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Discovery of 34 low-mass comoving systems using NOIRLab Source Catalog
DR2: We present the discovery of 34 comoving systems containing an ultra-cool
dwarf found by means of the NOIRLab Source Catalog (NSC) DR2. NSC's angular
resolution of $\sim$1" allows for the detection of small separation binaries
with significant proper motions. We used the catalog's accurate proper motion
measurements to identify the companions by cross-matching a previously compiled
list of brown dwarf candidates with NSC DR2. The comoving pairs consist of
either a very low-mass star and an ultra-cool companion, or a white dwarf and
an ultra-cool companion. The estimated spectral types of the primaries are in
the K and M dwarf regimes, those of the secondaries in the M, L and T dwarf
regimes. We calculated angular separations between $\sim$2 and $\sim$56",
parallactic distances between $\sim$43 and $\sim$261 pc and projected physical
separations between $\sim$169 and $\sim$8487 AU. The lowest measured total
proper motion is 97 mas yr$^{-1}$, the highest 314 mas yr$^{-1}$. Tangential
velocities range from $\sim$23 to $\sim$187 km s$^{-1}$. We also determined
comoving probabilities, estimated mass ratios and calculated binding energies
for each system. We found no indication of possible binarity for any component
of the 34 systems in the published literature. The discovered systems can
contribute to the further study of the formation and evolution of low-mass
systems as well as to the characterization of cool substellar objects. | astro-ph_SR |
Miras around the Galactic Center: We report results of our near-IR survey for variables in a field of view of
20 arcmin by 30 arcmin towards the Galactic Center (GC), where we detected 1364
long-period variables. We have established a method for the simultaneous
estimation of distances and extinctions using the period-luminosity relations
for the JHKs bands. Our method is applicable to Miras with periods in the range
100-350 days and mean magnitudes available in two or more filters. Here we
discuss 143 Miras whose distances and extinctions were obtained based on their
periods and H- and Ks-band magnitudes. We find that almost all of them are
located at the same distance to within our accuracy, and the distance modulus
of the GC is estimated to be 14.58+-0.02+-0.11 mag. The former error
corresponds to the statistical error and the latter to the systematic one which
includes the uncertainty of our assumed distance modulus of the LMC
(18.45+-0.05 mag). We also discuss the large and highly variable extinction
towards the GC. | astro-ph_SR |
Flickering in AGB stars: Probing the nature of accreting companions: Binary companions to asymptotic giant branch (AGB) stars are an important
aspect of their evolution. Few AGB companions have been detected, and in most
cases it is difficult to distinguish between main-sequence and white dwarf
companions. Detection of photometric flickering, a tracer of compact accretion
disks around white dwarfs, can help identify the nature of these companions. In
this work, we searched for flickering in four AGB stars suggested to have
likely accreting companions. We found no signs for flickering in two targets:
R~Aqr and V1016 Cyg. Flickering was detected in the other two stars: Mira and Y
Gem. We investigated the true nature of Mira's companion using three different
approaches. Our results for Mira strongly suggest that its companion is a white
dwarf. | astro-ph_SR |
MHD waves in sunspots: The review addresses the spatial frequency morphology of sources of sunspot
oscillations and waves, including their localization, size, oscillation
periods, height localization with the mechanism of cut-off frequency that forms
the observed emission variability. Dynamic of sunspot wave processes, provides
the information about the structure of wave fronts and their time variations,
investigates the oscillation frequency transformation depending on the wave
energy is shown. The initializing solar flares caused by trigger agents like
magnetoacoustic waves, accelerated particle beams, and shocks are discussed.
Special attention is paid to the relation between the flare reconnection
periodic initialization and the dynamics of sunspot slow magnetoacoustic waves.
A short review of theoretical models of sunspot oscillations is provided. | astro-ph_SR |
Simultaneous Multiwavelength Observations of Magnetic Activity in
Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS
J13142039+1320011): We present multi-epoch simultaneous radio, optical, H{\alpha}, UV, and X-ray
observations of the active, young, low-mass binary NLTT 33370 AB (blended
spectral type M7e). This system is remarkable for its extreme levels of
magnetic activity: it is the most radio-luminous ultracool dwarf (UCD) known,
and here we show that it is also one of the most X-ray luminous UCDs known. We
detect the system in all bands and find a complex phenomenology of both flaring
and periodic variability. Analysis of the optical light curve reveals the
simultaneous presence of two periodicities, 3.7859 $\pm$ 0.0001 and 3.7130
$\pm$ 0.0002 hr. While these differ by only ~2%, studies of differential
rotation in the UCD regime suggest that it cannot be responsible for the two
signals. The system's radio emission consists of at least three components:
rapid 100% polarized flares, bright emission modulating periodically in phase
with the optical emission, and an additional periodic component that appears
only in the 2013 observational campaign. We interpret the last of these as a
gyrosynchrotron feature associated with large-scale magnetic fields and a cool,
equatorial plasma torus. However, the persistent rapid flares at all rotational
phases imply that small-scale magnetic loops are also present and reconnect
nearly continuously. We present an SED of the blended system spanning more than
9 orders of magnitude in wavelength. The significant magnetism present in NLTT
33370 AB will affect its fundamental parameters, with the components' radii and
temperatures potentially altered by ~+20% and ~-10%, respectively. Finally, we
suggest spatially resolved observations that could clarify many aspects of this
system's nature. | astro-ph_SR |
6.7 GHz variability characteristics of new periodic methanol maser
sources: Discovery of periodic maser emission was an unexpected result from monitoring
observations of methanol transitions in high-mass young stellar objects. We
report on the detection of five new periodic sources from a monitoring program
with the Torun 32 m telescope. Variability with a period of 149 to 540 d and
different patterns from sinusoidal-like to intermittent was displayed. Three
dimensional structure of G59.633$-$0.192 determined from the time delays of
burst peaks of the spectral features and high angular resolution map implies
that the emission traces a disk. For this source the 6.7 GHz light curve
followed the infrared variability supporting a radiative scheme of pumping. An
unusual time delay of $\sim$80 d occurred in G30.400$-$0.296 could not be
explained by the light travel time and may suggest a strong differentiation of
physical conditions and excitation in this deeply embedded source. Our
observations suggest the intermittent variability may present a simple response
of maser medium to the underlying variability induced by the accretion
luminosity while other variability patterns may reflect more complex changes in
the physical conditions. | astro-ph_SR |
LMC Blue Supergiant Stars and the Calibration of the Flux-weighted
Gravity--Luminosity Relationship: High quality spectra of 90 blue supergiant stars in the Large Magellanic
Cloud are analyzed with respect to effective temperature, gravity, metallicity,
reddening, extinction and extinction law. An average metallicity, based on Fe
and Mg abundances, relative to the Sun of [Z] = -0.35 +/- 0.09 dex is obtained.
The reddening distribution peaks at E(B-V) = 0.08 mag, but significantly larger
values are also encountered. A wide distribution of the ratio of extinction to
reddening is found ranging from Rv = 2 to 6. The results are used to
investigate the blue supergiant relationship between flux-weighted gravity, and
absolute bolometric magnitude. The existence of a tight relationship, the FGLR,
is confirmed. However, in contrast to previous work the observations reveal
that the FGLR is divided into two parts with a different slope. For
flux-weighted gravities larger than 1.30 dex the slope is similar as found in
previous work, but the relationship becomes significantly steeper for smaller
values of the flux-weighted gravity. A new calibration of the FGLR for
extragalactic distance determinations is provided. | astro-ph_SR |
Two circumstellar nebulae discovered with the Wide-field Infrared Survey
Explorer and their massive central stars: We report the discovery of two mid-infrared nebulae in the northern
hemisphere with the Wide-field Infrared Survey Explorer and the results of
optical spectroscopy of their central stars, BD+60 2668 (composed of two
components, separated from each other by \approx 3 arcsec) and ALS 19653, with
the Calar Alto 3.5-m telescope and the Southern African Large Telescope (SALT),
respectively. We classify the components of BD+60 2668 as stars of spectral
types B0.5 II and B1.5 III. ALS 19653 is indicated in the SIMBAD data base as a
planetary nebula, while our observations show that it is a massive B0.5 Ib
star, possibly in a binary system. Using the stellar atmosphere code FASTWIND,
we derived fundamental parameters of the three stars as well as their surface
element abundances, implying that all of them are either on the main sequence
or only recently left it. This provides further evidence that massive stars can
produce circumstellar nebulae while they are still relatively unevolved. We
also report the detection of optical counterparts to the mid-infrared nebulae
and a second, more extended optical nebula around ALS 19653, and present the
results of SALT spectroscopy of both nebulae associated with this star. The
possible origin of the nebulae is discussed. | astro-ph_SR |
High-precision abundances of Sc, Mn, Cu, and Ba in solar twins. Trends
of element ratios with stellar age: A previous study of correlations between element abundance ratios, [X/Fe],
and ages of solar twin stars is extended to include Sc, Mn, Cu, and Ba. HARPS
spectra with S/N > 600 are used to derive very precise (+/- 0.01 dex)
differential abundances, and stellar ages with internal errors less than 1 Gyr
are obtained by interpolation in the logg - Teff diagram between isochrones
calculated with the Aarhus Stellar Evolution Code. For stars younger than 6
Gyr, [X/Fe] is tightly correlated with stellar age for all elements. For ages
between 6 and 9 Gyr, the [X/Fe] - age correlations break down and the stars
split up into two groups having respectively high and low [X/Fe] for the odd-Z
elements. It is concluded that while stars in the solar neighborhood younger
than about 6 Gyr were formed from interstellar gas with a smooth chemical
evolution, older stars have originated from regions enriched by supernovae with
different neutron excesses. Furthermore, the correlations between abundance
ratios and stellar age suggest that: i) Sc is made in Type II supernovae along
with the alpha-capture elements, ii) the Type II to Ia SNe yield ratio is about
the same for Mn and Fe, iii) Cu is mainly made by the weak s-process in massive
stars, iv) the Ba/Y yield ratio for AGB stars increases with decreasing stellar
mass, v) [Y/Mg] and [Y/Al] can be used as "chemical clocks" when determining
ages of solar metallicity stars. | astro-ph_SR |
The DDO Close Binary Spectroscopic Program: The survey of radial velocity orbits for short period (P < 1 day), bright (V
< 10, with a few fainter stars) conducted at the David Dunlap Observatory in
the last 9 years before its closure in 2008 included 162 binaries and resulted
in 150 SB2 orbits and 5 SB1 spectroscopic orbits thus becoming one of the main
legacies of DDO. The paper summarizes the main results from the survey. | astro-ph_SR |
Measurement of the core-collapse progenitor mass distribution of the
Small Magellanic Cloud: The physics of core-collapse (CC) supernovae (SNe) and how the explosions
depend on progenitor properties are central questions in astronomy. For only a
handful of SNe, the progenitor star has been identified in pre-explosion
images. Supernova remnants (SNRs), which are observed long after the original
SN event, provide a unique opportunity to increase the number of progenitor
measurements. Here, we systematically examine the stellar populations in the
vicinities of 23 known SNRs in the Small Magellanic Cloud (SMC) using the star
formation history (SFH) maps of Harris & Zaritsky (2004). We combine the
results with constraints on the SNR metal abundances and environment from X-ray
and optical observations. We find that 22 SNRs in the SMC have local SFHs and
properties consistent with a CC explosion, several of which are likely to have
been high-mass progenitors. This result supports recent theoretical findings
that high-mass progenitors can produce successful explosions. We estimate the
mass distribution of the CC progenitors and find that this distribution is
similar to a Salpeter IMF (within the uncertainties), while this result is
shallower than the mass distribution found in M31 and M33 by Jennings et al.
(2014) and D\'{\i}az-Rodr\'{\i}guez et al. (2018) using a similar approach.
Additionally, we find that a number of the SMC SNRs exhibit a burst of star
formation between 50-200 Myr ago. As these sources are likely CC, this
signature may be indicative of massive stars undergoing delayed CC as a
consequence of binary interaction, rapid rotation, or low metallicity. In
addition, the lack of Type Ia SNRs in the SMC is possibly a result of the short
visibility times of these sources as they may fall below the sensitivity limits
of current radio observations. | astro-ph_SR |
Image patch analysis of sunspots and active regions. I. Intrinsic
dimension and correlation analysis: The flare-productivity of an active region is observed to be related to its
spatial complexity. Mount Wilson or McIntosh sunspot classifications measure
such complexity but in a categorical way, and may therefore not use all the
information present in the observations. Moreover, such categorical schemes
hinder a systematic study of an active region's evolution for example. We
propose fine-scale quantitative descriptors for an active region's complexity
and relate them to the Mount Wilson classification. We analyze the local
correlation structure within continuum and magnetogram data, as well as the
cross-correlation between continuum and magnetogram data. We compute the
intrinsic dimension, partial correlation, and canonical correlation analysis
(CCA) of image patches of continuum and magnetogram active region images taken
from the SOHO-MDI instrument. We use masks of sunspots derived from continuum
as well as larger masks of magnetic active regions derived from the magnetogram
to analyze separately the core part of an active region from its surrounding
part. We find the relationship between complexity of an active region as
measured by Mount Wilson and the intrinsic dimension of its image patches.
Partial correlation patterns exhibit approximately a third-order Markov
structure. CCA reveals different patterns of correlation between continuum and
magnetogram within the sunspots and in the region surrounding the sunspots.
These results also pave the way for patch-based dictionary learning with a view
towards automatic clustering of active regions. | astro-ph_SR |
Close detached white dwarf + brown dwarf binaries: further evidence for
low values of the common envelope efficiency: Common envelope evolution is a fundamental ingredient in our understanding of
the formation of close binary stars containing compact objects which includes
the progenitors of type Ia supernovae, short gamma ray bursts and most stellar
gravitational wave sources. To predict the outcome of common envelope evolution
we still rely to a large degree on a simplified energy conservation equation.
Unfortunately, this equation contains a theoretically rather poorly constrained
efficiency parameter ($\alpha_{\mathrm{CE}}$) and, even worse, it is unclear if
energy sources in addition to orbital energy (such as recombination energy)
contribute to the envelope ejection process. In previous works we reconstructed
the evolution of observed populations of post common envelope binaries (PCEBs)
consisting of white dwarfs with main sequence star companions and found
indications that the efficiency is rather small
($\alpha_{\mathrm{CE}}\simeq0.2-0.3$) and that extra energy sources are only
required in very few cases. Here we used the same reconstruction tool to
investigate the evolutionary history of a sample of observed PCEBs with brown
dwarf companions. In contrast to previous works, we found that the evolution of
observationally well characterized PCEBs with brown dwarf companions can be
understood assuming a low common envelope efficiency
($\alpha_{\mathrm{CE}}=0.24-0.41$), similar to that required to understand
PCEBs with main sequence star companions, and that contributions from
recombination energy are not required. We conclude that the vast majority of
PCEBs form from common envelope evolution that can be parameterized with a
small efficiency and without taking into account additional energy sources. | astro-ph_SR |
Evolution of massive population III stars with rotation and magnetic
fields: [Abridged] We present a new grid of massive population III star models
including the effects of rotation on the stellar structure and chemical mixing,
and magnetic torques for the transport of angular momentum. Based on the grid,
we also present a phase diagram for the expected final fates of rotating
massive Pop III stars. Our non-rotating models become redder than the previous
models in the literature, given the larger overshooting parameter adopted in
this study. In particular, convective dredge-up of the helium core material
into the hydrogen envelope is observed in our non-rotating very massive star
models (>~200 Msun), which is potentially important for the chemical yields. On
the other hand, the stars become bluer and more luminous with a higher
rotational velocity. With the Spruit-Tayler dynamo, our models with a
sufficiently high initial rotational velocity can reach the critical rotation
earlier and lose more mass as a result, compared to the previous models without
magnetic fields. The most dramatic effect of rotation is found with the
so-called chemically homogeneous evolution (CHE), which is observed for a
limited mass and rotational velocity range. CHE has several important
consequences: 1) Both primary nitrogen and ionizing photons are abundantly
produced. 2) Conditions for gamma-ray burst progenitors are fulfilled for an
initial mass range of 13 - 84 Msun. 3) Pair instability supernovae of type Ibc
are expected for 84 -190 Msun and 4) Both a pulsational pair instability
supernova and a GRB may occur from the same progenitor of about 56 - 84 Msun,
which might significantly influence the consequent GRB afterglow. We find that
CHE does not occur for very massive stars (> 190 Msun), in which case the
hydrogen envelope expands to the red-supergiant phase and the final angular
momentum is too low to make any explosive event powered by rotation. | astro-ph_SR |
The Age and The Mass of The αHerculis Triple-Star System From A
MESA Grid of rotating stars with 1.3 <= M/Msun <= 8.0: \alpha^1 Her is the second closest Asymptotic Giant Branch (AGB) star to the
Sun, and the variable luminous M5 Ib-II member of a triple stellar system
containing G8 III and A9 IV-V components. However, the mass of this important
star was previously uncertain with published values ranging from ~2 - 15 Msun.
As shown by this study, its fortuitous membership in a nearby resolved triple
star system, makes it possible to determine its fundamental properties
including its mass and age. We present over twenty years of VRI photometry of
\alpha^1 Her as well as Wing intermediate-band near-IR TiO and NIR continuum
photometry. We introduce a new photometry-based calibration technique, and
extract the effective temperature and luminosity of \alpha^1 Her, in agreement
with recent interferometric measures. We find, Teff=3280 +/- 87 K and
log(L/Lsun)=3.92 +/- 0.14.
With the MESA code, we calculate a dense grid of evolutionary tracks for
Galactic low- to intermediate-mass (1.3 to 8 Msun) rotating stars from the
pre-main sequence phase to the advanced AGB phase. We include atomic diffusion
and rotation mechanisms to treat the effects of extra elemental mixing. Based
on the observed properties of the \alpha Herculis stars, we constrain the age
of the system to lie in the range 0.41 to 1.25 Gyr. Thus, the mass of \alpha^1
Her lies in the range 2.175 <= M/Msun <= 3.250. We compare our model-based age
inference with recent tracks of the Geneva and STAREVOL codes, and show their
agreement. In the prescribed mass range for \alpha^1 Her, the observed 12C/13C
and 16O/17O ratios are consistent (within 2\sigma) with the ratios predicted by
the MESA, Geneva and STAREVOL codes. | astro-ph_SR |
Optical photometric GTC/OSIRIS observations of the young massive
association Cygnus OB2: In order to fully understand the gravitational collapse of molecular clouds,
the star formation process and the evolution of circumstellar disks, these
phenomena must be studied in different Galactic environments with a range of
stellar contents and positions in the Galaxy. The young massive association
Cygnus OB2, in the Cygnus-X region, is an unique target to study how star
formation and the evolution of circumstellar disks proceed in the presence of a
large number of massive stars. We present a catalog obtained with recent
optical observations in r,i,z filters with OSIRIS, mounted on the $10.4\,m$ GTC
telescope, which is the deepest optical catalog of Cyg OB2 to date.
The catalog consist of 64157 sources down to M=0.15 solar masses at the
adopted distance and age of Cyg OB2. A total of 38300 sources have good
photometry in all three bands. We combined the optical catalog with existing
X-ray data of this region, in order to define the cluster locus in the optical
diagrams. The cluster locus in the r-i vs. i-z diagram is compatible with an
extinction of the optically selected cluster members in the 2.64<AV<5.57 range.
We derive an extinction map of the region, finding a median value of AV=4.33 in
the center of the association, decreasing toward the north-west. In the
color-magnitude diagrams, the shape of the distribution of main sequence stars
is compatible with the presence of an obscuring cloud in the foreground at
about 850+/-25 pc from the Sun. | astro-ph_SR |
Brightness of the Sun's small scale magnetic field: proximity effects: The net effect of the small scale magnetic field on the Sun's (bolometric)
brightness is studied with realistic 3D MHD simulations. The direct effect of
brightening within the magnetic field itself is consistent with measurements in
high-resolution observations. The high 'photometric accuracy' of the
simulations, however, reveal compensating brightness effects that are hard to
detect observationally. The influence of magnetic concentrations on the
surrounding nonmagnetic convective flows (a 'proximity effect') reduces the
brightness by an amount exceeding the brightening by the magnetic
concentrations themselves. The net photospheric effect of the small scale field
(~ -0.34% at a mean flux density of 50 G) is thus negative. We conclude that
the main contribution to the observed positive correlation between the magnetic
field and total solar irradiance must be magnetic dissipation in layers around
the temperature minimum and above (not included in the simulations). This
agrees with existing inferences from observations. | astro-ph_SR |
Block-induced complex structures building the flare-productive solar
active region 12673: Solar active region (AR) 12673 produced 4 X-class, 27 M-class, and numerous
lower class flares during its passage across the visible solar disk in
September 2017. Our study is to answer the questions why this AR was so
flare-productive and how the X9.3 flare, the largest one of the last decade,
took place. We find that there was a sunspot in the initial several days, and
then two bipolar regions emerged nearby it successively. Due to the standing of
the pre-existing sunspot, the movement of the bipoles was blocked, while the
pre-existing sunspot maintained its quasi-circular shaped umbra only with the
disappearance of a part of penumbra. Thus, the bipolar patches were
significantly distorted, and the opposite polarities formed two semi-circular
shaped structures. After that, two sequences of new bipolar regions emerged
within the narrow semi-circular zone, and the bipolar patches separated along
the curved channel. The new bipoles sheared and interacted with the previous
ones, forming a complex topological system, during which numerous flares
occurred. At the highly sheared region, a great deal of free energy was
accumulated. On September 6, one negative patch near the polarity inversion
line began to rapidly rotate and shear with the surrounding positive fields,
and consequently the X9.3 flare erupted. Our results reveal that the
block-induced complex structures built the flare-productive AR and the X9.3
flare was triggered by an erupting filament due to the kink instability. To
better illustrate this process, a block-induced eruption model is proposed for
the first time. | astro-ph_SR |
Properties of simulated sunspot umbral dots: Realistic 3D radiative MHD simulations reveal the magneto-convective
processes underlying the formation of the photospheric fine structure of
sunspots, including penumbral filaments and umbral dots. Here we provide
results from a statistical analysis of simulated umbral dots and compare them
with reports from high-resolution observations. A multi-level segmentation and
tracking algorithm has been used to isolate the bright structures in synthetic
bolometric and continuum brightness images. Areas, brightness, and lifetimes of
the resulting set of umbral dots are found to be correlated: larger umbral dots
tend to be brighter and live longer. The magnetic field strength and velocity
structure of umbral dots on surfaces of constant optical depth in the continuum
at 630 nm indicate that the strong field reduction and high velocities in the
upper parts of the upflow plumes underlying umbral dots are largely hidden from
spectro-polarimetric observations. The properties of the simulated umbral dots
are generally consistent with the results of recent high-resolution
observations. However, the observed population of small, short-lived umbral
dots is not reproduced by the simulations, possibly owing to insufficient
spatial resolution. | astro-ph_SR |
BANYAN. VII. A New Population of Young Substellar Candidate Members of
Nearby Moving Groups from the BASS Survey: [Abbreviated] We present the results of a near-infrared (NIR) spectroscopic
follow-up survey of 182 M4-L7 low-mass stars and brown dwarfs (BDs) from the
BANYAN All-Sky Survey (BASS) for candidate members of nearby, young moving
groups (YMGs). We confirm signs of low-gravity for 42 new BD discoveries with
estimated masses between 8-75 $M_{Jup}$ and identify previously unrecognized
signs of low gravity for 24 known BDs. This allows us to refine the fraction of
low-gravity dwarfs in the high-probability BASS sample to $\sim$82%. We use
this unique sample of 66 young BDs, supplemented with 22 young BDs from the
literature, to construct new empirical NIR absolute magnitude and color
sequences for low-gravity BDs. We obtain a spectroscopic confirmation of
low-gravity for 2MASS J14252798-3650229, which is a new $\sim$27 $M_{Jup}$, L4
$\gamma$ bona fide member of AB Doradus. We identify a total of 19 new
low-gravity candidate members of YMGs with estimated masses below 13 $M_{Jup}$,
seven of which have kinematically estimated distances within 40 pc. These
objects will be valuable benchmarks for a detailed atmospheric characterization
of planetary-mass objects with the next generation of instruments. We find 16
strong candidate members of the Tucana-Horologium association with estimated
masses between 12.5-14 $M_{Jup}$, a regime where our study was particularly
sensitive. This would indicate that for this association there is at least one
isolated object in this mass range for every $17.5_{-5.0}^{+6.6}$ main-sequence
stellar member, a number significantly higher than expected based on standard
log-normal initial mass function, however in the absence of radial velocity and
parallax measurements for all of them, it is likely that this over-density is
caused by a number of young interlopers from other moving groups. We identify
12 new L0-L5 field BDs, seven of which display peculiar properties. | astro-ph_SR |
Radio emission as a stellar activity indicator: Radio observations of stars trace the plasma conditions and magnetic field
properties of stellar magnetospheres and coronae. Depending on the plasma
conditions at the emitter site, radio emission in the metre- and decimetre-wave
bands is generated via different mechanisms such as gyrosynchrotron, electron
cyclotron maser instability, and plasma radiation processes. The ongoing LOFAR
Two-metre Sky Survey (LoTSS) and VLA Sky Survey (VLASS) are currently the most
sensitive wide-field radio sky surveys ever conducted. Because these surveys
are untargeted, they provide an opportunity to study the statistical properties
of the radio-emitting stellar population in an unbiased manner. Here, we
perform an untargeted search for stellar radio sources down to sub-mJy level
using these radio surveys. We find that the population of radio-emitting
stellar systems is mainly composed of two distinct categories:
chromospherically active stellar (CAS) systems and M dwarfs. We also seek to
identify signatures of a gradual transition within the M-dwarf population from
chromospheric/coronal acceleration close to the stellar surface similar to that
observed on the Sun, to magnetospheric acceleration occurring far from the
stellar surface similar to that observed on Jupiter. We determine that radio
detectability evolves with spectral type, and we identify a transition in radio
detectability around spectral type M4, where stars become fully convective.
Furthermore, we compare the radio detectability vs spectra type with X-ray and
optical flare (observed by TESS) incidence statistics. We find that the radio
efficiency of X-ray/optical flares, which is the fraction of flare energy
channelled into radio-emitting charges, increases with spectral type. These
results motivate us to conjecture that the emergence of large-scale magnetic
fields in CAS systems and later M dwarfs leads to an increase in radio
efficiency. | astro-ph_SR |
IC 4663: The first unambiguous [WN] Wolf-Rayet central star of a
planetary nebula: We report on the serendipitous discovery of the first central star of a
planetary nebula (PN) that mimics the helium- and nitrogen-rich WN sequence of
massive Wolf-Rayet (WR) stars. The central star of IC 4663 (PN G346.2-08.2) is
dominated by broad He II and N V emission lines which correspond to a [WN3]
spectral type. Unlike previous [WN] candidates, the surrounding nebula is
unambiguously a PN. At an assumed distance of 3.5 kpc, corresponding to a
stellar luminosity of 4000 Lsun, the V=16.9 mag central star remains 4-6 mag
fainter than the average luminosity of massive WN3 stars even out to an
improbable d=8 kpc. The nebula is typical of PNe with an elliptical morphology,
a newly discovered Asymptotic Giant Branch (AGB) halo, a relatively low
expansion velocity (v_exp=30 km/s) and a highly ionised spectrum with an
approximately Solar chemical abundance pattern. The [WN3] star is hot enough to
show Ne VII emission (T_*=140+/-20 kK) and exhibits a fast wind (v_infty=1900
km/s), which at d=3.5 kpc would yield a clumped mass loss rate of Mdot = 1.8 x
10^-8 Msun/yr with a small stellar radius (R_*=0.11 Rsun). Its atmosphere
consists of helium (95%), hydrogen (<2%), nitrogen (0.8%), neon (0.2%) and
oxygen (0.05%) by mass. Such an unusual helium-dominated composition cannot be
produced by any extant scenario used to explain the H-deficiency of post-AGB
stars. The O(He) central stars share a similar composition and the discovery of
IC 4663 provides the first evidence for a second He-rich/H-deficient post-AGB
evolutionary sequence [WN]->O(He). This suggests there is an alternative
mechanism responsible for producing the majority of H-deficient post-AGB stars
that may possibly be expanded to include other He-rich/H-deficient stars such
as R Coronae Borealis stars and AM Canum Venaticorum stars. The origin of the
unusual composition of [WN] and O(He) central stars remains unexplained. | astro-ph_SR |
Multi-epoch spectropolarimetry of SN 2009ip: direct evidence for
aspherical circumstellar material: We present spectropolarimetry of SN 2009ip throughout the evolution of its
2012 explosion. During the initial 2012a phase, when the source spectrum
exhibits broad P-Cygni lines, we measure a V-band polarization of P~0.9% at a
position angle of ~166 deg, indicating substantial asphericity for the 2012a
outflow. Near the subsequent peak of the 2012b phase, when the spectrum shows
signs of intense interaction with circumstellar material (CSM), we measure
P~1.7% at a position angle of 72 deg, indicating a separate physical component
of polarization, which has a higher degree of asphericity than the 2012a
outflow and an orthogonal axis of symmetry on the sky. Around 30 days past
peak, which is coincident with a bump in the declining light curve, we measure
P~0.7% and significant variations in P across some broad lines, particularly
HeI/NaI. By 60 days past peak the source appears to be approaching a low value
of interstellar polarization (P<0.2%). The results are consistent with a
scenario in which a potentially bipolar explosion during 2012a impacts a
toroidal distribution of CSM, thereby causing the 2012b brightening.
Orthogonality between the 2012a and 2012b geometries is inconsistent with the
hypothesis that the 2012a event launched the CSM that was hit 40 days later by
material from the 2012b event. Rather, the aspherical CSM probably has a
separate origin associated with the outbursts of the stellar progenitor during
prior years. Previous calculations that assumed spherical symmetry for the CSM
have underestimated the required explosion energy, as the results suggest that
<10% of the fast SN ejecta participated in strong CSM interaction during the
2012b phase. In light of the spectropolarimetric results, a kinetic energy of
1e51 erg for the ejecta is difficult to avoid, supporting the interpretation
that the 2012 outburst was the result of a core-collapse supernova explosion
(abridged) | astro-ph_SR |
Synthetic Light Curves for Born Again Events: Preliminary Results: The development of surveys which will be able to cover a large region of the
sky several times per year will allow the massive detection of transient events
taking place in timescales of years. In addition, the projected full
digitalization of the Harvard plate collection will open a new window to
identify slow transients taking place in timescales of centuries. In
particular, these projects will allow the detection of stars undergoing slow
eruptions as those expected during late helium flashes in the post-AGB
evolution. In order to identify those transients which correspond with late
helium flashes the development of synthetic light curves of those events is
mandatory. In this connection we present preliminary results of a project aimed
at computing grids of theoretical light curves of born again stars. | astro-ph_SR |
Spectroscopic survey of Kepler stars. II. FIES/NOT observations of A-
and F-type stars: We have analysed high-resolution spectra of 28 A and 22 F stars in the Kepler
field, observed with the FIES spectrograph at the Nordic Optical Telescope. We
provide spectral types, atmospheric parameters and chemical abundances for 50
stars. Balmer, Fe I, and Fe II lines were used to derive effective
temperatures, surface gravities, and microturbulent velocities. We determined
chemical abundances and projected rotational velocities using a spectrum
synthesis technique. Effective temperatures calculated by spectral energy
distribution fitting are in good agreement with those determined from the
spectral line analysis. The stars analysed include chemically peculiar stars of
the Am and Lambda Boo types, as well as stars with approximately solar chemical
abundances. The wide distribution of projected rotational velocity, Vsini, is
typical for A and F stars. The microturbulence velocities obtained are typical
for stars in the observed temperature and surface gravity ranges. Moreover, we
affirm the results of Niemczura et al., that Am stars do not have
systematically higher microturbulent velocities than normal stars of the same
temperature. | astro-ph_SR |
Observations of Reconnection Flows in a Flare on the Solar Disk: Magnetic reconnection is a well-accepted part of the theory of solar eruptive
events, though the evidence is still circumstantial. Intrinsic to the
reconnection picture of a solar eruptive event, particularly in the standard
model for two-ribbon flares ("CSHKP" model), are an advective flow of
magnetized plasma into the reconnection region, expansion of field above the
reconnection region as a flux rope erupts, retraction of heated
post-reconnection loops, and downflows of cooling plasma along those loops. We
report on a unique set of SDO/AIA imaging and Hinode/EIS spectroscopic
observations of the disk flare SOL2016-03-23T03:54 in which all four flows are
present simultaneously. This includes spectroscopic evidence for a plasma
upflow in association with large-scale expanding closed inflow field. The
reconnection inflows are symmetric, and consistent with fast reconnection, and
the post-reconnection loops show a clear cooling and deceleration as they
retract. Observations of coronal reconnection flows are still rare, and most
events are observed at the solar limb, obscured by complex foregrounds, making
their relationship to the flare ribbons, cusp field and arcades formed in the
lower atmosphere difficult to interpret. The disk location and favorable
perspective of this event have removed these ambiguities giving a clear picture
of the reconnection dynamics. | astro-ph_SR |
Using the Sun to estimate Earth-like planets detection capabilities. V.
Parameterizing the impact of solar activity components on radial velocities: Stellar activity induced by active structures (eg, spots, faculae) is known
to strongly impact the radial velocity time series. It then limits the
detection of small planetary RV signals (eg, an Earth-mass planet in the
habitable zone of a solar-like star). In previous papers, we studied the
detectability of such planets around the Sun seen as an edge-on star. For that
purpose, we computed the RV and photometric variations induced by solar
magnetic activity, using all active structures observed over one entire cycle.
Our goal is to perform similar studies on stars with different physical and
geometrical properties. As a first step, we focus on Sun-like stars seen with
various inclinations, and on estimating detection capabilities with forthcoming
instruments. To do so, we first parameterize the solar active structures with
the most realistic pattern so as to obtain results consistent with the observed
ones. We simulate the growth, evolution and decay of solar spots, faculae and
network, using parameters and empiric laws derived from solar observations and
literature. We generate the corresponding structure lists over a full solar
cycle. We then build the resulting spectra and deduce the RV and photometric
variations for a `Sun' seen with various inclinations. The produced RV signal
takes into account the photometric contribution of structures as well as the
attenuation of the convective blueshift. The comparison between our simulated
activity pattern and the observed one validates our model. We show that the
inclination of the stellar rotation axis has a significant impact on the time
series. RV long-term amplitudes as well as short-term jitters are significantly
reduced when going from edge-on to pole-on configurations. Assuming spin-orbit
alignment, the optimal configuration for planet detection is an inclined star
(i~45{\deg}). | astro-ph_SR |
A method to deconvolve stellar rotational velocities III. The
probability distribution function via Maximum Likelihood utilizing Finite
Distribution Mixtures: The study of accurate methods to estimate the distribution of stellar
rotational velocities is important for understanding many aspects of stellar
evolution. From such observations we obtain the projected rotational speed v
sin(i) in order to recover the true distribution of the rotational velocity. To
that end, we need to solve a difficult inverse problem that can be posed as a
Fredholm integral of the first kind. n this work we have used a novel approach
based on Maximum likelihood (ML) estimation to obtain an approximation of the
true rotational velocity probability density function expressed as a sum of
known distribution families. In our proposal, the measurements have been
treated as random variables drawn from the projected rotational velocity
probability density function. We analyzed the case of Maxwellian sum
approximation, where we estimated the parameters that define the sum of
distributions. The performance of the proposed method is analyzed using Monte
Carlo simulations considering two theoretical cases for the probability density
function of the true rotational stellar velocities: i) an unimodal Maxwellian
probability density distribution and ii) a bimodal Maxwellian probability
density distribution. The results show that the proposed method yielded more
accurate estimates in comparison with the Tikhonov regularization method,
especially for small sample length N=50. Our proposal was evaluated using real
data from three sets of measurements, and our findings were validated using
three statistical tests. The ML approach with Maxwellian sum approximation is a
accurate method to deconvolve the rotational velocity probability density
function, even when the sample length is small (N= 50) | astro-ph_SR |
On the Doppler Shift and Asymmetry of Stokes Profiles of Photospheric
FeI and Chromospheric MgI Lines: We analyzed the full Stokes spectra using simultaneous measurements of the
photospheric (FeI 630.15 and 630.25 nm) and chromospheric (MgI b2 517.27 nm)
lines. The data were obtained with the HAO/NSO Advanced Stokes Polarimeter,
about a near disc center sunspot region, NOAA AR 9661. We compare the
characteristics of Stokes profiles in terms of Doppler shifts and asymmetries
among the three spectral lines, which helps us to better understand the
chromospheric lines and the magnetic and flow fields in different magnetic
regions. The main results are: (1) For penumbral area observed by the
photospheric FeI lines, Doppler velocities derived from Stokes I (Vi) are very
close to those derived from linear polarization profiles (Vlp) but
significantly different from those derived from Stokes V profiles (Vzc), which
provides direct and strong evidence that the penumbral Evershed flows are
magnetized and mainly carried by the horizontal magnetic component. (2) The
rudimentary inverse Evershed effect observed by the MgI b2 line provides a
qualitative evidence on its formation height that is around or just above the
temperature minimum region. (3) Vzc and Vlp in penumbrae and Vzc in pores
generally approach their Vi observed by the chromospheric MgI line, which is
not the case for the photospheric FeI lines. (4) Outer penumbrae and pores show
similar behavior of the Stokes V asymmetries that tend to change from positive
values in the photosphere (FeI lines) to negative values in the low
chromosphere (MgI line). (5) The Stokes V profiles in plage regions are highly
asymmetric in the photosphere and more symmetric in the low chromosphere. (6)
Strong red shifts and large asymmetries are found around the magnetic polarity
inversion line within the common penumbra of the Delta spot. This study thus
emphasizes the importance of spectro-polarimetry using chromospheric lines. | astro-ph_SR |
The Role of Inverse Compton Scattering in Solar Coronal Hard X-ray and
Gamma-ray Sources: Coronal hard X-ray (HXR) and continuum gamma-ray sources associated with the
impulsive phase of solar flares have been the subject of renewed interest in
recent years. They have been interpreted in terms of thin-target, nonthermal
bremsstrahlung emission. This interpretation has led to rather extreme physical
requirements in some cases. For example, in one case, essentially all of the
electrons in the source must be accelerated to nonthermal energies to account
for the coronal HXR source. In other cases, the extremely hard photon spectra
of the coronal continuum gamma-ray emission suggest that the low energy cutoff
of the electron energy distribution lies in the MeV energy range. Here we
consider the role of inverse Compton scattering (ICS) as an alternate emission
mechanism in both the ultra- and mildly relativistic regimes. It is known that
relativistic electrons are produced during powerful flares; these are capable
of up-scattering soft photospheric photons to HXR and gamma-ray energies.
Previously overlooked is the fact that mildly relativistic electrons, generally
produced in much greater numbers in flares of all sizes, can up-scatter EUV/SXR
photons to HXR energies. We also explore ICS on anisotropic electron
distributions and show that the resulting emission can be significantly
enhanced over an isotropic electron distribution for favorable viewing
geometries. We briefly review results from bremsstrahlung emission and
reconsider circumstances under which nonthermal bremsstrahlung or ICS would be
favored. Finally, we consider a selection of coronal HXR and gamma-ray events
and find that in some cases the ICS is a viable alternative emission mechanism. | astro-ph_SR |
Planetary Nebulae in the Small Magellanic Cloud: We analyse the planetary nebulae (PNe) population of the Small Magellanic
Cloud (SMC), based on evolutionary models of stars with metallicities in the
range $10^{-3} \leq Z \leq 4\times 10^{-3}$ and mass $0.9 M\odot < M <
8M\odot$, evolved through the asymptotic giant branch (AGB) phase. The models
used account for dust formation in the circumstellar envelope. To characterise
the PNe sample of the SMC, we compare the observed abundances of the various
species with the final chemical composition of the AGB models: this study
allows us to identify the progenitors of the PNe observed, in terms of mass and
chemical composition. According to our interpretation, most of the PNe descend
from low-mass ($M < 2 M\odot$) stars, which become carbon rich, after
experiencing repeated third dredge-up episodes, during the AGB phase. A
fraction of the PNe showing the signature of advanced CNO processing are
interpreted as the progeny of massive AGB stars, with mass above $\sim 6
M\odot$, undergoing strong hot bottom burning. The differences with the
chemical composition of the PNe population of the Large Magellanic Cloud (LMC)
is explained on the basis of the diverse star formation history and
age-metallicity relation of the two galaxies. The implications of the present
study for some still highly debated points regarding the AGB evolution are also
commented. | astro-ph_SR |
The galactic unclassified B[e] star HD 50138 III. The short-term line
profile variability of its photospheric lines: HD 50138 presents the B[e] phenomenon, but its nature is not clear yet. This
star is known to present spectral variations, which have been associated with
outbursts and shell phases. We analyze the line profile variability of HD 50138
and its possible origin, which provide possible hints to its evolutionary
stage, so far said to be close to the end of (or slightly beyond) the main
sequence. New high-resolution spectra of HD 50138 obtained with the HERMES
spectrograph over several nights (five of them consecutively) were analyzed,
allowing us to confirm short-term line profile variability. Our new data show
short-term variations in the photospheric lines. On the other hand, purely
circumstellar lines (such as [O I] lines) do not show such rapid variability.
The rotational velocity of HD 50138, V_rot = 90.3 +- 4.3 km/s, and the rotation
period, P = 3.64 +- 1.16 d, were derived from the He II 4026A photospheric
line. Based on the moment method, we confirm that the origin of this short-term
line profile variability is not stellar spots, and it may be caused by
pulsations. In addition, we show that macroturbulence may affect the profiles
of photospheric lines, as is seen for B supergiants. The location of HD 50138
at the end of (or slightly beyond) the main sequence, the newly detected
presence of line profile variability resembling pulsating stars, and
macroturbulence make this star a fascinating object. | astro-ph_SR |
Verification of Asymptotic Relation for Mixed Modes in Red Giant Stars: High-precision space observations, such as made by the \kepler\ and \corot\
missions, allow us to detect mixed modes for $l = 1$ modes in their high
signal-to-noise photometry data. By means of asteroseismology, the inner
structure of red giant (RG) stars is revealed the first time with the help of
mixed modes. We analyse these mixed modes of a 1.3 M$_{\sun}$ RG model
theoretically from the approximate asymptotic descriptions of oscillations.
While fitting observed frequencies with the eigenvalue condition for mixed
modes, a good estimate of period spacing and coupling strength is also acquired
for more evolved models. We show that the behaviour of the mode inertia in a
given mode varies dramatically when the coupling is strong. An approximation of
period spacings is also obtained from the asymptotic dispersion relation, which
provides a good estimate of the coupling strength as well as period spacing
when g-mode-like mixed modes are sufficiently dense. By comparing the
theoretical coupling strength from the integral expression with the ones from
fitting methods, we confirmed that the theoretical asymptotic equation is
problematic in the evanescent region due to the potential singularities as well
as the use of the Cowling approximation. | astro-ph_SR |
The Age-Rotation-Activity Relation: From Myrs to Gyrs: Over the past 40 years, observational surveys have established the existence
of a tight relationship between a star's age, rotation period, and magnetic
activity. This age-rotation-activity relation documents the interplay between a
star's magnetic dynamo and angular momentum evolution, and provides a valuable
age estimator for isolated field stars. While the age-rotation-activity
relation has been studied extensively in clusters younger than 500 Myr,
empirically measured rotation periods are scarce for older ages. Using the
Palomar Transient Factory (PTF), we have begun a survey of stellar rotation to
map out the late-stage evolution of the age-rotation-activity relation: the
Columbia/Cornell/Caltech PTF (CCCP) survey of open clusters. The first CCCP
target is the nearby ~600 Myr Hyades-analog Praesepe, where PTF has produced
light curves spanning more than 3 months and containing >150 measurements for
~650 cluster members. Analyzing these light curves, we have measured rotation
periods for 40 K & M cluster members, filling the gap between the periods
previously reported for solar-type Hyads (Radick et al. 1987, Prosser et al.
1995) and for a handful of low-mass Praesepe members (Scholz et al. 2007). Our
measurements indicate that Praesepe's period-color relation undergoes at
transition at a characteristic spectral type of ~M1 --- from a well-defined
singular relation at higher mass, to a more scattered distribution of both fast
and slow-rotators at lower masses. The location of this transition is broadly
consistent with expectations based on observations of younger clusters and the
assumption that stellar-spin down is the dominant mechanism influencing angular
momentum evolution at ~600 Myr. In addition to presenting the results of our
photometric monitoring of Praesepe, we summarize the status and future of the
CCCP survey. | astro-ph_SR |
H-alpha as a Luminosity Class Diagnostic for K- and M-type Stars: We have identified the H-alpha absorption feature as a new spectroscopic
diagnostic of luminosity class in K- and M-type stars. From high-resolution
spectra of 19 stars with well-determined physical properties (including
effective temperatures and stellar radii), we measured equivalent widths for
H-alpha and the Ca II triplet and examined their dependence on both luminosity
class and stellar radius. H-alpha shows a strong relation with both luminosity
class and radius that extends down to late M spectral types. This behavior in
H-alpha has been predicted as a result of the density-dependent overpopulation
of the metastable 2S level in hydrogen, an effect that should become dominant
for Balmer line formation in non-LTE conditions. We conclude that this new
metallicity-insensitive diagnostic of luminosity class in cool stars could
serve as an effective means of discerning between populations such as Milky Way
giants and supergiant members of background galaxies. | astro-ph_SR |
The (B0+?)+O6 system FN CMa: A case for tidal-pulsational interaction?: FN CMa is visually double with a separation of about 0.6arcsec. Sixty
high-cadence VLT/UVES spectra permit the A and B components to be disentangled,
as the relative contribution of each star to the total light entering the
spectrograph fluctuates between exposures due to changes in seeing. Component A
exhibits rapid line-profile variations, leading us to attribute the photometric
variability seen by HIPPARCOS (with a derived P=0.08866d) to this component.
From a total of 122 archival and new echelle spectra it is shown that component
A is an SB1 binary with an orbital period of 117.55 days. The eccentricity of
0.6 may result in tidal modulation of the pulsation(s) of component Aa. | astro-ph_SR |
Atmospheric parameters of Cepheids from flux ratios with ATHOS: I. The
temperature scale: Context: The effective temperature is a key parameter governing the
properties of a star. For stellar chemistry, it has the strongest impact on the
accuracy of the abundances derived. Since Cepheids are pulsating stars,
determining their effective temperature is more complicated that in the case of
non-variable stars. Aims: We want to provide a new temperature scale for
classical Cepheids, with a high precision and full control of the systematics.
Methods: Using a data-driven machine learning technique employing observed
spectra, and taking great care to accurately phase single-epoch observations,
we have tied flux ratios to (label) temperatures derived using the infrared
surface brightness method. Results: We identified 143 flux ratios that allow us
to determine the effective temperature with a precision of a few K and an
accuracy better than 150 K, which is in line with the most accurate temperature
measures available to date. The method does not require a normalization of the
input spectra and provides homogeneous temperatures for low- and
high-resolution spectra, even at the lowest signal-to-noise ratios. Due to the
lack of a dataset of sufficient sample size for Small Magellanic Cloud
Cepheids, the temperature scale does not extend to Cepheids with [Fe/H] < -0.6
dex but nevertheless provides an exquisite, homogeneous means of characterizing
Galactic and Large Magellanic Cloud Cepheids. Conclusions: The temperature
scale will be extremely useful in the context of spectroscopic surveys for
Milky Way archaeology with the WEAVE and 4MOST spectrographs. It paves the way
for highly accurate and precise metallicity estimates, which will allow us to
assess the possible metallicity dependence of Cepheids' period-luminosity
relations and, in turn, to improve our measurement of the Hubble constant H0. | astro-ph_SR |
High-Precision Radio and Infrared Astrometry of LSPM J1314+1320AB - II:
Testing Pre--Main-Sequence Models at the Lithium Depletion Boundary with
Dynamical Masses: We present novel tests of pre$-$main-sequence models based on individual
dynamical masses for the M7 binary LSPM J1314+1320AB. Joint analysis of our
Keck adaptive optics astrometric monitoring along with Very Long Baseline Array
radio data from a companion paper yield component masses of $0.0885\pm0.0006$
$M_{\odot}$ and $0.0875\pm0.0010$ $M_{\odot}$ and a parallactic distance of
$17.249\pm0.013$ pc. We also derive component luminosities that are consistent
with the system being coeval at an age of $80.8\pm2.5$ Myr, according to BHAC15
evolutionary models. The presence of lithium is consistent with model
predictions, marking the first time the theoretical lithium depletion boundary
has been tested with ultracool dwarfs of known mass. However, we find that the
average evolutionary model-derived effective temperature ($2950\pm5$ K) is 180
K hotter than we derive from a spectral type$-$$T_{\rm eff}$ relation based on
BT-Settl models ($2770\pm100$ K). We suggest that the dominant source of this
discrepancy is model radii being too small by $\approx$13%. In a test that
mimics the typical application of evolutionary models by observers, we derive
masses on the H-R diagram using the luminosity and BT-Settl temperature. The
estimated masses are $46^{+16}_{-19}$% (2.0$\sigma$) lower than we measure
dynamically and would imply that this is a system of $\approx$50 $M_{\rm Jup}$
brown dwarfs, highlighting the large systematic errors possible when inferring
masses from the H-R diagram. This is first time masses have been measured for
ultracool ($\geq$M6) dwarfs displaying spectral signatures of low gravity.
Based on features in the infrared, LSPM J1314+1320AB appears higher gravity
than typical Pleiades and AB Dor members, opposite the expectation given its
younger age. The components of LSPM J1314+1320AB are now the nearest, lowest
mass pre$-$main-sequence stars with direct mass measurements. | astro-ph_SR |
Kinematic evidence for feedback-driven star formation in NGC 1893: OB associations are the prevailing star forming sites in the Galaxy. Up to
now, the process of how OB associations were formed remained a mystery. A
possible process is self-regulating star formation driven by feedback from
massive stars. However, although a number of observational studies uncovered
various signposts of feedback-driven star formation, the effectiveness of such
feedback has been questioned. Stellar and gas kinematics is a promising tool to
capture the relative motion of newborn stars and gas away from ionizing
sources. We present high-resolution spectroscopy of stars and gas in the young
open cluster NGC 1893. Our findings show that newborn stars and the tadpole
nebula Sim 130 are moving away from the central cluster containing two O-type
stars, and that the timescale of sequential star formation is about 1 Myr
within a 9 parsec distance. The newborn stars formed by feedback from massive
stars account for at least 18 per cent of the total stellar population in the
cluster, suggesting that this process can play an important role in the
formation of OB associations. These results support the self-regulating star
formation model. | astro-ph_SR |
The Kepler and Hale observations of V523 Lyr: We present new observations of the cataclysmic variable V523 Lyr, a member of
the open cluster NGC 6791. The Kepler Space telescope obtained photometric
observations of this source and we examine the nearly 3 year long light curve.
The observations show numerous small amplitude outbursts recurring on average
every 33 days, intermittent quasi-periodic oscillations, and a significant
fully coherent period of ~3.8 hr which we identify as the orbital period of the
binary. Contemporaneous optical spectroscopy of V523 Lyr reveals a faint blue
source with broad Balmer absorption lines containing narrow emission cores.
H$\alpha$ is in emission above the continuum. The low amplitude of the
photometric signal and no detected velocity motion suggest a low orbital
inclination. We discuss the properties of V523 Lyr and show that it is a member
of the growing group of anomalous Z Cam type CVs, systems which show stunted
outbursts, light curve standstills, and occasional deep drops in brightness. | astro-ph_SR |
Why variable AGB stars with Long Secondary Periods aren't binaries, but
are dusty: Roughly 30% of variable AGB stars show a Long Secondary Period, or LSP. These
LSPs have posed something of a problem in recent years and their cause remains
a mystery. By combining VLT-derived velocity curves with MACHO and OGLE light
curves we were able to examine many properties of these stars and test the
theory that LSPs are caused by binarity. We show why we concluded that the
binary model for LSPs is unlikely. Examining mid-infrared SAGE observations for
stars with LSPs shows that these stars are surrounded by a significant amount
of cool dust in a nonspherical distribution, e.g. a disk or clumps. The
unlikeliness of binarity in these stars forces us to conclude that the dust is
not in a disk. We are left without an acceptable explanation for Long Secondary
Periods in AGB stars. | astro-ph_SR |
Towards understanding dynamo action in M dwarfs: Recent progress in observational studies of magnetic activity in M dwarfs
urgently requires support from ideas of stellar dynamo theory. We propose a
strategy to connect observational and theoretical studies. In particular, we
suggest four magnetic configurations that appear relevant to dwarfs from the
viewpoint of the most conservative version of dynamo theory, and discuss
observational tests to identify the configurations observationally. As
expected, any such identification contains substantial uncertainties. However
the situation in general looks less pessimistic than might be expected. Several
identifications between the phenomenology of individual stars and dynamo models
are suggested. Remarkably, all models discussed predict substantial surface
magnetic activity at rather high stellar latitudes. This prediction looks
unexpected from the viewpoint of our experience observing the Sun (which of
course differs in some fundamental ways from these late-type dwarfs). We stress
that a fuller understanding of the topic requires a long-term (at least 15
years) monitoring of M dwarfs by Zeeman-Doppler imaging. | astro-ph_SR |
Detection of the Central Star of the Planetary Nebula NGC 6302: NGC 6302 is one of the highest ionization planetary nebulae known and shows
emission from species with ionization potential >300eV. The temperature of the
central star must be >200,000K to photoionize the nebula, and has been
suggested to be up to ~ 400,000K. On account of the dense dust and molecular
disc, the central star has not convincingly been directly imaged until now. NGC
6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as
part of the Servicing Mission 4 Early Release Observations. The central star is
directly detected for the first time, and is situated at the nebula centre on
the foreground side of the tilted equatorial disc. The magnitudes of the
central star have been reliably measured in two filters(F469N and F673N).
Assuming a hot black body, the reddening has been measured from the
(4688-6766\AA) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main
sequence binary companion can be excluded. The position of the star on the HR
diagram suggests a fairly massive PN central star of about 0.64,M_sun close to
the white dwarf cooling track. A fit to the evolutionary tracks for
(T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained;
however the luminosity and temperature remain uncertain. The model tracks
predict that the star is rapidly evolving, and fading at a rate of almost 1 %
per year. Future observations could test this prediction. | astro-ph_SR |
Evidence of a complex structure within the 2013 August 19 coronal mass
ejection. Radial and longitudinal evolution in the inner heliosphere: Context: Late on 2013 August 19, a coronal mass ejection (CME) erupted from
an active region located near the far-side central meridian from Earth's
perspective. The event and its accompanying shock were remotely observed by the
STEREO-A, STEREO-B and SOHO spacecraft. The interplanetary counterpart (ICME)
was intercepted by MESSENGER near 0.3 au, and by both STEREO-A and STEREO-B,
near 1 au, which were separated by 78{\deg} in heliolongitude. The main
objective of this study is to follow the radial and longitudinal evolution of
the ICME throughout the inner heliosphere, and to examine possible scenarios
for the different magnetic flux-rope configuration observed on the solar disk,
and measured in situ at the locations of MESSENGER and STEREO-A, separated by
15{\deg} in heliolongitude, and at STEREO-B, which detected the ICME flank.
Results: We find that the magnetic flux-rope structure detected at STEREO-B
belongs to the same ICME detected at MESSENGER and STEREO-A. The opposite
helicity deduced at STEREO-B, might be due to the spacecraft intercepting one
of the legs of the structure far from the flux-rope axis, while STEREO-A and
MESSENGER are crossing through the core of the magnetic flux rope. The
different flux-rope orientations measured at MESSENGER and STEREO-A arise
probably because the two spacecraft measure a curved, highly distorted and
rather complex magnetic flux-rope topology. The ICME may have suffered
additional distortion in its evolution in the inner heliosphere, such as the
west flank is propagating faster than the east flank when arriving 1 au.
Conclusions: This work illustrates how the ambient conditions can significantly
affect the expansion and propagation of the CME/ICME, introducing additional
irregularities to the already asymmetric eruption, and how these complex
structures cannot be directly reconstructed with the current models available. | astro-ph_SR |
The Be Star HD 215227: A Candidate Gamma-ray Binary: The emission-line Be star HD 215227 lies within the positional error circle
of the newly identified gamma-ray source AGL J2241+4454. We present new blue
spectra of the star, and we point out the morphological and variability
similarities to other Be binaries. An analysis of the available optical
photometry indicates a variation with a period of 60.37 +/- 0.04 d, which may
correspond to an orbital modulation of the flux from the disk surrounding the
Be star. The distance to the star of 2.6 kpc and its relatively large Galactic
latitude suggest that the binary was ejected from the plane by a supernova
explosion that created the neutron star or black hole companion. The binary and
runaway properties of HD 215227 make it an attractive candidate as the optical
counterpart of AGL J2241+4454 and as a new member of the small class of
gamma-ray emitting binaries. | astro-ph_SR |
Formation of the helium EUV resonance lines: Context: While classical models successfully reproduce intensities of many
transition region lines, they predict helium EUV line intensities roughly an
order of magnitude lower than the observed value.
Aims: To determine the relevant formation mechanism(s) of the helium EUV
resonance lines, capable of explaining the high intensities under quiet sun
conditions.
Methods: We synthesise and study the emergent spectra from a 3D
radiation-magnetohydrodynamics simulation model. The effects of coronal
illumination and non-equilibrium ionisation of hydrogen and helium are included
self-consistently in the numerical simulation.
Results: Radiative transfer calculations result in helium EUV line
intensities that are an order of magnitude larger than the intensities
calculated under the classical assumptions. The enhanced intensity of He I 584
is primarily caused by He II recombination cascades. The enhanced intensity of
He II 304 and He II 256 is caused primarily by non-equilibrium helium
ionisation.
Conclusion: The analysis shows that the long standing problem of the high
helium EUV line intensities disappears when taking into account optically thick
radiative transfer and non-equilibrium ionisation effects. | astro-ph_SR |
The solar wind in time: a change in the behaviour of older winds?: In the present paper, we model the wind of solar analogues at different ages
to investigate the evolution of the solar wind. Recently, it has been suggested
that winds of solar type stars might undergo a change in properties at old
ages, whereby stars older than the Sun would be less efficient in carrying away
angular momentum than what was traditionally believed. Adding to this, recent
observations suggest that old solar-type stars show a break in coronal
properties, with a steeper decay in X-ray luminosities and temperatures at
older ages. We use these X-ray observations to constrain the thermal
acceleration of winds of solar analogues. Our sample is based on the stars from
the `Sun in time' project with ages between 120-7000 Myr. The break in X-ray
properties leads to a break in wind mass-loss rates ($\dot{M}$) at roughly 2
Gyr, with $\dot{M}$ (t < 2 Gyr) $\propto t^{-0.74}$ and $\dot{M}$ (t > 2 Gyr)
$\propto$ $t^{-3.9}$. This steep decay in $\dot{M}$ at older ages could be the
reason why older stars are less efficient at carrying away angular momentum,
which would explain the anomalously rapid rotation observed in older stars. We
also show that none of the stars in our sample would have winds dense enough to
produce thermal emission above 1-2 GHz, explaining why their radio emissions
have not yet been detected. Combining our models with dynamo evolution models
for the magnetic field of the Earth we find that, at early ages ($\approx$100
Myr) our Earth had a magnetosphere that was 3 or more times smaller than its
current size. | astro-ph_SR |
Physical structure of the envelopes of intermediate-mass protostars: Context: Intermediate mass protostars provide a bridge between low- and
high-mass protostars. Furthermore, they are an important component of the UV
interstellar radiation field. Despite their relevance, little is known about
their formation process. Aims: We present a systematic study of the physical
structure of five intermediate mass, candidate Class 0 protostars. Our two
goals are to shed light on the first phase of intermediate mass star formation
and to compare these protostars with low- and high-mass sources. Methods: We
derived the dust and gas temperature and density profiles of the sample. We
analysed all existing continuum data on each source and modelled the resulting
SED with the 1D radiative transfer code DUSTY. The gas temperature was then
predicted by means of a modified version of the code CHT96. Results: We found
that the density profiles of five out of six studied intermediate mass
envelopes are consistent with the predictions of the "inside-out" collapse
theory.We compared several physical parameters, like the power law index of the
density profile, the size, the mass, the average density, the density at 1000
AU and the density at 10 K of the envelopes of low-, intermediate, and
high-mass protostars. When considering these various physical parameters, the
transition between the three groups appears smooth, suggesting that the
formation processes and triggers do not substantially differ. | astro-ph_SR |
Activity on a Li-rich giant: DI Psc revisited: We present a new Doppler imaging study for the Li-rich single K-giant DI Psc.
Surface temperature maps are reconstructed for two subsequent rotation cycles.
From the time evolution of the spot distribution antisolar-type differential
rotation pattern is revealed. We show marks of non-uniform Li-abundance as
well. The possible connection between the current evolutionary phase of the
star and its magnetic activity is briefly discussed. | astro-ph_SR |
Abundance ratios in GALAH DR2 and their implications for nucleosynthesis: Using a sample of 70 924 stars from the second data release of the GALAH
optical spectroscopic survey, we construct median sequences of [X/Mg] vs.
[Mg/H] for 21 elements, separating the high-$\alpha$/``low-Ia'' and
low-$\alpha$/``high-Ia'' stellar populations through cuts in [Mg/Fe]. Previous
work with the near-IR APOGEE survey has shown that such sequences are nearly
independent of location in the Galactic disk, implying that they are determined
by stellar nucleosynthesis yields with little sensitivity to other chemical
evolution aspects. The separation between the two [X/Mg] sequences indicates
the relative importance of prompt and delayed enrichment mechanisms, while the
sequences' slopes indicate metallicity dependence of the yields. GALAH and
APOGEE measurements agree for some of their common elements, but differ in
sequence separation or metallicity trends for others. GALAH offers access to
nine new elements. We infer that about $75\%$ of solar C comes from core
collapse supernovae and $25\%$ from delayed mechanisms. We find core collapse
fractions of $60-80\%$ for the Fe-peak elements Sc, Ti, Cu, and Zn, with strong
metallicity dependence of the core collapse Cu yield. For the neutron capture
elements Y, Ba, and La, we infer large delayed contributions with non-monotonic
metallicity dependence. The separation of the [Eu/Mg] sequences implies that at
least $\sim30\%$ of Eu enrichment is delayed with respect to star formation. We
compare our results to predictions of several supernova and AGB yield models;
C, Na, K, Mn, and Ca all show discrepancies with models that could make them
useful diagnostics of nucleosynthesis physics. | astro-ph_SR |
Characterising the observational properties of δ Sct stars in the
era of space photometry from the Kepler mission: The {\delta} Sct stars are a diverse group of intermediate-mass pulsating
stars located on and near the main sequence within the classical instability
strip in the Hertzsprung-Russell diagram. Many of these stars are hybrid stars
pulsating simultaneously with pressure and gravity modes that probe the physics
at different depths within a star's interior. Using two large ensembles of
{\delta} Sct stars observed by the Kepler Space Telescope, the instrumental
biases inherent to Kepler mission data and the statistical properties of these
stars are investigated. An important focus of this work is an analysis of the
relationships between the pulsational and stellar parameters, and their
distribution within the classical instability strip. It is found that a
non-negligible fraction of main sequence {\delta} Sct stars exist outside
theoretical predictions of the classical instability boundaries, which
indicates the necessity of a mass-dependent mixing length parameter to
simultaneously explain low- and high-radial order pressure modes in {\delta}
Sct stars within the Hertzsprung-Russell diagram. Furthermore, a search for
regularities in the amplitude spectra of these stars is also presented,
specifically the frequency difference between pressure modes of consecutive
radial order. In this work, it is demonstrated that an ensemble-based approach
using space photometry from the Kepler mission is not only plausible for
{\delta} Sct stars, but that it is a valuable method for identifying the most
promising stars for mode identification and asteroseismic modelling. The full
scientific potential of studying {\delta} Sct stars is as yet unrealised. The
ensembles discussed in this paper represent a high-quality data set for future
studies of rotation and angular momentum transport inside A and F stars using
asteroseismology. | astro-ph_SR |
Collisional relaxation of electrons in a warm plasma and accelerated
nonthermal electron spectra in solar flares: Extending previous studies of nonthermal electron transport in solar flares
which include the effects of collisional energy diffusion and thermalization of
fast electrons, we present an analytic method to infer more accurate estimates
of the accelerated electron spectrum in solar flares from observations of the
hard X-ray spectrum. Unlike for the standard cold-target model, the spatial
characteristics of the flaring region, especially the necessity to consider a
finite volume of hot plasma in the source, need to be taken into account in
order to correctly obtain the injected electron spectrum from the
source-integrated electron flux spectrum (a quantity straightforwardly obtained
from hard X-ray observations). We show that the effect of electron
thermalization can be significant enough to nullify the need to introduce an
{\it ad hoc} low-energy cutoff to the injected electron spectrum in order to
keep the injected power in non-thermal electrons at a reasonable value. Rather
the suppression of the inferred low-energy end of the injected spectrum
compared to that deduced from a cold-target analysis allows the inference from
hard X-ray observations of a more realistic energy in injected non-thermal
electrons in solar flares. | astro-ph_SR |
Absolute Parameters of Young Stars: PU Pup: We present combined photometric and spectroscopic analyses of the southern
binary star PU Pup. High-resolution spectra of this system were taken at the
University of Canterbury Mt. John Observatory in the years 2008 and again in
2014-15. We find the light contribution of the secondary component to be only
$\sim$2\% of the total light of the system in optical wavelengths, resulting in
a single-lined spectroscopic binary. Recent TESS data revealed grazing eclipses
within the light minima, though the tidal distortion, examined also from
HIPPARCOS data, remains the predominating light curve effect. Our model shows
PU Pup to have the more massive primary relatively close to filling its Roche
lobe. PU Pup is thus approaching the rare `fast phase' of interactive (Case B)
evolution. Our adopted absolute parameters are as follows: $M_1$ = 4.10
($\pm$0.20) M$_{\odot}$, $M_2$ = 0.65 ($\pm$0.05) M$_{\odot}$, $R_{1}$ = 6.60
($\pm$0.30) R$_{\odot}$, $R_2$ = 0.90 ($\pm$0.10) R$_{\odot}$; $T_{1}$ = 11500
($\pm$500) K, $T_{2}$ = 5000 ($\pm$350) K; photometric distance = 186 ($\pm$20)
pc, age = 170 ($\pm$20) My. The less-massive secondary component is found to be
significantly oversized and overluminous compared to standard Main Sequence
models. We discuss this discrepancy referring to heating from the reflection
effect. | astro-ph_SR |
The Automatic Identification and Tracking of Coronal Flux Ropes -- Part
I: Footpoints and Fluxes: Investigating the early-stage evolution of an erupting flux rope from the Sun
is important to understand the mechanisms of how it looses its stability and
its space weather impacts. Our aim is to develop an efficient scheme for
tracking the early dynamics of erupting solar flux ropes and use the algorithm
to analyse its early-stage properties. The algorithm is tested on a data-driven
simulation of an eruption that took place in active region AR12473. We
investigate the modelled flux rope's footpoint movement and magnetic flux
evolution and compare with observational data from the Solar Dynamics
Observatory's Atmospheric Imaging Assembly in the 211 $\unicode{x212B}$ and
1600 $\unicode{x212B}$ channels. To carry out our analysis, we use the
time-dependent data-driven magnetofrictional model (TMFM). We also perform
another modelling run, where we stop the driving of the TMFM midway through the
flux rope's rise through the simulation domain and evolve it instead with a
zero-beta magnetohydrodynamic (MHD) approach. The developed algorithm
successfully extracts a flux rope and its ascend through the simulation domain.
We find that the movement of the modelled flux rope footpoints showcases
similar trends in both TMFM and relaxation MHD run: they recede from their
respective central location as the eruption progresses and the positive
polarity footpoint region exhibits a more dynamic behaviour. The ultraviolet
brightenings and extreme ultraviolet dimmings agree well with the models in
terms of their dynamics. According to our modelling results, the toroidal
magnetic flux in the flux rope first rises and then decreases. In our
observational analysis, we capture the descending phase of toroidal flux. In
conclusion, the extraction algorithm enables us to effectively study the flux
rope's early dynamics and derive some of its key properties such as footpoint
movement and toroidal magnetic flux. | astro-ph_SR |
Gravitational settling in pulsating subdwarf B stars and their
progenitors: Diffusion of atoms can be important during quiescent phases of stellar
evolution. Particularly in the very thin inert envelopes of subdwarf B stars,
diffusive movements will considerably change the envelope structure and the
surface abundances on a short timescale. Also, the subdwarfs will inherit the
effects of diffusion in their direct progenitors, namely giants near the tip of
the red giant branch. This will influence the global evolution and the
pulsational properties of subdwarf B stars. We investigate the impact of
gravitational settling, thermal diffusion and concentration diffusion on the
evolution and pulsations of subdwarf B stars. Our diffusive stellar models are
compared with models evolved without diffusion. We constructed subdwarf B
models with a mass of 0.465 Msun from a 1 and 3 Msun ZAMS progenitor. The low
mass star ignited helium in an energetic flash, while the intermediate mass
star started helium fusion gently. For each progenitor type we computed series
with and without atomic diffusion. Atomic diffusion in red giants causes the
helium core mass at the onset of helium ignition to be larger. We find an
increase of 0.0015 Msun for the 1 Msun model and 0.0036 Msun for the 3 Msun
model. The effects on the red giant surface abundances are small after the
first dredge up. The evolutionary tracks of the diffusive subdwarf B models are
shifted to lower surface gravities and effective temperatures due to outward
diffusion of hydrogen. This affects both the frequencies of the excited modes
and the overall frequency spectrum. Especially the structure and pulsations of
the post-non-degenerate sdB star are drastically altered, proving that atomic
diffusion cannot be ignored in these stars. | astro-ph_SR |
The Ages of Stars: The age of an individual star cannot be measured, only estimated through
mostly model-dependent or empirical methods, and no single method works well
for a broad range of stellar types or for a full range in age. This review
presents a summary of the available techniques for age-dating stars and
ensembles of stars, their realms of applicability, and their strengths and
weaknesses. My emphasis is on low-mass stars because they are present from all
epochs of star formation in the Galaxy and because they present both special
opportunities and problems. The ages of open clusters are important for
understanding the limitations of stellar models and for calibrating empirical
age indicators. For individual stars, a hierarchy of quality for the available
age-dating methods is described. Although our present ability to determine the
ages of even the nearest stars is mediocre, the next few years hold great
promise as asteroseismology probes beyond stellar surfaces and starts to
provide precise interior properties of stars and as models continue to improve
when stressed by better observations. | astro-ph_SR |
Magnetic Fields of New CP Stars Discovered with Kepler Mission Data: The paper presents the first results of the ongoing spectropolarimetric
monitoring of magnetic fields of stars, whose chemically peculiar nature has
been previously revealed with the 1-m SAO RAS telescope. We selected the sample
candidates using the photometric data of the Kepler and TESS space missions.
The efficiency of the method of searching for new CP stars based on photometric
light curves has been confirmed. We present the magnetic field measurements and
estimate the atmospheric parameters of the objects under study. | astro-ph_SR |
Response of Granulation to Small Scale Bright Features in the Quiet Sun: We detected 2.8 bright points (BPs) per Mm$^2$ in the Quiet Sun (QS) with the
New Solar Telescope (NST) at Big Bear Solar Observatory; using the TiO 705.68
nm spectral line, at an angular resolution ~ 0.1'' to obtain 30 min data
sequence. Some BPs formed knots that were stable in time and influenced the
properties of the granulation pattern around them. The observed granulation
pattern within ~ 3'' of knots presents smaller granules than those observed in
a normal granulation pattern; i.e., around the knots a suppressed convection is
detected. Observed BPs covered ~ 5% of the solar surface and were not
homogeneously distributed. BPs had an average size of 0.22'', they were
detectable for 4.28 min in average, and had an averaged contrast of 0.1% in the
deep red TiO spectral line. | astro-ph_SR |
The Solar Neighborhood XXIV. Parallax Results from the CTIOPI 0.9-m
Program: Stars with $μ$ $\ge$ 1\farcs0 yr$^{-1}$ (MOTION Sample) and
Subdwarfs: We present 41 trigonometric parallaxes of 37 stellar systems, most of which
have proper motions greater than 1\farcs0 yr$^{-1}$. These are the first
trigonometric parallaxes for 24 systems. Overall, there are 15 red dwarf
systems and 22 red subdwarf systems in the sample. Five of the systems are
multiples with directly detected companions, and we have discovered
perturbations caused by unseen companions in two additional cases, the dwarf
LHS 501 and the subdwarf LHS 440. The latter system may eventually provide
important dynamical mass points on the subdwarf mass-luminosity relation. Two
additional stars of note are LHS 272, the third closest M-type subdwarf at a
distance of only 13.6 pc, and LHS 2734AB, a high velocity subdwarf binary with
$V_{tan}>$ 700 km/sec, which likely exceeds the escape velocity of the Milky
Way. We also report the first long term variability study of cool subdwarfs
indicating that cool subdwarfs are less photometrically variable than their
main sequence counterparts. | astro-ph_SR |
Gas and dust from solar metallicity AGB stars: We study the asymptotic giant branch (AGB) evolution of stars with masses
between $1~M_{\odot} - 8.5~M_{\odot}$. We focus on stars with a solar chemical
composition, which allows us to interpret evolved stars in the Galaxy. We
present a detailed comparison with models of the same chemistry, calculated
with a different evolution code and based on a different set of physical
assumptions. We find that stars of mass $\ge 3.5~M_{\odot}$ experience hot
bottom burning at the base of the envelope. They have AGB lifetimes shorter
than $\sim 3\times 10^5$ yr and eject into their surroundings gas contaminated
by proton-capture nucleosynthesis, at an extent sensitive to the treatment of
convection. Low mass stars with $1.5~M_{\odot} \le M \le 3~M_{\odot}$ become
carbon stars. During the final phases the C/O ratio grows to $\sim 3$. We find
a remarkable agreement between the two codes for the low-mass models and
conclude that predictions for the physical and chemical properties of these
stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB
phase. The dust produced is also dependent on the mass: low-mass stars produce
mainly solid carbon and silicon carbide dust, whereas higher mass stars produce
silicates and alumina dust. Possible future observations potentially able to
add more robustness to the present results are also discussed. | astro-ph_SR |
Stellar granulation as seen in disk-integrated intensity. II.
Theoretical scaling relations compared with observations: A large set of stars observed by CoRoT and Kepler shows clear evidence for
the presence of a stellar background, which is interpreted to arise from
surface convection, i.e., granulation. These observations show that the
characteristic time-scale (tau_eff) and the root-mean-square (rms) brightness
fluctuations (sigma) associated with the granulation scale as a function of the
peak frequency (nu_max) of the solar-like oscillations. We aim at providing a
theoretical background to the observed scaling relations based on a model
developed in the companion paper. We computed for each 3D model the theoretical
power density spectrum (PDS) associated with the granulation as seen in
disk-integrated intensity on the basis of the theoretical model. For each PDS
we derived tau_eff and sigma and compared these theoretical values with the
theoretical scaling relations derived from the theoretical model and the Kepler
measurements. We derive theoretical scaling relations for tau_eff and sigma,
which show the same dependence on nu_max as the observed scaling relations. In
addition, we show that these quantities also scale as a function of the
turbulent Mach number (Ma) estimated at the photosphere. The theoretical
scaling relations for tau_eff and sigma match the observations well on a global
scale. Our modelling provides additional theoretical support for the observed
variations of sigma and tau_eff with nu_m max. It also highlights the important
role of Ma in controlling the properties of the stellar granulation. However,
the observations made with Kepler on a wide variety of stars cannot confirm the
dependence of our scaling relations on Ma. Measurements of the granulation
background and detections of solar-like oscillations in a statistically
sufficient number of cool dwarf stars will be required for confirming the
dependence of the theoretical scaling relations with Ma. | astro-ph_SR |
Solar LImb Prominence CAtcher and Tracker (SLIPCAT): An Automated System
and Its Preliminary Statistical Results: In this paper, we present an automated system, which has the capability to
catch and track solar limb prominences based on observations from EUV 304
passband. The characteristic parameters and their evolution, including height,
position angle, area, length and brightness, are obtained without manual
interventions. By applying the system to the STEREO-B/SECCHI/EUVI 304 data
during 2007 April -2009 October, we obtain a total of 9477 well-tracked
prominences and a catalog of these events available online at
http://space.ustc.edu.cn/dreams/slipcat/. A detailed analysis of these
prominences suggests that the system has a rather good performance. We have
obtained several interesting statistical results based on the catalog. Most
prominences appear below the latitude of 60 degrees and at the height of about
26 Mm above the solar surface. Most of them are quite stable during the period
they are tracked. Nevertheless, some prominences have an upward speed of more
than 100 km/s, and some others show significant downward and/or azimuthal
speeds. There are strong correlations among the brightness, area and height.
The expansion of a prominence is probably one major cause of its fading during
the rising or erupting process. | astro-ph_SR |
A Survey of Stellar Families: Multiplicity of Solar-Type Stars: We present the results of a comprehensive assessment of companions to
solar-type stars. A sample of 454 stars, including the Sun, was selected from
the Hipparcos catalog with {\pi} > 40 mas, {\sigma}_{\pi}/{\pi} < 0.05, 0.5 < B
- V < 1.0 (~ F6-K3), and constrained by absolute magnitude and color to exclude
evolved stars. New observational aspects of this work include surveys for (1)
very close companions with long-baseline interferometry at the CHARA Array, (2)
close companions with speckle interferometry, and (3) wide proper motion
companions identified by blinking multi-epoch archival images. In addition, we
include the results from extensive radial-velocity monitoring programs and
evaluate companion information from various catalogs.
The overall observed fractions of single, double, triple, and higher order
systems are 56% \pm 2%, 33% \pm 2%, 8% \pm 1%, and 3% \pm 1%, respectively,
counting all confirmed stellar and brown dwarf companions. Our completeness
analysis indicates that only a few undiscovered companions remain in this
well-studied sample, implying that the majority (54% \pm 2%) of solar-type
stars are single, in contrast to the results of prior multiplicity studies. The
orbital-period distribution of companions is unimodal and roughly log-normal
with a peak of about 300 years. The period-eccentricity relation shows a
roughly flat distribution beyond the expected circularization for periods below
12 days. The mass-ratio distribution shows a preference for like-mass pairs,
which occur more frequently in relatively close pairs. The fraction of planet
hosts among single, binary, and multiple systems are statistically
indistinguishable, suggesting that planets are as likely to form around single
stars as they are around components of binary or multiple systems with
sufficiently wide separations. | astro-ph_SR |
Sigmoid-to-Flux-Rope Transition Leading to A Loop-Like Coronal Mass
Ejection: Sigmoids are one of the most important precursor structures for solar
eruptions. In this Letter, we study a sigmoid eruption on 2010 August 1 with
EUV data obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamic Observatory (SDO). In AIA 94 \AA\ (Fe XVIII; 6 MK), topological
reconfiguration due to tether-cutting reconnection is unambiguously observed
for the first time, i.e., two opposite J-shaped loops reconnect to form a
continuous S-shaped loop, whose central portion is dipped and aligned along the
magnetic polarity inversion line (PIL), and a compact loop crossing the PIL. A
causal relationship between photospheric flows and coronal tether-cutting
reconnections is evidenced by the detection of persistent converging flows
toward the PIL using line-of-sight magnetograms obtained by the Helioseismic
and Magnetic Imager (HMI) on board SDO. The S-shaped loop remains in
quasi-equilibrium in the lower corona for about 50 minutes, with the central
dipped portion rising slowly at ~10 km s-1. The speed then increases to ~60 km
s-1 about 10 minutes prior to the onset of a GOES-class C3.2 flare, as the
S-shaped loop speeds up its transformation into an arch-shaped loop, which
eventually leads to a loop-like coronal mass ejection (CME). The AIA
observations combined with H? filtergrams as well as hard X-ray (HXR) imaging
and spectroscopy are consistent with most flare loops being formed by
reconnection of the stretched legs of less-sheared J-shaped loops that
envelopes the rising flux rope, in agreement with the standard tether-cutting
scenario. | astro-ph_SR |
Subsonic structure and optically thick winds from Wolf--Rayet stars: Wolf-Rayet star's winds can be so dense and so optically thick that the
photosphere appears in the highly supersonic part of the outflow, veiling the
underlying subsonic part of the star, and leaving the initial acceleration of
the wind inaccessible to observations. We investigate the conditions and the
structure of the subsonic part of the outflow of Galactic WR stars, in
particular of the WNE subclass; our focus is on the conditions at the sonic
point. We compute 1D hydrodynamic stellar structure models for massive helium
stars adopting outer boundaries at the sonic point. We find that the outflows
of our models are accelerated to supersonic velocities by the radiative force
from opacity bumps either at temperatures of the order of 200kK by the Fe
opacity bump or of the order of 50kK by the HeII opacity bump. For a given
mass-loss rate, the conditions in the subsonic part of the outflow are
independent from the detailed physical conditions in the supersonic part. The
close proximity to the Eddington limit at the sonic point allows us to
construct a Sonic HR diagram, relating the sonic point temperature to the L/M
ratio and the stellar mass-loss rate, thereby constraining the sonic point
conditions, the subsonic structure, and the stellar wind mass-loss rates from
observations. The minimum mass-loss rate necessary to have the flow accelerated
to supersonic velocities by the Fe opacity bump is derived. A comparison of the
observed parameters of Galactic WNE stars to this minimum mass-loss rate
indicates that their winds are launched to supersonic velocities by the
radiation pressure arising from the Fe-bump. Conversely, models which do not
show transonic flows from the Fe opacity bump form inflated envelopes. We
derive an analytic criterion for the appearance of envelope inflation in the
subphotospheric layers. | astro-ph_SR |
Formation of Transient Coronal Holes during Eruption of a Quiescent
Filament and its Overlying Sigmoid: By using H$\alpha$, He I 10830, EUV and soft X-ray (SXR) data, we examined a
filament eruption that occurred on a quiet-sun region near the center of the
solar disk on 2006 January 12, which disturbed a sigmoid overlying the filament
channel observed by the $\emph{GOES-12}$ SXR Imager (SXI), and led to the
eruption of the sigmoid. The event was associated with a partial halo coronal
mass ejection (CME) observed by the Large Angle and Spectrometric Coronagraphs
(LASCO) on board the Solar and Heliospheric Observatory ($\emph{SOHO}$), and
resulted in the formation of two flare-like ribbons, post-eruption coronal
loops, and two transient coronal holes (TCHs), but there were no significantly
recorded $\emph{GOES}$ or H$\alpha$ flares corresponding to the eruption. The
two TCHs were dominated by opposite magnetic polarities and were located on the
two ends of the eruptive sigmoid. They showed similar locations and shapes in
He I 10830, EUV and SXR observations. During the early eruption phase,
brightenings first appeared on the locations of the two subsequent TCHs, which
could be clearly identified on He I 10830, EUV and SXR images. This eruption
event could be explained by the magnetic flux rope model, and the two TCHs were
likely to be the feet of the flux rope. | astro-ph_SR |
Anisotropic turbulent transport in stably stratified rotating stellar
radiation zones: Rotation is one of the key physical mechanisms that deeply impact the
evolution of stars. Helio- and asteroseismology reveal a strong extraction of
angular momentum from stellar radiation zones over the whole
Hertzsprung-Russell diagram. Turbulent transport in differentially rotating
stably stratified stellar radiation zones should be carefully modeled and its
strength evaluated. Stratification and rotation imply that this turbulent
transport is anisotropic. Only phenomenological prescriptions have been
proposed for the transport in the horizontal direction, which however
constitutes a cornerstone in current theoretical formalisms for stellar
hydrodynamics in evolution codes. We derive a new theoretical prescription for
the anisotropy of the turbulent transport in radiation zones using a spectral
formalism for turbulence that takes simultaneously stable stratification,
rotation, and a radial shear into account. Then, the horizontal turbulent
transport resulting from 3D turbulent motions sustained by the instability of
the radial differential rotation is derived. We implement this framework in the
stellar evolution code STAREVOL and quantify its impact on the rotational and
structural evolution of low-mass stars from the pre-main-sequence to the red
giant branch. The anisotropy of the turbulent transport scales as
$N^4\tau^2/\left(2\Omega^2\right)$, $N$ and $\Omega$ being the buoyancy and
rotation frequencies respectively and $\tau$ a time characterizing the source
of turbulence. This leads to a horizontal turbulent transport of similar
strength in average that those obtained with previously proposed prescriptions
even if it can be locally larger below the convective envelope. As a
consequence, a complementary transport mechanism like internal gravity waves or
magnetic fields is still needed to explain the observed strong transport of
angular momentum along stellar evolution. | astro-ph_SR |
Nonradial Oscillations in Classical Pulsating Stars. Predictions and
Discoveries: After a brief historical introduction and recalling basic concepts of stellar
oscillation theory, I focus my review on interpretation of secondary
periodicities found in RR Lyrae stars and Cepheids as a manifestation of
nonradial mode excitation. | astro-ph_SR |
Ejection of globular cluster interstellar media through ionization by
white dwarfs: UV radiation from white dwarfs can efficiently clear Galactic globular
clusters (GCs) of their intra-cluster medium (ICM). This solves the problem of
the missing ICM in clusters, which is otherwise expected to build up to easily
observable quantities. To show this, we recreate the ionizing flux in 47 Tuc,
following randomly generated stars through their AGB, post-AGB and white dwarf
evolution. Each white dwarf can ionize all the material injected into the
cluster by stellar winds for ~3 Myr of its evolution: ~40 such white dwarfs
exist at any point. Every GC's ICM should be ionized. The neutral cloud in M15
should be caused by a temporary overdensity. A pressure-supported ICM will
expand over the cluster's tidal radius, where it will be truncated, allowing
Jeans escape. The modelled Jeans mass-loss rate approximates the total stellar
mass-loss rate, allowing efficient clearing of ICM. Any cluster's ICM mass
should equal the mass injected by its stars over the sound-travel time between
the cluster core and tidal radius. We predict ~11.3 solar masses of ICM within
47 Tuc, cleared over ~4 Myr, compared to a dynamical timescale of 4.3 Myr. We
present a new mass hierarchy, discussing the transition between globular
clusters dwarf galaxies. | astro-ph_SR |
Multiple and changing cycles of active stars II. Results: We study the time variations of the cycles of 20 active stars based on
decades-long photometric or spectroscopic observations. A method of
time-frequency analysis, as discussed in a companion paper, is applied to the
data. Fifteen stars definitely show multiple cycles; the records of the rest
are too short to verify a timescale for a second cycle. The cycles typically
show systematic changes. For three stars, we found two cycles in each of them
that are not harmonics, and which vary in parallel, indicating that a common
physical mechanism arising from a dynamo construct. The positive relation
between the rotational and cycle periods is confirmed for the inhomogeneous set
of active stars. Stellar activity cycles are generally multiple and variable. | astro-ph_SR |
Type II supernovae from the Carnegie Supernova Project-I. I. Bolometric
light curves of 74 SNe II using uBgVriYJH photometry: The present study is the first of a series of three papers where we
characterise the type II supernovae (SNe~II) from the Carnegie Supernova
Project-I to understand their diversity in terms of progenitor and explosion
properties. In this first paper, we present bolometric light curves of 74
SNe~II. We outline our methodology to calculate the bolometric luminosity,
which consists of the integration of the observed fluxes in numerous
photometric bands ($uBgVriYJH$) and black-body (BB) extrapolations to account
for the unobserved flux at shorter and longer wavelengths. BB fits were
performed using all available broadband data except when line blanketing
effects appeared. Photometric bands bluer than $r$ that are affected by line
blanketing were removed from the fit, which makes near-infrared (NIR)
observations highly important to estimate reliable BB extrapolations to the
infrared. BB fits without NIR data produce notably different bolometric light
curves, and therefore different estimates of SN~II progenitor and explosion
properties when data are modelled. We present two methods to address the
absence of NIR observations: (a) colour-colour relationships from which NIR
magnitudes can be estimated using optical colours, and (b) new prescriptions
for bolometric corrections as a function of observed SN~II colours. Using our
74 SN~II bolometric light curves, we provide a full characterisation of their
properties based on several observed parameters. We measured magnitudes at
different epochs, as well as durations and decline rates of different phases of
the evolution. An analysis of the light-curve parameter distributions was
performed, finding a wide range and a continuous sequence of observed
parameters which is consistent with previous analyses using optical light
curves. | astro-ph_SR |
A Multiplicity Census of Intermediate-Mass Stars in Scorpius-Centaurus: Stellar multiplicity properties have been studied for much of the range from
the lowest to the highest stellar masses, but intermediate-mass stars from
F-type to late A-type have received relatively little attention. Here we report
on a Gemini/NICI snapshot imaging survey of 138 such stars in the young
Scorpius-Centaurus (Sco-Cen) region, for the purpose of studying multiplicity
with sensitivity down to planetary masses at wide separations. In addition to
two brown dwarfs and a companion straddling the hydrogen burning limit we
reported previously, here we present 26 new stellar companions and determine a
multiplicity fraction within 0.1"--5.0" of 21+/-4%. Depending on the adopted
semi-major axis distribution, our results imply a total multiplicity in the
range of ~60--80%, which further supports the known trend of a smoothly
continuous increase in the multiplicity fraction as a function of primary
stellar mass. A surprising feature in the sample is a distinct lack of nearly
equal-mass binaries, for which we discuss possible reasons. The survey yielded
no additional companions below or near the deuterium-burning limit, implying
that their frequency at >200 AU separations is not quite as high as might be
inferred from previous detections of such objects within the Sco-Cen region. | astro-ph_SR |
Rossby Wave Instability with Self-Gravity: The Rossby wave instability (RWI) in non-self-gravitating discs can be
triggered by a bump at a radius $r_0$ in the disc surface mass-density (which
is proportional to the inverse potential vorticity). It gives rise to a growing
non-axisymmetric perturbation [$\propto \exp(im\phi)$, $m=1,2..$] in the
vicinity of $r_0$ consisting of anticyclonic vortices which may facilitate
planetesimal growth in protoplanetary discs. Here, we analyze a continuum of
thin disc models ranging from self-gravitating to non-selfgravitating. The key
quantities determining the stability/instability are: (1) the parameters of the
bump (or depression) in the disc surface density, (2) the Toomre $Q$ parameter
of the disc (a non-self-gravitating disc has $Q\gg1$), and (3) the
dimensionless azimuthal wavenumber of the perturbation $\bar{k}_\phi =mQh/r_0$,
where $h$ is the half-thickness of the disc. For discs stable to axisymmetric
perturbations ($Q>1$), the self-gravity has a significant role for
$\bar{k}_\phi < \pi/2$ or $m<(\pi/2) (r_0/h)Q^{-1}$; instability may occur for
a depression or groove in the surface density if $Q\lesssim 2$. For
$\bar{k}_\phi > \pi/2$ the self-gravity is not important, and instability may
occur at a bump in the surface density. Thus, for all mode numbers $m \ge 1$,
the self-gravity is unimportant for $Q > (\pi/2)(r_0/h)$. We suggest that the
self-gravity be included in simulations for cases where $Q< (r_0/h)$. | astro-ph_SR |
Spectroscopic and Photometric Behaviour of LP Ori: We performed spectroscopic and photometric analyses on the early B-type LP
Ori young stellar object located in the Orion Nebula. The high-resolution
spectra of LP Ori was obtained at the Ankara University Kreiken Observatory in
2023, while all spectra recorded over the past 19 years were extracted from the
ESO and ESPaDOnS archives. In these spectra of LP Ori, there is typically an
emission observed in the core of the Balmer profile. This structure is
accompanied by a $\sim$14-year interval inverse P-Cygni repetition superimposed
on the Balmer profile. Additionally, an emission in the $\lambda$5875 He~I line
is visible in the spectra in the year 2023. When this emission is considered
together with the inverse P-Cygni structures, these observations suggest that
LP Ori is a Herbig Ae/Be star. The abundance pattern of LP Ori is close to
solar with the exception of a slightly rich helium and slightly poor Al
abundance. Additionally, the spectral energy distribution of LP Ori was
constructed to confirm the infrared excess caused by its circumstellar disk.
Furthermore, the photometric analysis performed on the TESS observations of LP
Ori shows significant photometric variability and the frequency analysis
reveals a $\beta$~Cephei star in its forthcoming evolution. | astro-ph_SR |
Coronal response to magnetically-suppressed CME events in M-dwarf stars: We report the results of the first state-of-the-art numerical simulations of
Coronal Mass Ejections (CMEs) taking place in realistic magnetic field
configurations of moderately active M-dwarf stars. Our analysis indicates that
a clear, novel, and observable, coronal response is generated due to the
collapse of the eruption and its eventual release into the stellar wind.
Escaping CME events, weakly suppressed by the large-scale field, induce a
flare-like signature in the emission from coronal material at different
temperatures due to compression and associated heating. Such flare-like
profiles display a distinctive temporal evolution in their Doppler shift signal
(from red to blue), as the eruption first collapses towards the star and then
perturbs the ambient magnetized plasma on its way outwards. For stellar fields
providing partial confinement, CME fragmentation takes place, leading to rise
and fall flow patterns which resemble the solar coronal rain cycle. In strongly
suppressed events, the response is better described as a gradual brightening,
in which the failed CME is deposited in the form of a coronal rain cloud
leading to a much slower rise in the ambient high-energy flux by relatively
small factors ($\sim2-3$). In all the considered cases (escaping/confined) a
fractional decrease in the emission from mid-range coronal temperature plasma
occurs, similar to the coronal dimming events observed on the Sun. Detection of
the observational signatures of these CME-induced features requires a sensitive
next generation X-ray space telescope. | astro-ph_SR |
Dynamics of the photosphere along the solar cycle from SDO/HMI: As the global magnetic field of the Sun has an activity cycle, one expects to
observe some variation of the dynamical properties of the flows visible in the
photosphere. We investigate the flow field during the solar cycle by analysing
SDO/HMI observations of continuum intensity, Doppler velocity and longitudinal
magnetic field. We first picked data at disk center during 6 years along the
solar cycle with a 48-hour time step in order to study the overall evolution of
the continuum intensity and magnetic field. Then we focused on thirty 6-hour
sequences of quiet regions without any remnant of magnetic activity separated
by 6 months, in summer and winter, when disk center latitude B0 is close to
zero. The horizontal velocity was derived from the local correlation tracking
technique over a field of view of 216.4Mm x 216.4Mm located at disk center. Our
measurements at disk center show the stability of the flow properties between
meso- and supergranular scales along the solar cycle. The network magnetic
field, produced locally at disk center independently from large scale dynamo,
together with continuum contrast, vertical and horizontal flows, seem to remain
constant during the solar cycle. | astro-ph_SR |
Evolution of the Radial Size and Expansion of Coronal Mass Ejections
Investigated by Combining Remote and In-Situ Observations: A fundamental property of coronal mass ejections (CMEs) is their radial
expansion, which determines the increase in the CME radial size and the
decrease in the CME magnetic field strength as the CME propagates. CME radial
expansion can be investigated either by using remote observations or by in-situ
measurements based on multiple spacecraft in radial conjunction. However, there
have been only few case studies combining both remote and in-situ observations.
It is therefore unknown if the radial expansion estimated remotely in the
corona is consistent with that estimated locally in the heliosphere. To address
this question, we first select 22 CME events between the years 2010 and 2013,
which were well observed by coronagraphs and by two or three spacecraft in
radial conjunction. We use the graduated cylindrical shell model to estimate
the radial size, radial expansion speed, and a measure of the dimensionless
expansion parameter of CMEs in the corona. The same parameters and two
additional measures of the radial-size increase and magnetic-field-strength
decrease with heliocentric distance of CMEs based on in-situ measurements are
also calculated. For most of the events, the CME radial size estimated by
remote observations is inconsistent with the in-situ estimates. We further
statistically analyze the correlations of these expansion parameters estimated
using remote and in-situ observations, and discuss the potential reasons for
the inconsistencies and their implications for the CME space weather
forecasting. | astro-ph_SR |
Ionization ratios and elemental abundances in the atmosphere of 68 Tauri: We have derived the ionization ratios of twelve elements in the atmosphere of
the star 68 Tauri (HD 27962) using an ATLAS9 model atmosphere with 72 layers
computed for the effective temperature and surface gravity of the star. We then
computed a grid of synthetic spectra generated by SYNSPEC49 based on an ATLAS9
model atmosphere in order to model one high resolution spectrum secured by one
of us (RM) with the echelle spectrograph SOPHIE at Observatoire de Haute
Provence. We could determine the abundances of several elements in their
dominant ionization stage, including those defining the Am phenomenon. We thus
provide new abundance determinations for 68 Tauri using updated accurate atomic
data retrieved from the NIST database which extend previous abundance works. | astro-ph_SR |
Asteroseismology of red giants from the first four months of Kepler
data: Fundamental parameters: Clear power excess in a frequency range typical for solar-type oscillations
in red giants has been detected in more than 1000 stars, which have been
observed during the first 138 days of the science operation of the NASA Kepler
satellite. This sample includes stars in a wide mass and radius range with
spectral types G and K, extending in luminosity from the bottom of the giant
branch up to high-luminous red giants. The high-precision asteroseismic
observations with Kepler provide a perfect source for testing stellar structure
and evolutionary models, as well as investigating the stellar population in our
Galaxy. We fit a global model to the observed frequency spectra, which allows
us to accurately estimate the granulation background signal and the global
oscillation parameters, such as the frequency of maximum oscillation power. We
find regular patterns of radial and non-radial oscillation modes and use a new
technique to automatically identify the mode degree and the characteristic
frequency separations between consecutive modes of the same spherical degree.
In most cases, we can also measure the small separation. The seismic parameters
are used to estimate stellar masses and radii and to place the stars in an H-R
diagram by using an extensive grid of stellar models that covers a wide
parameter range. Using Bayesian techniques throughout our analysis allows us to
determine reliable uncertainties for all parameters. We provide accurate
seismic parameters and their uncertainties for a large sample of red giants and
determine their asteroseismic fundamental parameters. We investigate the
influence of the stars' metallicities on their positions in the H-R diagram. We
study the red-giant populations in the red clump and bump and compare them to a
synthetic population and find a mass and metallicity gradient in the red clump
and clear evidence of a secondary-clump population. | astro-ph_SR |
Search for Alignment of Disk Orientations in Nearby Star-Forming
Regions: Lupus, Taurus, Upper Scorpius, $ρ$ Ophiuchi, and Orion: Spatial correlations among proto-planetary disk orientations carry unique
information on physics of multiple star formation processes. We select five
nearby star-forming regions that comprise a number of proto-planetary disks
with spatially-resolved images with ALMA and HST, and search for the mutual
alignment of the disk axes. Specifically, we apply the Kuiper test to examine
the statistical uniformity of the position angle (PA: the angle of the major
axis of the projected disk ellipse measured counter-clockwise from the north)
distribution. The disks located in the star-forming regions, except the Lupus
clouds, do not show any signature of the alignment, supporting the random
orientation. Rotational axes of 16 disks with spectroscopic measurement of PA
in the Lupus III cloud, a sub-region of the Lupus field, however, exhibit a
weak and possible departure from the random distribution at a $2\sigma$ level,
and the inclination angles of the 16 disks are not uniform as well.
Furthermore, the mean direction of the disk PAs in the Lupus III cloud is
parallel to the direction of its filament structure, and approximately
perpendicular to the magnetic field direction. We also confirm the robustness
of the estimated PAs in the Lupus clouds by comparing the different
observations and estimators based on three different methods including sparse
modeling. The absence of the significant alignment of the disk orientation is
consistent with the turbulent origin of the disk angular momentum. Further
observations are required to confirm/falsify the possible disk alignment in the
Lupus III cloud. | astro-ph_SR |
Spatio-kinematic models of five nova remnants: correlations between nova
shell axial ratio, expansion velocity, and speed class: We present long-slit intermediate-dispersion spectroscopic observations and
narrow-band direct imaging of four classical nova shells, namely TAur, HRDel,
DQHer and QUVul, and the nova-like source CKVul. These are used to construct
models of their nebular remnants using the morpho-kinematic modelling tool
Shape to reveal their 3D shape. All these nova remnants but CKVul can be
described by prolate ellipsoidal shells with different eccentricity degree,
from the spherical QUVul to the highly elongated shell with an equatorial
component HRDel. On the other hand, CKVul shows a more complex structure, with
two pairs of nested bipolar lobes. The spatio-kinematic properties of the
ellipsoidal nova shells derived from our models include their true axial
ratios. This parameter is expected to correlate with the expansion velocity and
decline time C3 (i.e., their speed class) of a nova as the result the
interaction of the ejecta with the circumstellar material and rotation speed
and magnetic field of the white dwarf. We have compared these three parameters
including data available in the literature for another two nova shells, V533
Her and FH Ser. There is an anti-correlation between the expansion velocity and
the axial ratio and decline time C3 for nova remnants with ellipsoidal
morphology, and a correlation between their axial ratios and decline times C3,
confirming theoretical expectations that the fastest expanding novae have the
smallest axial ratios. We note that the high expansion velocity of the nova
shell HRDel of 615 km/s is inconsistent with its long decline time C3 of 250
days. | astro-ph_SR |
Herschel-HIFI detections of hydrides towards AFGL 2591 (Envelope
emission versus tenuous cloud absorption): The Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel
Space Observatory allows the first observations of light diatomic molecules at
high spectral resolution and in multiple transitions. Here, we report deep
integrations using HIFI in different lines of hydrides towards the high-mass
star forming region AFGL 2591. Detected are CH, CH+, NH, OH+, H2O+, while NH+
and SH+ have not been detected. All molecules except for CH and CH+ are seen in
absorption with low excitation temperatures and at velocities different from
the systemic velocity of the protostellar envelope. Surprisingly, the CH(JF,P =
3/2_2,- - 1/2_1,+) and CH+(J = 1 - 0, J = 2 - 1) lines are detected in emission
at the systemic velocity. We can assign the absorption features to a foreground
cloud and an outflow lobe, while the CH and CH+ emission stems from the
envelope. The observed abundance and excitation of CH and CH+ can be explained
in the scenario of FUV irradiated outflow walls, where a cavity etched out by
the outflow allows protostellar FUV photons to irradiate and heat the envelope
at larger distances driving the chemical reactions that produce these
molecules. | astro-ph_SR |
General model of depolarization and transfer of polarization of singly
ionized atoms by collisions with hydrogen atoms: Simulations of the generation of the atomic polarization is necessary for
interpreting the second solar spectrum. For this purpose, it is important to
rigorously determine the effects of the isotropic collisions with neutral
hydrogen on the atomic polarization of the neutral atoms, ionized atoms and
molecules. Our aim is to treat in generality the problem of depolarizing
isotropic collisions between singly ionized atoms and neutral hydrogen in its
ground state. Using our numerical code, we computed the collisional
depolarization rates of the $p$-levels of ions for large number of values of
the effective principal quantum number $n^{*}$ and the Uns\"old energy $E_p$.
Then, genetic programming has been utilized to fit the available depolarization
rates. As a result, strongly non-linear relationships between the collisional
depolarization rates, $n^{*}$ and $E_p$ are obtained, and are shown to
reproduce the original data with accuracy clearly better than 10\%. These
relationships allow quick calculations of the depolarizing collisional rates of
any simple ion which is very useful for the solar physics community. In
addition, the depolarization rates associated to the complex ions and to the
hyperfine levels can be easily derived from our results. In this work we have
shown that by using powerful numerical approach and our collisional method,
general model giving the depolarization of the ions can be obtained to be
exploited for solar applications. | astro-ph_SR |
On the Temporal Evolution of the Disc Counterpart of Type II Spicules in
Quiet Sun: The type II spicule has been speculated to provide enough hot plasma to play
an important role in the mass loading and heating of the corona. We analyse the
disc counterpart to type II spicules, RBEs, in three high quality datasets from
CRISP at the SST. In a quiet Sun region at disc centre we find lower Doppler
velocities, 15-40km/s, and Doppler widths, 2-15km/s, of RBEs than in earlier
coronal hole studies, 30-50km/s and 7-23km/s, respectively. We examine the
spatial dependence of Doppler velocities and widths along RBE axes and conclude
that there is no clear trend over the FOV or in individual RBEs in quiet Sun at
disc centre. These differences with previous coronal hole studies are
attributed to the more varying magnetic field configuration in quiet Sun
conditions. Using an extremely high cadence dataset allowed us to improve
greatly on the determination of lifetimes of RBEs, found to range from 5 to 60s
with an average of 30s, as well as the transverse motions in RBEs, with
transverse velocities up to 55km/s and averaging 12km/s. Furthermore, our
measurements of the recurrence rates of RBEs provide important new constraints
on coronal heating by spicules. We also see many examples of a sinusoidal wave
pattern in the transverse motion with periods averaging 54s and amplitudes from
21.5 to 129km, agreeing well with previous studies of wave motion in limb
spicules. We interpret the appearance of RBEs over their full length within a
few seconds as the result of a combination of three kinds of motions as
reported earlier for spicules. Finally, we look at the temporal connection
between Ha and Ca 8542 RBEs and find Ca 8542 in addition to being located
closer to the footpoint also appear before the Ha RBE. This connection supports
the idea that heating occurs in spicules and contribute more weight to the
prominence of spicules as a source for heating and mass loading of the corona. | astro-ph_SR |
Korg: a modern 1D LTE spectral synthesis package: We present Korg, a new package for 1D LTE (local thermodynamic equilibrium)
spectral synthesis of FGK stars, which computes theoretical spectra from the
near-ultraviolet to the near-infrared, and implements both plane-parallel and
spherical radiative transfer. We outline the inputs and internals of Korg, and
compare synthetic spectra from Korg, MOOG, Turbospectrum, and SME. The
disagreements between Korg and the other codes are no larger than those between
the other codes, although disagreement between codes is substantial. We examine
the case of a C$_2$ band in detail, finding that uncertainties on physical
inputs to spectral synthesis account for a significant fraction of the
disagreement. Korg is 1-100 times faster than other codes in typical use,
compatible with automatic differentiation libraries, and easily extensible,
making it ideal for statistical inference and parameter estimation applied to
large data sets. Documentation and installation instructions are available at
https://ajwheeler.github.io/Korg.jl/stable/. | astro-ph_SR |
A review of the disc instability model for dwarf novae, soft X-ray
transients and related objects: I review the basics of the disc instability model (DIM) for dwarf novae and
soft-X-ray transients and its most recent developments, as well as the current
limitations of the model, focusing on the dwarf nova case. Although the DIM
uses the Shakura-Sunyaev prescription for angular momentum transport, which we
know now to be at best inaccurate, it is surprisingly efficient in reproducing
the outbursts of dwarf novae and soft X-ray transients, provided that some
ingredients, such as irradiation of the accretion disc and of the donor star,
mass transfer variations, truncation of the inner disc, etc., are added to the
basic model. As recently realized, taking into account the existence of winds
and outflows and of the torque they exert on the accretion disc may
significantly impact the model. I also discuss the origin of the superoutbursts
that are probably due to a combination of variations of the mass transfer rate
and of a tidal instability. I finally mention a number of unsolved problems and
caveats, among which the most embarrassing one is the modelling of the low
state. Despite significant progresses in the past few years both on our
understanding of angular momentum transport, the DIM is still needed for
understanding transient systems. | astro-ph_SR |
Learning about AGB stars by studying the stars polluted by their
outflows: A rich zoo of peculiar objects forms when Asymptotic Giant Branch (AGB)
stars, undergo interactions in a binary system. For example, Barium (Ba) stars
are main-sequence and red-giant stars that accreted mass from the outflows of a
former AGB companion, which is now a dim white dwarf (WD). Their orbital
properties can help us constrain AGB binary interaction mechanisms, and their
chemical abundances are a tracer of the nucleosynthesis processes that took
place inside the former AGB star. The observational constraints concerning the
orbital and stellar properties of Ba stars have increased in the past years,
but important uncertainties remained concerning their WD companions. In this
contribution, we used HD76225 to demonstrate that by combining radial-velocity
data with Hipparcos and Gaia astrometry, one can accurately constrain the
orbital inclinations of these systems and obtain the absolute masses of these
WDs, getting direct information about their AGB progenitors via initial-final
mass relationships. | astro-ph_SR |
The variable 6307A emission line in the spectrum of Eta Carinae:
blueshifted [S III] 6313A from the interacting winds: The 6307A emission line in the spectrum of Eta Car (Martin et al. 2006) is
blue-shifted [S III] 6313A emission originating from the outer wind structures
of the massive binary system. We realized the identification while analyzing
multiple forbidden emission lines not normally seen in the spectra of massive
stars. The high spatial and moderate spectral resolutions of HST/STIS resolve
forbidden lines of Fe+, N+, Fe++, S++, Ne++ and Ar++ into spatially and
velocity-resolved rope-like features originating from collisionally-excited
ions photo-ionized by UV photons or collisions. While the [Fe II] emission
extends across a velocity range of +/-500 km/s out to 0."7, more highly ionized
forbidden emissions are systematically blue-shifted (-500 to +200 km/s) and
extend only to 0."4. The [Fe II] defines the outer regions of the massive
primary wind. The [N II], [Fe III] emission define the the outer wind
interaction regions directly photo-ionized by far-UV radiation. Variations in
emission of [S III] 9533A, 9071A and 6313A suggest density ranges of 10^6 to
10^10 cm^-3 for electron temperatures ranging from 8,000 to 13,000K. Mapping
the temporal changes of the emission structure at critical phases of the
5.54-year period will provide important diagnostics of the interacting winds. | astro-ph_SR |
Time-resolved optical observations of five cataclysmic variables
detected by INTEGRAL: The ESA gamma-ray telescope, INTEGRAL, is detecting relatively more
intrinsically rare cataclysmic variables (CVs) than were found by surveys at
lower energies. Specifically, a large fraction of the CVs that are INTEGRAL
sources consists of asynchronous polars and intermediate polars (IPs). IP
classifications have been proposed for the majority of CVs discovered by
INTEGRAL, but, in many cases, there is very little known about these systems.
In order to address this, I present time-resolved optical data of five CVs
discovered through INTEGRAL observations. The white dwarf spin modulation is
detected in high-speed photometry of three of the new CVs (IGR J15094-6649, IGR
J16500-3307, and IGR J17195-4100), but two others (XSS J12270-4859 and IGR
J16167-4957) show no evidence of magnetism, and should be considered
unclassified systems. Spectroscopic orbital period (P_orb) measurements are
also given for IGR J15094-6649, IGR J16167-4957, IGR J16500-3307, and IGR
J17195-4100. | astro-ph_SR |
NOEMA maps the CO $J = 2-1$ environment of the red supergiant $μ$ Cep: Red supergiant stars are surrounded by a gaseous and dusty circumstellar
environment created by their mass loss which spreads heavy elements into the
interstellar medium. The structure and the dynamics of this envelope are
crucial to understand the processes driving the red supergiant mass loss and
the shaping of the pre-supernova ejecta. We have observed the emission from the
CO $J = 2-1$ line from the red supergiant star $\mu$~Cep with the NOEMA
interferometer. In the line the synthesized beam was $0.92 \times 0.72$~arcsec
($590 \times 462$~au at 641~pc). The continuum map shows only the unresolved
contribution of the free-free emission of the star chromosphere. The
continuum-subtracted channel maps reveal a very inhomogeneous and clumpy
circumstellar environment. In particular, we detected a bright CO clump, as
bright as the central source in the line, at 1.80~arcsec south-west from the
star, in the blue channel maps. After a deprojection of the radial velocity
assuming two different constant wind velocities, the observations were modelled
using the 3D radiative transfer code \textsc{lime} to derive the
characteristics of the different structures. We determine that the gaseous
clumps observed around $\mu$~Cep are responsible for a mass loss rate of $(4.9
\pm 1.0) \times 10^{-7}~{\rm M}_\odot\,{\rm yr}^{-1}$, in addition to a
spatially unresolved wind component with an estimated mass-loss rate of $2.0
\times 10^{-6}~{\rm M}_\odot\,{\rm yr}^{-1}$. Therefore, the clumps have a
significant role in $\mu$~Cep's mass loss ($\ge 25 \%$). We cannot exclude that
the unresolved central outflow may be made of smaller unresolved clumps. | astro-ph_SR |
A kinematic study of the disc-outflow system around a high-mass
protostar G59.783+0.065 probed by methanol and water masers: Class II CH3OH masers are used as a convenient tracer of disc-like structures
in high-mass star formation. However, more than half of them show a complex
distribution in Very Long Baseline Interferometry (VLBI) maps. The origin of
such a complex distribution is still unknown. We conducted VLBI monitoring
observations to unveil the origin of a complex class II CH3OH maser in the
high-mass star-forming region G59.783+0.065. We observed the CH3OH maser at 6.7
GHz and the H2O maser at 22 GHz to probe detailed circumstellar kinematics and
structures by the Japanese VLBI network and the VLBI Exploration of Radio
Astrometry. We found similar bipolar distributions in both masers, specifically
two clusters located 2000 au apart along the East-West direction. We detected a
linear distribution of CH3OH masers in the Western cluster. A position-velocity
diagram shows that the Western CH3OH masers trace a rotating disc-wind or
infalling component inside an edge-on disc-like structure. In contrast to the
simple bipolar expanding motions of the H2O masers, the CH3OH masers exhibited
complex motions despite their spatial coincidence. Some of the Eastern CH3OH
masers showed bipolar expansions similar to the H2O masers, while others
displayed random or even inward motions. Such complex kinematics and their
close association with the H2O maser could occur at the boundary between
outflow and inflow. We suggest that the complex distribution of class II CH3OH
masers, like G59.783+0.065 arises from several distinct circumstellar
structures that simultaneously achieve maser excitation. | astro-ph_SR |
Extreme Ultraviolet Late-Phase Flares: Before and During the Solar
Dynamics Observatory Mission: The SDO EUV observations have revealed interesting characteristics of warm
coronal emissions, which peak soon after the hot coronal X-ray emissions peak
during a flare and then sometimes peak for a second time hours after the X-ray
flare peak. This flare type, with two warm coronal emission peaks but only one
X-ray peak, has been named the EUV late phase. These flares have the distinct
properties of i) having a complex magnetic field structure with two initial
sets of coronal loops, with one upper set overlaying a lower set, ii) having an
eruptive flare initiated in the lower set and disturbing both loop sets, iii)
having the hot coronal emissions emitted only from the lower set, and iv)
having the first peak of the warm coronal emissions associated with the lower
set and its second peak emitted from the upper set much later. The disturbance
of the coronal loops by the eruption is at about the same time, but the
relaxation and cooling down of the heated coronal loops during the post-flare
reconnections have different time scales with the longer, upper loops being
significantly delayed from the lower loops. The difference in these cooling
time scales is related to the difference between the two peak times of the warm
coronal emission and is also apparent in the decay profile of the X-ray
emissions having two distinct decays, with the first decay slope being steeper
and the delayed decay slope being smaller during the time of the warm coronal
emission second peak. The frequency and relationship of the EUV late-phase
decay times between the Fe XVI two flare peaks and X-ray decay slopes are
examined using three years of SDO/EVE data, and the X-ray dual-decay character
is then exploited to estimate the frequency of EUV late-phase flares during the
past four solar cycles. This study indicates that the frequency of EUV
late-phase flares peaks before and after each solar cycle minimum. | astro-ph_SR |
Stellar magnetism: empirical trends with age and rotation: We investigate how the observed large-scale surface magnetic fields of
low-mass stars (~0.1 -- 2 Msun), reconstructed through Zeeman-Doppler imaging
(ZDI), vary with age t, rotation and X-ray emission. Our sample consists of 104
magnetic maps of 73 stars, from accreting pre-main sequence to main-sequence
objects (1 Myr < t < 10 Gyr). For non-accreting dwarfs we empirically find that
the unsigned average large-scale surface field <|Bv|> is related to age as
$t^{-0.655 \pm 0.045}$. This relation has a similar dependence to that
identified by Skumanich (1972), used as the basis for gyrochronology. Likewise,
our relation could be used as an age-dating method ("magnetochronology"). The
trends with rotation we find for the large-scale stellar magnetism are
consistent with the trends found from Zeeman broadening measurements (sensitive
to large- and small-scale fields). These similarities indicate that the fields
recovered from both techniques are coupled to each other, suggesting that
small- and large-scale fields could share the same dynamo field generation
processes. For the accreting objects, fewer statistically significant relations
are found, with one being a correlation between the unsigned magnetic flux and
rotation period. We attribute this to a signature of star-disc interaction,
rather than being driven by the dynamo. | astro-ph_SR |
Constraints on the distance moduli, helium and metal abundances, and
ages of globular clusters from their RR Lyrae and non-variable
horizontal-branch stars. II. Multiple stellar populations in 47Tuc, M3, and
M13: We present a new set of horizontal-branch (HB) models computed with the MESA
stellar evolution code. The models adopt $\alpha$-enhanced \cite{ags09} metals
mixtures and include the gravitational settling of He. They are used in our HB
population synthesis tool to generate theoretical distributions of HB stars in
order to describe the multiple stellar populations in the globular clusters
47Tuc, M3, and M13. The observed HB in 47Tuc is reproduced very well by our
simulations for [Fe/H] $= -0.70$ and [$\alpha$/Fe] $= +0.4$ if the initial
helium mass fraction varies by $\Delta Y_0 \sim 0.03$ and approximately 21%,
37%, and 42% of the stars have $Y_0 = 0.257$, 0.270, and 0.287, respectively.
These simulations yield $(m-M)_V = 13.27$, implying an age near 13.0 Gyr. In
the case of M3 and M13, our synthetic HBs for [Fe/H] $= -1.55$ and
[$\alpha$/Fe] $= 0.4$ match the observed ones quite well if M3 has $\Delta Y_0
\sim 0.01$ and $(m-M)_V = 15.02$, resulting in an age of 12.6 Gyr, whereas M13
has $\Delta Y_0 \sim 0.08$ and $(m-M)_V = 14.42$, implying an age of 12.9 Gyr.
Mass loss during giant-branch evolution and $\Delta Y_0$ appear to be the
primary second parameters for M3 and M13. New observations for 7 of the 9 known
RR Lyrae in M13 are also reported. Surprisingly, periods predicted for the
$c$-type variables tend to be too high (by up to $\sim 0.1$~d). | astro-ph_SR |
Multi-epoch high-resolution spectroscopy of SN2011fe - Linking the
progenitor to its environment: This study attempts to establish a link between the reasonably well known
nature of the progenitor of SN2011fe and its surrounding environment. This is
done with the aim of enabling the identification of similar systems in the vast
majority of the cases, when distance and epoch of discovery do not allow a
direct approach. To study the circumstellar environment of SN2011fe we have
obtained high-resolution spectroscopy of SN2011fe on 12 epochs, from 8 to 86
days after the estimated date of explosion, targeting in particular at the time
evolution of CaII and NaI. Three main absorption systems are identified from
CaII and NaI, one associated to the Milky Way, one probably arising within a
high-velocity cloud, and one most likely associated to the halo of M101. The
Galactic and host galaxy reddening, deduced from the integrated equivalent
widths (EW) of the NaI lines are E(B-V)=0.011+/-0.002 and E(B-V)=0.014+/-0.002
mag, respectively. The host galaxy absorption is dominated by a component
detected at the same velocity measured from the 21-cm HI line at the projected
SN position (~180 km/s). During the ~3 months covered by our observations, its
EW changed by 15.6+/-6.5 mA. This small variation is shown to be compatible
with the geometric effects produced by therapid SN photosphere expansion
coupled to the patchy fractal structure of the ISM. The observed behavior is
fully consistent with ISM properties similar to those derived for our own
Galaxy, with evidences for structures on scales <100 AU. SN2011fe appears to be
surrounded by a "clean" environment. The lack of blue-shifted, time-variant
absorption features is fully consistent with the progenitor being a binary
system with a main-sequence, or even another degenerate star. | astro-ph_SR |
The two-dimensional internal rotation of KIC11145123: The two-dimensional internal rotation of KIC11145123 has been inferred via
asteroseismology. Based on the Optimally Localized Averaging method and a
simple three-zone modeling of the internal rotation, we have found evidence for
a contrast between the internal rotation of the radiative region and that of
the convective core; the radiative region rotates almost uniformly throughout
the region, but the convective core may be rotating about 6 times faster than
the radiative region above. We have also found a marginally significant
evidence of latitudinal differential rotation in the outer envelope. These
newly indicated features of the internal rotation of the star can help us
further constrain the theory of angular momentum transport inside stars as well
as understand the complex physical properties of the star, which was once
thought to be a main-sequence A-type star but recently has been proposed to be
a blue straggler, based on spectroscopy. | astro-ph_SR |
Multicolor Photoelectric WBVR-Observations of the CBS HZ~Her=Her~X-1 in
1986--1988: We present results of four-color WBVR- photoelectric observations of the
close binary HZ~Her=HER~X-1 in 1986--1988. As a rule, the duration of the
observations exceeded two 35-day X-ray orbital periods in the 1986--1988
observing seasons. The accuracy and length of the photoelectric observations
facilitated multi-faceted studies, which enabled us to define several fine
photometric effects in the light curves of the binary more precisely and
attempt to interpret them in a model for the matter flow from the optical
component to the accretion disk around the neutron star. This model provides a
satisfactory explanation for the inhomogeneity of the gas flow and "hot spot",
as well as the existence of distinct "splashes" moving in their own Keplerian
orbits around the outer parts of the Keplerian disk. We present series of light
curves for all the observing seasons, as well as color--color diagrams that
reflect the physics of various photometric effects. The transformation
coefficients for each of the instrumental systems for the three observatories
at which the observations were carried out are given. Atmospheric extinction
was taken into account during multi-color observations of the object, with
subsequent correction for atmospheric effects with accuracies ranging from
0.003^m to 0.005^m for air masses up to M(z)=2. | astro-ph_SR |
Comparison of solar wind speed estimates from nearly simultaneous IPS
observations at 327 and 111 MHz: Results are presented of a comparison between solar wind speed estimates made
using the time delays between 3 pairs of 327 MHz antennas at ISEE and estimates
made by modeling the temporal power spectra observed with the 111 MHz BSA
antenna at LPI. The observations were made for 6 years in the descending phase
of solar cycle 24. More than 100 individual records were obtained for the
compact source 3C48 and the extended and anisotropic source 3C298. The
correlation between the daily speed estimates from 3C48 is 50%. Their annual
averages agree within the error estimates and show the expected solar cycle
variation. However the correlation between speeds from 3C298 is only 25% and
their annual averages do not agree well. We investigate possible causes of this
bias in the 3C298 estimated speeds. | astro-ph_SR |
Post-flare UV light curves explained with thermal instability of loop
plasma: In the present work we study the C8 flare occurred on September 26, 2000 at
19:49 UT and observed by the SOHO/SUMER spectrometer from the beginning of the
impulsive phase to well beyond the disappearance in the X-rays. The emission
first decayed progressively through equilibrium states until the plasma reached
2-3 MK. Then, a series of cooler lines, i.e. Ca x, Ca vii, Ne vi, O iv and Si
iii (formed in the temperature range log T = 4.3 - 6.3 under equilibrium
conditions), are emitted at the same time and all evolve in a similar way. Here
we show that the simultaneous emission of lines with such a different formation
temperature is due to thermal instability occurring in the flaring plasma as
soon as it has cooled below ~ 2 MK. We can qualitatively reproduce the relative
start time of the light curves of each line in the correct order with a simple
(and standard) model of a single flaring loop. The agreement with the observed
light curves is greatly improved, and a slower evolution of the line emission
is predicted, if we assume that the model loop consists of an ensemble of
subloops or strands heated at slightly different times. Our analysis can be
useful for flare observations with SDO/EVE. | astro-ph_SR |
The Evolution of Protoplanetary Disks: Probing the Inner Disk of Very
Low Accretors: We report FUV, optical, and NIR observations of three T Tauri stars in the
Orion OB1b subassociation with H$\alpha$ equivalent widths consistent with low
or absent accretion and various degrees of excess flux in the mid-infrared. We
aim to search for evidence of gas in the inner disk in HST ACS/SBC spectra, and
to probe the accretion flows onto the star using H$\alpha$ and He I
$\lambda$10830 in spectra obtained at the Magellan and SOAR telescopes. At the
critical age of 5 Myr, the targets are at different stages of disk evolution.
One of our targets is clearly accreting, as shown by redshifted absorption at
free-fall velocities in the He I line and wide wings in H$\alpha$; however, a
marginal detection of FUV H$_2$ suggests that little gas is present in the
inner disk, although the spectral energy distribution indicates that small dust
still remains close to the star. Another target is surrounded by a transitional
disk, with an inner cavity in which little sub-micron dust remains. Still, the
inner disk shows substantial amounts of gas, accreting onto the star at a
probably low, but uncertain rate. The third target lacks both a He I line or
FUV emission, consistent with no accretion or inner gas disk; its very weak IR
excess is consistent with a debris disk. Different processes occurring in
targets with ages close to the disk dispersal time suggest that the end of
accretion phase is reached in diverse ways. | astro-ph_SR |
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