gas-to-dust
mass ratio of $\sim$0.1. We do not detect substantial accretion, but we do see
H$\alpha$ in emission in one epoch, suggestive that there may be intermittent
dumping of small amounts of matter onto the star. Considering the low dust
mass, the presence of gas, and young age of T~54, we conclude that this system
is on the bridge between the protoplanetary and debris disk stages. | astro-ph_SR |
Massive star-formation toward G28.87+0.07 (IRAS 18411-0338) investigated
by means of maser kinematics and radio to infrared, continuum observations: We used the Very Long Baseline Array (VLBA) and the European VLBI Network
(EVN) to perform phase-referenced VLBI observations of the three most powerful
maser transitions associated with the high-mass star-forming region
G28.87+0.07: the 22.2 GHz H$_{2}$O, 6.7 GHz CH$_{3}$OH, and 1.665 GHz OH lines.
We also performed VLA observations of the radio continuum emission at 1.3 and
3.6 cm and Subaru observations of the continuum emission at 24.5 $\mu$m. Two
centimeter continuum sources are detected and one of them (named "HMC") is
compact and placed at the center of the observed distribution of H$_{2}$O,
CH$_{3}$OH and OH masers. The bipolar distribution of line-of-sight (l.o.s)
velocities and the pattern of the proper motions suggest that the water masers
are driven by a (proto)stellar jet interacting with the dense circumstellar
gas. The same jet could both excite the centimeter continuum source named "HMC"
(interpreted as free-free emission from shocked gas) and power the molecular
outflow observed at larger scales -- although one cannot exclude that the
free-free continuum is rather originating from a hypercompact \ion{H}{2}
region. At 24.5 $\mu$m, we identify two objects separated along the north-south
direction, whose absolute positions agree with those of the two VLA continuum
sources. We establish that $\sim$90% of the luminosity of the region
($\sim$\times10^{5} L_\sun$) is coming from the radio source "HMC", which
confirms the existence of an embedded massive young stellar object (MYSO)
exciting the masers and possibly still undergoing heavy accretion from the
surrounding envelope. | astro-ph_SR |
Deriving photospheric parameters and elemental abundances for a sample
of stars showing the FIP effect: One puzzling question in solar physics is the difference between elemental
abundances in the photosphere and the corona. Elements with low first
ionization potential (FIP) can be overabundant in the corona compared to the
photosphere under certain circumstances. The same phenomenon has been observed
on a handful of stars, while a few of them show the inverse effect. But not all
the stars in the original sample had precise photospheric abundances derived
from optical spectra, so for some the solar values were adopted. In this work
we make homogeneous abundance measurements from optical spectroscopy.
We collected spectra of 16 stars showing the FIP effect with the 1-m RCC
telescope of Konkoly Observatory, with resolution of $\lambda / \Delta \lambda
\sim 21\,000$. We determine the fundamental astrophysical parameters
($T_\mathrm{eff}$, $\log g$, $[M/H]$, $\xi_\mathrm{mic}$, $v \sin i$) and
individual elemental abundances with the SME spectral synthesis code using
MARCS2012 model atmosphere and spectral line parameters from the Vienna Atomic
Line Database (VALD). | astro-ph_SR |
Pre-flare activity and magnetic reconnection during the evolutionary
stages of energy release in a solar eruptive flare: In this paper, we present a multi-wavelength analysis of an eruptive
white-light M3.2 flare which occurred in active region NOAA 10486 on November
1, 2003. Excellent set of high resolution observations made by RHESSI and TRACE
provide clear evidence of significant pre-flare activities for ~9 minutes in
the form of an initiation phase observed at EUV/UV wavelengths followed by the
X-ray precursor phase. During the initiation phase, we observed localized
brightenings in the highly sheared core region close to the filament and
interactions among short EUV loops overlying the filament which led to the
opening of magnetic field lines. The X-ray precursor phase is manifested in
RHESSI measurements below ~30 keV and coincided with the beginning of flux
emergence at the flaring location along with early signatures of the eruption.
From the RHESSI observations, we conclude that both plasma heating and electron
acceleration occurred during the precursor phase. The main flare is consistent
with the standard flare model. However, after the impulsive phase, intense HXR
looptop source was observed without significant footpoint emission. More
intriguingly, for a brief period the looptop source exhibited strong HXR
emission with energies up to 100 keV and significant non-thermal
characteristics. The present study indicates a causal relation between the
activities in the preflare and main flare. We also conclude that pre-flare
activities, occurred in the form of subtle magnetic reorganization along with
localized magnetic reconnection, played a crucial role in destabilizing the
active region filament leading to solar eruptive flare and associated
large-scale phenomena. | astro-ph_SR |
Magnetohydrodynamic wave mode identification in circular and elliptical
sunspot umbrae: evidence for high order modes: In this paper we provide clear direct evidence of multiple concurrent higher
order magnetohydrodynamic (MHD) modes in circular and elliptical sunspots by
applying both Proper Orthogonal Decomposition (POD) and Dynamic Mode
Decomposition (DMD) techniques on solar observational data. These techniques
are well documented and validated in the areas of fluid mechanics, hydraulics,
and granular flows, yet are relatively new to the field of solar physics. While
POD identifies modes based on orthogonality in space and it provides a clear
ranking of modes in terms of their contribution to the variance of the signal,
DMD resolves modes that are orthogonal in time. The clear presence of the
fundamental slow sausage and kink body modes, as well as higher order slow
sausage and kink body modes have been identified using POD and DMD analysis of
the chromospheric H$\alpha$ line at 6562.808~{\AA} for both the circular and
elliptical sunspots. Additionally, to the various slow body modes, evidence for
the presence of the fast surface kink mode was found in the circular sunspot.
All the MHD modes patterns were cross-correlated with their theoretically
predicted counterparts and we demonstrated that ellipticity cannot be neglected
when interpreting MHD wave modes. The higher-order MHD wave modes are even more
sensitive to irregularities in umbral cross-sectional shapes, hence this must
be taken into account for more accurate modelling of the modes in sunspots and
pores. | astro-ph_SR |
The frequency of large variations in the near-infrared fluxes of T Tauri
stars: Variability is a characteristic feature of young stellar objects (YSOs) and
could contribute to the large scatter observed in HR diagrams for star forming
regions. For typical YSOs, however, the long-term effects of variability are
poorly constrained. Here I use archived near-infrared photometry from 2MASS,
UKIDSS, and DENIS to investigate the long-term variability of high-confidence
members of the four star forming regions Rho-Oph, ONC, IC348, and NGC1333. The
total sample comprises more than 600 objects, from which ~320 are considered to
have a disk. The dataset covers timescales up to 8 yr. About half of the YSOs
are variable on a 2sigma level, with median amplitudes of 5-20%. The fraction
of highly variable objects with amplitudes >0.5 mag in at least two
near-infrared bands is very low - 2% for the entire sample and 3% for objects
with disks. These sources with strong variability are mostly objects with disks
and are prime targets for follow-up studies. The variability amplitudes are
largest in NGC1333, presumably because it is the youngest sample of YSOs. The
frequency of highly variable objects also increases with the time window of the
observations (from weeks to years). These results have three implications: 1)
When deriving luminosities for YSOs from near-infrared magnitudes, the typical
error introduced by variability is in the range of 5-20% percent and depends on
disk fraction and possibly age. 2) Variability is a negligible contribution to
the scatter in HR diagrams of star forming regions (except for a small number
of extreme objects), if luminosities are derived from near-infrared magnitudes.
3) Accretion outbursts with an increase in mass accretion rate by several order
of magnitudes, as required in scenarios for episodic accretion, occur with a
duty cycle of >2000-2500 yr in the Class II phase. (abridged) | astro-ph_SR |
A flash of polarized optical light points to an aspherical "cow": The astronomical transient AT2018cow is the closest example of the new class
of luminous, fast blue optical transients (FBOTs). Liverpool Telescope RINGO3
observations of AT2018cow are reported here, which constitute the earliest
polarimetric observations of an FBOT. At 5.7 days post-explosion, the optical
emission of AT2018cow exhibited a chromatic polarization spike that reached ~7%
at red wavelengths. This is the highest intrinsic polarization recorded for a
non-relativistic explosive transient, and is observed in multiple bands and at
multiple epochs over the first night of observations, before rapidly declining.
The apparent wavelength dependence of the polarization may arise through
depolarization or dilution of the polarized flux, due to conditions in
AT~2018cow at early times. A second ``bump" in the polarization is observed at
blue wavelengths at ~12 days. Such a high polarization requires an extremely
aspherical geometry that is only apparent for a brief period (<1 day), such as
shock breakout through an optically thick disk. For a disk-like configuration,
the ratio of the thickness to radial extent must be ~10%. | astro-ph_SR |
Correlation Analysis of Mode Frequencies with Activity Proxies at
Different Phases of the Solar Cycle: We analyze intermediate degree p- and f-mode eigenfrequencies measured by
GONG and MDI/SOHO for a complete solar cycle to study their correlation with
solar activity. We demonstrate that the frequencies do vary linearly with the
activity, however the degree of correlation differs from phase to phase of the
cycle. During the rising and the declining phases, the mode frequencies are
strongly correlated with the activity proxies whereas during the low- and
high-activity periods, the frequencies have significantly lower correlation
with all the activity proxies considered here. | astro-ph_SR |
Theoretical seismology in 3D : nonlinear simulations of internal gravity
waves in solar-like stars: Internal gravity waves (hereafter IGWs) are studied for their impact on the
angular momentum transport in stellar radiation zones and the information they
provide about the structure and dynamics of deep stellar interiors. We here
present the first 3D nonlinear numerical simulations of IGWs excitation and
propagation in a solar-like star. The aim is to study the behavior of waves in
a realistic 3D nonlinear time dependent model of the Sun and to characterize
their properties. We compare our results with theoretical and 1D predictions.
It allows us to point out the complementarity between theory and simulation and
to highlight the convenience but also the limits of the asymptotic and linear
theories. We show that a rich spectrum of IGWs is excited by the convection,
representing about 0.4% of the total solar luminosity. We study the spatial and
temporal properties of this spectrum, the effect of thermal damping and
nonlinear interactions between waves. We give quantitative results about the
modes frequencies, evolution with time and rotational splitting and we discuss
the amplitude of IGWs considering different regimes of parameters. This work
points out the importance of high performance simulation for its
complementarity with observation and theory. It opens a large field of
investigation concerning IGWs propagating nonlinearly in 3D spherical
structures. The extension of this work to other types of stars, with different
masses, structures and rotation rates will lead to a deeper and more accurate
comprehension of IGWs in stars. | astro-ph_SR |
High-resolution spectroscopic view of planet formation sites: Theories of planet formation predict the birth of giant planets in the inner,
dense, and gas-rich regions of the circumstellar disks around young stars.
These are the regions from which strong CO emission is expected. Observations
have so far been unable to confirm the presence of planets caught in formation.
We have developed a novel method to detect a giant planet still embedded in a
circumstellar disk by the distortions of the CO molecular line profiles
emerging from the protoplanetary disk's surface. The method is based on the
fact that a giant planet significantly perturbs the gas velocity flow in
addition to distorting the disk surface density. We have calculated the
emerging molecular line profiles by combining hydrodynamical models with
semianalytic radiative transfer calculations. Our results have shown that a
giant Jupiter-like planet can be detected using contemporary or future
high-resolution near-IR spectrographs such as VLT/CRIRES or ELT/METIS. We have
also studied the effects of binarity on disk perturbations. The most
interesting results have been found for eccentric circumprimary disks in
mid-separation binaries, for which the disk eccentricity - detectable from the
asymmetric line profiles - arises from the gravitational effects of the
companion star. Our detailed simulations shed new light on how to constrain the
disk kinematical state as well as its eccentricity profile. Recent findings by
independent groups have shown that core-accretion is severely affected by disk
eccentricity, hence detection of an eccentric protoplanetary disk in a young
binary system would further constrain planet formation theories. | astro-ph_SR |
Characterising face-on accretion onto and the subsequent contraction of
protoplanetary discs: Observations indicate that stars generally lose their protoplanetary discs on
a timescale of about 5 Myr. Which mechanisms are responsible for the disc
dissipation is still debated. Here we investigate the movement through an
ambient medium as a possible cause of disc dispersal. The ram pressure exerted
by the flow can truncate the disc and the accretion of material with no
azimuthal angular momentum leads to further disc contraction. We derive a
theoretical model from accretion disc theory that describes the evolution of
the disc radius, mass, and surface density profile as a function of the density
and velocity of the ambient medium. We test our model by performing
hydrodynamical simulations of a protoplanetary disc embedded in a flow with
different velocities and densities. We find that our model gives an adequate
description of the evolution of the disc radius and accretion rate onto the
disc. The total disc mass in the simulations follows the theoretically expected
trend, except at the lowest density where our simulated discs lose mass owing
to continuous stripping. This stripping may be a numerical rather than a
physical effect. Some quantitative differences exist between the model
predictions and the simulations. These are at least partly caused by numerical
viscous effects in the disc and depend on the resolution of the simulation. Our
model can be used as a conservative estimate for the process of face-on
accretion onto protoplanetary discs, as long as viscous processes in the disc
can be neglected. The model predicts that in dense gaseous environments, discs
can shrink substantially in size and can, in theory, sweep up an amount of gas
of the order of their initial mass. This process could be relevant for planet
formation in dense environments. | astro-ph_SR |
Period and amplitude variations in post-common-envelope eclipsing
binaries observed with SuperWASP: Period or amplitude variations in eclipsing binaries may reveal the presence
of additional massive bodies in the system, such as circumbinary planets. Here,
we have studied twelve previously-known eclipsing post-common-envelope binaries
for evidence of such light curve variations, on the basis of multi-year
observations in the SuperWASP archive. The results for HW Vir provided strong
evidence for period changes consistent with those measured by previous studies,
and help support a two-planet model for the system. ASAS J102322-3737.0
exhibited plausible evidence for a period increase not previously suggested;
while NY Vir, QS Vir and NSVS 14256825 afforded less significant support for
period change, providing some confirmation to earlier claims. In other cases,
period change was not convincingly observed; for AA Dor and NSVS 07826147,
previous findings of constant period were confirmed. This study allows us to
present hundreds of new primary eclipse timings for these systems, and further
demonstrates the value of wide-field high-cadence surveys like SuperWASP for
the investigation of variable stars. | astro-ph_SR |
CoBiToM Project -- II: Evolution of contact binary systems close to the
orbital period cut-off: Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the
smallest and least massive stars. These systems are faint and rare, and it is
believed that they have reached a contact configuration after several Gyrs of
evolution via angular momentum loss, mass transfer and mass loss through
stellar wind processes. This study is conducted in the frame of Contact
Binaries Towards Merging (CoBiToM) Project and presents the results from light
curve and orbital analysis of 30 ultra-short orbital period contact binaries,
with the aim to investigate the possibility of them being red nova progenitors,
eventually producing merger events. Approximately half of the systems exhibit
orbital period modulations, as a result of mass transfer or mass loss
processes. Although they are in contact, their fill-out factor is low (less
than 30 per cent), while their mass ratio is larger than the one in longer
period contact binaries. The present study investigates the orbital stability
of these systems and examines their physical and orbital parameters in
comparison to those of the entire sample of known and well-studied contact
binaries, based on combined spectroscopic and photometric analysis. It is found
that ultra-short orbital period contact binaries have very stable orbits, while
very often additional components are gravitationally bound in wide orbits
around the central binary system. We confirmed that the evolution of such
systems is very slow, which explains why the components of ultra-short orbital
period systems are still Main Sequence stars after several Gyrs of evolution. | astro-ph_SR |
Nearby Young, Active, Late-type Dwarfs in Gaia's First Data Release: The Galex Nearby Young Star Survey (GALNYSS) has yielded a sample of
$\sim$2000 UV-selected objects that are candidate nearby ($D
\stackrel{<}{\sim}$150 pc), young (age $\sim$10--100 Myr), late-type stars.
Here, we evaluate the distances and ages of the subsample of (19) GALNYSS stars
with Gaia Data Release 1 (DR1) parallax distances $D \le 120$ pc. The overall
youth of these 19 mid-K to early-M stars is readily apparent from their
positions relative to the loci of main sequence stars and giants in Gaia-based
color-magnitude and color-color diagrams constructed for all Galex- and
WISE-detected stars with parallax measurements included in DR1. The isochronal
ages of all 19 stars lie in the range $\sim$10--100 Myr. Comparison with
Li-based age estimates indicates a handful of these stars may be young
main-sequence binaries rather than pre-main sequence stars. Nine of the 19
objects have not previously been considered as nearby, young stars, and all but
one of these are found at declinations north of $+$30$^\circ$. The Gaia DR1
results presented here indicate that the GALNYSS sample includes several
hundred nearby, young stars, a substantial fraction of which have not been
previously recognized as having ages $\stackrel{<}{\sim}$100 Myr. | astro-ph_SR |
Corrected Weight Functions for Stellar Oscillation Eigenfrequencies: Kawaler et al. (1985) present a variational expression for the
eigenfrequencies associated with stellar oscillations. We highlight and correct
a typographical error in the weight functions appearing in these expressions,
and validate the correction numerically. | astro-ph_SR |
Stellar atmospheres, atmospheric extension and fundamental parameters:
weighing stars using the stellar mass index: One of the great challenges in understanding stars is measuring their masses.
The best methods for measuring stellar masses include binary interaction,
asteroseismology and stellar evolution models, but these methods are not ideal
for red giant and supergiant stars. In this work, we propose a novel method for
inferring stellar masses of evolved red giant and supergiant stars using
interferometric and spectrophotometric observations combined with spherical
model stellar atmospheres to measure what we call the stellar mass index,
defined as the ratio between the stellar radius and mass. The method is based
on the correlation between different measurements of angular diameter, used as
a proxy for atmospheric extension, and fundamental stellar parameters. For a
given star, spectrophotometry measures the Rosseland angular diameter while
interferometric observations generally probe a larger limb-darkened angular
diameter. The ratio of these two angular diameters is proportional to the
relative extension of the stellar atmosphere, which is strongly correlated to
the star's effective temperature, radius and mass. We show that these
correlations are strong and can lead to precise measurements of stellar masses. | astro-ph_SR |
Isochronal age scale of young moving groups in the solar neighbourhood: We present a self-consistent, absolute isochronal age scale for young (< 200
Myr), nearby (< 100 pc) moving groups, which is consistent with recent lithium
depletion boundary ages for both the Beta Pic and Tucana-Horologium moving
groups. This age scale was derived using a set of semi-empirical
pre-main-sequence model isochrones that incorporate an empirical colour-Teff
relation and bolometric corrections based on the observed colours of Pleiades
members, with theoretical corrections for the dependence on log g. Absolute
ages for young, nearby groups are vital as these regions play a crucial role in
our understanding of the early evolution of low- and intermediate-mass stars,
as well as providing ideal targets for direct imaging and other measurements of
dusty debris discs, substellar objects and, of course, extrasolar planets. | astro-ph_SR |
Understanding the origin of the magnetic field morphology in the
wide-binary protostellar system BHR 71: We present 1.3 mm ALMA observations of polarized dust emission toward the
wide-binary protostellar system BHR 71 IRS1 and IRS2. IRS1 features what
appears to be a natal, hourglass-shaped magnetic field. In contrast, IRS2
exhibits a magnetic field that has been affected by its bipolar outflow. Toward
IRS2, the polarization is confined mainly to the outflow cavity walls. Along
the northern edge of the redshifted outflow cavity of IRS2, the polarized
emission is sandwiched between the outflow and a filament of cold, dense gas
traced by N$_2$D$^+$, toward which no dust polarization is detected. This
suggests that the origin of the enhanced polarization in IRS2 is the
irradiation of the outflow cavity walls, which enables the alignment of dust
grains with respect to the magnetic field -- but only to a depth of ~300 au,
beyond which the dust is cold and unpolarized. However, in order to align
grains deep enough in the cavity walls, and to produce the high polarization
fraction seen in IRS2, the aligning photons are likely to be in the mid- to
far-infrared range, which suggests a degree of grain growth beyond what is
typically expected in very young, Class 0 sources. Finally, toward IRS1 we see
a narrow, linear feature with a high (10-20%) polarization fraction and a well
ordered magnetic field that is not associated with the bipolar outflow cavity.
We speculate that this feature may be a magnetized accretion streamer; however,
this has yet to be confirmed by kinematic observations of dense-gas tracers. | astro-ph_SR |
A 9-Month Hubble Space Telescope Near-UV Survey of M87. I. Light and
Color Curves of 94 Novae, and a Re-determination of the Nova Rate: M87 has been monitored with a cadence of 5 days over a 9 month-long span
through the near-ultraviolet (NUV:F275W) and optical (F606W) filters of the
Wide Field Camera 3 (WFC3) of the $\textit{Hubble Space Telescope}$. This
unprecedented dataset yields the NUV and optical light and color curves of 94
M87 novae, characterizing the outburst and decline properties of the largest
extragalactic nova dataset in the literature (after M31 and M81). We test and
confirm nova modelers' prediction that recurrent novae cannot erupt more
frequently that once every 45 days; show that there are zero rapidly recurring
novae in the central $\sim$ 1/3 of M87 with recurrence times $ < $ 130 days;
demonstrate that novae closely follow the K-band light of M87 to within a few
arcsec of the galaxy nucleus; show that nova NUV light curves are as
heterogeneous as their optical counterparts, and usually peak 5 to 30 days
after visible light maximum; determine our observations' annual detection
completeness to be 71 - 77\%; and measure the rate Rnova of nova eruptions in
M87 as $352_{-37}^{+37}$/yr. The corresponding luminosity-specific classical
nova rate for this galaxy is $7.91_{-1.20}^{+1.20}/yr/10^{10}L_\odot,_{K}$.
These rates confirm that ground-based observations of extragalactic novae miss
most faint, fast novae and those near the centers of galaxies. An annual M87
nova rate of 300 or more seems inescapable. A luminosity-specific nova rate of
$\sim$ $7 - 10/yr/10^{10}L_\odot,_{K}$ in ${\it all}$ types of galaxies is
indicated by the data available in 2023. | astro-ph_SR |
Atomic diffusion in solar-like stars with MESA. Comparison with the
Montreal/Montpellier and CESTAM stellar evolution codes: The stellar evolution code Modules for Experiments in Stellar Astrophysics
(MESA) is public and is widely used by the community. It includes the
possibility of taking several non-standard processes such as atomic diffusion
into account. Even if the effect of gravitational settling is considered a
standard ingredient in stellar modelling today, this is not the case for
radiative accelerations. The specific treatment of atomic diffusion along with
the radiative accelerations has never been compared with other stellar
evolution codes. Benchmarking these codes is important because improved
accuracy is required in order to analyse data from present and future space
missions, such as the \textit{Kepler}, Transiting Exoplanet Survey Satellite
(TESS), and PLAnetary Transits and Oscillations of stars (PLATO) missions.
The aim of this paper is to compare MESA models including atomic diffusion
(with radiative accelerations) with models computed with the
Montreal/Montpellier stellar evolution code and with the Code d'Evolution
Stellaire Adaptatif et Modulaire (CESTAM). Additionally, we assess the impact
of some MESA options related to atomic diffusion.
We calculated atomic diffusion, including radiative accelerations, following
the abundance profiles of 14 elements with MESA models. This was then compared
with 1.1 and 1.4~$M_{\odot}$ models computed with the Montreal/Montpellier and
CESTAM codes. Various tests of MESA options for atomic diffusion were also
carried out by varying only one of them at a time.
We find that the abundance profiles of the considered elements in the MESA
models compare rather well with the models computed with the two other codes
when atomic diffusion options are carefully set. We also show that some options
in MESA are crucial for a proper treatment of atomic diffusion. | astro-ph_SR |
Spectroscopic and Photometric Study of the Asymptotic Giant Branch Star
T Cephei: Spectroscopy and photometry of the Asymptotic Giant Branch star T Cephei were
recorded concurrently on 36 nights during its 387 day pulsation cycle in 2022.
Photometry was used to calibrate all spectra in absolute flux. We report on the
variation of B and V magnitudes, B-V colour index, spectral type, effective
temperature and Balmer emission line flux during one complete pulsation cycle. | astro-ph_SR |
Short term period variable stars observed at OAUNI: We present the first scientific results of the program on short term period
variable stars observed using the OAUNI facility at the peruvian Andes. These
results include good quality light curves of delta Scuti stars, rapidly
oscillating stars along with eclipsing and cataclysmic binaries. The
photometric precision reached by the available instrumental and equipment, and
used in the several scientific subprograms, has satisfied the initial
expectations. | astro-ph_SR |
Wide-angle protostellar outflows driven by narrow jets in stratified
cores: Most simulations of outflow feedback on star formation are based on the
assumption that outflows are driven by a wide angle "X-wind," rather than a
narrow jet. However, the arguments initially raised against pure jet-driven
flows were based on steady ejection in a uniform medium, a notion that is no
longer supported based on recent observations. We aim to determine whether a
pulsed narrow jet launched in a density-stratified, self-gravitating core could
reproduce typical molecular outflow properties, without the help of a
wide-angle wind component. We performed axisymmetric hydrodynamic simulations
using the MPI-AMRVAC code with optically thin radiative cooling on timescales
up to 10000 yrs. Then we computed and compared the predicted properties with
observational data. First, the jet-driven shell expands faster and wider
through a core with steeply decreasing density than through an uniform core.
Second, when blown into the same singular flattened core, a jet-driven shell
has a similar width as a wide-angle wind-driven shell in the first few hundred
years, but a decelerating expansion on long timescales. The flow adopts a
conical shape and a base opening angle reaching up to $90\unicode{xb0}$. Third,
after $\sim$ 10000 yrs, a pulsed jet-driven shell shows fitting features and a
qualitative resemblance with recent observations of protostellar outflows with
the Atacama Large Millimeter Array (ALMA), such as HH46-47 and CARMA-7. In
particular, similarities are seen in the shell widths, opening angles,
position-velocity diagrams, and mass-velocity distribution, with some showing a
closer resemblance than in simulations based on a wide-angle "X-wind" model.
Therefore, a realistic ambient density stratification in addition to
millenia-long integration times are equally essential to reliably predict the
properties of outflows driven by a pulsed jet and to confront them with the
observations. | astro-ph_SR |
Long-term variability of T Tauri stars using WASP: We present a reference study of the long-term optical variability of young
stars using data from the WASP project. Our primary sample is a group of
well-studied classical T Tauri stars (CTTS), mostly in Taurus-Auriga. WASP
lightcurves cover timescales up to 7 years and typically contain 10000-30000
datapoints. We quantify the variability as function of timescale using the
time-dependent standard deviation 'pooled sigma'. We find that the overwhelming
majority of CTTS has low-level variability with sigma<0.3mag dominated by
timescales of a few weeks, consistent with rotational modulation. Thus, for
most young stars monitoring over a month is sufficient to constrain the total
amount of variability over timescales up to a decade. The fraction of stars
with strong optical variability (sigma>0.3mag) is 21% in our sample and 21% in
an unbiased control sample. An even smaller fraction (13% in our sample, 6% in
the control) show evidence for an increase in variability amplitude as a
function of timescale from weeks to months or years. The presence of long-term
variability correlates with the spectral slope at 3-5mu, which is an indicator
of inner disk geometry, and with the U-B band slope, which is an accretion
diagnostics. This shows that the long-term variations in CTTS are predominantly
driven by processes in the inner disk and in the accretion zone. Four of the
stars with long-term variations show periods of 20-60d, significantly longer
than the rotation periods and stable over months to years. One possible
explanation are cyclic changes in the interaction between the disk and the
stellar magnetic field. | astro-ph_SR |
Magnetic OB[A] Stars with TESS: probing their Evolutionary and
Rotational properties -- The MOBSTER Collaboration: In this contribution, we present the MOBSTER Collaboration, a large community
effort to leverage high-precision photometry from the Transiting Exoplanet
Survey Satellite (\textit{TESS}) in order to characterize the variability of
magnetic massive and intermediate-mass stars. These data can be used to probe
the varying column density of magnetospheric plasma along the line of sight for
OB stars, thus improving our understanding of the interaction between surface
magnetic fields and massive star winds. They can also be used to map out the
brightness inhomogeneities present on the surfaces of Ap/Bp stars, informing
present models of atomic diffusion in their atmospheres. Finally, we review our
current and ongoing studies, which lead to new insights on this topic. | astro-ph_SR |
Understanding the origin of the [OI] low-velocity component from T Tauri
stars: The formation time, masses, and location of planets are strongly impacted by
the physical mechanisms that disperse protoplanetary disks and the timescale
over which protoplanetary material is cleared out. Accretion of matter onto the
central star, protostellar winds/jets, magnetic disk winds, and
photoevaporative winds operate concurrently. Hence, disentangling their
relative contribution to disk dispersal requires identifying diagnostics that
trace different star-disk environments. Here, we analyze the low velocity
component (LVC) of the Oxygen optical forbidden lines, which is found to be
blueshifted by a few km/s with respect to the stellar velocity. We find that
the [OI] LVC profiles are different from those of [NeII] at 12.81\mu m and CO
at 4.7\mu m lines pointing to different origins for these gas lines. We report
a correlation between the luminosity of the [OI] LVC and the accretion
luminosity Lacc. We do not find any correlation with the X-ray luminosity,
while we find that the higher is the stellar FUV luminosity, the higher is the
luminosity of the [OI] LVC. In addition, we show that the [OI]6300\AA/5577\AA\
ratio is low (ranging between 1 and 8). These findings favor an origin of the
[OI] LVC in a region where OH is photodissociated by stellar FUV photons and
argue against thermal emission from an X-ray-heated layer. Detailed modeling of
two spectra with the highest S/N and resolution shows that there are two
components within the LVC: a broad, centrally peaked component that can be
attributed to gas arising in a warm disk surface in Keplerian rotation (with
FWHM between ~40 and ~60 km/s), and a narrow component (with FWHM ~10 km/s and
small blueshifts of ~2 km/s) that may arise in a cool (<1,000 K) molecular
wind. | astro-ph_SR |
Shearing box simulations in the Rayleigh unstable regime: We study the stability properties of Rayleigh unstable flows both in the
purely hydrodynamic and magnetohydrodynamic (MHD) regimes for two different
values of the shear $q=2.1, 4.2$ ($q = - d\ln\Omega / d\ln r$) and compare it
with the Keplerian case $q=1.5$. We find that the $q>2$ regime is unstable both
in the hydrodynamic and in the MHD limit (with an initially weak magnetic
field). In this regime, the velocity fluctuations dominate the magnetic
fluctuations. In contrast, in the $q<2$ (magnetorotational instability (MRI))
regime the magnetic fluctuations dominate. This highlights two different paths
to MHD turbulence implied by the two regimes, suggesting that in the $q>2$
regime the instability produces primarily velocity fluctuations that cause
magnetic fluctuations, with the causality reversed for the $q<2$ MRI unstable
regime. We also find that the magnetic field correlation is increasingly
localized as the shear is increased in the Rayleigh unstable regime. In
calculating the time evolution of spatial averages of different terms in the
MHD equations, we find that the $q>2$ regime is dominated by terms which are
nonlinear in the fluctuations, whereas for $q<2$, the linear terms play a more
significant role. | astro-ph_SR |
Solar supergranular fractal dimension dependence on the Solar cycle
phase: We study the complexity of the supergranular network through fractal
dimension by using Ca II K digitized data archive obtained from Kodaikanal
solar observatory. The data consists of 326 visually selected supergranular
cells spread across the 23rd solar cycle. Only cells that were well-defined
were chosen for the analysis and we discuss the potential selection effect
thereof, mainly that it favors cells of a smaller size (< 20 Mm). Within this
sample, we analyzed the fractal dimension of supergranules across the Solar
cycle and find that it is anticorrelated with the activity level. | astro-ph_SR |
FUSE, STIS, and Keck spectroscopic analysis of the UV-bright star vZ
1128 in M3 (NGC 5272): We present a spectral analysis of the UV-bright star vZ 1128 in M3 based on
observations with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Space
Telescope Imaging Spectrograph (STIS), and the Keck HIRES echelle spectrograph.
By fitting the H I, He I, and He II lines in the Keck spectrum with non-LTE
H-He models, we obtain Teff = 36,600 K, log g = 3.95, and log N(He)/N(H) =
-0.84. The star's FUSE and STIS spectra show photospheric absorption from C, N,
O, Al, Si, P, S, Fe, and Ni. No stellar features from elements beyond the iron
peak are observed. Both components of the N V 1240 doublet exhibit P~Cygni
profiles, indicating a weak stellar wind, but no other wind features are seen.
The star's photospheric abundances appear to have changed little since it left
the red giant branch (RGB). Its C, N, O, Al, Si, Fe, and Ni abundances are
consistent with published values for the red-giant stars in M3, and the
relative abundances of C, N, and O follow the trends seen on the cluster RGB.
In particular, its low C abundance suggests that the star left the asymptotic
giant branch before the onset of third dredge-up. | astro-ph_SR |
Discovery of electron cyclotron MASER emission from the magnetic Bp star
HD 133880 with the Giant Metrewave Radio Telescope: We report the discovery of coherent radio emission from the young,
rapidly-rotating magnetic Bp star HD 133880 at a frequency of 610 MHz with the
Giant Metrewave Radio Telescope (GMRT). This is only the second magnetic star
in which coherent radio emission has been detected. In our observations of HD
133880 covering the full rotational cycle of the star (except for a phase
window 0.17-0.24), we witness an abrupt order-of-magnitude flux enhancement
along with $\approx100$ percent right circular polarization. We attribute this
phenomenon to coherent Electron Cyclotron MASER Emission. We attribute the lack
of left circularly polarised emission to the asymmetric topology of the star's
magnetic field. The phase of enhancement, $0.73$, differs from the previously
reported phase of enhancement, $0.16$, (at 610 MHz Chandra et al. 2015) by
one-half cycle. However, no flux enhancement is found at phase $0.16$ in our
data, which could be due to an unstable or drifting emission region, or a
consequence of the reported changes of the star's rotational period. Either of
these factors could have shifted the enhancement to the above-mentioned phase
window not sampled by our observations. | astro-ph_SR |
Constraining the neutrino magnetic dipole moment from white dwarf
pulsations: Pulsating white dwarf stars can be used as astrophysical laboratories to
constrain the properties of weakly interacting particles. Comparing the cooling
rates of these stars with the expected values from theoretical models allows us
to search for additional sources of cooling due to the emission of axions,
neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive
an upper bound to the neutrino magnetic dipole moment using an estimate of the
rate of period change of the pulsating DB white dwarf star PG 1351+489. By
comparing the theoretical rate of change of period expected for this star with
the rate of change of period with time of PG 1351+489, we assess the possible
existence of additional cooling by neutrinos with magnetic dipole moment. Our
models suggest the existence of some additional cooling in this pulsating DB
white dwarf, consistent with a non-zero magnetic dipole moment. Our upper limit
for the neutrino magnetic dipole moment is somewhat less restrictive than, but
still compatible with, other limits inferred from the white dwarf luminosity
function or from the color-magnitude diagram of the Globular cluster M5.
Further improvements of the measurement of the rate of period change of the
dominant pulsation mode of PG 1351+489 will be necessary to confirm our bound. | astro-ph_SR |
Generalized multi-polytropic Rankine-Hugoniot relations and the entropy
condition: The study aims at a derivation of generalized \RH relations, especially that
for the entropy, for the case of different upstream/downstream polytropic
indices and their implications. We discuss the solar/stellar wind interaction
with the interstellar medium for different polytropic indices and concentrate
on the case when the polytropic index changes across hydrodynamical shocks. We
use first a numerical mono-fluid approach with constant polytropic index in the
entire integration region to show the influence of the polytropic index on the
thickness of the helio-/astrosheath and on the compression ratio. Second, the
Rankine-Hugonoit relations for a polytropic index changing across a shock are
derived analytically, particularly including a new form of the entropy
condition. In application to the/an helio-/astrosphere, we find that the size
of the helio-/astrosheath as function of the polytropic index decreases in a
mono-fluid model for indices less than $\gamma=5/3$ and increases for higher
ones and vice versa for the compression ratio. Furthermore, we demonstrate that
changing polytropic indices across a shock are physically allowed only for
sufficiently high Mach numbers and that in the hypersonic limit the compression
ratio depends only on the downstream polytropic index, while the ratios of the
temperature and pressure as well as the entropy difference depend on both, the
upstream and downstream polytropic indices. | astro-ph_SR |
Search for systemic mass loss in Algols with bow shocks: Aims. Various studies indicate that interacting binary stars of Algol type
evolve non-conservatively. However, direct detections of systemic mass loss in
Algols have been scarce so far. We study the systemic mass loss in Algols by
looking for the presence of infrared excesses originating from the thermal
emission of dust grains, which is linked to the presence of a stellar wind.
Methods. In contrast to previous studies, we make use of the fact that
stellar and interstellar material is piled up at the edge of the astrosphere
where the stellar wind interacts with the interstellar medium. We analyse WISE
W3 $12\,\mu$m and WISE W4 $22\,\mu$m data of Algol-type binary Be and B[e]
stars and the properties of their bow shocks. From the stand-off distance of
the bow shock we are able to determine the mass-loss rate of the binary system.
Results. Although the velocities of the stars with respect to the
interstellar medium are quite low, we find bow shocks present in two systems,
namely $\pi$ Aqr, and $\varphi$ Per; a third system, CX Dra, shows a more
irregular circumstellar environment morphology which might somehow be related
to systemic mass loss. The properties of the two bow shocks point to mass-loss
rates and wind velocities typical of single B stars, which do not support an
enhanced systemic mass loss. | astro-ph_SR |
The Factory and The Beehive I. Rotation Periods For Low-Mass Stars in
Praesepe: Stellar rotation periods measured from single-age populations are critical
for investigating how stellar angular momentum content evolves over time, how
that evolution depends on mass, and how rotation influences the stellar dynamo
and the magnetically heated chromosphere and corona. We report rotation periods
for 40 late-K to mid-M stars members of the nearby, rich, intermediate-age
(~600 Myr) open cluster Praesepe. These rotation periods were derived from ~200
observations taken by the Palomar Transient Factory of four cluster fields from
2010 February to May. Our measurements indicate that Praesepe's mass-period
relation transitions from a well-defined singular relation to a more scattered
distribution of both fast and slow rotators at ~0.6 Msun. The location of this
transition is broadly consistent with expectations based on observations of
younger clusters and the assumption that stellar-spin down is the dominant
mechanism influencing angular momentum evolution at 600 Myr. However, a
comparison to data recently published for the Hyades, assumed to be coeval to
Praesepe, indicates that the divergence from a singular mass-period relation
occurs at different characteristic masses, strengthening the finding that
Praesepe is the younger of the two clusters. We also use previously published
relations describing the evolution of rotation periods as a function of color
and mass to evolve the sample of Praesepe periods in time. Comparing the
resulting predictions to periods measured in M35 and NGC 2516 (~150 Myr) and
for kinematically selected young and old field star populations suggests that
stellar spin-down may progress more slowly than described by these relations. | astro-ph_SR |
OGLE-2016-BLG-1003: First Resolved Caustic-crossing Binary-source Event
Discovered by Second-generation Microlensing Surveys: We report the analysis of the first resolved caustic-crossing binary-source
microlensing event OGLE-2016-BLG-1003. The event is densely covered by the
round-the-clock observations of three surveys. The light curve is characterized
by two nested caustic-crossing features, which is unusual for typical
caustic-crossing perturbations. From the modeling of the light curve, we find
that the anomaly is produced by a binary source passing over a caustic formed
by a binary lens. The result proves the importance of high-cadence and
continuous observations, and the capability of second-generation microlensing
experiments to identify such complex perturbations that are previously unknown.
However, the result also raises the issues of the limitations of current
analysis techniques for understanding lens systems beyond two masses and of
determining the appropriate multiband observing strategy of survey experiments. | astro-ph_SR |
A Study of the Orbital Periods of Deeply Eclipsing SW Sextantis Stars: Results are presented of a five-year project to study the orbital periods of
eighteen deeply eclipsing novalike cataclysmic variables, collectively known as
SW Sextantis stars, by combining new measurements of eclipse times with
published measurements stretching back in some cases over fifty years. While
the behaviour of many of these binary systems is consistent with a constant
orbital period, it is evident that in several cases this is not true. Although
the time span of these observations is relatively short, evidence is emerging
that the orbital periods of some of these stars show cyclical variation with
periods in the range 10-40 years. The two stars with the longest orbital
periods, V363 Aur and BT Mon, also show secular period reduction with rates of
-6.6 \times 10^{-8} days/year and -3.3 \times 10^{-8} days/year. New
ephemerides are provided for all eighteen stars to facilitate observation of
future eclipses. | astro-ph_SR |
Conditions for Photospherically Driven Alfvenic Oscillations to Heat the
Solar Chromosphere by Pedersen Current Dissipation: A magnetohydrodynamic model that includes a complete electrical conductivity
tensor is used to estimate conditions for photospherically driven, linear,
non-plane Alfvenic oscillations extending from the photosphere to the lower
corona to drive a chromospheric heating rate due to Pedersen current
dissipation that is comparable to the net chromospheric net radiative loss of
$\sim 10^7$ ergs-cm$^{-2}$-sec$^{-1}$. The heating rates due to electron
current dissipation in the photosphere and corona are also computed. The wave
amplitudes are computed self-consistently as functions of an inhomogeneous
background (BG) atmosphere. The effects of the conductivity tensor are resolved
numerically using a resolution of 3.33 m. The oscillations drive a
chromospheric heating flux $F_{Ch} \sim 10^7 - 10^8$ ergs-cm$^{-2}$-sec$^{-1}$
at frequencies $\nu \sim 10^2 - 10^3$ mHz for BG magnetic field strengths $B
\gtrsim 700$ G and magnetic field perturbation amplitudes $\sim 0.01 - 0.1$
$B$. The total resistive heating flux increases with $\nu$. Most heating occurs
in the photosphere. Thermalization of Poynting flux in the photosphere due to
electron current dissipation regulates the Poynting flux into the chromosphere,
limiting $F_{Ch}$. $F_{Ch}$ initially increases with $\nu$, reaches a maximum,
and then decreases with increasing $\nu$ due to increasing electron current
dissipation in the photosphere. The resolution needed to resolve the
oscillations increases from $\sim 10$ m in the photosphere to $\sim 10$ km in
the upper chromosphere, and is proportional to $\nu^{-1/2}$. Estimates suggest
that these oscillations are normal modes of photospheric flux tubes with
diameters $\sim 10-20$ km, excited by magnetic reconnection in current sheets
with thicknesses $\sim 0.1$ km. | astro-ph_SR |
Precision southern hemisphere VLBI pulsar astrometry II: Measurement of
seven parallaxes: Accurate measurement of pulsar distances via astrometry using very long
baseline interferometry enables the improvement of Galactic electron density
distribution models, improving distance estimates for the vast majority of
pulsars for which parallax measurements are unavailable. However, pulsars at
southern declinations have been under-represented in previous interferometric
astrometry campaigns. In order to redress this imbalance, we have conducted a
two-year astrometric campaign targeting eight southern pulsars with the
Australian Long Baseline Array. The program summarized in this paper has
resulted in the measurement of seven new pulsar parallaxes, with success on
objects down to a mean flux density of 0.8 mJy at 1600 MHz. Our results
highlight the substantial uncertainties that remain when utilizing free
electron density models for individual pulsar distances. Until this study, PSR
J0630-2834 was believed to convert 16% of its spin-down energy into x-rays, but
our measured parallax distance of 332 (+52 -40) pc has revised this value to
<1%. In contrast, PSR J0108-1431 was found to be almost a factor of two more
distant than previously thought, making its conversion of spin-down energy to
x-rays the most efficient known (>1%). The 8.5 second radio pulsar J2144-3933
was found to be closer than previously predicted, making its apparent 1400 MHz
radio luminosity the lowest of any known pulsar (20 microJy kpc^2). We have
examined the growing population of neutron stars with accurate parallaxes to
determine the effect of distance errors on the underlying neutron star velocity
distribution, and find that typical distance errors may be biasing the
estimated mean pulsar velocity upwards by 5%, and are likely to exaggerate the
distribution's high-velocity tail. | astro-ph_SR |
Estimating Stellar Atmospheric Parameters by Automated Methods Using
SSLs: Libraries of stellar spectra, such as ELODIE (Prugniel & Soubiran 2001),
CFLIB (Valdes et al. 2004), or MILES (S\'anchez-Bl\'azquez et al. 2006), are
used for a variety of applications, and especially in modelling stellar
populations (e. g. Le Borgne et al. (2004)). In that context, apart from the
completeness and quality of these spectral databases (Singh et al. 2006), the
accurate calibration of stellar atmospheric parameters, temperature (Teff),
surface gravity (log g), and metallicity ([Fe/H]), is known to be critical
(Prugniel et al. 2007; Percival & Salaris 2009). We discuss the technique of
determining stellar atmospheric parameters accurately by `full spectrum
fitting'. | astro-ph_SR |
Validation of the Exoplanet Kepler-21b using PAVO/CHARA Long-Baseline
Interferometry: We present long-baseline interferometry of the Kepler exoplanet host star
HD179070 (Kepler-21) using the PAVO beam combiner at the CHARA Array. The
visibility data are consistent with a single star and exclude stellar
companions at separations ~1-1000 mas (~ 0.1-113 AU) and contrasts < 3.5
magnitudes. This result supports the validation of the 1.6 R_{earth} exoplanet
Kepler-21b by Howell et al. (2012) and complements the constraints set by
adaptive optics imaging, speckle interferometry, and radial velocity
observations to rule out false-positives due to stellar companions. We conclude
that long-baseline interferometry has strong potential to validate transiting
extrasolar planets, particularly for future projects aimed at brighter stars
and for host stars where radial velocity follow-up is not available. | astro-ph_SR |
Accretion process, magnetic fields, and apsidal motion in the pre-main
sequence binary DQ Tau: Classical T Tauri stars (CTTSs) are young stellar objects that accrete
materials from their accretion disc influenced by their strong magnetic field.
The magnetic pressure truncates the disc at a few stellar radii and forces the
material to leave the disc plane and fall onto the stellar surface by following
the magnetic field lines. However, this global scheme may be disturbed by the
presence of a companion interacting gravitationally with the accreting
component. This work is aiming to study the accretion and the magnetic field of
the tight eccentric binary DQ Tau, composed of two equal-mass ($\sim$ 0.6 \msun
) CTTSs interacting at different orbital phases. We investigated the
variability of the system using a high-resolution spectroscopic and
spectropolarimetric monitoring performed with ESPaDOnS at the CFHT. We provide
the first ever magnetic field analysis of this system, the Zeeman-Doppler
imaging revealed a stronger magnetic field for the secondary than the primary
(1.2 kG and 0.5 kG, respectively), but the small-scale fields analysed through
Zeeman intensification yielded similar strengths (about 2.5 kG). The magnetic
field topology and strengths are compatible with the accretion processes on
CTTSs. Both components of this system are accreting, with a change of the main
accretor during the orbital motion. In addition, the system displays a strong
enhancement of the mass accretion rate at periastron and apastron. We also
discovered, for the first time in this system, the apsidal motion of the
orbital ellipse. | astro-ph_SR |
Modelling the occurrence of grand minima in sun-like stars using a
dynamo model: In this work, we have studied the variability and frequency of occurrence of
the grand minima using kinematic dynamo models of one solar mass star with
different rotation rates and depths of convection zones. We specify the
large-scale flows (differential rotations and meridional circulations) from
corresponding hydrodynamic models. We include stochastic fluctuations in the
Babcock-Leighton source for the poloidal field to produce variable stellar
cycles. We observe that the rapidly rotating stars produce highly irregular
cycles with strong magnetic fields and rarely produce Maunder-like grand
minima, whereas the slowly rotating stars (Sun and longer rotation period)
produce smooth cycles of weaker strength and occasional grand minima. In
general, the number of the grand minima increases with the decrease in rotation
rate. These results can be explained by the fact that with the increase of
rotation period, the supercriticality of the dynamo decreases, and the dynamo
is more prone to produce extended grand minima in this regime. | astro-ph_SR |
Magnetic field and prominences of the young, solar-like, ultra-rapid
rotator V530 Per: We investigate signatures of magnetic fields and activity at the surface and
in the prominence system of the ultra-rapid rotator V530 Per, a G-type
solar-like member of the young open cluster $\alpha$~Persei. This object has a
rotation period shorter than all stars with available magnetic maps. With a
time-series of spectropolarimetric observations gathered with ESPaDOnS over 2
nights on the CFHT, we reconstruct the surface brightness and large-scale
magnetic field of V530 Per using the Zeeman-Doppler imaging method, assuming an
oblate stellar surface. We also estimate the short term evolution of the
brightness distribution through latitudinal differential rotation. Using the
same data set, we finally map the spatial distribution of prominences through
tomography of the H\alpha emission. The brightness map is dominated by a large,
dark spot near the pole, accompanied by a complex distribution of bright and
dark features at lower latitudes. The magnetic field map is reconstructed as
well, most of the large-scale magnetic field energy is stored in the toroidal
field component. The main radial field structure is a positive region of about
500 G, at the location of the dark polar spot. The brightness map of V530 Per
is sheared by solar-like differential rotation, with a roughly solar value for
the difference in rotation rate between the pole and equator. \halpha~is
observed in emission, and is mostly modulated by the stellar rotation period.
The prominence system is organized in a ring at the approximate location of the
co-rotation radius, with significant evolution between the two observing
nights. V530 Per is the first example of a solar-type star to have its surface
magnetic field and prominences mapped together, which will bring important
observational constraints to better understand the role of slingshot
prominences in the angular momentum evolution of the most active stars. | astro-ph_SR |
An Observational Perspective of Transitional Disks: Transitional disks are objects whose inner disk regions have undergone
substantial clearing. The Spitzer Space Telescope produced detailed spectral
energy distributions (SEDs) of transitional disks that allowed us to infer
their radial dust disk structure in some detail, revealing the diversity of
this class of disks. The growing sample of transitional disks also opened up
the possibility of demographic studies, which provided unique insights. There
now exist (sub)millimeter and infrared images that confirm the presence of
large clearings of dust in transitional disks. In addition, protoplanet
candidates have been detected within some of these clearings. Transitional
disks are thought to be a strong link to planet formation around young stars
and are a key area to study if further progress is to be made on understanding
the initial stages of planet formation. Here we provide a review and synthesis
of transitional disk observations to date with the aim of providing timely
direction to the field, which is about to undergo its next burst of growth as
ALMA reaches its full potential. We discuss what we have learned about
transitional disks from SEDs, color-color diagrams, and imaging in the (sub)mm
and infrared. We then distill the observations into constraints for the main
disk clearing mechanisms proposed to date (i.e., photoevaporation, grain
growth, and companions) and explore how the expected observational signatures
from these mechanisms, particularly planet-induced disk clearing, compare to
actual observations. Lastly, we discuss future avenues of inquiry to be pursued
with ALMA, JWST, and next generation of ground-based telescopes. | astro-ph_SR |
Extension and validation of the pendulum model for longitudinal solar
prominence oscillations: Longitudinal oscillations in prominences are common phenomena on the Sun.
These oscillations can be used to infer the geometry and intensity of the
filament magnetic field. Previous theoretical studies of longitudinal
oscillations made two simplifying assumptions: uniform gravity and
semi-circular dips on the supporting flux tubes. However, the gravity is not
uniform and realistic dips are not semi-circular. To understand the effects of
including the nonuniform solar gravity on longitudinal oscillations, and
explore the validity of the pendulum model with different flux-tube geometries.
We first derive the equation describing the motion of the plasma along the flux
tube including the effects of nonuniform gravity, yielding corrections to the
original pendulum model. We also compute the full numerical solutions for the
normal modes, and compare them with the new pendulum approximation. We have
found that the nonuniform gravity introduces a significant modification in the
pendulum model. We have also found a cut-off period, i.e. the longitudinal
oscillations cannot have a period longer than 167 minutes. In addition,
considering different tube geometries, the period depends almost exclusively on
the radius of curvature at the bottom of the dip. We conclude that nonuniform
gravity significantly modifies the pendulum model. These corrections are
important for prominence seismology, because the inferred values of the radius
of curvature and minimum magnetic-field strength differ substantially from
those of the old model. However, we find that the corrected pendulum model is
quite robust and is still valid for non-circular dips. | astro-ph_SR |
Destabilization of a Solar Prominence/Filament Field System by a Series
of Eight Homologous Eruptive Flares: Homologous flares are flares that occur repetitively in the same active
region, with similar structure and morphology. A series of at least eight
homologous flares occurred in active region NOAA 11237 over 16 - 17 June 2011.
A nearby prominence/filament was rooted in the active region, and situated near
the bottom of a coronal cavity. The active region was on the southeast solar
limb as seen from SDO/AIA, and on the disk as viewed from STEREO/EUVI-B. The
dual perspective allows us to study in detail behavior of the
prominence/filament material entrained in the magnetic field of the
repeatedly-erupting system. Each of the eruptions was mainly confined, but
expelled hot material into the prominence/filament cavity system (PFCS). The
field carrying and containing the ejected hot material interacted with the PFCS
and caused it to inflate, resulting in a step-wise rise of the PFCS
approximately in step with the homologous eruptions. The eighth eruption
triggered the PFCS to move outward slowly, accompanied by a weak coronal
dimming. As this slow PFCS eruption was underway, a final ejective flare
occurred in the core of the active region, resulting in strong dimming in the
EUVI-B images and expulsion of a coronal mass ejection (CME). A plausible
scenario is that the repeated homologous flares could have gradually
destabilized the PFCS, and its subsequent eruption removed field above the
acitive region and in turn led to the ejective flare, strong dimming, and CME. | astro-ph_SR |
Generalized three body problem and the instability of the core-halo
objects in binary systems: Goal of the presented research is to construct simplified model of the
core-halo structures in binary systems. Examples are provided by Thorne-Zytkov
objects, hot Jupiters, protoplanets with large moons, red supergiants in
binaries and globular clusters with central black hole. Instability criteria
due to resonance between internal and orbital frequencies in such a systems has
been derived. To achieve assumed goals, generalized planar circular restricted
three body problem is investigated with one of the point masses, $M$, replaced
with spherical body of finite size. Mechanical system under consideration
includes two large masses $m$ and $M$ and the test body with small mass $\mu$.
Only gravitational interactions are considered. Equations of motion are
presented, and linear instability criteria are derived using quantifier
elimination.
Motion of the test mass $\mu$ is shown to be unstable due to resonance
between orbital and internal frequencies if $\frac{M}{d^3} < \frac{4}{3} \pi
\rho < \frac{ M + 3 m \left( 1+\mu/M \right)^{-1}}{d^3}$, where $\rho$ is the
central density of mass $M$, and $d$ distance between masses $m$ and $M$
(circular orbit diameter).
The above result is important for core-collapse supernova theory, with mass
$\mu$ identified with helium core of the exploding massive star. The
instability cause off-center supernova "ignition" relative to the
center-of-mass of the hydrogen envelope. The instability is also inevitable
during protoplanet growth, with hypothetical ejection of the rocky core from
gas giants and formation of the "puffy planets" due to resonance with orbital
frequency. Hypothetical central intermediate black holes of the globular
clusters are also in unstable position with respect to perturbations caused by
the Galaxy. | astro-ph_SR |
The Clusters AgeS Experiment (CASE): Variable Stars in the Globular
Cluster M4: Based on over 3000 BV images of M4 collected in years 1995-2009 we obtain
light curves of 22 variables, 10 of which are newly detected objects. We
identify four detached eclipsing binaries and eight contact binaries. Accurate
periods are found for all but two variables. Nineteen variables are
proper-motion members of the cluster, and the remaining three are field stars.
Five variables are optical counterparts of X-ray sources. For one of the
variables unassociated with X-ray sources we report a flare lasting for about
90 min and reaching an amplitude of 0.11 mag in V. One of the new contact
binaries has a record-low mass ratio q=0.06. Another four such systems show
season-to season luminosity variations probably related to magnetic activity
cycles, whose lengths are surprisingly similar to that of the solar cycle
despite a huge difference in rotational periods. The location of contact
binaries on the color-magnitude diagram of M4 strongly suggests that at least
in globular clusters the principal factor enabling EW systems to form from
close but detached binaries is stellar evolution. We identify 46 blue and
yellow stragglers in M4 and discuss their properties. We also derive a map of
the differential extinction in the central part of M4, and determine the
reddening of a selected reference region, E(B-V)= 0.392 mag. | astro-ph_SR |
Sunspot rotation. II. Effects of varying the field strength and twist of
an emerging flux tube: Context. Observations of flux emergence indicate that rotational velocities
may develop within sunspots. However, the dependence of this rotation on
sub-photospheric field strength and twist remains largely unknown.
Aims. We investigate the effects of varying the initial field strength and
twist of an emerging sub-photospheric magnetic flux tube on the rotation of the
sunspots at the photosphere.
Methods. We consider a simple model of a stratified domain with a
sub-photospheric interior layer and three overlying atmospheric layers. A
twisted arched flux tube is inserted in the interior and is allowed to rise
into the atmosphere. To achieve this, the MHD equations are solved using the
Lagrangian-remap code, Lare3d. We perform a parameter study by independently
varying the sub-photospheric magnetic field strength and twist.
Results. Altering the initial field strength and twist significantly affects
the tube's evolution and the rotational motions that develop at the
photosphere. The rotation angle, vorticity, and current show a direct
dependence on the initial field strength. We find that an increase in field
strength increases the angle through which the fieldlines rotate, the length of
fieldlines extending into the atmosphere, and the magnetic energy transported
to the atmosphere. This also affects the amount of residual twist in the
interior. The length of the fieldlines is crucial as we predict the twist per
unit length equilibrates to a lower value on longer fieldlines. No such direct
dependence is found when we modify the twist owing to the complex effect this
has on the tension force acting on the tube. However, there is still a clear
ordering in quantities such as the rotation angle, helicity, and free energy
with higher initial twist cases being related to sunspots that rotate more
rapidly, transporting more helicity and magnetic energy to the atmosphere. | astro-ph_SR |
FIP Bias Evolution in a Decaying Active Region: Solar coronal plasma composition is typically characterized by first
ionization potential (FIP) bias. Using spectra obtained by Hinode's EUV Imaging
Spectrometer (EIS) instrument, we present a series of large-scale, spatially
resolved composition maps of active region (AR) 11389. The composition maps
show how FIP bias evolves within the decaying AR from 2012 January 4-6.
Globally, FIP bias decreases throughout the AR. We analyzed areas of
significant plasma composition changes within the decaying AR and found that
small-scale evolution in the photospheric magnetic field is closely linked to
the FIP bias evolution observed in the corona. During the AR's decay phase,
small bipoles emerging within supergranular cells reconnect with the
pre-existing AR field, creating a pathway along which photospheric and coronal
plasmas can mix. The mixing time scales are shorter than those of plasma
enrichment processes. Eruptive activity also results in shifting the FIP bias
closer to photospheric in the affected areas. Finally, the FIP bias still
remains dominantly coronal only in a part of the AR's high-flux density core.
We conclude that in the decay phase of an AR's lifetime, the FIP bias is
becoming increasingly modulated by episodes of small-scale flux emergence, i.e.
decreasing the AR's overall FIP bias. Our results show that magnetic field
evolution plays an important role in compositional changes during AR
development, revealing a more complex relationship than expected from previous
well-known Skylab results showing that FIP bias increases almost linearly with
age in young ARs (Widing $\&$ Feldman, 2001, ApJ, 555, 426). | astro-ph_SR |
The Branchings of the Main s-process: Their Sensitivity to alpha-induced
Reactions on 13C and 22Ne and to the Uncertainties of the Nuclear Network: This paper provides a detailed analysis of the main component of the slow
neutron capture process (the s-process), which accounts for the solar
abundances of half of the nuclei with 90 <~ A <~ 208. We examine the impact of
the uncertainties of the two neutron sources operating in low-mass asymptotic
giant branch (AGB) stars: the 13C(alpha, n)16O reaction, which releases
neutrons radiatively during interpulse periods (kT ~ 8 keV), and the
22Ne(alpha, n)25Mg reaction, partially activated during the convective thermal
pulses (TPs). We focus our attention on the branching points that mainly
influence the abundance of s-only isotopes. In our AGB models, the 13C is fully
consumed radiatively during interpulse. In this case, we find that the present
uncertainty associated to the 13C(alpha, n)16O reaction has marginal effects on
s-only nuclei. On the other hand, a reduction of this rate may increase the
amount of residual (or unburned) 13C at the end of the interpulse: in this
condition, the residual 13C is burned at higher temperature in the convective
zone powered by the following TP. The neutron burst produced by the 22Ne(alpha,
n)25Mg reaction has major effects on the branches along the s path. The
contributions of s-only isotopes with 90 <~ A <= 204 are reproduced within
solar and nuclear uncertainties, even if the 22Ne(alpha, n)25Mg rate is varied
by a factor of two. Improved beta-decay and neutron capture rates of a few key
radioactive nuclides would help to attain a comprehensive understanding of the
solar main component. | astro-ph_SR |
Toward understanding the early stages of an impulsively accelerated
coronal mass ejection: The expanding magnetic flux in coronal mass ejections (CMEs) often forms a
cavity. A spherical model is simultaneously fit to STEREO EUVI and COR1 data of
an impulsively accelerated CME on 25 March 2008, which displays a well-defined
extreme ultraviolet (EUV) and white-light cavity of nearly circular shape
already at low heights ~ 0.2 Rs. The center height h(t) and radial expansion
r(t) of the cavity are obtained in the whole height range of the main
acceleration. We interpret them as the axis height and as a quantity
proportional to the minor radius of a flux rope, respectively. The
three-dimensional expansion of the CME exhibits two phases in the course of its
main upward acceleration. From the first h and r data points, taken shortly
after the onset of the main acceleration, the erupting flux shows an
overexpansion compared to its rise, as expressed by the decrease of the aspect
ratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident
with the impulsive rise of the acceleration and is followed by a phase of very
gradual change of the aspect ratio (a nearly self-similar expansion) toward k ~
1.5 at h ~ 10 Rs. The initial overexpansion of the CME cavity can be caused by
flux conservation around a rising flux rope of decreasing axial current and by
the addition of flux to a growing, or even newly forming,flux rope by magnetic
reconnection. Further analysis will be required to decide which of these
contributions is dominant. The data also suggest that the horizontal component
of the impulsive cavity expansion (parallel to the solar surface) triggers the
associated EUV wave, which subsequently detaches from the CME volume. | astro-ph_SR |
Radio Measurements of the stellar proper motions in the core of the
Orion Nebula Cluster: Using multi-epoch VLA observations, covering a time baseline of 29.1 years,
we have measured the proper motions of 88 young stars with compact radio
emission in the core of the Orion Nebula Cluster (ONC) and the neighboring
BN/KL region. Our work increases the number of young stars with measured proper
motion at radio frequencies by a factor of 2.5 and enables us to perform a
better statistical analysis of the kinematics of the region than was previously
possible. Most stars (79 out of 88) have proper motions consistent with a
Gaussian distribution centered on
$\overline{\mu_{\alpha}\cos{\delta}}=1.07\pm0.09\quad{\rm mas\,yr^{-1}}$, and
$\overline{\mu_{\delta}}=-0.84\pm0.16\quad{\rm mas\,yr^{-1}}$, with velocity
dispersions of $\sigma_{\alpha}=1.08\pm0.07\quad{\rm mas\,\,yr^{-1}},$
$\sigma_{\delta}=1.27\pm0.15\quad{\rm mas\,\,yr^{-1}}$. We looked for organized
movements of these stars but found no clear indication of radial
expansion/contraction or rotation. The remaining nine stars in our sample show
peculiar proper motions that differ from the mean proper motions of the ONC by
more than 3-$\sigma$. One of these stars, V 1326 Ori, could have been expelled
from the Orion Trapezium 7,000 years ago. Two could be related to the
multi-stellar disintegration in the BN/KL region, in addition to the previously
known sources BN, I and n. The others either have high uncertainties (so their
anomalous proper motions are not firmly established) or could be foreground
objects. | astro-ph_SR |
Horizontal flow fields observed in Hinode G-band images. I. Methods: Context: The interaction of plasma motions and magnetic fields is an
important mechanism, which drives solar activity in all its facets. For
example, photospheric flows are responsible for the advection of magnetic flux,
the redistribution of flux during the decay of sunspots, and the built-up of
magnetic shear in flaring active regions. Aims: Systematic studies based on
G-band data from the Japanese Hinode mission provide the means to gather
statistical properties of horizontal flow fields. This facilitates comparative
studies of solar features, e.g., G-band bright points, magnetic knots, pores,
and sunspots at various stages of evolution and in distinct magnetic
environments, thus, enhancing our understanding of the dynamic Sun. Methods: We
adapted Local Correlation Tracking (LCT) to measure horizontal flow fields
based on G-band images obtained with the Solar Optical Telescope on board
Hinode. In total about 200 time-series with a duration between 1-16 h and a
cadence between 15-90 s were analyzed. Selecting both a high-cadence (dt = 15
s) and a long-duration (dT = 16 h) time-series enabled us to optimize and
validate the LCT input parameters, hence, ensuring a robust, reliable, uniform,
and accurate processing of a huge data volume. Results: The LCT algorithm
produces best results for G-band images having a cadence of 60-90 s. If the
cadence is lower, the velocity of slowly moving features will not be reliably
detected. If the cadence is higher, the scene on the Sun will have evolved too
much to bear any resemblance with the earlier situation. Consequently, in both
instances horizontal proper motions are underestimated. The most reliable and
yet detailed flow maps are produced using a Gaussian kernel with a size of 2560
km x 2560 km and a full-width-at-half-maximum (FWHM) of 1200 km (corresponding
to the size of a typical granule) as sampling window. | astro-ph_SR |
From Protoplanetary Disks to Extrasolar Planets: Understanding the Life
Cycle of Circumstellar Gas with Ultraviolet Spectroscopy: Few scientific discoveries have captured the public imagination like the
explosion of exoplanetary science during the past two decades. This work has
fundamentally changed our picture of Earth's place in the Universe and led NASA
to make significant investments towards understanding the demographics of
exoplanetary systems and the conditions that lead to their formation. The story
of the formation and evolution of exoplanetary systems is essentially the story
of the circumstellar gas and dust that are initially present in the
protostellar environment; in order to understand the variety of planetary
systems observed, we need to understand the life cycle of circumstellar gas
from its initial conditions in protoplanetary disks to its endpoint as planets
and their atmospheres. In this white paper response to NASA's Request for
Information "Science Objectives and Requirements for the Next NASA UV/Visible
Astrophysics Mission Concepts (NNH12ZDA008L)", we describe scientific programs
that would use the unique capabilities of a future NASA ultraviolet
(UV)/visible space observatory to make order-of-magnitude advances in our
understanding of the life cycle of circumstellar gas. | astro-ph_SR |
WR 63: A multiple system (O+O)+WR ?: The spectrum of the Wolf-Rayet (WR) star WR 63 contains spectral lines of two
different O stars that show regular radial velocity (RV) variations with
amplitudes of ~160 and ~225 km/s on a ~4.03 d period. The light-curve shows two
narrow eclipses that are 0.2 mag deep on the same period as the RV changes. On
the other hand, our data show no significant RV variations for the WR spectral
lines. Those findings are compatible with WR 63 being a triple system composed
of two non-interacting late O stars orbiting a WR star on a period larger than
1000 days. The amplitude of the WR spectral line-profile variability reaches
7-8% of the line intensity and seems related to a 0.04 mag periodic photometric
variation. Large wind density structures are a possible origin of this
variability, but our data are not sufficient to verify this. Our analysis shows
that, should the three stars be bound, they would be coeval with an age of
about 5.9+/-1.4 Myrs. The distance to the O stars is estimated to be 3.4+/-0.5
kpc. Their dynamical masses are 14.3+/-0.1 and 10.3+/-0.1 M_sol. Using
rotating, single star evolutionary tracks, we estimate their initial masses to
be 18+/-2 and 16+/-2 M_sol for the primary and the secondary, respectively.
Regular spectral monitoring is required in the future to detect RV variations
of the WR star that would prove that it is gravitationally bound to the close
O+OB system and to determine its mass. | astro-ph_SR |
A deep catalogue of classical Be stars in the direction of the Perseus
Arm: spectral types and interstellar reddenings: We present a catalogue of 247 photometrically and spectroscopically confirmed
fainter classical Be stars (13 < r < 16) in the direction of the Perseus Arm of
the Milky Way (-1 < b < +4, 120 < l < 140). The catalogue consists of 181
IPHAS-selected new classical Be stars, in addition to 66 objects that were
studied by Raddi et al. (2013) more closely, and 3 stars identified as
classical Be stars in earlier work. This study more than doubles the number
known in the region. Photometry spanning 0.6 to 5 micron, spectral types, and
interstellar reddenings are given for each object. The spectral types were
determined from low-resolution spectra (lambda / Delta-lambda ~ 800-2000), to a
precision of 1-3 subtypes. The interstellar reddenings are derived from the (r
- i) colour, using a method that corrects for circumstellar disc emission. The
colour excesses obtained range from E(B-V) = 0.3 up to 1.6 - a distribution
that modestly extends the range reported in the literature for Perseus-Arm open
clusters. For around half the sample, the reddenings obtained are compatible
with measures of the total sightline Galactic extinction. Many of these are
likely to lie well beyond the Perseus Arm. | astro-ph_SR |
Microlensing Binaries Discovered through High-Magnification Channel: Microlensing can provide a useful tool to probe binary distributions down to
low-mass limits of binary companions. In this paper, we analyze the light
curves of 8 binary lensing events detected through the channel of
high-magnification events during the seasons from 2007 to 2010. The
perturbations, which are confined near the peak of the light curves, can be
easily distinguished from the central perturbations caused by planets. However,
the degeneracy between close and wide binary solutions cannot be resolved with
a $3\sigma$ confidence level for 3 events, implying that the degeneracy would
be an important obstacle in studying binary distributions. The dependence of
the degeneracy on the lensing parameters is consistent with a theoretic
prediction that the degeneracy becomes severe as the binary separation and the
mass ratio deviate from the values of resonant caustics. The measured mass
ratio of the event OGLE-2008-BLG-510/MOA-2008-BLG-369 is $q\sim 0.1$, making
the companion of the lens a strong brown-dwarf candidate. | astro-ph_SR |
Hubble Space Telescope reveals multiple Sub-Giant Branch in eight
Globular Clusters: In the last few years many globular clusters (GCs) have revealed complex
color-magnitude diagrams, with the presence of multiple main sequences (MSs),
broaden or multiple sub-giant branches (SGBs) and MS turn offs, and broad or
split red giant branches (RGBs). After a careful correction for differential
reddening, high accuracy photometry with the Hubble Space Telescope presented
in this paper reveals a broadened or even split SGB in five additional Milky
Way GCs: NGC 362, NGC 5286, NGC 6656, NGC 6715, and NGC 7089. In addition, we
confirm (with new and archival HST data) the presence of a split SGB in 47Tuc,
NGC 1851, and NGC 6388. The fraction of faint SGB stars with respect to the
entire SGB population varies from one cluster to another and ranges from
$\sim$0.03 for NGC 362 to ~0.50 for NGC 6715. The average magnitude difference
between the bright SGB and the faint SGB is almost the same at different
wavelengths. This peculiarity is consistent with the presence of two groups of
stars with either an age difference of about 1-2 Gyrs, or a significant
difference in their overall C+N+O content. | astro-ph_SR |
Evidence of Electron Acceleration around the Reconnection X-point in a
Solar Flare: Particle acceleration is one of the most significant features that are
ubiquitous among space and cosmic plasmas. It is most prominent during flares
in the case of the Sun, with which huge amount of electromagnetic radiation and
high-energy particles are expelled into the interplanetary space through
acceleration of plasma particles in the corona. Though it has been well
understood that energies of flares are supplied by the mechanism called
magnetic reconnection based on the observations in X-rays and EUV with space
telescopes, where and how in the flaring magnetic field plasmas are accelerated
has remained unknown due to the low plasma density in the flaring corona. We
here report the first observational identification of the energetic non-thermal
electrons around the point of the ongoing magnetic reconnection (X-point); with
the location of the X-point identified by soft X-ray imagery and the localized
presence of non-thermal electrons identified from imaging-spectroscopic data at
two microwave frequencies. Considering the existence of the reconnection
outflows that carries both plasma particles and magnetic fields out from the
X-point, our identified non-thermal microwave emissions around the X-point
indicate that the electrons are accelerated around the reconnection X-point.
Additionally, the plasma around the X-point was also thermally heated up to 10
MK. The estimated reconnection rate of this event is ~0.017. | astro-ph_SR |
High resolution soft X-ray spectroscopy and the quest for the hot (5-10
MK) plasma in solar active regions: We discuss the diagnostics available to study the 5-10 MK plasma in the solar
corona, which is key to understanding the heating in the cores of solar active
regions. We present several simulated spectra, and show that excellent
diagnostics are available in the soft X-rays, around 100 Angstroms, as six
ionisation stages of Fe can simultaneously be observed, and electron densities
derived, within a narrow spectral region. As this spectral range is almost
unexplored, we present an analysis of available and simulated spectra, to
compare the hot emission with the cooler component. We adopt recently designed
multilayers to present estimates of count rates in the hot lines, with a
baseline spectrometer design. Excellent count rates are found, opening up the
exciting opportunity to obtain high-resolution spectroscopy of hot plasma. | astro-ph_SR |
Cepheid Abundances: Multiphase Results and Spatial Gradients: Parameters and abundances have been derived for 435 Cepheids based on an
analysis of 1127 spectra. Results from five or more phases are available for 52
of the program stars. The latter set of stars span periods between 1.5 and 68
days. The parameters and abundances show excellent consistency across phase.
For iron, the average range in the determined abundance is 0.11 from these 52
stars. For 163 stars with more than one phase available the average range is
0.07. The variation in effective temperature tracks well with phase, as does
the total broadening velocity. The gravity and microturbulent velocity follow
phase, but with less variation and regularity.
Abundance gradients have been derived using GAIA DR2 parallax data (GAIA
Collaboration et al. 2016, 2018), as well as Bayesian distance estimates based
upon GAIA DR2 from Bailer-Jones et al. (2018). The abundance gradient derived
for iron is d[Fe/H]/dR = -0.05 dex/kpc, similar to gradients derived in
previous studies. | astro-ph_SR |
Measurements of Non-Thermal Line Widths in Solar Active Regions: Spectral line widths are often observed to be larger than can be accounted
for by thermal and instrumental broadening alone. This excess broadening is a
key observational constraint for both nanoflare and wave dissipation models of
coronal heating. Here we present a survey of non-thermal velocities measured in
the high temperature loops (1--5MK) often found in the cores of solar active
regions. This survey of $\textit{Hinode}$ Extreme Ultraviolet Imaging
Spectrometer (EIS) observations covers 15 non-flaring active regions that span
a wide range of solar conditions. We find relatively small non-thermal
velocities, with a mean value of 17km s$^{-1}$, and no significant trend with
temperature or active region magnetic flux. These measurements appear to be
inconsistent with those expected from reconnection jets in the corona,
chromospheric evaporation induced by coronal nanoflares, and Alfv\'en wave
turbulence models. Furthermore, because the observed non-thermal widths are
generally small their measurements are difficult and susceptible to systematic
effects. | astro-ph_SR |
Space, time and velocity association of successive coronal mass
ejections: Our aim is to investigate the possible physical association between
consecutive coronal mass ejections (CMEs). Through a statistical study of the
main characteristics of 27761 CMEs observed by SOHO/LASCO during the past 20
years. We found the waiting time (WT) or time elapsed between two consecutive
CMEs is $< 5$ hrs for 59\% and $< 25$ hrs for 97\% of the events, and the CME
WTs follow a Pareto Type IV statistical distribution. The difference of the
position-angle of a considerable population of consecutive CME pairs is less
than $30^\circ$, indicating the possibility that their source locations are in
the same region. The difference between the speed of trailing and leading
consecutive CMEs follows a generalized Student t-distribution. The fact that
the WT and the speed difference have heavy-tailed distributions along with a
detrended fluctuation analysis shows that the CME process has a long-range
dependence. As a consequence of the long-range dependence, we found a small but
significative difference between the speed of consecutive CMEs, with the speed
of the trailing CME being higher than the speed of the leading CME. The
difference is largest for WTs < 2 hrs and tends to be zero for WTs > 10 hrs,
and it is more evident during the ascending and descending phases of the solar
cycle. We suggest that this difference may be caused by a drag force acting
over CMEs closely related in space and time. | astro-ph_SR |
First Keck Nulling Observations of a Young Stellar Object: Probing the
Circumstellar Environment of the Herbig Ae star MWC 325: We present the first N-band nulling plus K- and L-band V2 observations of a
young stellar object, MWC325, taken with the 85 m baseline Keck Interferometer.
The Keck nuller was designed for the study of faint dust signatures associated
with debris disks, but it also has a unique capability for studying the
temperature and density distribution of denser disks found around young stellar
objects. Interferometric observations of MWC 325 at K, L and N encompass a
factor of five in spectral range and thus, especially when spectrally dispersed
within each band, enable characterization of the structure of the inner disk
regions where planets form. Fitting our observations with geometric models such
as a uniform disk or a Gaussian disk show that the apparent size increases
monotonically with wavelength in the 2-12 um wavelength region, confirming the
widely held assumption based on radiative transfer models, now with spatially
resolved measurements over broad wavelength range, that disks are extended with
a temperature gradient. The effective size is a factor of about 1.3 and 2
larger in the L-band and N-band, respectively, compared to that in the K-band.
The existing interferometric measurements and the spectral energy distribution
can be reproduced by a flat disk or a weakly-shadowed nearly flat-disk model,
with only slight flaring in the outer regions of the disk, consisting of
representative "sub-micron" (0.1 um) and "micron" (2 um) grains of a 50:50
ratio of silicate and graphite. This is marked contrast with the disks
previously found in other Herbig Ae/Be stars suggesting a wide variety in the
disk properties among Herbig Ae/Be stars. | astro-ph_SR |
Perpendicular Diffusion of Energetic Particles in Collisionless Plasmas: A fundamental problem in plasma and astrophysics is the interaction between
energetic particles and magnetized plasmas. In the current paper we focus on
particle diffusion across the guide magnetic field. It is shown that the
perpendicular diffusion coefficient depends only on the parallel diffusion
coefficient and the Kubo number. Therefore, one can find four asymptotic limits
depending on the values of these two parameters. These regimes are the
quasilinear limit, the Kadomtsev & Pogutse limit, the scaling of Rechester &
Rosenbluth, and the scaling found by Zybin & Istomin. In the current article we
focus on the Rechester & Rosenbluth scenario because this was not discovered
before in the context of collisionless plasmas. Examples and applications are
discussed as well. We show that an energy independent ratio of perpendicular
and parallel diffusion coefficients can be found and that this ratio can be
very small but also close to unity. This is exactly what one observes in the
solar wind. | astro-ph_SR |
Solar Flare Prediction and Feature Selection using Light Gradient
Boosting Machine Algorithm: Solar flares are among the most severe space weather phenomena, and they have
the capacity to generate radiation storms and radio disruptions on Earth. The
accurate prediction of solar flare events remains a significant challenge,
requiring continuous monitoring and identification of specific features that
can aid in forecasting this phenomenon, particularly for different classes of
solar flares. In this study, we aim to forecast C and M class solar flares
utilising a machine-learning algorithm, namely the Light Gradient Boosting
Machine. We have utilised a dataset spanning 9 years, obtained from the
Space-weather Helioseismic and Magnetic Imager Active Region Patches (SHARP),
with a temporal resolution of 1 hour. A total of 37 flare features were
considered in our analysis, comprising of 25 active region parameters and 12
flare history features. To address the issue of class imbalance in solar flare
data, we employed the Synthetic Minority Oversampling Technique (SMOTE). We
used two labeling approaches in our study: a fixed 24-hour window label and a
varying window that considers the changing nature of solar activity. Then, the
developed machine learning algorithm was trained and tested using forecast
verification metrics, with an emphasis on evaluating the true skill statistic
(TSS). Furthermore, we implemented a feature selection algorithm to determine
the most significant features from the pool of 37 features that could
distinguish between flaring and non-flaring active regions. We found that
utilising a limited set of useful features resulted in improved prediction
performance. For the 24-hour prediction window, we achieved a TSS of 0.63
(0.69) and accuracy of 0.90 (0.97) for $\geq$C ($\geq$M) class solar flares. | astro-ph_SR |
Penumbral micro-jets at high spatial and temporal resolution: Sunspot observations in chromospheric spectral lines have revealed the
existence of short-lived linear bright transients, commonly referred to as
penumbral micro-jets (PMJs). Details on the origin and physical nature of PMJs
are to large extend still unkown. We aim to characterize the dynamical nature
of PMJs to provide guidance for future modelling efforts. We analyze high
spatial (0.1 arcsec) and temporal resolution (1 s) Ca II H filtergram (0.1 nm
bandwidth) observations of a sunspot obtained on two consecutive days with the
Swedish 1-m Solar Telescope. We find that PMJs appear to be the rapid
brightening of an already existing (faint) fibril. The rapid brightening is the
fast increase (typically less than 10 s) in intensity over significant length
(several 100s of km) of the existing fibril. For most PMJs, we cannot identify
a clear root or source from where the brightening appears to originate. After
the fast onset, about half of the PMJs have a top that is moving with an
apparent velocity between 5 and 14 km/s, most of them upwards. For the other
PMJs, there is no significant motion of the top. For about a third of the PMJs
we observe a splitting into two parallel and co-evolving linear features during
the later phases of the lifetime of the PMJ. We conclude that mass flows can
play only limited role in the onset phase of PMJs and that it is more likely
that we see the effect of a fast heating front. | astro-ph_SR |
High-cadence spectroscopy of M-dwarfs. I. Analysis of systematic effects
in HARPS-N line profile measurements on the bright binary GJ 725A+B: Understanding the sources of instrumental systematic noise is a must to
improve the design of future spectrographs. In this study, we alternated
observations of the well-suited pair of M-stars GJ 725A+B to delve into the
sub-night HARPS-N response. Besides the possible presence of a low-mass planet
orbiting GJ 725B, our observations reveal changes in the spectral energy
distribution (SED) correlated with measurements of the width of the
instrumental line profile and, to a lower degree, with the Doppler
measurements. To study the origin of these effects, we searched for
correlations among several quantities defined and measured on the spectra and
on the acquisition images.
We find that the changes in apparent SED are very likely related to flux
losses at the fibre input. Further tests indicate that such flux losses do not
seriously affect the shape of the instrumental point spread function of
HARPS-N, but identify an inefficient fitting of the continuum as the most
likely source of the systematic variability observed in the FWHM. This index,
accounting for the HARPS-N cross-correlation profiles width, is often used to
decorrelate Doppler time-series. We show that the Doppler measurement obtained
by a parametric least-squares fitting of the spectrum accounting for continuum
variability is insensitive to changes in the slope of the SED, suggesting that
forward modeling techniques to measure moments of the line profile are the
optimal way to achieve higher accuracy. Remaining residual variability at ~1
m/s suggests that for M-stars Doppler surveys the current noise floor still has
an instrumental origin. | astro-ph_SR |
Statistical Analysis of the Horizontal Divergent Flow in Emerging Solar
Active Regions: Solar active regions (ARs) are thought to be formed by magnetic fields from
the convection zone. Our flux emergence simulations revealed that a strong
horizontal divergent flow (HDF) of unmagnetized plasma appears at the
photosphere before the flux begins to emerge. In our earlier study, we analyzed
HMI data for a single AR and confirmed presence of this precursor plasma flow
in the actual Sun. In this paper, as an extension of our earlier study, we
conducted a statistical analysis of the HDFs to further investigate their
characteristics and better determine the properties. From SDO/HMI data, we
picked up 23 flux emergence events over a period of 14 months, the total flux
of which ranges from 10^{20} to 10^{22} Mx. Out of 23 selected events, 6 clear
HDFs were detected by the method we developed in our earlier study, and 7 HDFs
detected by visual inspection were added to this statistic analysis. We found
that the duration of the HDF is on average 61 minutes and the maximum HDF speed
is on average 3.1 km s^{-1}. We also estimated the rising speed of the
subsurface magnetic flux to be 0.6-1.4 km s^{-1}. These values are highly
consistent with our previous one-event analysis as well as our simulation
results. The observation results lead us to the conclusion that the HDF is
rather a common feature in the earliest phase of AR emergence. Moreover, our
HDF analysis has capability of determining the subsurface properties of
emerging fields that cannot be directly measured. | astro-ph_SR |
The inner environment of protoplanetary disks with near infrared
spectro-interferometry: In this paper, I review how optical spectro-interferometry has become a
particularly well suited technique to study the close environment of young
stars, by spatially resolving both their IR continuum and line emission
regions. I summarize in which ways optical interferometers have brought major
insights about our understanding of the inner part of circumstellar disks, a
region in which the first stages of planet formation are thought to occur. In
particular, I emphasize how new methods are now enabling to probe the hot gas
emission, in addition to the circumstellar dust. | astro-ph_SR |
The rotation rate of solar active and ephemeral regions -- II. Temporal
variations of the rotation rates: Systematic studies of the rotation rate of sunspot groups using white-light
images yield controversial results on the variations of the rotation rate:
sunspot groups were found to either accelerate or decelerate systematically.
This disagreement might be related to shortcomings of the method used to probe
the rotation rate of sunspot groups. In contrast to previous works, in this
study we use magnetic field maps to analyse the variations of the rotation rate
of active regions. We found that an active region may exhibit either
acceleration or deceleration during the emergence while the rotation rate
remains almost unchanged during decay. Hence, we suppose that there is no
systematic geometrical inclination to the radial direction of the apex of the
subsurface magnetic flux loop forming an active region. A thorough comparison
of the rotation rate of unipolar and bi/multipolar active regions revealed no
significant changes in the rotation rate of decaying active regions. In
contrast to previous works, we presume the rotation rate to keep constant
(within the expected uncertainties) during the evolution of an active region
after emergence. | astro-ph_SR |
A new ephemeris and an orbital solution of ε Aurigae: We collected rich series of RV measurements covering last 110 years and
photometric observations from the past 6 primary eclipses, complemented them by
our new observations and derived a new precise ephemeris and an orbital
solution of epsilon Aur. | astro-ph_SR |
Flux-Rope Twist in Eruptive Flares and CMEs: due to Zipper and
Main-Phase Reconnection: The nature of three-dimensional reconnection when a twisted flux tube erupts
during an eruptive flare or coronal mass ejection is considered. The
reconnection has two phases: first of all, 3D "zipper reconnection" propagates
along the initial coronal arcade, parallel to the polarity inversion line
(PIL), then subsequent quasi-2D "main phase reconnection" in the low corona
around a flux rope during its eruption produces coronal loops and chromospheric
ribbons that propagate away from the PIL in a direction normal to it.
One scenario starts with a sheared arcade: the zipper reconnection creates a
twisted flux rope of roughly one turn ($2\pi$ radians of twist), and then main
phase reconnection builds up the bulk of the erupting flux rope with a
relatively uniform twist of a few turns. A second scenario starts with a
pre-existing flux rope under the arcade. Here the zipper phase can create a
core with many turns that depend on the ratio of the magnetic fluxes in the
newly formed flare ribbons and the new flux rope. Main phase reconnection then
adds a layer of roughly uniform twist to the twisted central core. Both phases
and scenarios are modeled in a simple way that assumes the initial magnetic
flux is fragmented along the PIL. The model uses conservation of magnetic
helicity and flux, together with equipartition of magnetic helicity, to deduce
the twist of the erupting flux rope in terms the geometry of the initial
configuration.
Interplanetary observations show some flux ropes have a fairly uniform twist,
which could be produced when the zipper phase and any pre-existing flux rope
possess small or moderate twist (up to one or two turns). Other interplanetary
flux ropes have highly twisted cores (up to five turns), which could be
produced when there is a pre-existing flux rope and an active zipper phase that
creates substantial extra twist. | astro-ph_SR |
The nature of Elsässer variables in compressible MHD: The Els\"{a}sser variables are often used in studies of plasma turbulence, in
helping differentiate between MHD waves propagating parallel or anti-parallel
to the main magnetic field. While for pure Alfv\'en waves in a homogeneous
plasma the method is strictly valid, we show that compressible, magnetoacoustic
waves are in general described by both Els\"{a}sser variables. Furthermore, in
a compressible and inhomogeneous plasma, the pure MHD waves (Alfv\'en, fast and
slow) are no longer normal modes, but waves become linearly coupled or display
mixed properties of Alfv\'en and magnetoacoustic nature. These waves are
necessarily described by both Els\"{a}sser variables and therefore the
Els\"{a}sser formalism cannot be used to strictly separate parallel and
anti-parallel propagating waves. Nevertheless, even in an inhomogeneous plasma,
for a highly Alfv\'enic wave the Els\"{a}sser variable corresponding to the
propagation direction appears still dominating. We suggest that for Alfv\'enic
waves, the relative amplitude of Els\"{a}sser variables depends on the local
degree of inhomogeneity and other plasma and wave properties. This finding has
implications for turbulence studies in inhomogeneous and compressible plasmas,
such as the solar corona and solar wind. | astro-ph_SR |
Summary of IAU GA SpS5 - I. Obscured and distant clusters: This first part of Special Session 5 explored the current status of
infrared-based observations of obscured and distant stellar clusters in the
Milky Way galaxy. Recent infrared surveys, either serendipitously or using
targeted searches, have uncovered a rich population of young and massive
clusters. However, cluster characterization is more challenging as it must be
obtained often entirely in the infrared due to high line-of-sight extinction.
Despite this, much is to be gained through the identification and careful
analysis of these clusters, as they allow for the early evolution of massive
stars to be better constrained. Further, they act as beacons delineating the
Milky Way's structure and as nearby, resolved analogues to the distant
unresolved massive clusters studied in distant galaxies. | astro-ph_SR |
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