text
stringlengths 89
2.49k
| category
stringclasses 19
values |
|---|---|
SN 2011ht: Confirming a Class of Interacting Supernovae with Plateau
Light Curves (Type IIn-P): We present photometry and spectroscopy of the Type IIn supernova (SN) 2011ht,
identified previously as a SN impostor. The light curve exhibits an abrupt
transition from a well-defined ~120 day plateau to a steep bolometric decline.
Leading up to peak brightness, a hot emission-line spectrum exhibits signs of
interaction with circumstellar material (CSM), in the form of relatively narrow
P-Cygni features of H I and He I superimposed on broad Lorentzian wings. For
the remainder of the plateau phase the spectrum exhibits strengthening P-Cygni
profiles of Fe II, Ca II, and H-alpha. By day 147, after the plateau has ended,
the SN entered the nebular phase, heralded by the appearance of forbidden
transitions of [O I], [O II], and [Ca II] over a weak continuum. At this stage,
the light curve exhibits a low luminosity that is comparable to that
sub-luminous Type II-P supernovae, and a relatively fast visual-wavelength
decline that is significantly steeper than the Co-56 decay rate. However, the
total bolometric decline, including the IR luminosity, is consistent with Co-56
decay, and implies a low Ni-56 mass of ~0.01 M(Sun). We therefore characterize
SN 2011ht as a bona-fide core-collapse SN very similar to the peculiar SNe IIn
1994W and 2009kn. These three SNe define a subclass, which are Type IIn based
on their spectrum, but that also exhibit well-defined plateaus and produce low
Ni-56 yields. We therefore suggest Type IIn-P as a name for this subclass.
Possible progenitors of SNe IIn-P, consistent with the available data, include
8-10 M(Sun) stars, which undergo core collapse as a result of electron capture
after a brief phase of enhanced mass loss, or more massive M>25 M(Sun)
progenitors, which experience substantial fallback of the metal-rich
radioactive ejecta. In either case, the energy radiated by these three SNe
during their plateau must be dominated by CSM interaction (abridged). | astro-ph_SR |
On the rotation periods of the components of the triple system
TYC9300-0891-1AB/TYC9300-0529-1 in the Octans Association: Stellar rotation depends on different parameters. The range of values of
these parameters causes the dispersion in the rotation period distributions
observed in young stellar clusters/associations. We focus our investigation on
the effects of different circumstellar environments on stellar rotation. More
specifically, we are searching in stellar Associations for visual triple
systems where all stellar parameters are similar, with the only exceptions of
the unknown initial rotation period, and of the circum-stellar environment, in
the sense that one of the two about equal-mass components has a close-by third
'perturber' component. In the present study we analyse the 35-Myr old visual
triple system TYC 9300-0891-1AB + TYC 9300-0529-1 in the young Octans stellar
association consisting of three equal-mass K0V components. We collected from
the literature all information that allowed us to infer that the three
components are actually physically bound forming a triple system and are
members of the Octans Association. We collected broad-band photometric
timeseries in two observation seasons. We discovered that all the components
are variable, magnetically active, and from periodogram analysis we found the
unresolved components TYC 9300-0891-1AB to have a rotation period P = 1.383d
and TYC 9300-0529-1 a rotation period P = 1.634d. TYC 9300-0891-1A, TYC
9300-0891-1B, and TYC 9300-0529-1 have same masses, ages, and initial chemical
compositions. The relatively small 16% rotation period difference measured by
us indicates that all components had similar initial rotation periods and disc
lifetimes, and the separation of 157AU between the component A and the
'perturber' component B (or vice-versa) has been sufficiently large to prevent
any significant perturbation/shortening of the accretion-disc lifetime. | astro-ph_SR |
Unveiling the nature of red novae cool explosions using archive plate
photometry: Based on archive photographic photometry and recent CCD photometric data for
red novae V4332 Sgr and V838 Mon, we established their stellar composition,
exploded components, and the nature of explosions. Low temperature in the
outburst maximum is due to quasi-adiabatic expansion of a massive stellar
envelope after the central energy surge preceded the outburst. | astro-ph_SR |
Period-Colour and Amplitude-Colour relations for OGLE-$δ$ Scuti
stars in the Galactic Bulge and LMC: We present an analysis on the behaviour of the Galactic bulge and the Large
Magellanic Cloud (LMC) $\delta$ Scuti stars in terms of period-colour and
amplitude-colour (PCAC) relations at maximum, mean and minimum light. The
publicly available Optical Gravitational Lensing Experiment-IV (OGLE-IV) light
curves for Galactic bulge and OGLE-III light curves for LMC $\delta$ Scuti
stars are exploited for the analysis. It has been found that the Galactic bulge
$\delta$ Scuti stars obey flat PC relations at maximum/mean/minimum light while
the LMC $\delta$ Scutis have sloped/sloped/flat PC relations at
maximum/mean/minimum light. Both the Galactic bulge and the LMC $\delta$ Scutis
have sloped/flat/sloped AC relations at maximum/mean/minimum. These relations
also show that Galactic $\delta$ Scutis are hotter as compared to their LMC
counterparts. The period-amplitude (PA) relations for $\delta$ Scutis exhibit
different behaviour in the Galactic bulge and the LMC. The LMC variables are
found to have higher amplitudes at a given period. The amplitude of the
Galactic bulge $\delta$ Scuti shows a bimodal distribution which can be
modelled using a two-component Gaussian Mixture Model: one component with a
lower amplitude and another with a higher amplitude. The observed behaviour of
the $\delta$ Scuti PCAC relations can be explained using the theory of the
interaction of hydrogen ionization front (HIF) and stellar photosphere as well
as the PA diagram. We use MESA-RSP to calculate theoretical non-linear
hydrodynamical pulsation models for $\delta$ Scuti stars with input
metallicities of $Z=0.02$ and $Z=0.008$ appropriate for the Galactic bulge and
LMC, respectively. The observed PCAC relations and theoretical calculations
support the HIF-photosphere interaction theory. | astro-ph_SR |
Brightness Oscillations in Models of Young Binary Systems with Low-Mass
Secondary Components: We consider a model for the cyclic brightness variations of a young star with
a low-mass companion that accretes matter from the remnants of a protostellar
cloud. At small inclinations of the binary orbit to the line of sight, the
streams of matter and the density waves excited in the circumbinary disk can
screen the primary component of the binary from the observer. To study these
phenomena, we have computed grids of hydrodynamic models for binary systems by
the SPH method based on which we have constructed the phase light curves as a
function of the rotation angle of the apsidal line relative to the observer.
The model parameters were varied within the following ranges: the component
mass ratio q = 0.01-0.1 and the eccentricity e = 0-0.5. We adopted optical
grain characteristics typical of circumstellar dust. Our computations have
shown that the brightness oscillations with orbital phase can have a complex
structure. The amplitudes and shapes of the light curves depend strongly on the
inclination of the binary orbit and its orientation relative to the observer
and on the accretion rate. The results of our computations are used to analyze
the cyclic activity of UX Ori stars. | astro-ph_SR |
The $κ$-cookbook: a novel generalizing approach to unify
$κ$-like distributions for plasma particle modeling: In the literature different so-called $\kappa$-distribution functions are
discussed to fit and model the velocity (or energy) distributions of solar wind
species, pickup ions or magnetospheric particles. Here we introduce a
generalized (isotropic) $\kappa$-distribution as a "cookbook", which admits as
special cases, or "recipes", all the other known versions of $\kappa$-models. A
detailed analysis of the generalized distribution function is performed,
providing general analytical expressions for the velocity moments, Debye
length, and entropy, and pointing out a series of general requirements that
plasma distribution functions should satisfy. From a contrasting analysis of
the recipes found in the literature, we show that all of them lead to almost
the same macroscopic parameters with a small standard deviation between them.
However, one of these recipes called the regularized $\kappa$-distribution
provides a functional alternative for macroscopic parameterization without any
constraint for the power-law exponent $\kappa$. | astro-ph_SR |
Two-sided-loop jets associated with magnetic reconnection between
emerging loops and twisted filament threads: Coronal jets are always produced by magnetic reconnection between emerging
flux and pre-existing overlying magnetic fields. When the overlying field is
vertical/obilique or horizontal, the coronal jet will appear as anemone type or
two-sided-loop type. Most of observational jets are of the anemone type, and
only a few of two-sided-loop jets have been reported. Using the high-quality
data from New Vacuum Solar Telescope, Interface Region Imaging Spectrograph,
and Solar Dynamics Observatory, we present an example of two-sided-loop jets
simultaneously observed in the chromosphere, transition region, and corona. The
continuous emergence of magnetic flux brought in successively emerging of
coronal loops and the slowly rising of an overlying horizontal filament
threads. Sequentially, there appeared the deformation of the loops, the
plasmoids ejection from the loop top, and pairs of loop brightenings and jet
moving along the untwisting filament threads. All the observational results
indicate there exist magnetic reconnection between the emerging loops and
overlying horizontal filament threads, and it is the first example of
two-sided-loop jets associated with ejected plasmoids and twisted overlying
fields. | astro-ph_SR |
Survey of multiple populations in globular clusters among very low-mass
stars: Recent work has shown that NIR Hubble Space Telescope (HST) photometry allows
us to disentangle multiple populations (MPs) among M dwarfs of globular
clusters (GCs) and investigate this phenomenon in very low-mass (VLM) stars.
Here, we present the color-magnitude diagrams (CMDs) of nine GCs and the open
cluster NGC 6791 in the F110W and F160W bands of HST, showing that the main
sequences (MSs) below the knee are either broadened or split thus providing
evidence of MPs among VLM stars. In contrast, the MS of NGC 6791 is consistent
with a single population. The color distribution of M-dwarfs dramatically
changes between different GCs and the color width correlates with the cluster
mass. We conclude that the MP ubiquity, variety, and dependence on GC mass are
properties common to VLM and more-massive stars. We combined UV, optical, and
NIR observations of NGC 2808 and NGC 6121 (M 4) to identify MPs along with a
wide range of stellar masses (~ 0.2 - 0.8M ), from the MS turn off to the VLM
regime, and measured, for the first time, their mass functions (MFs). We find
that the fraction of MPs does not depend on the stellar mass and that their MFs
have similar slopes. These findings indicate that the properties of MPs do not
depend on stellar mass. In a scenario where the second generations formed in
higher-density environments than the first generations, the possibility that
the MPs formed with the same initial MF would suggest that it does not depend
on the environment. | astro-ph_SR |
Data processing of Visible Emission Line Coronagraph Onboard ADITYA L1: ADITYA-L1 is India's first dedicated mission to observe the sun and its
atmosphere from a halo orbit around L1 point. Visible emission line coronagraph
(VELC) is the prime payload on board at Aditya-L1 to observe the sun's corona.
VELC is designed as an internally occulted reflective coronagraph to meet the
observational requirements of wide wavelength band and close to the solar limb
(1.05 Ro). Images of the solar corona in continuum and spectra in three
emission lines 5303{\AA} [Fe xiv], 7892{\AA} [Fe xi] and 10747 [Fe xiii]
obtained with high cadence to be analyzed using software algorithms
automatically. A reasonable part of observations will be made in synoptic mode,
those, need to be analyzed and results made available for public use. The
procedure involves the calibration of instrument and detectors, converting the
images into fits format, correcting the images and spectra for the instrumental
effects, align the images etc. Then, develop image processing algorithms to
detect the occurrence of energetic events using continuum images. Also derive
physical parameters, such as temperature and velocity structure of solar corona
using emission line observations. Here, we describe the calibration of
detectors and the development of software algorithms to detect the occurrence
of CMEs and analyze the spectroscopic data. | astro-ph_SR |
A large stellar evolution database for population synthesis studies. V.
Stellar models and isochrones with CNONa abundance anticorrelations: We present a new grid of stellar models and isochrones for old stellar
populations, covering a large range of [Fe/H] values, for an heavy element
mixture characterized by CNONa abundance anticorrelations as observed in
Galactic globular cluster stars. The effect of this metal abundance pattern on
the evolutionary properties of low mass stars, from the main sequence to the
horizontal branch phase is analyzed. We perform comparisons between these new
models, and our reference alpha-enhanced calculations, and discuss briefly
implications for CMDs showing multiple main sequence or subgiant branches. A
brief qualitative discussion of the effect of CN abundances on color-T_{eff}
transformations is also presented, highlighting the need to determine
theoretical color transformations for the appropriate metal mixture, if one
wants to interpret observations in the Stroemgren system, or broadband filters
blueward of the Johnson V-band. | astro-ph_SR |
Chromospheric Activity in 55 Cancri: II. Theoretical Wave Studies versus
Observations: In this study, we consider chromospheric heating models for 55 Cancri in
conjunction with observations. The theoretical models, previously discussed in
Paper I, are self-consistent, nonlinear and time-dependent ab-initio
computations encompassing the generation, propagation, and dissipation of
waves. Our focus is the consideration of both acoustic waves and longitudinal
flux tube waves amounting to two-component chromosphere models. 55 Cancri, a
K-type orange dwarf, is a star of low activity, as expected by its age, which
also implies a relatively small magnetic filling factor. The Ca II K fluxes are
computed (multi-ray treatment) assuming partial redistribution and
time-dependent ionization. The theoretical Ca II H+K fluxes are subsequently
compared with observations. It is found that for stages of lowest chromospheric
activity the observed Ca II fluxes are akin, though not identical, to those
obtained by acoustic heating, but agreement can be obtained if low levels of
magnetic heating - consistent with the assumed photospheric magnetic filling
factor - are considered as an additional component; this idea is in alignment
with previous proposals conveyed in the literature. | astro-ph_SR |
Exploring B4: A Pulsating sdB star, in a Binary, in the Open Cluster NGC
6791: We report on Kepler photometry of the hot sdB star B4 in the open cluster NGC
6791. We confirm that B4 is a reflection effect binary with an sdB component
and a low-mass main sequence companion with a circular 0.3985 d orbit. The sdB
star is a g-mode pulsator (a V1093 Her star) with periods ranging from 2384 s
to 7643 s. Several of the pulsation modes show symmetric splitting by 0.62
microHz. Attributing this to rotational splitting, we conclude that the sdB
component has a rotation period of approximately 9.63 d, indicating that tidal
synchronization has not been achieved in this system. Comparison with
theoretical synchronization time provides a discriminant between various
theoretical models. | astro-ph_SR |
Metallicities from high resolution spectra of 49 RR Lyrae Variables: Accurate metallicities of RR Lyrae are extremely important in constraining
period-luminosity-metallicity relationships (PLZ), particularly in the
near-infrared. We analyse 69 high-resolution spectra of Galactic RR Lyrae stars
from the Southern African Large Telescope (SALT). We measure metallicities of
58 of these RR Lyrae stars with typical uncertainties of 0.13 dex. All but one
RR Lyrae in this sample has accurate ({\sigma}_parallax ~ 10%) parallax from
Gaia. Combining these new high resolution spectroscopic abundances with similar
determinations from the literature for 93 stars, we present new PLZ
relationships in WISE W1 and W2 magnitudes, and the Wesenheit magnitudes
W(W1,V-W1) and W(W2,V-W2). | astro-ph_SR |
Observation of "Topological" Microflares in the Solar Atmosphere: We report on observation of the unusual kind of solar microflares, presumably
associated with the so-called "topological trigger" of magnetic reconnection,
which was theoretically suggested long time ago by Gorbachev et al. (Sov. Ast.
1988, v.32, p.308) but has not been clearly identified so far by observations.
As can be seen in pictures by Hinode SOT in CaII line, there may be a bright
loop connecting two sunspots, which looks at the first sight just as a magnetic
field line connecting the opposite poles. However, a closer inspection of SDO
HMI magnetograms shows that the respective arc is anchored in the regions of
the same polarity near the sunspot boundaries. Yet another peculiar feature is
that the arc flashes almost instantly as a thin strip and then begins to expand
and decay, while the typical chromospheric flares in CaII line are much wider
and propagate progressively in space. A qualitative explanation of the unusual
flare can be given by the above-mentioned model of topological trigger. Namely,
there are such configurations of the magnetic sources on the surface of
photosphere that their tiny displacements result in the formation and fast
motion of a 3D null point along the arc located well above the plane of the
sources. So, such a null point can quickly ignite a magnetic reconnection along
the entire its trajectory. Pictorially, this can be presented as flipping the
so-called two-dome magnetic-field structure (which is just the reason why such
mechanism was called topological). The most important prerequisite for the
development of topological instability in the two-dome structure is a cruciform
arrangement of the magnetic sources in its base, and this condition is really
satisfied in the case under consideration. | astro-ph_SR |
Investigation of the Progenitors of the Type Ia Supernovae Associated
With the LMC Supernova Remnants 0505-67.9 and 0509-68.7: Although Type Ia supernovae have been heavily scrutinized due to their use in
making cosmological distance estimates, we are still unable to definitively
identify the progenitors for the entire population. While answers have been
presented for certain specific systems, a complete solution remains elusive. We
present observations of two supernova remnants (SNRs) in the Large Magellanic
Cloud, SNR 0505-67.9 and SNR 0509-68.7, for which we have identified the center
of the remnant and the 99.73% containment central region in which any companion
star left over after the supernova must be located. Both remnants have a number
of potential ex-companion stars near their centers; all possible single and
double degenerate progenitor models remain viable for these two supernovae.
Future observations may be able to identify the true ex-companions for both
remnants. | astro-ph_SR |
Models for Metal-Poor Stars with Different Initial Abundances of C, N,
O, Mg, and Si. III. Grids of Isochrones for $-2.5 \le$ [Fe/H] $\le -0.5$ and
Helium Abundances $Y = 0.25$ and $0.29$ at Each Metallicity: Stellar evolutionary tracks for $0.12 \le M/M_\odot \le 1.0$ have been
computed for each of several variations in the abundances of C, N, and O,
assuming mass-fraction helium abundances $Y = 0.25$ and $0.29$, and 11
metallicities in the range $-1.5 \le$ [Fe/H] $\le -0.5$, in 0.2 dex increments.
Such computations are provided for mixtures with [O/Fe] between $+0.4$ and
$+0.8$, for different C:N:O ratios at a fixed value of [CNO/Fe], and for
enhanced C. Computer codes are provided to interpolate within these grids to
produce isochrones for ages $> 7$ Gyr and to generate magnitudes and colours
for many broad-band filters using bolometric corrections based on MARCS model
atmospheres and synthetic spectra. The models are compared with (i) similar
computations produced by other workers, (ii) observed UV, optical, and IR
colour-magnitude diagrams (CMDs), (iii) the effective temperatures, $(V-I_C)_0$
and $(V-K_S)_0$ colours of Pop. II stars in the solar neighbourhood, and (iv)
empirical data for the absolute magnitude of the tip of the giant branch
(TRGB). The isochrones are especially successful in reproducing the observed
morphologies of observed CMDs and in satisfying the TRGB constraints. They also
fare quite well in explaining the IR colours of low mass stars in globular
clusters, indicating that they have [O/Fe] $\approx +0.6$, though some
challenges remain. | astro-ph_SR |
Towards solar measurements of nuclear reaction rates: Nuclear reaction rates are a fundamental yet uncertain ingredient in stellar
evolution models. The astrophysical S-factor pertaining to the initial reaction
in the proton-proton chain is uncertain at the 1% level, which contributes a
systematic but generally unpropagated error of similar order in the theoretical
ages of stars. In this work, we study the prospect of improving the measurement
of this and other reaction rates in the pp chain and CNO cycle using
helioseismology and solar neutrinos. We show that when other aspects of the
solar model are improved, then it shall be possible using current solar data to
improve the precision of this measurement by nearly an order of magnitude, and
hence the corresponding uncertainty on the ages of low-mass stars by a similar
amount. | astro-ph_SR |
Revisiting the Cygnus OB associations: OB associations play an important role in Galactic evolution, though their
origins and dynamics remain poorly studied, with only a small number of systems
analysed in detail. In this paper we revisit the existence and membership of
the Cygnus OB associations. We find that of the historical OB associations only
Cyg OB2 and OB3 stand out as real groups. We search for new OB stars using a
combination of photometry, astrometry, evolutionary models and an SED fitting
process, identifying 4680 probable OB stars with a reliability of $>$90\%. From
this sample we search for OB associations using a new and flexible clustering
technique, identifying 6 new OB associations. Two of these are similar to the
associations Cyg OB2 and OB3, though the others bear no relationship to any
existing systems. We characterize the properties of the new associations,
including their velocity dispersions and total stellar masses, all of which are
consistent with typical values for OB associations. We search for evidence of
expansion and find that all are expanding, albeit anistropically, with stronger
and more significant expansion in the direction of Galactic longitude. We also
identify two large-scale (160 pc and 25 km s$^{-1}$) kinematic expansion
patterns across the Cygnus region, each including three of our new
associations, and attribute this to the effects of feedback from a previous
generation of stars. This work highlights the need to revisit the existence and
membership of the historical OB associations, if they are to be used to study
their properties and dynamics. | astro-ph_SR |
A stellar relic filament in the Orion star forming region: We report the discovery of the oldest stellar substructure in the Orion star
forming region (OSFR), the Orion relic filament. The relic filament is
physically associated with the OSFR as demonstrated by Gaia DR2 photometry and
astrometry, as well as targeted radial velocity follow-up observations of a
bright sub-sample of proper-motion selected candidate members. Gaia DR2
parallaxes place the Orion relic filament in the more distant part of the OSFR,
approx. 430pc from the Sun. Given its age, velocity dispersion, spatial extent,
and shape, it is not possible for the Orion relic filament to have formed as a
single stellar cluster, even taking into account residual gas expulsion. The
relic filament is also too young to be a tidal stream, since Galactic tides act
on much longer time scales of order 100 Myr. It therefore appears likely that
the structure formed from a molecular cloud filament similar to Orion A in the
OSFR and retained its morphology despite decoupling from its natal gas. Hence,
the Orion relic filament bears witness to the short-lived evolutionary phase
between gas removal and dispersion due to shears and tides, and provides
crucial new insights into how stars are formed in molecular clouds. | astro-ph_SR |
Detailed opacity calculations for astrophysical applications: Nowadays, several opacity codes are able to provide data for stellar
structure models, but the computed opacities may show significant differences.
In this work, we present state-of-the-art precise spectral opacity
calculations, illustrated by stellar applications. The essential role of
laboratory experiments to check the quality of the computed data is underlined.
We review some X-ray and XUV laser and Z-pinch photo-absorption measurements as
well as X-ray emission spectroscopy experiments involving hot dense plasmas
produced by ultra-high-intensity laser irradiation. The measured spectra are
systematically compared with the fine-structure opacity code SCO-RCG. Focus is
put on iron, due to its crucial role in understanding asteroseismic
observations of $\beta$ Cephei-type and Slowly Pulsating B stars, as well as of
the Sun. For instance, in $\beta$ Cephei-type stars, the iron-group opacity
peak excites acoustic modes through the "kappa-mechanism". A particular
attention is paid to the higher-than-predicted iron opacity measured at the
Sandia Z-machine at solar interior conditions. We discuss some theoretical
aspects such as density effects, photo-ionization, autoionization or the
"filling-the-gap" effect of highly excited states. | astro-ph_SR |
A $ΔR\sim 9.5$ mag Super Flare of An Ultracool Star Detected by
$\text{SVOM/GWAC}$ System: In this paper, we report the detection and follow-ups of a super stellar
flare GWAC\,181229A with an amplitude of $\Delta R\sim$9.5 mag on a M9 type
star by $\text{SVOM/GWAC}$ and the dedicated follow-up telescopes. The
estimated bolometric energy $E_{bol}$ is $(5.56-9.25)\times10^{34}$ ergs, which
places the event to be one of the most powerful flares on ultracool stars. The
magnetic strength is inferred to be (3.6-4.7) kG. Thanks to the sampling with a
cadence of 15 seconds, a new component near the peak time with a very steep
decay is detected in the $R$-band light curve, followed by the two-component
flare template given by Davenport et al. (2014). An effective temperature of
$5340\pm40$ K is measured by a blackbody shape fitting to the spectrum at the
shallower phase during the flare. The filling factors of the flare are
estimated to be $\sim$30\% and 19\% at the peak time and at 54 min after the
first detection. The detection of the particular event with large amplitude,
huge-emitted energy and a new component demonstrates that a high cadence sky
monitoring cooperating with fast follow-up observations is very essential for
understanding the violent magnetic activity. | astro-ph_SR |
Extended radio emission associated with a breakout eruption from the
back side of the Sun: Context. Coronal mass ejections (CMEs) on the Sun are the largest explosions
in the Solar System that can drive powerful plasma shocks. The eruptions,
shocks, and other processes associated to CMEs are efficient particle
accelerators and the accelerated electrons in particular can produce radio
bursts through the plasma emission mechanism. Aims. Coronal mass ejections and
associated radio bursts have been well studied in cases where the CME
originates close to the solar limb or within the frontside disc. Here, we study
the radio emission associated with a CME eruption on the back side of the Sun
on 22 July 2012. Methods. Using radio imaging from the Nan\c{c}ay
Radioheliograph, spectroscopic data from the Nan\c{c}ay Decametric Array, and
extreme-ultraviolet observations from the Solar Dynamics Observatory and Solar
Terrestrial Relations Observatory spacecraft, we determine the nature of the
observed radio emission as well as the location and propagation of the CME.
Results. We show that the observed low-intensity radio emission corresponds to
a type II radio burst or a short-duration type IV radio burst associated with a
CME eruption due to breakout reconnection on the back side of the Sun, as
suggested by the pre-eruptive magnetic field configuration. The radio emission
consists of a large, extended structure, initially located ahead of the CME,
that corresponds to various electron acceleration locations. Conclusions. The
observations presented here are consistent with the breakout model of CME
eruptions. The extended radio emission coincides with the location of the
current sheet and quasi-separatrix boundary of the CME flux and the overlying
helmet streamer and also with that of a large shock expected to form ahead of
the CME in this configuration. | astro-ph_SR |
Spectroscopic subsystems in nearby wide binaries: Radial velocity (RV) monitoring of solar-type visual binaries has been
conducted at the CTIO/SMARTS 1.5-m telescope to study short-period systems.
Data reduction is described, mean and individual RVs of 163 observed objects
are given. New spectroscopic binaries are discovered or suspected in 17
objects, for some of them orbital periods could be determined. Subsystems are
efficiently detected even in a single observation by double lines and/or by the
RV difference between the components of visual binaries. The potential of this
detection technique is quantified by simulation and used for statistical
assessment of 96 wide binaries within 67pc. It is found that 43 binaries
contain at least one subsystem and the occurrence of subsystems is equally
probable in either primary or secondary components. The frequency of subsystems
and their periods match the simple prescription proposed by the author (2014,
AJ, 147, 87). The remaining 53 simple wide binaries with a median projected
separation of 1300AU have the distribution of the RV difference between their
components that is not compatible with the thermal eccentricity distribution
f(e)=2e but rather matches the uniform eccentricity distribution. | astro-ph_SR |
A catalogue of 323 cataclysmic variables from LAMOST DR6: In this work, we present a catalog of cataclysmic variables (CVs) identified
from the Sixth Data Release (DR6) of the Large Sky Area Multi-Object Fiber
Spectroscopic Telescope (LAMOST). To single out the CV spectra, we introduce a
novel machine-learning algorithm called UMAP to screen out a total of 169,509
H$\alpha$-emission spectra, and obtain a classification accuracy of the
algorithm of over 99.6$\%$ from the cross-validation set. We then apply the
template matching program PyHammer v2.0 to the LAMOST spectra to obtain the
optimal spectral type with metallicity, which helps us identify the
chromospherically active stars and potential binary stars from the 169,509
spectra. After visually inspecting all the spectra, we identify 323 CV
candidates from the LAMOST database, among them 52 objects are new. We further
discuss the new CV candidates in subtypes based on their spectral features,
including five DN subtype during outbursts, five NL subtype and four magnetic
CVs (three AM Her type and one IP type). We also find two CVs that have been
previously identified by photometry, and confirm their previous classification
by the LAMOST spectra. | astro-ph_SR |
Forward asteroseismic modeling of stars with a convective core from
gravity-mode oscillations: parameter estimation and stellar model selection: We propose a methodological framework to perform forward asteroseismic
modeling of stars with a convective core, based on gravity-mode oscillations.
These probe the near-core region in the deep stellar interior. The modeling
relies on a set of observed high-precision oscillation frequencies of
low-degree coherent gravity modes with long lifetimes and their observational
uncertainties. Identification of the mode degree and azimuthal order is assumed
to be achieved from rotational splitting and/or from period spacing patterns.
This paper has two major outcomes. The first is a comprehensive list and
discussion of the major uncertainties of theoretically predicted gravity-mode
oscillation frequencies based on linear pulsation theory, caused by fixing
choices of the input physics for evolutionary models. Guided by a hierarchy
among these uncertainties of theoretical frequencies, we subsequently provide a
global methodological scheme to achieve forward asteroseismic modeling. We
properly take into account correlations amongst the free parameters included in
stellar models. Aside from the stellar mass, metalicity and age, the major
parameters to be estimated are the near-core rotation rate, the amount of
convective core overshooting, and the level of chemical mixing in the radiative
zones. This modeling scheme allows for maximum likelihood estimation of the
stellar parameters for fixed input physics of the equilibrium models, followed
by stellar model selection considering various choices of the input physics.
Our approach uses the Mahalanobis distance instead of the often used $\chi^2$
statistic and includes heteroscedasticity. It provides estimation of the
unknown variance of the theoretically predicted oscillation frequencies. | astro-ph_SR |
The Gaia DR1 Mass-Radius Relation for White Dwarfs: The Gaia Data Release 1 (DR1) sample of white dwarf parallaxes is presented,
including 6 directly observed degenerates and 46 white dwarfs in wide binaries.
This data set is combined with spectroscopic atmospheric parameters to study
the white dwarf mass-radius relationship (MRR). Gaia parallaxes and G
magnitudes are used to derive model atmosphere dependent white dwarf radii,
which can then be compared to the predictions of a theoretical MRR. We find a
good agreement between Gaia DR1 parallaxes, published effective temperatures
(Teff) and surface gravities (log g), and theoretical MRRs. As it was the case
for Hipparcos, the precision of the data does not allow for the
characterisation of hydrogen envelope masses. The uncertainties on the
spectroscopic atmospheric parameters are found to dominate the error budget and
current error estimates for well-known and bright white dwarfs may be slightly
optimistic. With the much larger Gaia DR2 white dwarf sample it will be
possible to explore the MRR over a much wider range of mass, Teff, and spectral
types. | astro-ph_SR |
Sunspot splitting triggering an eruptive flare: We investigate how the splitting of the leading sunspot and associated flux
emergence and cancellation in active region NOAA 11515 caused an eruptive M5.6
flare on 2012 July 2. Our study employs multi-wavelength observations from HMI,
AIA and ChroTel. Emerging flux formed a neutral line ahead of the leading
sunspot and new satellite spots. The sunspot splitting caused a long-lasting
flow toward this neutral line, where a filament formed. Further flux emergence,
partly of mixed-polarity, as well as episodes of flux cancellation occurred
repeatedly at the neutral line. Following a nearby C-class precursor flare with
signs of interaction with the filament, the filament erupted nearly
simultaneously with the onset of the M5.6 flare and evolved into a coronal mass
ejection. The sunspot stretched without forming a light bridge, splitting
unusually fast (within about a day, complete approximately 6 hours after the
eruption) in two nearly equal parts. The front part separated strongly from the
active region to approach the neighbouring active region where all its coronal
magnetic connections were rooted. It also rotated rapidly (by 4.9 degree/hr)
and caused significant shear flows at its edge. The eruption resulted from a
complex sequence of processes in the (sub-)photosphere and corona. The
persistent flows toward the neutral line likely caused the formation of a flux
rope which held the filament. These flows and their associated flux
cancellation, the emerging flux, and the precursor flare all contributed to the
destabilization of the flux rope. We interpret the sunspot splitting as the
separation of two flux bundles differently rooted in the convection zone and
only temporarily joined in the spot. This explains the rotation as continued
rise of the separating flux and implies that at least this part of the sunspot
was still connected to its roots deep in the convection zone. | astro-ph_SR |
High Resolution Optical and NIR Spectra of HBC 722: We present the results of high resolution (R$\ge$30,000) optical and near-IR
spectroscopic monitoring observations of HBC 722, a recent FU Orionis object
that underwent an accretion burst in 2010. We observed HBC 722 in
optical/near-IR with the BOES, HET-HRS, and IGRINS spectrographs, at various
points in the outburst. We found atomic lines with strongly blueshifted
absorption features or P Cygni profiles, both evidence of a wind driven by the
accretion. Some lines show a broad double-peaked absorption feature, evidence
of disk rotation. However, the wind-driven and disk-driven spectroscopic
features are anti-correlated in time; the disk features became strong as the
wind features disappeared. This anti-correlation might indicate that the
rebuilding of the inner disk was interrupted by the wind pressure during the
first two years. The Half-Width at Half-Depth (HWHD) of the double-peaked
profiles decreases with wavelength, indicative of the Keplerian rotation; the
optical spectra with the disk feature are fitted by a G5 template stellar
spectrum convolved with a rotation velocity of 70 km s$^{-1}$ while the near-IR
disk features are fitted by a K5 template stellar spectrum convolved with a
rotation velocity of 50 km s$^{-1}$. Therefore, the optical and near-IR spectra
seem to trace the disk at 39 and 76 $\textit{R}_{\odot}$, respectively. We fit
a power-law temperature distribution in the disk, finding an index of 0.8,
comparable to optically thick accretion disk models. | astro-ph_SR |
OH maser toward IRAS 06056+2131: polarization parameters and evolution
status: We present high angular resolution observations of OH maser emission towards
the high-mass star forming region IRAS 06056+2131. The observations were
carried out using the UK radio interferometer array, Multi-Element Radio Linked
Interferometer Network (MERLIN) in the OH main lines at 1665- and 1667-MHz, in
addition to the OH satellite line at 1720-MHz. The results of this study
revealed emission in the 1665 MHz line with an estimated total intensity of
$\sim 4$ Jy. We did not detect any emission from the 1667-MHz and 1720-MHz
lines.
The full polarization mode of MERLIN enables us to investigate the magnetic
field in the OH maser region. Our results show that IRAS 06056+2131 is a highly
circularly polarized source. In this transition, a Zeeman pair is identified
from which a magnetic strength of $\sim -1.5$ mG is inferred. The orientation
of the linear polarization vectors suggests that the magnetic field lines at
the location of the OH maser emission \textbf{might be} in agreement with the
orientation of the outflow thought to be associated with this source. The star
forming evolutionary status of the embedded proto-stellar object is discussed. | astro-ph_SR |
The Mass of the Compact Object in the X-Ray Binary Her X-1/HZ Her: We have obtained the first estimates of the masses of the components of the
Her X-1/HZ Her X-ray binary system taking into account non-LTE effects in the
formation of the H_gamma absorption line: mx=1.8Msun and mv=2.5Msun. These mass
estimates were made in a Roche model based on the observed radial-velocity
curve of the optical star, HZ Her. The masses for the X-ray pulsar and optical
star obtained for an LTE model lie are mx=0.85\pm0.15Msun and
mv=1.87\pm0.13Msun. These mass estimates for the components of Her X-1/HZ Her
derived from the radial-velocity curve should be considered tentative. Further
mass estimates from high-precision observations of the orbital variability of
the absorption profiles in a non-LTE model for the atmosphere of the optical
component should be made. | astro-ph_SR |
Chemical mixing in low mass stars: I. Rotation against atomic diffusion
including radiative accelerations: When modelling stars with masses larger than 1.2Msun with no observed
chemical peculiarity, atomic diffusion is often neglected because, on its own,
it causes unrealistic surface abundances compared with those observed. The
reality is that atomic diffusion is in competition with other transport
processes. The purpose of this study is to quantify the opposite or conjugated
effects of atomic diffusion and rotationally induced mixing in stellar models
of low mass stars. Our second goal is to estimate the impact of neglecting both
rotational mixing and atomic diffusion in stellar parameter inferences for
stars with masses larger than 1.3Msun. Using the AIMS code, we infer the masses
and ages of a set of representative artificial stars for which models were
computed with the CESTAM evolution code, taking into account rotationally
induced mixing and atomic diffusion, including radiative accelerations. We show
that for masses lower than 1.3Msun, rotation dominates the transport of
chemical elements, and strongly reduces the effect of atomic diffusion, with
net surface abundance modifications similar to solar ones. At larger mass,
atomic diffusion and rotation are competing equally. Above 1.44Msun, atomic
diffusion dominates in stellar models with initial rotation smaller than
80km.s-1 producing a chemical peculiarity which is not observed in
Kepler-legacy stars. This indicates that a transport process of chemical
elements is missing. Importantly, neglecting rotation and atomic diffusion
(including radiative accelerations) in the models, when inferring the
parameters of F-type stars, may lead to errors of 5%, 2.5% and 25% respectively
for stellar masses, radii and ages. Atomic diffusion (including radiative
accelerations) and rotational mixing should be taken into account in stellar
models in order to determine accurate stellar parameters. | astro-ph_SR |
Correlation between the optical veiling and accretion properties: A case
study of the classical T Tauri star DK Tau: Classical T Tauri stars (cTTs) accrete from their circumstellar disk. The
material falls onto the stellar surface, producing an accretion shock, which
generates veiling in a star's spectra. In addition, the shock causes a
localized accretion spot at the level of the chromosphere. Our goal is to
investigate the accretion, particularly the mass accretion rates (Macc), for
the cTTs DK Tau, over two periods of 17 and 29 days, using two different
procedures for comparison purposes. The first method relies on the derivation
of the accretion luminosity via accretion-powered emission lines. The second
compares the variability of the optical veiling with accretion shock models to
determine mass accretion rates. We used observations taken in 2010 and 2012
with the ESPaDOnS spectropolarimeter at the CFHT. We find peak values of the
veiling (at 550 nm) ranging from 0.2 to 1.3, with a steeper trend across the
wavelength range for higher peak values. When using the accretion-powered
emission lines, we find mass accretion rate values ranging from
log(Macc[Msol/yr]) = -8.20 to log(Macc[Msol/yr]) = -7.40. This agrees with the
values found in the literature, as well as the values calculated using the
accretion shock models and the veiling. In addition, we identify a power-law
correlation between the values of the accretion luminosity and the optical
veiling. For the 2010 observations, using the values of the filling factors
(which represent the area of the star covered by an accretion spot) derived
from the shock models, we infer that the accretion spot was located between +45
degrees and +75 degrees in latitude. We show that both methods of determining
the mass accretion rate yield similar results. We also present a helpful means
of confirming the accretion luminosity values by measuring the veiling at a
single wavelength in the optical. | astro-ph_SR |
Coronal Polarization: We present an overview of the physical mechanisms responsible for the coronal
polarization at different wavelength regimes. We also review different
techniques using coronal polarization to determine various quantities necessary
for understanding the thermodynamic properties of the solar coronal plasma.
This includes the coronal magnetic field, electronic densities, temperatures,
velocities, etc. The future needs to acquire better information on the solar
corona using polarization will be outlined. | astro-ph_SR |
The first stars: CEMP--no stars and signatures of spinstars: (Abridged) The CEMP--no stars are "carbon-enhanced-metal-poor" stars that in
principle show no evidence of s-- and r--elements from neutron captures. We try
to understand the origin and nucleosynthetic site of their peculiar CNO,
Ne--Na, and Mg--Al abundances. We compare the observed abundances to the
nucleosynthetic predictions of AGB models and of models of rotating massive
stars with internal mixing and mass loss. We also analyze the different
behaviors of $\alpha$-- and CNO--elements, as well the abundances of elements
involved in the Ne--Na and Mg--Al cycles. We show that CEMP-no stars exhibit
products of He--burning that have gone through partial mixing and processing by
the CNO cycle, producing low $^{12}$C/$^{13}$C and a broad variety of [C/N] and
[O/N] ratios. From a $^{12}$C/$^{13}$C vs. [C/N] diagram, we conclude that
neither the yields of AGB stars (in binaries or not) nor the yields of classic
supernovae can fully account for the observed CNO abundances in CEMP-no stars.
Better agreement is obtained once the chemical contribution by stellar winds of
fast-rotating massive stars is taken into account, where partial mixing takes
place, leading to various amounts of CNO being ejected. CEMP--no stars present
a wide variety in the [C/Fe], [N/Fe], [O/Fe], [Na/Fe], [Mg/Fe], [Al/Fe], and
[Sr/Fe] ratios. We show that back-and-forth, partial mixing between the He--
and H--regions may account for this variety. Some s--elements, mainly of the
first peak, may even be produced by these processes in a small fraction of the
CEMP--no stars. We propose a classification scheme for the CEMP--no and low--s
stars, based on the changes in composition produced by these successive
back-and-forth mixing motions. | astro-ph_SR |
The Infrared Evolution of Dust in V838 Monocerotis: Luminous Red Variables (LRVs) are most likely eruptions that are the outcome
of stellar mergers. V838 Mon is one of the best-studied members of this class,
representing an archetype for stellar mergers resulting from B-type stars. As
result of the merger event, nova-like eruptions occur driving mass-loss from
the system. As the gas cools considerable circumstellar dust is formed. V838
Mon erupted in 2002 and is undergoing very dynamic changes in its dust
composition, geometry, and infrared luminosity providing a real-time laboratory
to validate mineralogical condensation sequences in stellar mergers and
evolutionary scenarios. We discuss recent NASA Stratospheric Observatory for
Infrared Astronomy SOFIA 5 to 38 micron observations combined with archival
NASA Spitzer spectra that document the temporal evolution of the freshly formed
(within the last 20 yrs) circumstellar material in the environs of V838 Mon.
Changes in the 10 micron spectral region are strong evidence that we are
witnessing a classical dust condensation sequence expected to occur in
oxygen-rich environments where alumina formation is followed by that of
silicates at the temperature cools. | astro-ph_SR |
Candidate Coronagraphic Detections of Protoplanetary Disks around Four
Young Stars: We present potential detections of H-band scattered light emission around
four young star, selected from a total sample of 45 young stars observed with
the CIAO coronagraph of the Subaru telescope. Two CTTS, CI Tau and DI Cep, and
two WTTS, LkCa 14 and RXJ 0338.3+1020 were detected. In all four cases, the
extended emission is within the area of the residual PSF halo, and is revealed
only through careful data reduction. We compare the observed extended emission
with simulations of the scattered light emission, to evaluate the plausibility
and nature of the detected emission. | astro-ph_SR |
An independent analysis of the brown dwarf atmosphere monitoring (BAM)
data: large-amplitude variability is rare outside the L/T transition: Observations of variability can provide valuable information about the
processes of cloud formation and dissipation in brown dwarf atmospheres. Here
we report the results of an independent analysis of archival data from the
Brown dwarf Atmosphere Monitoring (BAM) program. Time series data for 14 L and
T dwarfs reported to be significantly variable over timescales of hours were
analyzed. We confirm large-amplitude variability (amplitudes $>$2\%) for 4/13
targets and place upper limits of 0.7\%-1.6\% on variability in the remaining
sample. For two targets we find evidence of weak variability at amplitudes of
1.3\% and 1.6\%. Based on our revised classification of variable objects in the
BAM study, we find strong variability outside the L/T transition to be rare at
near infrared wavelengths. From a combined sample of 81 L0-T9 dwarfs from the
revised BAM sample and the variability survey of Radigan et al. we infer an
overall observed frequency for large-amplitude variability outside the L/T
transition of 3.2$_{-1.8}^{+2.8}$\%, in contrast to 24$^{+11}_{-9}$\% for
L9-T3.5 spectral types. We conclude that while strong variability is not
limited to the L/T transition, it occurs more frequently in this spectral type
range, indicative of larger or more highly contrasting cloud features at these
spectral types. | astro-ph_SR |
H2O Maser Polarization of the Water Fountains IRAS 15445-5449 and IRAS
18043-2116: We present the morphology and linear polarization of the 22-GHz H2O masers in
the high-velocity outflow of two post-AGB sources, d46 (IRAS 15445-5449) and
b292 (IRAS 18043-2116). The observations were performed using The Australia
Telescope Compact Array. Different levels of saturated maser emission have been
detected for both sources. We also present the mid-infrared image of d46
overlaid with the distribution of the maser features that we have observed in
the red-shifted lobe of the bipolar structure. The relative position of the
observed masers and a previous radio continuum observation suggests that the
continnum is produced along the blue-shifted lobe of the jet. It is likely due
to synchrontron radiation, implying the presence of a strong magnetic field in
the jet. The fractional polarization levels measured for the maser features of
d46 indicate that the polarization vectors are tracing the poloidal component
of the magnetic field in the emitting region. For the H2O masers of b292 we
have measured low levels of fractional linear polarization. The linear
polarization in the H2O maser region of this source likely indicates a dominant
toroidal or poloidal magnetic field component. Since circular polarization was
not detected it is not possible to determine the magnetic field strength.
However, we present a 3-sigma evaluation of the upper limit intensity of the
magnetic field in the maser emitting regions of both observed sources. | astro-ph_SR |
Basal magnetic flux and the local solar dynamo: The average unsigned magnetic flux density in magnetograms of the quiet Sun
is generally dominated by instrumental noise. Due to the entirely different
scaling behavior of the noise and the solar magnetic pattern it has been
possible to determine the standard deviation of the Gaussian noise distribution
and remove the noise contribution from the average unsigned flux density for
the whole 15-yr SOHO/MDI data set and for a selection of SDO/HMI magnetograms.
There is a very close correlation between the MDI disk-averaged unsigned
vertical flux density and the sunspot number, and regression analysis gives a
residual level of 2.7 G when the sunspot number is zero. The selected set of
HMI magnetograms, which spans the most quiet phase of solar activity, has a
lower limit of 3.0 G to the noise-corrected average flux density. These
apparently cycle-independent levels may be identified as a basal flux density,
which represents an upper limit to the possible flux contribution from a local
dynamo, but not evidence for its existence. The 3.0 G HMI level, when scaled to
the Hinode spatial resolution, translates to 3.5 G, which means that the much
higher average flux densities always found by Hinode in quiet regions do not
originate from a local dynamo. The contributions to the average unsigned flux
density come almost exclusively from the extended wings of the probability
density function (PDF), also in the case of HMI magnetograms with only
basal-level magnetic flux. These wings represent intermittent magnetic flux.
While the global dynamo appears to dominate the magnetic energy spectrum at all
the resolved spatial scales, there are indications from the observed Hanle
depolarization in atomic lines that the local dynamo may dominate the spectrum
at scales of order 1-10 km and below. | astro-ph_SR |
Modelling convection in A star atmospheres. Bisectors and lineshapes of
HD108642: We present a code, VeDyn, for modelling envelopes and atmospheres of A to F
stars focusing on accurate treatment of convective processes. VeDyn implements
the highly sophisticated convection model of Canuto and Dubovikov (1998) but is
fast and handy enough to be used in practical astrophysical applications. We
developed the HME envelope solver for this convection model furtheron to
consistently model the envelope together with the stellar atmosphere. The
synthesis code SynthV was extended to account for the resulting velocity
structure. Finally, we tested our approach on atomic line bisectors. It is
shown that our synthetic line bisectors of HD108642 bend towards the blue and
are of a magnitude comparable to the observed ones. Even though this approach
of modelling convection requires the solution of a coupled system of nonlinear
differential equations, it is fast enough to be applicable to many of the
investigation techniques relying on model atmospheres. | astro-ph_SR |
M dwarf luminosity, radius, and $α$-enrichment from $I$-band
spectral features: Despite the ubiquity of M dwarfs and their growing importance to studies of
exoplanets, Galactic evolution, and stellar structure, methods for precisely
measuring their fundamental stellar properties remain elusive. Existing
techniques for measuring M dwarf luminosity, mass, radius, or composition are
calibrated over a limited range of stellar parameters or require expensive
observations. We find a strong correlation between the $K_S$-band luminosity
($M_K$), the observed strength of the $I$-band sodium doublet absorption
feature, and [Fe/H] in M dwarfs without strong H$\alpha$ emission. We show that
the strength of this feature, coupled with [Fe/H] and spectral type, can be
used to derive M dwarf $M_K$ and radius without requiring parallax.
Additionally, we find promising evidence that the strengths of the $I$-band
sodium doublet and the nearby $I$-band calcium triplet may jointly indicate
$\alpha$-element enrichment. The use of these $I$-band features requires only
moderate-resolution near-infrared spectroscopy to provide valuable information
about the potential habitability of exoplanets around M dwarfs, and surface
gravity and distance for M dwarfs throughout the Galaxy. This technique has
immediate applicability for both target selection and candidate planet host
system characterization for exoplanet missions such as \textit{TESS} and
\textit{K2}. | astro-ph_SR |
New Low-Mass Stars in the 25 Orionis Stellar Group and Orion OB1a
Sub-association from SDSS-III/BOSS Spectroscopy: The Orion OB1a sub-association is a rich low mass star (LMS) region. Previous
spectroscopic studies have confirmed 160 LMSs in the 25 Orionis stellar group
(25 Ori), which is the most prominent overdensity of Orion OB1a. Nonetheless,
the current census of the 25 Ori members is estimated to be less than 50%
complete, leaving a large number of members to be still confirmed. We retrieved
172 low-resolution stellar spectra in Orion OB1a observed as ancillary science
in the SDSS-III/BOSS survey, for which we classified their spectral types and
determined physical parameters. To determine memberships, we analyzed the
H$_\alpha$ emission, LiI$\lambda$6708 absorption, and NaI$\lambda\lambda$8183,
8195 absorption as youth indicators in stars classified as M-type. We report 50
new LMSs spread across the 25 Orionis, ASCC 18, and ASCC 20 stellar groups with
spectral types from M0 to M6, corresponding to a mass range of 0.10$\le
m/\textrm{M}_\odot \le$0.58. This represents an increase of 50% in the number
of known LMSs in the area and a net increase of 20% in the number of 25 Ori
members in this mass range. Using parallax values from the Gaia DR1 catalog, we
estimated the distances to these three stellar groups and found that they are
all co-distant, at 338$\pm$66 pc. We analyzed the spectral energy distributions
of these LMSs and classified their disks by evolutionary classes. Using H-R
diagrams, we found a suggestion that 25 Ori could be slightly older that the
other two observed groups in Orion OB1a. | astro-ph_SR |
A Granulation "Flicker"-based Measure of Stellar Surface Gravity: In Bastien et al. (2013) we found that high quality light curves, such as
those obtained by Kepler, may be used to measure stellar surface gravity via
granulation-driven light curve "flicker". Here, we update and extend the
relation originally presented in Bastien et al. (2013) after calibrating
flicker against a more robust set of asteroseismically derived surface
gravities. We describe in detail how we extract the flicker signal from the
light curves, including how we treat phenomena, such as exoplanet transits and
shot noise, that adversely affect the measurement of flicker. We examine the
limitations of the technique, and, as a result, we now provide an updated
treatment of the flicker-based logg error. We briefly highlight further
applications of the technique, such as astrodensity profiling or its use in
other types of stars with convective outer layers. We discuss potential uses in
current and upcoming space-based photometric missions. Finally, we supply
flicker-based logg values, and their uncertainties, for 27 628 Kepler stars not
identified as transiting-planet hosts, with 4500<teff<7150 K, 2.5<logg<4.6,
Kepler magnitude <13.5, and overall photometric amplitudes <10 parts per
thousand. | astro-ph_SR |
Recipe for inferring sub-surface solar magnetism via local mode-coupling
using Slepian basis functions: Direct seismic imaging of sub-surface flow, sound-speed and magnetic field is
crucial for predicting flux tube emergence on the solar surface, an important
ingredient for space weather. The sensitivity of helioseismic mode-amplitude
cross-correlation to $p$- and $f$-mode oscillations enable formal inversion of
such sub-photospheric perturbations. It is well-known that such problems are
written in the form of an integral equation that connects the perturbations to
the observations via ``sensitivity kernels". While the sensitivity kernels for
flow and sound-speed have been known for decades and have been used
extensively, formulating kernels for general magnetic perturbations had been
elusive. A recent study proposed sensitivity kernels for Lorentz-stresses
corresponding to global magnetic fields of general geometry. The present study
is devoted to proposing kernels for inferring Lorentz-stresses as well as the
solenoidal magnetic field in a local patch on the Sun via Cartesian
mode-coupling. Moreover, for the first time in solar physics, Slepian functions
are employed to parameterize perturbations in the horizontal dimension. This is
shown to increase the number of data constraints in the inverse problem,
implying an increase in the precision of inferred parameters. This paves the
path to reliably imaging sub-surface solar magnetic features in, e.g.,
supergranules, sunspots and (emerging) active regions. | astro-ph_SR |
Quiet-Sun intensity contrasts in the near ultraviolet: We present high-resolution images of the Sun in the near ultraviolet spectral
range between 214 nm and 397 nm as obtained from the first science flight of
the 1-m Sunrise balloon-borne solar telescope. The quiet-Sun rms intensity
contrasts found in this wavelength range are among the highest values ever
obtained for quiet-Sun solar surface structures - up to 32.8% at a wavelength
of 214 nm. We compare with theoretical intensity contrasts obtained from
numerical magneto-hydrodynamic simulations. For 388 nm and 312 nm the
observations agree well with the numerical simulations whereas at shorter
wavelengths discrepancies between observed and simulated contrasts remain. | astro-ph_SR |
Observational and model analysis of a two-ribbon flare possibly induced
by a neighbouring blowout jet: In this paper, we present unique observations of a blowout coronal jet that
possibly triggered a two-ribbon confined C1.2 flare in a bipolar solar active
region NOAA 12615 on 2016 December 5. The jet activity initiates at
chromospheric/transition-region heights with a small brightening that
eventually grows in a larger volume with well developed standard morphological
jet features, viz., base and spire. The spire widens up with a collimated
eruption of cool and hot plasma components, observed in the 304 and 94 A
channels of AIA, respectively. The speed of the plasma ejection, which forms
the jet's spire, was higher for the hot component (~200 km/s) than the cooler
one (~130 km/s). The NLFF model of coronal fields at pre- and post-jet phases
successfully reveal opening of previously closed magnetic field lines with a
rather inclined/low-lying jet structure. The peak phase of the jet emission is
followed by the development of a two-ribbon flare that shows coronal loop
emission in HXRs up to ~25 keV energy. The coronal magnetic fields rooted at
the location of EUV flare ribbons, derived from the NLFF model, demonstrate the
pre-flare phase to exhibit an "X-type" configuration while the magnetic fields
at the post-flare phase are more or less parallel oriented. The comparisons of
multi-wavelength measurements with the magnetic field extrapolations suggest
that the jet activity likely triggered the two-ribbon flare by perturbing the
field in the interior of the active region. | astro-ph_SR |
The Gaia-ESO Survey: Carbon abundance in the Galactic thin and thick
disks: This paper focuses on carbon that is one of the most abundant elements in the
Universe and is of high importance in the field of nucleosynthesis and galactic
and stellar evolution. Even nowadays, the origin of carbon and the relative
importance of massive and low- to intermediate-mass stars in producing it is
still a matter of debate. In this paper we aim at better understanding the
origin of carbon by studying the trends of [C/H], [C/Fe],and [C/Mg] versus
[Fe/H], and [Mg/H] for 2133 FGK dwarf stars from the fifth Gaia-ESO Survey
internal data release (GES iDR5). The availability of accurate parallaxes and
proper motions from Gaia DR2 and radial velocities from GES iDR5 allows us to
compute Galactic velocities, orbits and absolute magnitudes and, for 1751
stars, ages via a Bayesian approach. Three different selection methodologies
have been adopted to discriminate between thin and thick disk stars. In all the
cases, the two stellar groups show different abundance ratios, [C/H], [C/Fe],
and [C/Mg], and span different age intervals, with the thick disk stars being,
on average, older than those in the thin disk. The behaviours of [C/H], [C/Fe],
and [C/Mg] versus [Fe/H], [Mg/H], and age all suggest that C is primarily
produced in massive stars like Mg. The increase of [C/Mg] for young thin disk
stars indicates a contribution from low-mass stars or the increased C
production from massive stars at high metallicities due to the enhanced mass
loss. The analysis of the orbital parameters Rmed and |Zmax| support an
"inside-out" and "upside-down" formation scenario for the disks of Milky Way. | astro-ph_SR |
Proton and electron temperatures in the solar wind and their
correlations with the solar wind speed: The heating and acceleration of the solar wind remains one of the fundamental
unsolved problems in heliophysics. It is usually observed that the proton
temperature $T_i$ is highly correlated with the solar wind speed $V_{SW}$,
while the electron temperature $T_e$ shows anti-correlation or no clear
correlation with the solar wind speed. Here we inspect both Parker Solar Probe
(PSP) and WIND data and compare the observations with simulation results. PSP
observations below 30 solar radii clearly show a positive correlation between
proton temperature and wind speed and a negative correlation between electron
temperature and wind speed. One year (2019) of WIND data confirm that proton
temperature is positively correlated with solar wind speed, but the electron
temperature increases with the solar wind speed for slow wind while it
decreases with the solar wind speed for fast wind. Using a one-dimensional
Alfv\'en-wave-driven solar wind model with different proton and electron
temperatures, we for the first time find that if most of the dissipated
Alfv\'en wave energy heats the ions instead of electrons, a positive
$T_i-V_{SW}$ correlation and a negative $T_e-V_{SW}$ correlation arise
naturally. If the electrons gain a small but finite portion of the dissipated
wave energy, the $T_e-V_{SW}$ correlation evolves with radial distance to the
Sun such that the negative correlation gradually turns positive. The model
results show that Alfv\'en waves are one of the possible explanations of the
observed evolution of proton and electron temperatures in the solar wind. | astro-ph_SR |
Exploring B4: A Pulsating sdB star, in a Binary, in the Open Cluster NGC
6791: We report on Kepler photometry of the hot sdB star B4 in the open cluster NGC
6791. We confirm that B4 is a reflection effect binary with an sdB component
and a low-mass main sequence companion with a circular 0.3985 d orbit. The sdB
star is a g-mode pulsator (a V1093 Her star) with periods ranging from 2384 s
to 7643 s. Several of the pulsation modes show symmetric splitting by 0.62
microHz. Attributing this to rotational splitting, we conclude that the sdB
component has a rotation period of approximately 9.63 d, indicating that tidal
synchronization has not been achieved in this system. Comparison with
theoretical synchronization time provides a discriminant between various
theoretical models. | astro-ph_SR |
Outbursts and stellar properties of the classical Be star HD 6226: The bright and understudied classical Be star HD 6226 has exhibited multiple
outbursts in the last several years during which the star grew a viscous
decretion disk. We analyze 659 optical spectra of the system collected from
2017-2020, along with a UV spectrum from the Hubble Space Telescope and high
cadence photometry from both TESS and the KELT survey. We find that the star
has a spectral type of B2.5IIIe, with a rotation rate of 74% of critical. The
star is nearly pole-on with an inclination of $13.4$ degree. We confirm the
spectroscopic pulsational properties previously reported, and report on three
photometric oscillations from KELT photometry. The outbursting behavior is
studied with equivalent width measurements of H$\alpha$ and H$\beta$, and the
variations in both of these can be quantitatively explained with two
frequencies through a Fourier analysis. One of the frequencies for the emission
outbursts is equal to the difference between two photometric oscillations,
linking these pulsation modes to the mass ejection mechanism for some
outbursts. During the TESS observation time period of 2019 October 7 to 2019
November 2, the star was building a disk. With a large dataset of H$\alpha$ and
H$\beta$ spectroscopy, we are able to determine the timescales of dissipation
in both of these lines, similar to past work on Be stars that has been done
with optical photometry. HD 6226 is an ideal target with which to study the Be
disk-evolution given its apparent periodic nature, allowing for targeted
observations with other facilities in the future. | astro-ph_SR |
Correlation between lithium abundances and ages of solar twin stars: We want to determine the lithium abundances of solar twin stars as a function
of stellar age to provide constraints for stellar evolutions models and to
investigate whether there is a connection between low Li abundance and the
occurrence of planets. For a sample of 21 solar twins observed with the HARPS
spectrograph at high spectral resolution (R~115.000) and very high
signal-to-noise ratio (600 < S/N < 2400), precise lithium abundances were
obtained by spectral synthesis of the LiI 6707.8 A line and compared to stellar
ages, masses, and metallicities determined from a spectroscopic analysis of the
same set of HARPS spectra. We show that for the large majority of the solar
twins there is a strong correlation between lithium abundance and stellar age.
As the age increases from 1 to 9 Gyr, the Li abundance decreases by a factor of
~ 50. The relation agrees fairly well with predictions from non-standard
stellar evolution models of Li destruction at the bottom of the upper
convection zone. Two stars deviate from the relation by having Li abundances
enhanced by a factor of ~ 10, which may be due to planet engulfment. On the
other hand, we find no indication of a link between planet hosting stars and
enhanced lithium depletion. | astro-ph_SR |
Simultaneous X-ray and Radio Observations of Rotating Radio Transient
J1819-1458: We present the results of simultaneous radio and X-ray observations of PSR
J1819-1458. Our 94-ks XMM-Newton observation of the high magnetic field 5*10^13
G pulsar reveals a blackbody spectrum (kT~130 eV) with a broad absorption
feature, possibly composed of two lines at ~1.0 and ~1.3 keV. We performed a
correlation analysis of the X-ray photons with radio pulses detected in 16.2
hours of simultaneous observations at 1-2 GHz with the Green Bank, Effelsberg,
and Parkes telescopes, respectively. Both the detected X-ray photons and radio
pulses appear to be randomly distributed in time. We find tentative evidence
for a correlation between the detected radio pulses and X-ray photons on
timescales of less than 10 pulsar spin periods, with the probability of this
occurring by chance being 0.46%. This suggests that the physical process
producing the radio pulses may also heat the polar-cap. | astro-ph_SR |
V405 Peg (RBS 1955): A Nearby, Low-Luminosity Cataclysmic Binary: (Abridged). The cataclysmic binary V405 Peg, originally discovered as ROSAT
Bright Source (RBS) 1955 (= 1RXS J230949.6+213523), shows a strong contribution
from a late-type secondary star in its optical spectrum, which led Schwope et
al. to suggest it to be among the nearest cataclysmic binaries. We present
extensive optical observations of V405 Peg. Time-series spectroscopy shows the
orbital period, Porb, to be 0.1776469(7) d (= 4.2635 hr), or 5.629 cycle/d. We
classify the secondary as M3 - M4.5. Astrometry with the MDM 2.4m telescope
gives a parallax 7.2 +- 1.1 milli-arcsec, and a relative proper motion of 58
mas/yr. Our best estimate of the distance yields d = 149 (+26, -20) pc. The
secondary stars's radial velocity has K2 = 92 +- 3 km/s, indicating a fairly
low orbital inclination if the masses are typical. Extensive I-band time-series
observations in the show the system varying between a minimum brightness level
of I = 14.14 and states of enhanced activity about 0.2 mag brighter. While the
low-state shows an ellipsoidal modulation, an additional photometric modulation
appears in the high state, with 0.1 mag amplitude and period 220-280 min. The
frequency of this modulation appears to be stable for a month or so, but no
single period was consistently detected from one observing season to the next.
We estimate the system luminosity by combining optical measurements with the
archival X-ray spectrum. The implied mass accretion rate is orders of
magnitudes below the predictions for the standard angular momentum loss above
the period gap. The system may possibly belong to a largely undiscovered
population of hibernating CVs. | astro-ph_SR |
Warm Breeze from the starboard bow: a new population of neutral helium
in the heliosphere: We investigate the signals from neutral He atoms observed from Earth orbit in
2010 by IBEX. The full He signal observed during the 2010 observation season
can be explained as a superposition of pristine neutral interstellar He gas and
an additional population of neutral He that we call the Warm Breeze. The Warm
Breeze is approximately two-fold slower and 2.5 times warmer than the primary
interstellar He population, and its density in front of the heliosphere is ~7%
that of the neutral interstellar helium. The inflow direction of the Warm
Breeze differs by ~19deg from the inflow direction of interstellar gas. The
Warm Breeze seems a long-term feature of the heliospheric environment. It has
not been detected earlier because it is strongly ionized inside the
heliosphere, which brings it below the threshold of detection via pickup ion
and heliospheric backscatter glow observations, as well as by the direct
sampling of GAS/Ulysses. Possible sources for the Warm Breeze include (1) the
secondary population of interstellar helium, created via charge exchange and
perhaps elastic scattering of neutral interstellar He atoms on interstellar He+
ions in the outer heliosheath, or (2) a gust of interstellar He originating
from a hypothetic wave train in the Local Interstellar Cloud. A secondary
population is expected from models, but the characteristics of the Warm Breeze
do not fully conform to modeling results. If, nevertheless, this is the
explanation, IBEX-Lo observations of the Warm Breeze provide key insights into
the physical state of plasma in the outer heliosheath. If the second hypothesis
is true, the source is likely to be located within a few thousand of AU from
the Sun, which is the propagation range of possible gusts of interstellar
neutral helium with the Warm Breeze characteristics against dissipation via
elastic scattering in the Local Cloud. | astro-ph_SR |
Active region upflows: 1. Multi-instrument observations: Upflows at the edges of active regions (ARs) are studied by spatially and
temporally combining multi-instrument observations obtained with EIS/Hinode,
AIA and HMI/SDO and IBIS/NSO, to derive their plasma parameters. This
information is used for benchmarking data-driven modelling of the upflows
(Galsgaard et al., 2015). The studied AR upflow displays blueshifted emission
of 5-20 km/s in Fe XII and Fe XIII and its average electron density is 1.8x10^9
cm^3 at 1 MK. The time variation of the density shows no significant change (in
a 3sigma error). The plasma density along a single loop drops by 50% over a
distance of 20000 km. We find a second velocity component in the blue wing of
the Fe XII and Fe XIII lines at 105 km/s. This component is persistent during
the whole observing period of 3.5 hours with variations of only 15 km/s. We
also study the evolution of the photospheric magnetic field and find that
magnetic flux diffusion is responsible for the formation of the upflow region.
High cadence Halpha observations of the chromosphere at the footpoints of the
upflow region show no significant jet-like (spicule/rapid blue excursion)
activity to account for several hours/days of plasma upflow. Using an image
enhancement technique, we show that the coronal structures seen in the AIA 193A
channel is comparable to the EIS Fe XII images, while images in the AIA 171A
channel reveals additional loops that are a result of contribution from cooler
emission to this channel. Our results suggest that at chromospheric heights
there are no signatures that support the possible contribution of spicules to
AR upflows. We suggest that magnetic flux diffusion is responsible for the
formation of the coronal upflows. The existence of two velocity components
possibly indicate the presence of two different flows which are produced by two
different physical mechanisms, e.g. magnetic reconnection and pressure-driven. | astro-ph_SR |
HD66051: the first eclipsing binary hosting an early-type magnetic star: Early-type magnetic stars are rarely found in close binary systems. No such
objects were known in eclipsing binaries prior to this study. Here we
investigated the eclipsing, spectroscopic double-lined binary HD66051, which
exhibits out-of-eclipse photometric variations suggestive of surface brightness
inhomogeneities typical of early-type magnetic stars. Using a new set of
high-resolution spectropolarimetric observations, we discovered a weak magnetic
field on the primary and found intrinsic, element-dependent variability in its
spectral lines. The magnetic field structure of the primary is dominated by a
nearly axisymmetric dipolar component with a polar field strength $B_{\rm
d}\approx600$ G and an inclination with respect to the rotation axis of
$\beta_{\rm d}=13^{\rm o}$. A weaker quadrupolar component is also likely to be
present. We combined the radial velocity measurements derived from our spectra
with archival optical photometry to determine fundamental masses (3.16 and 1.75
$M_\odot$) and radii (2.78 and 1.39 $R_\odot$) with a 1-3% precision. We also
obtained a refined estimate of the effective temperatures (13000 and 9000 K)
and studied chemical abundances for both components with the help of
disentangled spectra. We demonstrate that the primary component of HD66051 is a
typical late-B magnetic chemically peculiar star with a non-uniform surface
chemical abundance distribution. It is not an HgMn-type star as suggested by
recent studies. The secondary is a metallic-line star showing neither a strong,
global magnetic field nor intrinsic spectral variability. Fundamental
parameters provided by our work for this interesting system open unique
possibilities for probing interior structure, studying atomic diffusion, and
constraining binary star evolution. | astro-ph_SR |
Direct MD simulation of liquid-solid phase equilibria for two-component
plasmas: We determine the liquid-solid phase diagram for carbon-oxygen and
oxygen-selenium plasma mixtures using two-phase MD simulations. We identified
liquid, solid, and interface regions using a bond angle metric. To study finite
size effects, we perform 27648 and 55296 ion simulations. To help monitor
non-equilibrium effects, we calculate diffusion constants $D_i$. For the
carbon-oxygen system we find that $D_O$ for oxygen ions in the solid is much
smaller than $D_C$ for carbon ions and that both diffusion constants are 80 or
more times smaller than diffusion constants in the liquid phase. There is
excellent agreement between our carbon-oxygen phase diagram and that predicted
by Medin and Cumming. This suggests that errors from finite size and
non-equilibrium effects are small and that the carbon-oxygen phase diagram is
now accurately known. The oxygen-selenium system is a simple two-component
model for more complex rapid proton capture nucleosynthesis ash compositions
for an accreting neutron star. Diffusion of oxygen, in a predominately selenium
crystal, is remarkably fast, comparable to diffusion in the liquid phase. We
find a somewhat lower melting temperature for the oxygen-selenium system than
that predicted by Medin and Cumming. This is probably because of electron
screening effects. | astro-ph_SR |
On the role of tachoclines in solar and stellar dynamos: Rotational shear layers at the boundary between radiative and convective
zones, tachoclines, play a key role in the process of magnetic field generation
in solar-like stars. We present two sets of global simulations of rotating
turbulent convection and dynamo. The first set considers a stellar convective
envelope only; the second one, aiming at the formation of a tachocline,
considers also the upper part of the radiative zone. Our results indicate that
the resulting mean-flows and dynamo properties like the growth rate, saturation
energy and mode depend on the Rossby (Ro) number. For the first set of models
either oscillatory (with ~2 yr period) or steady dynamo solutions are obtained.
The models in the second set naturally develop a tachocline which, in turn,
leads to the generation of strong mean magnetic field. Since the field is also
deposited into the stable deeper layer, its evolutionary time-scale is much
longer than in the models without a tachocline. Surprisingly, the magnetic
field in the upper turbulent convection zone evolves in the same time scale as
the deep field. These models result in either an oscillatory dynamo with ~30 yr
period or in a steady dynamo depending on Ro. In terms of the mean-field dynamo
coefficients computed using FOSA, the field evolution in the oscillatory models
without a tachocline seems to be consistent with dynamo waves propagating
according to the Parker-Yoshimura sign rule. In the models with tachoclines the
dynamics is more complex involving other transport mechanisms as well as
tachocline instabilities. | astro-ph_SR |
No evidence of chemical anomalies in the bimodal turnoff cluster NGC
1806 in the LMC: We have studied the chemical composition of NGC 1806, a massive,
intermediate-age globular cluster that shows a double main sequence turnoff. We
analyzed a sample of high-resolution spectra (secured with FLAMES at the Very
Large Telescope) for 8 giant stars, members of the cluster, finding an average
iron content of [Fe/H]=--0.60 +- 0.01 dex and no evidence of intrinsic
star-to-star variations in the abundances of light elements (Na, O, Mg, Al).
Also, the (m_(F814W); m_(F336W)-m_(F814W)) color-magnitude diagram obtained by
combining optical and near-UV Hubble Space Telescope photometry exhibits a
narrow red giant branch, thus ruling out intrinsic variations of C and N
abundances in the cluster. These findings demonstrate that NGC 1806 does not
harbor chemically distinct sub-populations, at variance with what was found in
old globular clusters. In turn, this indicates that the double main sequence
turnoff phenomenon cannot be explained in the context of the self-enrichment
processes usually invoked to explain the chemical anomalies observed in old
globulars. Other solutions (i.e., stellar rotation, merging between clusters or
collisions with giant molecular clouds) should be envisaged to explain this
class of globulars. | astro-ph_SR |
Rapidly varying A-type stars in the SuperWASP archive: The searches for transiting exoplanets have produced a vast amount of
time-resolved photometric data of many millions of stars. One of the leading
ground-based surveys is the SuperWASP project. We present the initial results
of a survey of over 1.5 million A-type stars in the search for high frequency
pulsations using SuperWASP photometry. We are able to detect pulsations down to
the 0.5 mmag level in the broad-band photometry. This has enabled the discovery
of several rapidly oscillating Ap stars and over 200 delta Scuti stars with
frequencies above 50 d$^{-1}$, and at least one pulsating sdB star. Such a
large number of results allows us to statistically study the frequency overlap
between roAp and delta Scuti stars and probe to higher frequency regimes with
existing data. | astro-ph_SR |
A census of Coronal Mass Ejections on solar-like stars: Coronal Mass Ejections (CMEs) may have major importance for planetary and
stellar evolution. Stellar CME parameters, such as mass and velocity, have yet
not been determined statistically. So far only a handful of stellar CMEs has
been detected mainly on dMe stars using spectroscopic observations. We
therefore aim for a statistical determination of CMEs of solar-like stars by
using spectroscopic data from the ESO phase 3 and Polarbase archives. To
identify stellar CMEs we use the Doppler signal in optical spectral lines being
a signature of erupting filaments which are closely correlated to CMEs. We
investigate more than 3700 hours of on-source time of in total 425 dF-dK stars.
We find no signatures of CMEs and only few flares. To explain this low level of
activity we derive upper limits for the non detections of CMEs and compare
those with empirically modelled CME rates. To explain the low number of
detected flares we adapt a flare power law derived from EUV data to the
H{\alpha} regime, yielding more realistic results for H{\alpha} observations.
In addition we examine the detectability of flares from the stars by extracting
Sun-as-a-star H{\alpha} light curves. The extrapolated maximum numbers of
observable CMEs are below the observationally determined upper limits, which
indicates that the on-source times were mostly too short to detect stellar CMEs
in H{\alpha}. We conclude that these non detections are related to
observational biases in conjunction with a low level of activity of the
investigated dF-dK stars. | astro-ph_SR |
Shock-Cloud Interaction in the Solar Corona: Flare associated coronal shock waves sometimes interact with solar
prominences leading to large amplitude prominence oscillations. Such prominence
activation gives us unique opportunity to track time evolution of shock-cloud
interaction in cosmic plasmas. Although the dynamics of interstellar
shock-cloud interaction is extensively studied, coronal shock-solar prominence
interaction is rarely studied in the context of shock-cloud interaction.
Associated with X5.4 class solar flare occurred on 7 March, 2012, a globally
propagated coronal shock wave interacted with a polar prominence leading to
large amplitude prominence oscillation. In this paper, we studied bulk
acceleration and excitation of internal flow of the shocked prominence using
three-dimensional MHD simulations. We studied eight magnetohydrodynamic (MHD)
simulation runs with different mass density structure of the prominence, and
one hydrodynamic simulation run, and compared the result. In order to compare
observed motion of activated prominence with corresponding simulation, we also
studied prominence activation by injection of triangular shaped coronal shock.
We found that magnetic tension force mainly accelerate (and then decelerate)
the prominence. The internal flow, on the other hand, is excited during the
shock front sweeps through the the prominence and damps almost exponentially.
We construct phenomenological model of bulk momentum transfer from shock to the
prominence, which agreed quantitatively with all the simulation results. Based
on the phenomenological prominence-activation model, we diagnosed physical
parameters of coronal shock wave. The estimated energy of the coronal shock is
several percent of total energy released during the X5.4 flare. | astro-ph_SR |
The Pair-Instability Mass Gap for Black Holes: Stellar evolution theory predicts a "gap" in the black hole birth function
caused by the pair instability. Presupernova stars that have a core mass below
some limiting value, Mlo, after all pulsational activity is finished, collapse
to black holes, whereas more massive ones, up to some limiting value, Mhi,
explode, promptly and completely, as pair-instability supernovae. Previous work
has suggested Mlo is approximately 50 solar masses and Mhi is approximately 130
solar masses. These calculations have been challenged by recent LIGO
observations that show many black holes merging with individual masses, Mlo is
least some 65 solar masses. Here we explore four factors affecting the
theoretical estimates for the boundaries of this mass gap: nuclear reaction
rates, evolution in detached binaries, rotation, and hyper-Eddington accretion
after black hole birth. Current uncertainties in reaction rates by themselves
allow Mlo to rise to 64 solar masses and Mhi as large as 161 solar masses.
Rapid rotation could further increase Mlo to about 70 solar masses, depending
on the treatment of magnetic torques. Evolution in detached binaries and
super-Eddington accretion can, with great uncertainty, increase Mlo still
further. Dimensionless Kerr parameters close to unity are allowed for the more
massive black holes produced in close binaries, though they are generally
smaller. | astro-ph_SR |
Coevality in Young Eclipsing Binaries: The ages of the components in very short period pre-main sequence (PMS)
binaries are essential to an understanding of their formation. We considered a
sample of 7 PMS eclipsing binaries (EBs) with ages 1 to 6.3 MY and component
masses 0.2 to 1.4 Msun The very high precision with which their masses and
radii have been measured, and the capability provided by the {\it Modules for
Experiments in Stellar Astrophysics (MESA)} to calculate their evolutionary
tracks at exactly the measured masses, allows the determination of age
differences of the components independent of their luminosities and effective
temperatures. We found that the components of 5 EBs, ASAS J052821+0338.5,
Parenago 1802, JW 380, CoRoT 223992193, and UScoCTIO 5, formed within 0.3 MY of
each other. The parameters for the components of V1174 Ori, imply an
implausible large age difference of 2.7 MY and should be reconsidered. The 7th
EB in our sample, RX J0529.4+0041 fell outside the applicability of our
analysis. | astro-ph_SR |
Identification, mass and age of primary red clump stars from spectral
features derived with the LAMOST DR7: Although red clump (RC) stars are easy to identify due to their stability of
luminosity and color, about 20-50% are actually red giant branch (RGB) stars in
the same location on the HR diagram. In this paper, a sample of 210,504 spectra
for 184 318 primary RC (PRC) stars from the LAMOST DR7 is identified, which has
a purity of higher than 90 percent. The RC and the RGB stars are successfully
distinguished through LAMOST spectra(R~1800 and SNR>10) by adopting the XGBoost
ensemble learning algorithm, and the secondary RC stars are also removed. The
SHapley Additive exPlanations (SHAP) value is used to explain the top features
that the XGBoost model selected. The features are around Fe5270, MgH & MgIb,
Fe4957, Fe4207, Cr5208, and CN, which can successfully distinguish RGB and RC
stars. The XGBoost is also used to estimate the ages and masses of PRC stars by
training their spectra with Kepler labeled asteroseismic parameters. The
uncertainties of mass and age are 13 and 31 percent, respectively. Verifying
the feature attribution model, we find the age-sensitive elements XGBoost gets
are consistent with the literature. Distances of the PRC stars are derived by
$K_{S}$ absolute magnitude calibrated by Gaia EDR3, which has an uncertainty of
about 6 percent and shows the stars mainly locate at the Galactic disk. We also
test the XGBoost with R$\sim$250, which is the resolution of the Chinese Space
Station Telescope(CSST) under construction, it is still capable of finding
sensitive features to distinguish RC and RGB. | astro-ph_SR |
The low-mass stellar mass functions of rich, compact clusters in the
Large Magellanic Cloud: Context. We use Hubble Space Telescope photometry of six rich, compact star
clusters in the Large Magellanic Cloud (LMC), with ages ranging from 0.01 to
1.0 Gyr, to derive the clusters' stellar mass functions (MFs) at their
half-mass radii.
Aims. The LMC is an ideal environment to study stellar MFs, because it
contains a large population of compact clusters at different evolutionary
stages. We aim to obtain constraints on the initial MFs (IMFs) of our sample
clusters on the basis of their present-day MFs, combined with our understanding
of their dynamical and photometric evolution.
Methods. We derive the clusters' present-day MFs below 1.0 Msun using deep
observations with the Space Telescope Imaging Spectrograph and updated stellar
population synthesis models.
Results. Since the relaxation timescales of low-mass stars are very long,
dynamical evolution will not have affected the MFs below 1.0 Msun
significantly, so that - within the uncertainties - the derived MFs are
consistent with the solar-neighbourhood IMF, at least for the younger clusters.
Conclusions. The IMF in the low-density, low-metallicity environment of the
LMC disk is not significantly different from that in the solar neighbourhood. | astro-ph_SR |
On the Lira law and the nature of extinction towards Type Ia Supernovae: We have studied the relation between the color evolution of Type Ia
Supernovae (SNe Ia) from maximum light to the Lira law regime and the presence
of narrow absorption features. Based on a nearby sample of 89 SNe Ia we have
found that the rate of change of B-V colors at late phases (between 35 and 80
days after maximum) varies significantly among different SNe Ia. At maximum
light, faster Lira law B-V decliners have significantly higher equivalent
widths of blended Na I D1 & D2 narrow absorption lines, redder colors and lower
R_V reddening laws. We do not find faster Lira law B-V decliners to have a
strong preference for younger galaxy environments, where higher interstellar
material (ISM) column densities would be expected. We interpret these results
as evidence for the presence of circumstellar material (CSM). The differences
in colors and reddening laws found at maximum light are also present 55 days
afterward, but unlike the colors at maximum, they show a significant variation
among different host galaxy morphological types. This suggests that the effect
of ISM on the colors is more apparent at late times. Finally, we discuss how
the transversal expansion of the ejecta in an inhomogeneous ISM could mimic
some of these findings. | astro-ph_SR |
Stellar mid-life crisis: subcritical magnetic dynamos of solar-like
stars and the breakdown of gyrochronology: Recent observations reveal the surprising breakdown of stellar gyrochronology
relations at about the Sun's age hinting that middle-aged, solar-like stars
transition to a magnetically inactive future. We provide a theoretical basis
for these intriguing observations inspired by simulations with a mathematical
dynamo model that can explore long-term solar cycle fluctuations. We reproduce
the observed bimodal distribution of sunspot numbers, but only for subcritical
dynamos. Based on a bifurcation analysis, we argue that ageing of solar-like
stars makes the magnetically-weak dynamo regime readily accessible. Weak
magnetic field production in this regime compromises wind-driven angular
momentum losses thus disrupting the hegemony of magnetic braking on stellar
rotational spin-down. This hypothesis of {\emph{subcritical magnetic dynamos}}
of solar-like stars provides a self-consistent, unifying physical basis for a
diversity of solar-stellar phenomena such as why stars beyond their mid-life do
not spin-down as fast as in their youth, the break-down of stellar
gyrochronology relations, the observed bimodal distribution of long-term
sunspot observations and recent findings suggesting that the Sun may be
transitioning to a magnetically inactive future. | astro-ph_SR |
The GRAVITY Young Stellar Object survey -- I. Probing the disks of
Herbig Ae/Be stars in terrestrial orbits: The formation and the evolution of protoplanetary disks are important stages
in the lifetime of stars. The processes of disk evolution and planet formation
are intrinsically linked. We spatially resolve with GRAVITY/VLTI in the K-band
the sub au-scale region of 27 stars to gain statistical understanding of their
properties. We look for correlations with stellar parameters, such as
luminosity, mass, temperature and age. Our sample also cover a range of various
properties in terms of reprocessed flux, flared or flat morphology, and gaps.
We developed semi-physical geometrical models to fit our interferometric data.
Our best models correspond to smooth and wide rings, implying that wedge-shaped
rims at the dust sublimation edge are favored, as found in the H-band. The
closure phases are generally non-null with a median value of ~10 deg,
indicating spatial asymmetries of the intensity distributions. Multi-size grain
populations could explain the closure phase ranges below 20-25 deg but other
scenarios should be invoked to explain the largest ones. Our measurements
extend the Radius-Luminosity relation to ~1e4 Lsun and confirm the significant
spread around the mean relation observed in the H-band. Gapped sources exhibit
a large N-to-K band size ratio and large values of this ratio are only observed
for the members of our sample that would be older than 1 Ma, less massive, and
with lower luminosity. In the 2 Ms mass range, we observe a correlation in the
increase of the relative age with the transition from group II to group I, and
an increase of the N-to-K size ratio. However, the size of the current sample
does not yet permit us to invoke a clear universal evolution mechanism across
the HAeBe mass range. The measured locations of the K-band emission suggest
that these disks might be structured by forming young planets, rather than by
depletion due to EUV, FUV, and X-ray photo-evaporation. | astro-ph_SR |
Evolution of the post merger remnants from the coalescence of
oxygen-neon and carbon-oxygen white dwarf pairs: Although multidimensional simulations have investigated the processes of
double WD mergers, post-merger evolution only focused on the carbon-oxygen (CO)
WD or helium (He) WD merger remnants. In this work, we investigate for the
first time the evolution of the remnants stemmed from the merger of oxygen-neon
(ONe) WDs with CO WDs. Our simulation results indicate that the merger remnants
can evolve to hydrogen- and helium-deficient giants with maximum radius of
about 300Rsun. Our models show evidence that merger remnants more massive than
1.95Msun can ignite Ne before significant mass-loss ensues, and they thus would
become electron-capture supernovae (ECSNe). However, remnants with initial
masses less than 1.90Msun will experience further core contraction and longer
evolutionary time before reaching at the conditions for Ne-burning. Therefore
their fates are more dependent on mass-loss rates due to stellar winds, and
thus more uncertain. Relatively high mass-loss rates would cause such remnants
to end their lives as ONe WDs. Our evolutionary models can naturally explain
the observational properties of the double WD merger remnant IRAS 00500+6713
(J005311). As previously suggested in the literature, we propose and justify
that J005311 may be the remnant from the coalescence of an ONe WD and an CO WD.
We deduce that the final outcome of J005311 would be a massive ONe WD rather
than a supernova explosion. Our investigations may be able to provide possible
constraints on the wind mass-loss properties of the giants which have
CO-dominant envelopes. | astro-ph_SR |
FUV variability of HD 189733. Is the star accreting material from its
hot Jupiter?: Hot Jupiters are subject to strong irradiation from the host stars and, as a
consequence, they do evaporate. They can also interact with the parent stars by
means of tides and magnetic fields. Both phenomena have strong implications for
the evolution of these systems. Here we present time resolved spectroscopy of
HD~189733 observed with the Cosmic Origin Spectrograph (COS) on board to HST.
The star has been observed during five consecutive HST orbits, starting at a
secondary transit of the planet ($\phi$ ~0.50-0.63). Two main episodes of
variability of ion lines of Si, C, N and O are detected, with an increase of
line fluxes. Si IV lines show the highest degree of variability. The FUV
variability is a signature of enhanced activity in phase with the planet
motion, occurring after the planet egress, as already observed three times in
X-rays. With the support of MHD simulations, we propose the following
interpretation: a stream of gas evaporating from the planet is actively and
almost steadily accreting onto the stellar surface, impacting at $70-90\deg$
ahead of the sub-planetary point. | astro-ph_SR |
G-band and Hard X-ray Emissions of the 2006 December 14 flare observed
by Hinode/SOT and RHESSI: We report on G-band emission observed by the Solar Optical Telescope onboard
the Hinode satellite in association with the X1.5-class flare on 2006 December
14. The G-band enhancements originate from the footpoints of flaring coronal
magnetic loops, coinciding with non-thermal hard X-ray bremsstrahlung sources
observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. At the
available 2 minute cadence, the G-band and hard X-ray intensities are
furthermore well correlated in time. Assuming that the G-band enhancements are
continuum emission from a blackbody, we derived the total radiative losses of
the white-light flare (white-light power). If the G-band enhancements
additionally have a contribution from lines, the derived values are
overestimates. We compare the white-light power with the power in hard X-ray
producing electrons using the thick target assumption. Independent of the
cutoff energy of the accelerated electron spectrum, the white-light power and
the power of accelerated electrons are roughly proportional. Using the observed
upper limit of ~30 keV for the cutoff energy, the hard X-ray producing
electrons provide at least a factor of 2 more power than needed to produce the
white-light emission. For electrons above 40 keV, the powers roughly match for
all four of the time intervals available during the impulsive phase. Hence, the
flare-accelerated electrons contain enough energy to produce the white-light
flare emissions. The observed correlation in time, space, and power strongly
suggests that electron acceleration and white-light production in solar flares
are closely related. However, the results also call attention to the
inconsistency in apparent source heights of the hard X-ray (chromosphere) and
white-light (upper photosphere) sources. | astro-ph_SR |
Magnetic feature tracking, what determines the speed?: Recent observations revealed that small magnetic elements abundant at the
solar surface move poleward with a velocity which seems to be lower than the
plasma velocity $U_{\theta}$. Guerrero et al. (2011) explained this discrepancy
as a consequence of diffusive spreading of the magnetic elements due to a
positive radial gradient of $|U_{\theta}|$. As the gradient's sign (inferred by
local helioseismology) is still unclear, cases with a negative gradient are
studied in this paper. Under this condition, the velocity of the magnetic
tracers turns out to be larger than the plasma velocity, in disagreement with
the observations. Alternative mechanisms for explaining them independently are
proposed. For the turbulent magnetic pumping it is shown that it has to be
unrealistically strong to reconcile the model with the observations. | astro-ph_SR |
The AstraLux Multiplicity Survey: Extension to Late M-dwarfs: The distribution of multiplicity among low-mass stars is a key issue to
understanding the formation of stars and brown dwarfs, and recent surveys have
yielded large enough samples of nearby low-mass stars to study this issue
statistically to good accuracy. Previously, we have presented a multiplicity
study of ~700 early/mid M-type stars observed with the AstraLux high-resolution
Lucky Imaging cameras. Here, we extend the study of multiplicity in M-type
stars through studying 286 nearby mid/late M-type stars, bridging the gap
between our previous study and multiplicity studies of brown dwarfs. Most of
the targets have been observed more than once, allowing us to assess common
proper motion to confirm companionship. We detect 68 confirmed or probable
companions in 66 systems, of which 41 were previously undiscovered. Detections
are made down to the resolution limit of ~100 mas of the instrument. The raw
multiplicity in the AstraLux sensitivity range is 17.9%, leading to a total
multiplicity fraction of 21-27% depending on the mass ratio distribution, which
is consistent with being flat down to mass ratios of ~0.4, but cannot be
stringently constrained below this value. The semi-major axis distribution is
well represented by a log-normal function with mu_a = 0.78 and sigma_a = 0.47,
which is narrower and peaked at smaller separations than for a Sun-like sample.
This is consistent with a steady decrease in average semi-major axis from the
highest-mass binary stars to the brown dwarf binaries. | astro-ph_SR |
J-type carbon stars: A dominant source of 14N-rich presolar SiC grains
of type AB: We report Mo isotopic data of 27 new presolar SiC grains, including 12
14N-rich AB (14N/15N>440, AB2) and 15 mainstream (MS) grains, and their
correlated Sr and Ba isotope ratios when available. Direct comparison of the
data for the MS grains, which came from low-mass asymptotic giant branch (AGB)
stars with large s-process isotope enhancements, with the AB2 grain data
demonstrates that AB2 grains show near-solar isotopic compositions and lack
s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of
AB2 grains clearly exclude born-again AGB stars, where the intermediate
neutron-capture process (i-process) takes place, as their stellar source. On
the other hand, low-mass CO novae, and early R- and J-type carbon stars show
13C and 14N excesses but no s-process enhancements and are thus potential
stellar sources of AB2 grains. Since both early R-type carbon stars and CO
novae are rare objects, the abundant J-type carbon stars (10-15% of all carbon
stars) are thus likely to be a dominant source of AB2 grains. | 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 |
Triple evolution: an important channel in the formation of type Ia
supernovae: Type Ia supernovae (SNe Ia) are thought to be the result of thermonuclear
explosions in white dwarfs (WDs). Commonly considered formation pathways
include two merging WDs (the double degenerate channel), and a single WD
accreting material from a H or He donor (the single degenerate channel). Since
the predicted SNe Ia rates from WD in binaries are thought to be insufficient
to explain the observed SNe Ia rate, it is important to study similar
interactions in higher-order multiple star systems such as triple systems. We
use the evolutionary population synthesis code Multiple Stellar Evolution (MSE)
to study stellar evolution, binary interactions and gravitational dynamics of
the triple-star systems. Also, unlike previous studies, prescriptions are
included to simultaneously take into account the single and double degenerate
channels, and we consider triples across the entire parameter space (including
those with tight inner binaries). We explore the impact of typically ignored or
uncertain physics such as fly-bys and CE prescription parameters on our
results. The majority of systems undergo circular mergers to explode as SNe Ia,
while eccentric collisions contribute to $0.4-4$ per cent of SNe Ia events. The
time-integrated SNe Ia rate from the triple channel is found to be $(3.60 \pm
0.04) \times {10^{-4}}\,\mathrm{M_{\odot}}^{-1}$ which is, surprisingly,
similar to that of the isolated binary channel where the SNe Ia rate is $(3.2
\pm 0.1) \times {10^{-4}}\,\mathrm{M_{\odot}}^{-1}$. This implies that triples,
when considering their entire parameter space, yield an important contribution
to the overall SNe Ia rate. | astro-ph_SR |
Observation of flux tube crossings in the solar wind: Current sheets are ubiquitous in the solar wind.They are a major source of
the solar wind MHD turbulence intermittency. They may result from non-linear
interactions of the solar wind MHD turbulence or are the boundaries of flux
tubes that originate from the solar surface. Some current sheets appear in
pairs and are the boundaries of transient structures such as magnetic holes and
reconnection exhausts, or the edges of pulsed Alfv\'{e}n waves. For an
individual current sheet, discerning whether it is a flux tube boundary or due
to non-linear interactions, or the boundary of a transient structure is
difficult. In this work, using data from the {\sl Wind} spacecraft, we identify
two three-current-sheet events. Detailed examination of these two events
suggest that they are best explained by the flux tube crossing scenario. Our
study provides a convincing evidence supporting the scenario that the solar
wind consists of flux tubes where distinct plasmas reside. | astro-ph_SR |
Chromospheric swirls I. Automated detection in H$α$ observations
and their statistical properties: Chromospheric swirls are considered to play a significant role in the
dynamics and heating of the upper solar atmosphere. It is important to
automatically detect and track them in chromospheric observations and determine
their properties. We applied a recently developed automated chromospheric swirl
detection method to time-series observations of a quiet region of the solar
chromosphere obtained in the H$\alpha$-0.2 \r{A} wavelength of the H$\alpha$
spectral line by the CRISP instrument at the Swedish 1-m Solar Telescope. The
algorithm exploits the morphological characteristics of swirling events in high
contrast chromospheric observations and results in the detection of these
structures in each frame of the time series and their tracking over time. We
conducted a statistical analysis to determine their various properties,
including a survival analysis for deriving the mean lifetime. A mean number of
146 $\pm$ 9 swirls was detected within the FOV at any given time. The mean
surface density is found equal to $\sim$0.08 swirls$ $Mm$^{-2}$ and the
occurrence rate is $\sim$10$^{-2}$ swirls$ $Mm$^{-2}$ min$^{-1}$. These values
are much higher than those previously reported from chromospheric observations.
The radii of the detected swirls range between 0.5 and 2.5 Mm, with a mean
value equal to 1.3 $\pm$ 0.3 Mm, which is slightly higher than previous
reports. The lifetimes range between 1.5 min and 33.7 min with an arithmetic
mean value of $\sim$8.5 min. A survival analysis of the lifetimes, however,
using the Kaplan-Meier estimator in combination with a parametric model results
in a mean lifetime of 10.3 $\pm$ 0.6 min. An automated method sheds more light
on their abundance than visual inspection, while higher cadence, higher
resolution observations will most probably result in the detection of a higher
number of such features on smaller scales and with shorter lifetimes. | astro-ph_SR |
Evolved Massive Stars at Low-metallicity II. Red Supergiant Stars in the
Small Magellanic Cloud: We present the most comprehensive RSG sample for the SMC up to now, including
1,239 RSG candidates. The initial sample is derived based on a source catalog
for the SMC with conservative ranking. Additional spectroscopic RSGs are
retrieved from the literature, as well as RSG candidates selected from the
inspection of CMDs. We estimate that there are in total $\sim$ 1,800 or more
RSGs in the SMC. We purify the sample by studying the infrared CMDs and the
variability of the objects, though there is still an ambiguity between AGBs and
RSGs. There are much less RSGs candidates ($\sim$4\%) showing PAH emission
features compared to the Milky Way and LMC ($\sim$15\%). The MIR variability of
RSG sample increases with luminosity. We separate the RSG sample into two
subsamples ("risky" and "safe") and identify one M5e AGB star in the "risky"
subsample. Most of the targets with large variability are also the bright ones
with large MLR. Some targets show excessive dust emission, which may be related
to previous episodic mass loss events. We also roughly estimate the total gas
and dust budget produced by entire RSG population as $\rm
\sim1.9^{+2.4}_{-1.1}\times10^{-6}~M_{\odot}/yr$ in the most conservative case.
Based on the MIST models, we derive a linear relation between $T_{\rm eff}$ and
observed $\rm J-K_S$ color with reddening correction for the RSG sample. By
using a constant bolometric correction and this relation, the Geneva
evolutionary model is compared with our RSG sample, showing a good agreement
and a lower initial mass limit of $\sim$7 $\rm M_\odot $ for the RSG
population. Finally, we compare the RSG sample in the SMC and the LMC. Despite
the incompleteness of LMC sample in the faint end, the result indicates that
the LMC sample always shows redder color (except for the $\rm IRAC1-IRAC2$ and
$\rm WISE1-WISE2$ colors due to CO absorption) and larger variability than the
SMC sample. | astro-ph_SR |
Primeval very low-mass stars and brown dwarfs - VI. Population
properties of metal-poor degenerate brown dwarfs: We presented 15 new T dwarfs that were selected from UKIRT Infrared Deep Sky
Survey, Visible and Infrared Survey Telescope for Astronomy, and Wide-field
Infrared Survey Explorer surveys, and confirmed with optical to near infrared
spectra obtained with the Very Large Telescope and the Gran Telescopio
Canarias. One of these new T dwarfs is mildly metal-poor with slightly
suppressed $K$-band flux. We presented a new X-shooter spectrum of a known
benchmark sdT5.5 subdwarf, HIP 73786B. To better understand observational
properties of brown dwarfs, we discussed transition zones (mass ranges) with
low-rate hydrogen, lithium, and deuterium burning in brown dwarf population.
The hydrogen burning transition zone is also the substellar transition zone
that separates very low-mass stars, transitional, and degenerate brown dwarfs.
Transitional brown dwarfs have been discussed in previous works of the Primeval
series. Degenerate brown dwarfs without hydrogen fusion are the majority of
brown dwarfs. Metal-poor degenerate brown dwarfs of the Galactic thick disc and
halo have become T5+ subdwarfs. We selected 41 T5+ subdwarfs from the
literature by their suppressed $K$-band flux. We studied the spectral-type -
colour correlations, spectral-type - absolute magnitude correlations,
colour-colour plots, and HR diagrams of T5+ subdwarfs, in comparison to these
of L-T dwarfs and L subdwarfs. We discussed the T5+ subdwarf discovery
capability of deep sky surveys in the 2020s. | astro-ph_SR |
Twenty-Five Sub-Arcsecond Binaries Discovered By Lunar Occultations: We report on 25 sub-arcsecond binaries, detected for the first time by means
of lunar occultations in the near-infrared as part of a long-term program using
the ISAAC instrument at the ESO Very Large Telescope. The primaries have
magnitudes in the range K=3.8 to 10.4, and the companions in the range K=6.4 to
12.1. The magnitude differences have a median value of 2.8, with the largest
being 5.4. The projected separations are in the range 6 to 748 milliarcseconds
and with a median of 18 milliarcseconds, or about 3 times less than the
diffraction limit of the telescope. Among our binary detections are a pre-main
sequence star and an enigmatic Mira-like variable previously suspected to have
a companion. Additionally, we quote an accurate first-time near-IR detection of
a previously known wider binary.
We discuss our findings on an individual basis as far as made possible by the
available literature, and we examine them from a statistical point of view. We
derive a typical frequency of binarity among field stars of ~10%, in the
resolution and sensitivity range afforded by the technique (~0.003" to ~0.5",
and K~12mag, respectively). This is in line with previous results by the same
technique but we point out interesting differences that we can trace up to
sensitivity, time sampling, and average distance of the targets. Finally, we
discuss the prospects for further follow-up studies. | astro-ph_SR |
Derivations and Observations of Prominence Bulk Motions and Mass: In this chapter we review observations and techniques for measuring both bulk
flows in prominences and prominence mass. Measuring these quantities is
essential to development and testing of models discussed throughout this book.
Prominence flows are complex and various, ranging from the relatively linear
flows along prominence spines to the complex, turbulent patterns exhibited by
hedgerow prominences. Techniques for measuring flows include time slice and
optical flow techniques used for motions in the plane of the sky and the use of
spectral line profiles to determine Doppler velocities along the line of sight.
Prominence mass measurement is chiefly done via continuum absorption
measurements, but mass has also been estimated using cloud modeling and white
light measurements. | astro-ph_SR |
Saturation of large-scale dynamo in anisotropically forced turbulence: Turbulent dynamo theories have faced difficulties in obtaining evolution of
large-scale magnetic fields on short dynamical time-scales due to the
constraint imposed by magnetic helicity balance. This has critical implications
for understanding the large-scale magnetic field evolution in astrophysical
systems like the Sun, stars and galaxies. Direct numerical simulations (DNS) in
the past with isotropically forced helical turbulence have shown that
large-scale dynamo saturation time-scales are dependent on the magnetic
Reynolds number (Rm). In this work, we have carried out periodic box DNS of
helically forced turbulence leading to a large-scale dynamo with two kinds of
forcing function, an isotropic one based on that used in PENCIL-CODE and an
anisotropic one based on Galloway-Proctor flows. We show that when the
turbulence is forced anisotropically, the nonlinear (saturation) behaviour of
the large-scale dynamo is only weakly dependent on Rm. In fact the magnetic
helicity evolution on small and large scales in the anisotropic case is
distinctly different from that in the isotropic case. This result possibly
holds promise for the alleviation of important issues like catastrophic
quenching. | astro-ph_SR |
Mapping the Local Halo: Statistical Parallax Analysis of SDSS Low-Mass
Subdwarfs: We present a statistical parallax study of nearly 2,000 M subdwarfs with
photometry and spectroscopy from the Sloan Digital Sky Survey. Statistical
parallax analysis yields the mean absolute magnitudes, mean velocities and
velocity ellipsoids for homogenous samples of stars. We selected homogeneous
groups of subdwarfs based on their photometric colors and spectral appearance.
We examined the color-magnitude relations of low-mass subdwarfs and quantified
their dependence on the newly-refined metallicity parameter, zeta. We also
developed a photometric metallicity parameter, delta(g-r), based on the g-r and
r-z colors of low-mass stars and used it to select stars with similar
metallicities. The kinematics of low-mass subdwarfs as a function of color and
metallicity were also examined and compared to main sequence M dwarfs. We find
that the SDSS subdwarfs share similar kinematics to the inner halo and thick
disk. The color-magnitude relations derived in this analysis will be a powerful
tool for identifying and characterizing low-mass metal-poor subdwarfs in future
surveys such as GAIA and LSST, making them important and plentiful tracers of
the stellar halo. | astro-ph_SR |
Type Ia Single Degenerate Survivors Must Be Overluminous: In the single-degenerate (SD) channel of a Type Ia supernovae (SN Ia)
explosion, a main-sequence (MS) donor star survives the explosion but it is
stripped of mass and shock heated. An essentially unavoidable consequence of
mass loss during the explosion is that the companion must have an overextended
envelope after the explosion. While this has been noted previously, it has not
been strongly emphasized as an inevitable consequence. We calculate the future
evolution of the companion by injecting 2-6 10^47 ergs into the stellar
evolution model of a 1 Msun donor star based on the post-explosion progenitors
seen in simulations. We find that, due to the Kelvin-Helmholtz collapse of the
envelope, the companion must become significantly more luminous (10 - 10^3
Lsun) for a long period of time (10^3 - 10^4 years). The lack of such a
luminous "leftover" star in the LMC supernova remnant SNR 0609-67.5 provides
another piece of evidence against the SD scenario. We also show that none of
the stars proposed as the survivors of the Tycho supernova, including Tycho G,
could plausibly be the donor star. Additionally, luminous donors closer than
~10 Mpc should be observable with the Hubble Space Telescope starting ~2 years
post-peak. Such systems include SN 1937C, SN 1972E, SN 1986G, and SN 2011fe.
Thus, the SD channel is already ruled out for at least two nearby SNe Ia and
can be easily tested for a number of additional ones. We also discuss similar
implications for the companions of core-collapse SNe. | astro-ph_SR |
A Search for Interstellar Carbon Chain Alcohol HC4OH in Star-Forming
Region L1527 and Dark Cloud TMC-1: We report a sensitive search for the rotational transitions of the carbon
chain alcohol HC4OH in the frequency range of 21.2-46.7 GHz in the star-forming
region L1527 and the dark cloud TMC-1. The motivation was laboratory detection
of HC4OH by microwave spectroscopy. Despite achieving rms noise levels of
several millikelvin in the antenna temperature using the 45 m telescope at
Nobeyama Radio Observatory, the detection was not successful, leading to 3
sigma upper limits corresponding to the column densities of 2.0 \times 1012 and
5.6 \times 1012 cm-2 in L1527 and TMC-1, respectively. These upper limits
indicate that [HC4OH]/[HC5N] ratios are less than 0.3 and 0.1 in L1527 and
TMC-1, respectively, where HC5N is an HC4-chain cyanide and HC4OH is a
hydroxide. These ratios suggest that the cyano carbon chain molecule dominates
the hydroxyl carbon chain molecule in L1527 and TMC-1. This is contrary to the
case of saturated compounds in hot cores, e.g., CH3OH and CH3CN, and can be a
chemical feature of carbon chain molecules in L1527 and TMC-1. In addition, the
column densities of the "unsubstituted" carbon chain molecule C4H and the
sulfur-bearing molecules SO and HCS+ were determined from detected lines in
L1527. | astro-ph_SR |
Investigating Gaia EDR3 parallax systematics using asteroseismology of
Cool Giant Stars observed by Kepler, K2, and TESS I. Asteroseismic distances
to 12,500 red-giant stars: Gaia EDR3 has provided unprecedented data that generate a lot of interest in
the astrophysical community, despite the fact that systematics affect the
reported parallaxes at the level of ~ 10 muas. Independent distance
measurements are available from asteroseismology of red-giant stars with
measurable parallaxes, whose magnitude and colour ranges more closely reflect
those of other stars of interest. In this paper, we determine distances to
nearly 12,500 red-giant branch and red clump stars observed by Kepler, K2, and
TESS. This is done via a grid-based modelling method, where global
asteroseismic observables, constraints on the photospheric chemical
composition, and on the unreddened photometry are used as observational inputs.
This large catalogue of asteroseismic distances allows us to provide a first
comparison with Gaia EDR3 parallaxes. Offset values estimated with
asteroseismology show no clear trend with ecliptic latitude or magnitude, and
the trend whereby they increase (in absolute terms) as we move towards redder
colours is dominated by the brightest stars. The correction model proposed by
Lindegren et al. (2021) is not suitable for all the fields considered in this
study. We find a good agreement between asteroseismic results and model
predictions of the red clump magnitude. We discuss possible trends with the
Gaia scan law statistics, and show that two magnitude regimes exist where
either asteroseismology or Gaia provides the best precision in parallax. | astro-ph_SR |
Diffraction-limited Visible Light Images of Orion Trapezium Cluster With
the Magellan Adaptive Secondary AO System (MagAO): We utilized the new high-order (250-378 mode) Magellan Adaptive Optics system
(MagAO) to obtain very high spatial resolution observations in "visible light"
with MagAO's VisAO CCD camera. In the good-median seeing conditions of Magellan
(0.5-0.7") we find MagAO delivers individual short exposure images as good as
19 mas optical resolution. Due to telescope vibrations, long exposure (60s) r'
(0.63 micron) images are slightly coarser at FWHM=23-29 mas (Strehl ~28%) with
bright (R<9 mag) guide stars. These are the highest resolution filled-aperture
images published to date. Images of the young (~1 Myr) Orion Trapezium Theta 1
Ori A, B, and C cluster members were obtained with VisAO. In particular, the 32
mas binary Theta 1 Ori C1/C2 was easily resolved in non-interferometric images
for the first time. Relative positions of the bright trapezium binary stars
were measured with ~0.6-5 mas accuracy. We now are sensitive to relative proper
motions of just ~0.2 mas/yr (~0.4 km/s at 414 pc) - this is a ~2-10x
improvement in orbital velocity accuracy compared to previous efforts. For the
first time, we see clear motion of the barycenter of Theta 1 Ori B2/B3 about
Theta 1 Ori B1. All five members of the Theta 1 Ori B system appear likely a
gravitationally bound "mini-cluster", but we find that not all the orbits can
be both circular and co-planar. The lowest mass member of the Theta 1 Ori B
system (B4; mass ~0.2 Msun) has a very clearly detected motion (at 4.1+/-1.3
km/s; correlation=99.9%) w.r.t B1 and will likely be ejected in the future.
This "ejection" process of the lowest mass member of a "mini-cluster" could
play a major role in the formation of low mass stars and brown dwarfs.(slightly
abridged abstract) | astro-ph_SR |
Models of Individual Blue Stragglers: This chapter describes the current state of models of individual blue
stragglers. Stellar collisions, binary mergers (or coalescence), and partial or
ongoing mass transfer have all been studied in some detail. The products of
stellar collisions retain memory of their parent stars and are not fully mixed.
Very high initial rotation rates must be reduced by an unknown process to allow
the stars to collapse to the main sequence. The more massive collision products
have shorter lifetimes than normal stars of the same mass, while products
between low mass stars are long-lived and look very much like normal stars of
their mass. Mass transfer can result in a merger, or can produce another binary
system with a blue straggler and the remnant of the original primary. The
products of binary mass transfer cover a larger portion of the colour-magnitude
diagram than collision products for two reasons: there are more possible
configurations which produce blue stragglers, and there are differing
contributions to the blended light of the system. The effects of rotation may
be substantial in both collision and merger products, and could result in
significant mixing unless angular momentum is lost shortly after the formation
event. Surface abundances may provide ways to distinguish between the formation
mechanisms, but care must be taking to model the various mixing mechanisms
properly before drawing strong conclusions. Avenues for future work are
outlined. | astro-ph_SR |
The quest for stable circumbinary companions to post-common envelope sdB
eclipsing binaries Does the observational evidence support their existence?: Context. Period variations have been detected in a number of eclipsing close
compact binary subdwarf B stars (sdBs) and these have often been interpreted as
caused by circumbinary massive planets or brown dwarfs. Various evolutionary
scenarios have been proposed for these stars, but a definite mechanism remains
to be established. Equally puzzling is the formation of these putative
circumbinary objects which must have formed either from the remaining post
common envelope circumbinary disk or survived its evolution. Aims. In this
paper we review the eclipse time variations (ETVs) exhibited by seven such
systems and explore if there is conclusive evidence that the ETVs observed over
the last two decades can reliably predict the presence of circumbinary bodies.
Methods. We report 246 new observations of the seven sdB systems made between
2013 September and 2017 July using a worldwide network of telescopes. We
combined our new data with previously published measurements to analyse the
ETVs of these systems. Results. Our data shows that period variations cannot be
modelled simply on the basis of circumbinary objects. This implies that more
complex processes may be taking place in these systems. From eclipse time
variations, it has historically been suggested that five of the seven binary
systems reported herein had circumbinary objects. Based on our recent
observations and analysis only three systems remain serious contenders. We find
agreement with other observers that at least a decade of observations is
required to establish reliable ephemeris. With longer observational baselines
it is quite conceivable that the data will support the circumbinary object
hypothesis of these binary systems. Also we generally agree with other
observers that larger values of (O-C) residuals are found with secondary
companions of spectral type M5/6 or earlier as a result of an Applegate type
mechanism | astro-ph_SR |
Zeeman Doppler maps. II: the perils of eschewing physics: For the observational modeling of horizontal abundance distributions and of
magnetic geometries in chemically peculiar (CP) stars, Zeeman Doppler mapping
(ZDM) has become the method of choice. Comparisons between abundance maps
obtained for CP stars and predictions from numerical simulations of atomic
diffusion have always proved unsatisfactory, with the blame routinely put on
theory. Expanding a previous study aimed at clarifying the question of the
uniqueness of ZDM maps, this paper inverts the roles between observational
modeling and time-dependent diffusion results, casting a cold eye on essential
assumptions and algorithms underlying ZDM, in particular the Tikhonov-style
regularization functionals, from 1D to 3D. We show that these have been
established solely for mathematical convenience, but that they in no way
reflect the physical reality in the atmospheres of magnetic CP stars.
Recognizing that the observed strong magnetic fields in most well-mapped stars
require the field geometry to be force-free, we demonstrate that many published
maps do not meet this condition. There follows a discussion of the frequent
changes in magnetic and abundance maps of well observed stars and a caveat
concerning the use of least squares deconvolution in ZDM analyses. It emerges
that because of the complexity and non-linearity of the field-dependent
chemical stratifications, Tikhonov based ZDM inversions cannot recover the true
abundance and magnetic geometries. As our findings additionally show, there is
no way to define a physically meaningful 3D regularization functional instead.
ZDM remains dysfunctional and does not provide any observational constraints
for the modeling of atomic diffusion. | astro-ph_SR |
The Herschel Cold Debris Disks: Confusion with the Extragalactic
Background at 160 mu: The Herschel "DUst around NEarby Stars (DUNES)" survey has found a number of
debris disk candidates that are apparently very cold, with temperatures near
22K. It has proven difficult to fit their spectral energy distributions with
conventional models for debris disks. Given this issue we carefully examine the
alternative explanation, that the detections arise from confusion with IR
cirrus and/or background galaxies that are not physically associated with the
foreground star. We find that such an explanation is consistent with all of
these detections. | astro-ph_SR |
Survey of Nearby FGK Stars at 160 microns with Spitzer: The Spitzer Space Telescope has advanced debris disk science tremendously
with a wealth of information on debris disks around nearby A, F, G, K and M
stars at 24 and 70 microns with the MIPS photometer and at 8-34 microns with
IRS. Here we present 160 micron observations of a small sub-set of these stars.
At this wavelength, the stellar photospheric emission is negligible and any
detected emission corresponds to cold dust in extended Kuiper belt analogs.
However, the Spitzer 160 micron observations are limited in sensitivity by the
large beam size which results in significant ''noise'' due to cirrus and
extragalactic confusion. In addition, the 160 micron measurements suffer from
the added complication of a light leak next to the star's position whose flux
is proportional to the near-infrared flux of the star. We are able to remove
the contamination from the leak and report 160 micron measurements or upper
limits for 24 stars. Three stars (HD 10647, HD 207129, and HD 115617) have
excesses at 160 micron that we use to constrain the properties of the debris
disks around them. A more detailed model of the spectral energy distribution of
HD 10647 reveals that the 70 and 160 micron emission could be due to small
water ice particles at a distance of 100 AU consistent with Hubble Space
Telescope optical imaging of circumstellar material in the system. | astro-ph_SR |
Binarity and Accretion in AGB Stars: HST/STIS Observations of UV
Flickering in Y Gem: Binarity is believed to dramatically affect the history and geometry of mass
loss in AGB and post-AGB stars, but observational evidence of binarity is
sorely lacking. As part of a project to search for hot binary companions to
cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to
be a source of strong and variable UV and X-ray emission. Here we report UV
spectroscopic observations of Y Gem obtained with the Hubble Space Telescope
that show strong flickering in the UV continuum on time-scales of <~20 s,
characteristic of an active accretion disk. Several UV lines with P-Cygni-type
profiles from species such as Si IV and C IV are also observed, with emission
and absorption features that are red- and blue- shifted by velocities of ~500
km/s from the systemic velocity. Our model for these (and previous)
observations is that material from the primary star is gravitationally captured
by a companion, producing a hot accretion disk. The latter powers a fast
outflow that produces blue-shifted features due to absorption of UV continuum
emitted by the disk, whereas the red-shifted emission features arise in heated
infalling material from the primary. The outflow velocities support a previous
inference by Sahai et al. (2015) that Y Gem's companion is a low-mass
main-sequence star. Blackbody fitting of the UV continuum implies an accretion
luminosity of about 13 Lsun and thus a mass-accretion rate >5e-7 Msun/yr; we
infer that Roche lobe overflow is the most likely binary accretion mode for Y
Gem. | astro-ph_SR |
Magnetic field variations and the seismicity of solar active regions: Dynamical changes in the solar corona have proven to be very important in
inducing seismic waves into the photosphere. Different mechanisms for their
generation have been proposed. In this work, we explore the magnetic field
forces as plausible mechanisms to generate sunquakes as proposed by Hudson,
Fisher and Welsch. We present a spatial and temporal analysis of the
line-of-sight magnetic field variations induced by the seismically active 2003
October 29 and 2005 January 15 solar flares and compare these results with
other supporting observations. | astro-ph_SR |
Properties of ion-cyclotron waves in the open solar corona: Remote observations of coronal holes have strongly implicated the resonant
interactions of ion-cyclotron waves with ions as a principal mechanism for
plasma heating and acceleration of the fast solar wind. In order to study these
waves, a WKB (Wentzel-Kramers-Brillouin) linear perturbation analysis is used
in the work frame of the collisionless multi-fluid model where we consider in
addition to the protons a second ion component made of alpha particles. We
consider a non-uniform background plasma describing a funnel region in the open
coronal holes and we use the ray tracing Hamiltonian type equations to compute
the ray path of the waves and the spatial variation of their properties. | astro-ph_SR |
A Multi-Survey Approach to White Dwarf Discovery: By selecting astrometric and photometric data from the Sloan Digital Sky
Survey (SDSS), the L{\'e}pine & Shara Proper Motion North Catalog (LSPM-North),
the Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a
succession of methods to isolate white dwarf candidates for follow-up
spectroscopy. Our methods include: reduced proper motion diagram cuts, color
cuts, and atmospheric model adherence. We present spectroscopy of 26 white
dwarfs obtained from the CTIO 4m and APO 3.5m telescopes. Additionally, we
confirm 28 white dwarfs with spectra available in the SDSS DR7 database but
unpublished elsewhere, presenting a total of 54 WDs. We label one of these as a
recovered WD while the remaining 53 are new discoveries. We determine physical
parameters and estimate distances based on atmospheric model analyses. Three
new white dwarfs are modeled to lie within 25 pc. Two additional white dwarfs
are confirmed to be metal-polluted (DAZ). Follow-up time series photometry
confirms another object to be a pulsating ZZ Ceti white dwarf. | astro-ph_SR |
Most Lithium-rich Low-mass Evolved Stars Revealed as Red Clump stars by
Asteroseismology and Spectroscopy: Lithium has confused scientists for decades at almost each scale of the
universe. Lithium-rich giants are peculiar stars with lithium abundances over
model prediction. A large fraction of lithium-rich low-mass evolved stars are
traditionally supposed to be red giant branch (RGB) stars. Recent studies,
however, report that red clump (RC) stars are more frequent than RGB. Here, we
present a uniquely large systematic study combining the direct asteroseismic
analysis with the spectroscopy on the lithium-rich stars. The majority of
lithium-rich stars are confirmed to be RCs, whereas RGBs are minor. We reveal
that the distribution of lithium-rich RGBs steeply decline with the increasing
lithium abundance, showing an upper limit around 2.6 dex, whereas the Li
abundances of RCs extend to much higher values. We also find that the
distributions of mass and nitrogen abundance are notably different between RC
and RGB stars. These findings indicate that there is still unknown process that
significantly affects surface chemical composition in low-mass stellar
evolution. | astro-ph_SR |
Characteristics of Flight Delays during Solar Flares: Solar flare is one of the severest solar activities on the sun, and it has
many important impacts on the near-earth space. It has been found that flight
arrival delays will increase during solar flare. However, the detailed
intrinsic mechanism of how solar flares influence the delays is still unknown.
Based on 5-years huge amount of flight data, here we comprehensively analyze
the flight departure delays during 57 solar flares. It is found that the
averaged flight departure delay time during solar flares increased by 20.68%
(7.67 min) compared to those during quiet periods. It is also shown that solar
flare related flight delays reveal apparent time and latitude dependencies.
Flight delays during dayside solar flares are more serious than those during
nightside flares, and the longer (shorter) delays tend to occur in the lower
(higher) latitude airport. Further analyses suggest that flight delay time and
delay rate would be directly modulated by the solar intensity (soft X-ray flux)
and the Solar Zenith Angle. For the first time, these results indicate that the
communication interferences caused by solar flares will directly affect flight
departure delay time and delay rate. This work also expands our conventional
understandings to the impacts of solar flares on human society, and it could
also provide us with brand new views to help prevent or cope with flight
delays. | astro-ph_SR |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.