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A Sun-like star orbiting a boson star: The high-precision astrometric mission GAIA recently reported the remarkable
discovery of a Sun-like star closely orbiting a dark object, with a semi-major
axis and period of $1.4\, \rm{AU}$ and $187.8$ days respectively. While the
plausible expectation for the central dark object is a black hole, the
evolutionary mechanism leading to the formation of such a two-body system is
highly challenging. Here, we challenge the scenario of a central black hole and
show that the observed orbital dynamics can be explained under fairly general
assumptions if the central dark object is a stable clump of bosonic particles
of spin-0, or spin-1, known as a boson star. We further explain how future
astrometric measurements of similar systems will provide an exciting
opportunity to probe the fundamental nature of compact objects and test compact
alternatives to black holes. | astro-ph_SR |
Multi-scale magnetic field investigation of the M-dwarf eclipsing binary
CU Cancri: We aim to characterise the magnetic field of the eclipsing binary CU Cnc. The
determination of magnetic field parameters of this target enables comparisons
with both observations of similar stars and theoretical predictions of the
magnetic field strength for CU Cnc. The target is therefore providing an
excellent opportunity to test our understanding of the generation of magnetic
fields in low-mass stars and its impact on stellar structure. We use
spectropolarimetric observations obtained with ESPaDOnS to investigate the
magnetic properties of CU Cnc. We generate average line profiles with LSD,
which are used to extract information about the radial velocities of the
components, expanding the number of radial velocity measurements available and
allowing for a determination of orbital parameters. Stokes V LSD profiles are
used with ZDI to obtain large-scale magnetic field structures on both
components. We also use polarised radiative transfer modelling to investigate
the small-scale fields by utilising Zeeman splitting of magnetically sensitive
Ti I lines in non-polarised spectra. The large-scale fields are dominantly
poloidal and have an average strength of ~100 G on both components. This
analysis of the large-scale fields likely suffers from some amount of
hemisphere degeneracy due to the high inclination of the target. Both
components also show unusual magnetic field configurations compared to stars
with similar parameters, the primary is weakly axisymmetric (~10%) and the
secondary has a strong torroidal contribution (~20%). The small-scale fields
are significantly stronger, at 3.1 and 3.6 kG for the primary and secondary
respectively. This measurement is in excellent agreement with surface field
strength predictions for CU Cnc from magnetoconvective stellar evolution
models. These results indicates that magnetic fields play a significant role in
radius inflation of active stars. | astro-ph_SR |
ALMA and VLA observations of the outflows in IRAS 16293-2422: We present ALMA and VLA observations of the molecular and ionized gas at
0.1-0.3 arcsec resolution in the Class 0 protostellar system IRAS 16293-2422.
These data clarify the origins of the protostellar outflows from the deeply
embedded sources in this complex region. Source A2 is confirmed to be at the
origin of the well known large scale north-east--south-west flow. The most
recent VLA observations reveal a new ejection from that protostar,
demonstrating that it drives an episodic jet. The central compact part of the
other known large scale flow in the system, oriented roughly east-west, is well
delineated by the CO(6-5) emission imaged with ALMA and is confirmed to be
driven from within component A. Finally, a one-sided blueshifted bubble-like
outflow structure is detected here for the first time from source B to the
north-west of the system. Its very short dynamical timescale (~ 200 yr), low
velocity, and moderate collimation support the idea that source B is the
youngest object in the system, and possibly one of the youngest protostars
known. | astro-ph_SR |
The Anatomy of an Unusual Edge-on Protoplanetary Disk I. Dust Settling
in a Cold Disk: As the earliest stage of planet formation, massive, optically thick, and gas
rich protoplanetary disks provide key insights into the physics of star and
planet formation. When viewed edge-on, high resolution images offer a unique
opportunity to study both the radial and vertical structures of these disks and
relate this to vertical settling, radial drift, grain growth, and changes in
the midplane temperatures. In this work, we present multi-epoch HST and Keck
scattered light images, and an ALMA 1.3 mm continuum map for the remarkably
flat edge-on protoplanetary disk SSTC2DJ163131.2-242627, a young solar-type
star in $\rho$ Ophiuchus. We model the 0.8 $\mu$m and 1.3 mm images in separate
MCMC runs to investigate the geometry and dust properties of the disk using the
MCFOST radiative transfer code. In scattered light, we are sensitive to the
smaller dust grains in the surface layers of the disk, while the sub-millimeter
dust continuum observations probe larger grains closer to the disk midplane. An
MCMC run combining both datasets using a covariance-based log-likelihood
estimation was marginally successful, implying insufficient complexity in our
disk model. The disk is well characterized by a flared disk model with an
exponentially tapered outer edge viewed nearly edge-on, though some degree of
dust settling is required to reproduce the vertically thin profile and lack of
apparent flaring. A colder than expected disk midplane, evidence for dust
settling, and residual radial substructures all point to a more complex radial
density profile to be probed with future, higher resolution observations. | astro-ph_SR |
Science with the ngVLA: Planetary Nebulae: Planetary nebulae (PNe) represent the near endpoints of evolution for stars
of initial mass $\sim$1-8 $M_\odot$, wherein the envelope of an asymptotic
giant branch (AGB) star becomes photodissociated and ionized by high-energy
radiation from a newly emerging white dwarf that was the progenitor star's
core. It is increasingly evident that most PNe are descended from binary
systems. PNe hence provide unique insight into a diverse range of astrophysical
phenomena, including the influence of companion stars on the late stages of
stellar evolution; stellar wind interactions and shocks; the physics and
chemistry of photoionized plasmas and photon-dominated regions (PDRs); and
enrichment of the ISM in the products of intermediate-mass stellar
nucleosynthesis. We describe specific examples of the potential impact of the
ngVLA in each of these areas. | astro-ph_SR |
A Separable Solution for the Oscillatory Structure of Plasma in
Accretion Disks: We provide a new analysis of the system of partial differential equations
describing the radial and vertical equilibria of the plasma in accretion disks.
In particular, we show that the partial differential system can be separated
once a definite, oscillatory (or hyperbolic) form for the radial dependence of
the relevant physical quantities is assumed. The system is thus reduced to an
ordinary differential system in the vertical dimensionless coordinate. The
resulting equations can be integrated analytically in the limit of small
magnetic pressure. We complete our analysis with a direct numerical integration
of the more general case. The main result is that a ring-like density profile
(i.e., radial oscillations in the mass density) can appear even in the limit of
small magnetic pressure. | astro-ph_SR |
Signature of Collision of Magnetic Flux Tubes in the Quiet Solar
Photosphere: Collision of the magnetic flux tubes in the Quiet Sun was proposed as one of
the possible sources for the heating of the solar atmosphere (Furusawa and
Sakai, 2000). The solar photosphere was observed using the New Solar Telescope
ad Big Bear Solar Observatory. In TiO spectral line at 705.68 nm we approached
resolution of 0.1". The horizontal plasma wave was observed spreading from the
larger bright point. Shorty after this wave an increase in the oscillatory
power appeared at the same location as the observed bright point. This behavior
matches some of the results from the simulation of the collision of the two
flux tubes with a weak current. | astro-ph_SR |
A Kepler Study of Starspot Lifetimes with Respect to Light Curve
Amplitude and Spectral Type: Wide-field high precision photometric surveys such as Kepler have produced
reams of data suitable for investigating stellar magnetic activity of cooler
stars. Starspot activity produces quasi-sinusoidal light curves whose phase and
amplitude vary as active regions grow and decay over time. Here we investigate,
firstly, whether there is a correlation between the size of starspots - assumed
to be related to the amplitude of the sinusoid - and their decay timescale and,
secondly, whether any such correlation depends on the stellar effective
temperature. To determine this, we computed the autocorrelation functions of
the light curves of samples of stars from Kepler and fitted them with apodised
periodic functions. The light curve amplitudes, representing spot size were
measured from the root-mean-squared scatter of the normalised light curves. We
used a Monte Carlo Markov Chain to measure the periods and decay timescales of
the light curves. The results show a correlation between the decay time of
starspots and their inferred size. The decay time also depends strongly on the
temperature of the star. Cooler stars have spots that last much longer, in
particular for stars with longer rotational periods. This is consistent with
current theories of diffusive mechanisms causing starspot decay. We also find
that the Sun is not unusually quiet for its spectral type - stars with
solar-type rotation periods and temperatures tend to have (comparatively)
smaller starspots than stars with mid-G or later spectral types. | astro-ph_SR |
Seismic detection of acoustic sharp features in the CoRoT target HD49933: The technique of determining the acoustic location of layers of sharp changes
in the sound speed inside a star from the oscillatory signal in its frequencies
is applied on a solar-type star, the CoRoT target, HD49933. We are able to
determine the acoustic depth of the second helium ionisation zone of HD49933 to
be 794 +55/-68 seconds. The acoustic depth of the base of the convective zone
is found to be 1855 +173/-412 seconds where the large error bars reflect the
ambiguity in the result, which is difficult to determine with present precision
on the frequencies because of the intrinsically weak nature of the signal. The
positions of both these layers are consistent with those in a representative
stellar model of HD49933. | astro-ph_SR |
The Origin of Major Solar Activity - Collisional Shearing Between
Nonconjugated Polarities of Multiple Bipoles Emerging Within Active Regions: Active Regions (ARs) that exhibit compact Polarity Inversion Lines (PILs) are
known to be very flare-productive. However, the physical mechanisms behind this
statistical inference have not been demonstrated conclusively. We show that
such PILs can occur due to the collision between two emerging flux tubes nested
within the same AR. In such multipolar ARs, the flux tubes may emerge
simultaneously or sequentially, each initially producing a bipolar magnetic
region (BMR) at the surface. During each flux tube's emergence phase, the
magnetic polarities can migrate such that opposite polarities belonging to
different BMRs collide, resulting in shearing and cancellation of magnetic
flux. We name this process 'collisional shearing' to emphasize that the
shearing and flux cancellation develops due to the collision. Collisional
shearing is a process different from the known concept of flux cancellation
occurring between polarities of a single bipole, a process that has been
commonly used in many numerical models. High spatial and temporal resolution
observations from the Solar Dynamics Observatory for two emerging ARs, AR11158
and AR12017, show the continuous cancellation of up to 40% of the unsigned
magnetic flux of the smallest BMR, which occurs at the collisional PIL for as
long as the collision persists. The flux cancellation is accompanied by a
succession of solar flares and CMEs, products of magnetic reconnection along
the collisional PIL. Our results suggest that the quantification of magnetic
cancellation driven by collisional shearing needs to be taken into
consideration in order to improve the prediction of solar energetic events and
space weather. | astro-ph_SR |
Carbon, nitrogen and $α$-element abundances determine the formation
sequence of the Galactic thick and thin disks: Using the DR12 public release of APOGEE data, we show that thin and thick
disk separate very well in the space defined by [$\alpha$/Fe], [Fe/H] and
[C/N]. Thick disk giants have both higher [C/N] and higher [$\alpha$/Fe] than
do thin disk stars with similar [Fe/H]. We deduce that the thick disk is
composed of lower mass stars than the thin disk. Considering the fact that at a
given metallicity there is a one-to-one relation between stellar mass and age,
we are then able to infer the chronology of disk formation. Both the thick and
the thin disks - defined by [$\alpha$/Fe] -- converge in their dependance on
[C/N] and [C+N/Fe] at [Fe/H]$\approx$-0.7. We conclude that 1) the majority of
thick disk stars formed earlier than did the thin disk stars 2) the formation
histories of the thin and thick disks diverged early on, even when the [Fe/H]
abundances are similar 3) that the star formation rate in the thin disk has
been lower than in the thick disk, at all metallicities. Although these general
conclusions remain robust, we also show that current stellar evolution models
cannot reproduce the observed C/N ratios for thick disk stars. Unexpectedly,
reduced or inhibited canonical extra-mixing is very common in field stars.
While subject to abundance calibration zeropoint uncertainties, this implies a
strong dependence of non canonical extra-mixing along the red giant branch on
the initial composition of the star and in particular on the $\alpha$ elemental
abundance. | astro-ph_SR |
On the photometric signature of fast rotators: Rapidly rotating stars have been recently recognized as having a major role
in the interpretation of colour-magnitude diagrams of young and
intermediate-age star clusters in the Magellanic Clouds and in the Milky Way.
In this work, we evaluate the distinctive spectra and distributions in
colour-colour space that follow from the presence of a substantial range in
effective temperatures across the surface of fast rotators. The calculations
are inserted in a formalism similar to the one usually adopted for non-rotating
stars, which allows us to derive tables of bolometric corrections as a function
not only of a reference effective temperature, surface gravity and metallicity,
but also of the rotational speed with respect to the break-up value, $\omega$,
and the inclination angle, $i$. We find that only very fast rotators
($\omega>0.95$) observed nearly equator-on ($i>45^\circ$) present sizable
deviations from the colour-colour relations of non-rotating stars. In light of
these results, we discuss the photometry of the $\sim$ 200-Myr-old cluster NGC
1866 and its split main sequence, which has been attributed to the simultaneous
presence of slow and fast rotators. The small dispersion of its stars in
colour-colour diagrams allow us to conclude that fast rotators in this cluster
either have rotational velocities $\omega<0.95$, or are all observed nearly
pole-on. Such geometric colour-colour effects, although small, might be
potentially detectable in the huge, high-quality photometric samples in the
post-Gaia era, in addition to the evolutionary effects caused by
rotation-induced mixing. | astro-ph_SR |
Predictive Capabilities of Avalanche Models for Solar Flares: We assess the predictive capabilities of various classes of avalanche models
for solar flares. We demonstrate that avalanche models cannot generally be used
to predict specific events due to their high sensitivity to their embedded
stochastic process. We show that deterministically driven models can
nevertheless alleviate this caveat and be efficiently used for large events
predictions. Our results promote a new approach for large (typically X-class)
solar flares predictions based on simple and computationally inexpensive
avalanche models. | astro-ph_SR |
Kepler observations of the beaming binary KPD 1946+4340: The Kepler Mission has acquired 33.5d of continuous one-minute photometry of
KPD 1946+4340, a short-period binary system that consists of an sdB and a white
dwarf. In the light curve, eclipses are clearly seen, with the deepest
occurring when the compact white dwarf crosses the disc of the sdB (0.4%) and
the more shallow ones (0.1%) when the sdB eclipses the white dwarf. As
expected, the sdB is deformed by the gravitational field of the white dwarf,
which produces an ellipsoidal modulation of the light curve. Spectacularly, a
very strong Doppler beaming (aka Doppler boosting) effect is also clearly
evident at the 0.1% level. This originates from the sdB's orbital velocity,
which we measure to be 164.0\pm1.9 km/s from supporting spectroscopy. We
present light curve models that account for all these effects, as well as
gravitational lensing. We derive system parameters and uncertainties from the
light curve using Markov Chain Monte Carlo simulations. Adopting a theoretical
white dwarf mass-radius relation, the mass of the subdwarf is found to be
0.47\pm0.03 Msun and the mass of the white dwarf 0.59\pm0.02 Msun. The
effective temperature of the white dwarf is 15 900\pm300K. With a spectroscopic
effective temperature of Teff = 34 730\pm250K and a surface gravity of log g =
5.43\pm0.04, the sdB is in a shell He burning stage. The detection of Doppler
beaming in Kepler light curves potentially allows one to measure radial
velocities without the need of spectroscopic data. For the first time, a
photometrically observed Doppler beaming amplitude is compared to a
spectroscopically established value. The sdB's radial velocity amplitude
derived from the photometry 168\pm4 km/s is in perfect agreement with the
spectroscopic value. After subtracting our best model for the orbital effects,
we searched the residuals for stellar oscillations but did not find any
significant pulsation frequencies. | astro-ph_SR |
The Araucaria Project: High-precision orbital parallax and masses of the
eclipsing binary TZ~Fornacis: Context: Independent distance estimates are particularly useful to check the
precision of other distance indicators, while accurate and precise masses are
necessary to constrain evolution models. Aim: The goal is to measure the masses
and distance of the detached eclipsing-binary TZ~For with a precision level
lower than 1\,\% using a fully geometrical and empirical method. Method: We
obtained the first interferometric observations of TZ~For with the VLTI/PIONIER
combiner, which we combined with new and precise radial velocity measurements
to derive its three-dimensional orbit, masses, and distance. Results: The
system is well resolved by PIONIER at each observing epoch, which allowed a
combined fit with eleven astrometric positions. Our derived values are in a
good agreement with previous work, but with an improved precision. We measured
the mass of both components to be $M_1 = 2.057 \pm 0.001\,M_\odot$ and $M_2 =
1.958 \pm 0.001\,M_\odot$. The comparison with stellar evolution models gives
an age of the system of $1.20 \pm 0.10$\,Gyr. We also derived the distance to
the system with a precision level of 1.1\,\%: $d = 185.9 \pm 1.9$\,pc. Such
precise and accurate geometrical distances to eclipsing binaries provide a
unique opportunity to test the absolute calibration of the surface
brightness-colour relation for late-type stars, and will also provide the best
opportunity to check on the future Gaia measurements for possible systematic
errors. | astro-ph_SR |
Multi-wavelength Spectral Analysis of Ellerman Bombs Observed by FISS
and IRIS: Ellerman bombs (EBs) are a kind of solar activities that is suggested to
occur in the lower atmosphere. Recent observations using the Interface Region
Imaging Spectrograph (IRIS) show connections of EBs and IRIS bombs (IBs),
implying that EBs might be heated to a much higher temperature ($8\times10^{4}$
K) than previous results. Here we perform a spectral analysis of the EBs
simultaneously observed by the Fast Imaging Solar Spectrograph (FISS) and IRIS.
The observational results show clear evidence of heating in the lower
atmosphere, indicated by the wing enhancement in H$\alpha$, Ca II 8542 \r{A}
and Mg II triplet lines, and also by brightenings in the images of 1700 \r{A}
and 2832 \r{A} ultraviolet continuum channels. Additionally, the Mg II triplet
line intensity is correlated with that of H$\alpha$ when the EB occurs,
indicating the possibility to use the triplet as an alternative way to identify
EBs. However, we do not find any signal in IRIS hotter lines (C II and Si IV).
For further analysis, we employ a two-cloud model to fit the two chromospheric
lines (H$\alpha$ and Ca II 8542 \r{A}) simultaneously, and obtain a temperature
enhancement of 2300 K for a strong EB. This temperature is among the highest of
previous modeling results while still insufficient to produce IB signatures at
ultraviolet wavelengths. | 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 |
Heavy Elements Nucleosynthesis On Accreting White Dwarfs: building seeds
for the p-process: The origin of the proton-rich trans-iron isotopes in the solar system is
still uncertain. Single-degenerate thermonuclear supernovae (SNIa) with
n-capture nucleosynthesis seeds assembled in the external layers of the
progenitor's rapidly accreting white dwarf phase may produce these isotopes. We
calculate the stellar structure of the accretion phase of five white dwarf
models with initial masses >~ 0.85Msun using the stellar code MESA. The
near-surface layers of the 1, 1.26, 1.32 and 1.38Msun models are most
representative of the regions in which the bulk of the p nuclei are produced
during SNIa explosions, and for these models we also calculate the
neutron-capture nucleosynthesis in the external layers. Contrary to previous
rapidly-accreting white dwarf models at lower mass, we find that the H-shell
ashes are the main site of n-capture nucleosynthesis. We find high neutron
densities up to several 10^15 cm^-3 in the most massive WDs. Through the
recurrence of the H-shell ashes these intermediate neutron densities can be
sustained effectively for a long time leading to high neutron exposures with a
strong production up to Pb. Both the neutron density and the neutron exposure
increase with increasing the mass of the accreting WD. Finally, the SNIa
nucleosynthesis is calculated using the obtained abundances as seeds. We obtain
solar to super-solar abundances for p-nuclei with A>96. Our models show that
SNIa are a viable p-process production site. | astro-ph_SR |
An empirical clock to measure the dynamical age of stellar systems: Blue Straggler Stars (BSS) are among the brightest and more massive stars in
globular clusters (GCs). For this reason they represent an ideal tool to probe
the dynamical evolution of these stellar systems. Here I show, following the
results by Ferraro et al. (2012), that the BSS radial distribution can be used
as a powerful indicator of the cluster dynamical age. In fact on the basis of
their BSS radial distribution shape, GCs can be efficiently grouped in
different families corresponding to the different dynamical stages reached by
the stellar systems. This allows to define a first empirical clock, the
dynamical clock, able to measure the dynamical age of a stellar system from
pure observational quantities. | astro-ph_SR |
Faint warm debris disks around nearby bright stars explored by AKARI and
IRSF: Context: Debris disks are important observational clues for understanding
planetary-system formation process. In particular, faint warm debris disks may
be related to late planet formation near 1 AU. A systematic search of faint
warm debris disks is necessary to reveal terrestrial planet formation. Aims:
Faint warm debris disks show excess emission that peaks at mid-IR wavelengths.
Thus we explore debris disks using the AKARI mid-IR all-sky point source
catalog (PSC), a product of the second generation unbiased IR all-sky survey.
Methods : We investigate IR excess emission for 678 isolated main-sequence
stars for which there are 18 micron detections in the AKARI mid-IR all-sky
catalog by comparing their fluxes with the predicted fluxes of the photospheres
based on optical to near-IR fluxes and model spectra. The near-IR fluxes are
first taken from the 2MASS PSC. However, 286 stars with Ks<4.5 in our sample
have large flux errors in the 2MASS photometry due to saturation. Thus we have
measured accurate J, H, and Ks band fluxes, applying neutral density (ND)
filters for Simultaneous InfraRed Imager for Unbiased Survey (SIRIUS) on IRSF,
the \phi 1.4 m near-IR telescope in South Africa, and improved the flux
accuracy from 14% to 1.8% on average. Results: We identified 53 debris-disk
candidates including eight new detections from our sample of 678 main-sequence
stars. The detection rate of debris disks for this work is ~8%, which is
comparable with those in previous works by Spitzer and Herschel. Conclusion:
The importance of this study is the detection of faint warm debris disks around
nearby field stars. At least nine objects have a large amount of dust for their
ages, which cannot be explained by the conventional steady-state collisional
cascade model. | astro-ph_SR |
The IACOB spectroscopic database of Northern Galactic OB stars: We present the IACOB spectroscopic database, an homogeneous set of high
quality, high resolution spectra of Galactic O- and B-type stars obtained with
the FIES spectrograph attached to the Nordic Optical Telescope. We also present
some results from ongoing projects using the IACOB database. | astro-ph_SR |
Absolute Properties of the Eclipsing $γ$ Dor Star V404 Lyrae: We present the first high-resolution spectra for the eclipsing binary V404
Lyr showing $\gamma$ Dor pulsations, which we use to study its absolute
properties. By fitting models to the disentangling spectrum of the primary
star, we found that it has an effective temperature of $T_{\rm eff,1}=7,330 \pm
150$ K and a rotational velocity of $v_1\sin$$i=148\pm18$ km s$^{-1}$. The
simultaneous analysis of our double-lined radial velocities and the
pulsation-subtracted ${\it Kepler}$ data gives us accurate stellar and system
parameters of V404 Lyr. The masses, radii, and luminosities are $M_1$ =
2.17$\pm$0.06 M$_\odot$, $R_1$ = 1.91$\pm$0.02 R$_\odot$, and $L_1$ =
9.4$\pm$0.8 L$_\odot$ for the primary, and $M_2$ = 1.42$\pm$0.04 M$_\odot$,
$R_2$ = 1.79$\pm$0.02 R$_\odot$, and $L_2$ = 2.9$\pm$0.2 L$_\odot$ for the
secondary. The tertiary component orbiting the eclipsing pair has a mass of
$M_{\rm 3b}$ = 0.71$\pm$0.15 $M_\odot$ in an orbit of $P_{\rm 3b}$ = 642$\pm$3
days, $e_{\rm 3b}$ = 0.21$\pm$0.04, and $a_{\rm 3b}$ = 509$\pm$2 R$_\odot$. The
third light of $l_3=4.1\pm0.2\%$ could be partly attributable to the K-type
circumbinary object. By applying a multiple frequency analysis to the
eclipse-subtracted light residuals, we detected 45 frequencies with signal to
noise amplitude ratios larger than 4.0. Identified as independent pulsation
modes, seven frequencies ($f_1-f_6$, $f_9$), their new pulsation constants, and
the location in the Hertzsprung-Russell diagram indicate that the pulsating
primary is a $\gamma$ Dor-type variable star. | astro-ph_SR |
STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar
observations: The inference of the underlying state of the plasma in the solar chromosphere
remains extremely challenging because of the nonlocal character of the observed
radiation and plasma conditions in this layer. Inversion methods allow us to
derive a model atmosphere that can reproduce the observed spectra by
undertaking several physical assumptions.
The most advanced approaches involve a depth-stratified model atmosphere
described by temperature, line-of-sight velocity, turbulent velocity, the three
components of the magnetic field vector, and gas and electron pressure. The
parameters of the radiative transfer equation are computed from a solid ground
of physical principles. To apply these techniques to spectral lines that sample
the chromosphere, NLTE effects must be included in the calculations.
We developed a new inversion code STiC to study spectral lines that sample
the upper chromosphere. The code is based the RH synthetis code, which we
modified to make the inversions faster and more stable. For the first time,
STiC facilitates the processing of lines from multiple atoms in non-LTE, also
including partial redistribution effects. Furthermore, we include a
regularization strategy that allows for model atmospheres with a complex
stratification, without introducing artifacts in the reconstructed physical
parameters, which are usually manifested in the form of oscillatory behavior.
This approach takes steps toward a node-less inversion, in which the value of
the physical parameters at each grid point can be considered a free parameter.
In this paper we discuss the implementation of the aforementioned techniques,
the description of the model atmosphere, and the optimizations that we applied
to the code. We carry out some numerical experiments to show the performance of
the code and the regularization techniques that we implemented. We made STiC
publicly available to the community. | astro-ph_SR |
On the nature of massive helium star winds and Wolf-Rayet-type mass loss: The mass-loss rates of massive helium stars are one of the major
uncertainties in modern astrophysics. Regardless of whether they were stripped
by a binary companion or managed to peel off their outer layers by themselves,
the influence and final fate of helium stars -- in particular the resulting
black hole mass -- highly depends on their wind mass loss as stripped-envelope
objects. While empirical mass-loss constraints for massive helium stars have
improved over the last decades, the resulting recipes are limited to
metallicities with the observational ability to sufficiently resolve individual
stars. Yet, theoretical efforts have been hampered by the complexity of
Wolf-Rayet (WR) winds arising from the more massive helium stars. In an
unprecedented effort, we calculate next-generation stellar atmosphere models
resembling massive helium main sequence stars with Fe-bump driven winds up to
$500\,M_\odot$ over a wide metallicity range between $2.0$ and $0.02\,Z_\odot$.
We uncover a complex $\Gamma_\text{e}$-dependency of WR-type winds and their
metallicity-dependent breakdown. The latter can be related to the onset of
multiple scattering, requiring higher $L/M$-ratios at lower metallicity. Based
on our findings, we derive the first ever theoretically-motivated mass-loss
recipe for massive helium stars. We also provide estimates for LyC and He II
ionizing fluxes, finding stripped helium stars to contribute considerably at
low metallicity. In sharp contrast to OB-star winds, the mass loss for helium
stars scales with the terminal velocity. While limited to the helium main
sequence, our study marks a major step towards a better theoretical
understanding of helium star evolution. | astro-ph_SR |
Photoionization and electron-ion recombination in astrophysical plasmas: Photoionization and its inverse, electron-ion recombination, are key
processes that influence many astrophysical plasmas (and gasses), and the
diagnostics that we use to analyse the plasmas. In this review we provide a
brief overview of the importance of photoionization and recombination in
astrophysics. We highlight how the data needed for spectral analyses, and the
required accuracy, varies considerably in different astrophysical environments.
We then discuss photoionization processes, highlighting resonances in their
cross-sections. Next we discuss radiative recombination, and low and high
temperature dielectronic recombination. The possible suppression of low
temperature dielectronic recombination (LTDR) and high temperature dielectronic
recombination (HTDR) due to the radiation field and high densities is
discussed. Finally we discuss a few astrophysical examples to highlight
photoionization and recombination processes. | astro-ph_SR |
Evidence of a Plasmoid-Looptop Interaction and Magnetic Inflows During a
Solar Flare/CME Eruptive Event: Observational evidence is presented for the merging of a downward-propagating
plasmoid with a looptop kernel during an occulted limb event on 2007 January
25. RHESSI lightcurves in the 9-18 keV energy range, as well as that of the 245
MHz channel of the Learmonth Solar Observatory, show enhanced nonthermal
emission in the corona at the time of the merging suggesting that additional
particle acceleration took place. This was attributed to a secondary episode of
reconnection in the current sheet that formed between the two merging sources.
RHESSI images were used to establish a mean downward velocity of the plasmoid
of 12 km/s. Complementary observations from the SECCHI suite of instruments
onboard STEREO-Behind showed that this process occurred during the acceleration
phase of the associated CME. From wavelet-enhanced EUVI, images evidence of
inflowing magnetic field lines prior to the CME eruption is also presented. The
derived inflow velocity was found to be 1.5 km/s. This combination of
observations supports a recent numerical simulation of plasmoid formation,
propagation and subsequent particle acceleration due to the tearing mode
instability during current sheet formation. | astro-ph_SR |
On the Distribution of Orbital Eccentricities for Very Low-Mass Binaries: We have compiled a sample of 16 orbits for very low-mass stellar (<0.1 Msun)
and brown dwarf binaries, enabling the first comprehensive study of the
eccentricity distribution for such objects. We find that very low-mass binaries
span a broad range of eccentricities (0.03<e<0.83), with a median eccentricity
of 0.34. We examine potential observational biases in this sample, and for
visual binaries we show through Monte Carlo simulations that appropriate
selection criteria result in all eccentricities being equally represented (<5%
difference between input and output e distributions). The orbits of this sample
of very low-mass binaries show some significant differences from their
solar-type counterparts. They lack a correlation between orbital period and
eccentricity and display a much higher fraction of near-circular orbits (e<0.1)
than solar-type stars, which together suggest a different formation mechanism
or dynamical history for these two populations. Very low-mass binaries also do
not follow the e^2 distribution of Ambartsumian (1937), which would be expected
if their orbits were distributed in phase space according to a function of
energy alone (e.g., the Boltzmann distribution). We find that numerical
simulations of very low-mass star formation do not completely reproduce the
observed properties of our binary sample. The cluster formation model of Bate
(2009) agrees very well with the overall e distribution, but lack any high-e
(>0.6) binaries at orbital periods comparable to our sample. In contrast, the
circumstellar disk fragmentation model of Stamatellos & Whitworth (2009)
predicts only high-e binaries and thus is highly inconsistent with our sample.
These discrepancies could be explained if multiple formation processes have
produced the field population. | astro-ph_SR |
The intimate relation between the low T/W instability and the
co-rotation point: We study the low T/W instability associated with the f-mode of differentially
rotating stars. Our stellar models are described by a polytropic equation of
state and the rotation profile is given by the standard j-constant law. The
properties of the relevant oscillation modes, including the instability growth
time, are determined from time evolutions of the linearised dynamical equations
in Newtonian gravity. In order to analyse the instability we monitor also the
canonical energy and angular momentum. Our results demonstrate that the l=m=2
f-mode becomes unstable as soon as a co-rotation point develops inside the star
(i.e. whenever there is a point where the mode's pattern speed matches the bulk
angular velocity). Considering various degrees of differential rotation, we
show that the instability grows faster deep inside the co-rotation region and
deduce an empirical relation that correlates the mode frequency and the star's
parameters, which captures the main features of the l=m=2 f-mode growth time.
This function is proportional to the product of the kinetic to gravitational
energy ratio and the gradient of the star's spin, strengthening further the
relationship between the co-rotation point and the low T/W instability. We
briefly consider also the l=m=2 r-mode and demonstrate that it never moves far
inside the co-rotation region even for significant differential rotation. | astro-ph_SR |
Flux emergence and coronal eruption: Our aim is to study the photospheric flux distribution of a twisted flux tube
that emerges from the solar interior. We also report on the eruption of a new
flux rope when the emerging tube rises into a pre-existing magnetic field in
the corona. To study the evolution, we use 3D numerical simulations by solving
the time-dependent and resistive MHD equations. We qualitatively compare our
numerical results with MDI magnetograms of emerging flux at the solar surface.
We find that the photospheric magnetic flux distribution consists of two
regions of opposite polarities and elongated magnetic tails on the two sides of
the polarity inversion line (PIL), depending on the azimuthal nature of the
emerging field lines and the initial field strength of the rising tube. Their
shape is progressively deformed due to plasma motions towards the PIL. Our
results are in qualitative agreement with observational studies of magnetic
flux emergence in active regions (ARs). Moreover, if the initial twist of the
emerging tube is small, the photospheric magnetic field develops an undulating
shape and does not possess tails. In all cases, we find that a new flux rope is
formed above the original axis of the emerging tube that may erupt into the
corona, depending on the strength of the ambient field. | astro-ph_SR |
Exploring Bistability in the Cycles of the Solar Dynamo through Global
Simulations: The calling card of solar magnetism is the sunspot cycle, during which
sunspots regularly reverse their polarity sense every 11 years. However, a
number of more complicated time-dependent behaviors have also been identified.
In particular, there are temporal modulations associated with active longitudes
and hemispheric asymmetry, when sunspots appear at certain solar longitudes or
else in one hemisphere preferentially. So far, a direct link between between
this asymmetric temporal behavior and the underlying solar dynamo has remained
elusive. In this work, we present results from global, 3D magnetohydrodynamic
(MHD) simulations, which for the first time display both behavior reminiscent
of the sunspot cycle (regular polarity reversals and equatorward migration of
internal magnetic field) and asymmetric, irregular behavior that in the
simulations we interpret as active longitudes and hemispheric asymmetry. The
simulations are thus bistable, in that the turbulent convection can stably
support two distinct flavors of magnetism at different times, in superposition,
or with smooth transitions from one state to the other. We discuss this new
family of dynamo models in the context of the extensive observations of the
Sun's surface magnetic field with the Solar and Heliospheric Observatory (SOHO)
and the Solar Dynamics Observatory (SDO), as well as earlier observations of
sunspot number and synoptic maps. We suggest that the solar dynamo itself may
be bistable in nature, exhibiting two types of temporal behavior in the
magnetic field. | astro-ph_SR |
High spatial resolution imaging of SO and H2CO in AB Auriga: the first
SO image in a transitional disk: Transitional disks are structures of dust and gas around young stars with
large inner cavities in which planet formation may occur. Lopsided dust
distributions are observed in the dust continuum emission at millimeter
wavelengths. These asymmetrical structures can be explained as the result of an
enhanced gas density vortex where the dust is trapped potentially promoting the
rapid growth to the planetesimal scale. AB Aur hosts a transitional disk with a
clear horseshoe morphology which strongly suggests the presence of a dust trap.
Our goal is to investigate its formation and the possible effects on the gas
chemistry. We used the NOEMA interferometer to image the 1mm continuum dust
emission and the 13CO J=2->1, C18O J=2->1, SO J=56->45 and H2CO J=303->202
rotational lines. Line integrated intensity ratio images are built to
investigate the chemical changes within the disk. We have used a single point
(n,T) chemical model to investigate the lifetime of gaseous CO, H2CO and SO in
the dust trap. Our model shows that for densities >10^7 cm^-3, the SO molecules
are depleted (directly frozen or converted into SO2 and then frozen out) in
less than 0.1 Myr. The lower SO abundance towards the dust trap could indicate
that a larger fraction of the gas is in a high density environment. Gas
dynamics, grain growth and gas chemistry are coupled in the planet formation
process. Because of the strong dependence of SO abundance on the gas density,
the sulfur chemistry can be used as a chemical diagnostic to detect the
birthsites of future planets. However, the large uncertainties inherent to
chemical models and the limited knowledge of the disk physical structure and
initial conditions are important drawbacks. | astro-ph_SR |
Driving solar coronal MHD simulations on high-performance computers: The quality of today's research is often tightly limited to the available
computing power and scalability of codes to many processors. For example,
tackling the problem of heating the solar corona requires a most realistic
description of the plasma dynamics and the magnetic field. Numerically solving
such a magneto-hydrodynamical (MHD) description of a small active region (AR)
on the Sun requires millions of computation hours on current high-performance
computing (HPC) hardware. The aim of this work is to describe methods for an
efficient parallelization of boundary conditions and data input/output (IO)
strategies that allow for a better scaling towards thousands of processors
(CPUs). The Pencil Code is tested before and after optimization to compare the
performance and scalability of a coronal MHD model above an AR. We present a
novel boundary condition for non-vertical magnetic fields in the photosphere,
where we approach the realistic pressure increase below the photosphere. With
that, magnetic flux bundles become narrower with depth and the flux density
increases accordingly. The scalability is improved by more than one order of
magnitude through the HPC-friendly boundary conditions and IO strategies. This
work describes also the necessary nudging methods to drive the MHD model with
observed magnetic fields from the Sun's photosphere. In addition, we present
the upper and lower atmospheric boundary conditions (photospheric and towards
the outer corona), including swamp layers to diminish perturbations before they
reach the boundaries. Altogether, these methods enable more realistic 3D MHD
simulations than previous models regarding the coronal heating problem above an
AR -- simply because of the ability to use a large amount of CPUs efficiently
in parallel. | astro-ph_SR |
Hydrogen Balmer Line Broadening in Solar and Stellar Flares: The broadening of the hydrogen lines during flares is thought to result from
increased charge (electron, proton) density in the flare chromosphere. However,
disagreements between theory and modeling prescriptions have precluded an
accurate diagnostic of the degree of ionization and compression resulting from
flare heating in the chromosphere. To resolve this issue, we have incorporated
the unified theory of electric pressure broadening of the hydrogen lines into
the non-LTE radiative transfer code RH. This broadening prescription produces a
much more realistic spectrum of the quiescent, A0 star Vega compared to the
analytic approximations used as a damping parameter in the Voigt profiles. We
test recent radiative-hydrodynamic (RHD) simulations of the atmospheric
response to high nonthermal electron beam fluxes with the new broadening
prescription and find that the Balmer lines are over-broadened at the densest
times in the simulations. Adding many simultaneously heated and cooling model
loops as a "multithread" model improves the agreement with the observations. We
revisit the three-component phenomenological flare model of the YZ CMi
Megaflare using recent and new RHD models. The evolution of the broadening,
line flux ratios, and continuum flux ratios are well-reproduced by a
multithread model with high-flux nonthermal electron beam heating, an extended
decay phase model, and a "hot spot" atmosphere heated by an ultrarelativistic
electron beam with reasonable filling factors: 0.1%, 1%, and 0.1% of the
visible stellar hemisphere, respectively. The new modeling motivates future
work to understand the origin of the extended gradual phase emission. | astro-ph_SR |
An AMR Study of the Common Envelope Phase of Binary Evolution: The hydrodynamic evolution of the common envelope phase of a low mass binary
composed of a 1.05 Msun red giant and a 0.6 Msun companion has been followed
for five orbits of the system using a high resolution method in three spatial
dimensions. During the rapid inspiral phase, the interaction of the companion
with the red giant's extended atmosphere causes about 25% of the common
envelope to be ejected from the system, with mass continuing to be lost at the
end of the simulation at a rate ~ 2 Msun/yr. In the process the resulting loss
of angular momentum and energy reduces the orbital separation by a factor of
seven. After this inspiral phase the eccentricity of the orbit rapidly
decreases with time. The gravitational drag dominates hydrodynamic drag at all
times in the evolution, and the commonly-used Bondi-Hoyle-Lyttleton
prescription for estimating the accretion rate onto the companion significantly
overestimates the true rate. On scales comparable to the orbital separation,
the gas flow in the orbital plane in the vicinity of the two cores is subsonic
with the gas nearly corotating with the red giant core and circulating about
the red giant companion. On larger scales, 90% of the outflow is contained
within 30 degrees of the orbital plane, and the spiral shocks in this material
leave an imprint on the density and velocity structure. Of the energy released
by the inspiral of the cores, only about 25% goes toward ejection of the
envelope. | astro-ph_SR |
Physical and chemical properties of Wolf-Rayet planetary nebulae: Wolf-Rayet ([WR]) and weak emission-line ($wels$) central stars of planetary
nebulae (PNe) have hydrogen-deficient atmospheres, whose origins are not well
understood. In the present study, we have conducted plasma diagnostics and
abundance analyses of 18 Galactic PNe surrounding [WR] and $wels$ nuclei, using
collisionally excited lines (CELs) and optical recombination lines (ORLs)
measured with the Wide Field Spectrograph on the ANU 2.3-m telescope at the
Siding Spring Observatory complemented with optical archival data. Our plasma
diagnostics imply that the electron densities and temperatures derived from
CELs are correlated with the intrinsic nebular H$\beta$ surface brightness and
excitation class, respectively. Self-consistent plasma diagnostics of heavy
element ORLs of N${}^{2+}$ and O${}^{2+}$ suggest that a small fraction of cool
($\lesssim 7000$ K), dense ($\sim 10^4-10^5$ cm$^{-3}$) materials may be
present in some objects, though with large uncertainties. Our abundance
analyses indicate that the abundance discrepancy factors (ADF$\equiv$ORLs/CELs)
of O${}^{2+}$ are correlated with the dichotomies between forbidden-line and He
I temperatures. Our results likely point to the presence of a tiny fraction of
cool, oxygen-rich dense clumps within the diffuse warm ionized nebulae.
Moreover, our elemental abundances derived from CELs are mostly consistent with
AGB models in the range of initial masses from 1.5 to 5M$_{\odot}$. Further
studies are necessary to understand better the origins of abundance
discrepancies in PNe around [WR] and $wels$ stars. | astro-ph_SR |
Submillimeter continuum observations of Sagittarius B2 at subarcsecond
spatial resolution: We report the first high spatial resolution submillimeter continuum
observations of the Sagittarius B2 cloud complex using the Submillimeter Array
(SMA). With the subarcsecond resolution provided by the SMA, the two massive
star-forming clumps Sgr B2(N) and Sgr B2(M) are resolved into multiple compact
sources. In total, twelve submillimeter cores are identified in the Sgr B2(M)
region, while only two components are observed in the Sgr B2(N) clump. The gas
mass and column density are estimated from the dust continuum emission. We find
that most of the cores have gas masses in excess of 100 M$_{\odot}$ and column
densities above 10$^{25}$ cm$^{-2}$. The very fragmented appearance of Sgr
B2(M), in contrast to the monolithic structure of Sgr B2 (N), suggests that the
former is more evolved. The density profile of the Sgr B2(N)-SMA1 core is well
fitted by a Plummer density distribution. This would lead one to believe that
in the evolutionary sequence of the Sgr B2 cloud complex, a massive star forms
first in an homogeneous core, and the rest of the cluster forms subsequently in
the then fragmenting structure. | astro-ph_SR |
Collision strengths for FIR and UV transtions in PIII and the phosphorus
abundance: Phosphorus abundance is crucial for DNA-based extraterrestrial life in
exoplanets. Atomic data for observed spectral lines of P-ions are needed for
its accurate determination. We present the first calculations for collision
strengths for the forbidden PIII fine structure transition $3s^23p
(^2P^o_{1/2-3/2})$ within the ground state at 17.9 $\mu$m, as well as allowed
UV transitions in the $3s^23p (^2P^o_{1/2,3/2}) \rightarrow 3s3p^2
(^2D_{3/2,5/2}, ^2S_{1/2}, ^2P_{1/2,3/2})$ multiplets between 915-1345 $\AA$.
Collision strengths are computed using the Breit-Pauli R-Matrix method
including the first 18 levels, and they exhibit extensive auto-ionizing
resonance structures. In particular, the Maxwellian averaged effective
collision strength for the FIR 17.9 $\mu$m transition shows a factor 3
temperature variation broadly peaking at typical nebular temperatures. Its
theoretical emissivity with solar phosphorus abundance is computed relative to
H$\beta$ and found to be similar to observed intensties from planetary nebulae;
the abundances derived in earlier works are 3-5 times sub-solar. The results
pertain to the reported paucity of phosphorus from preferred production sites
in supernovae, and abundances in planetary nebulae and supernova remnants. | astro-ph_SR |
MAVKA: Software for Statistically Optimal Determination of Extrema: We introduce the program MAVKA for determination of characteristics of
extrema using observations in the adjacent data intervals, with intended
applications to variable stars, but it may be used for signals of arbitrary
nature. We have used a dozen of basic functions, some of them use the interval
near extremum without splitting the interval (algebraic polynomial in general
form, "Symmetrical" algebraic polynomial using only even degrees of time
(phase) deviation from the position of symmetry argument), others split the
interval into 2 subintervals (a Taylor series of the "New Algol Variable", "the
function of Prof. Z. Mikul\'a\v{s}ek"), or even 3 parts ("Asymptotic Parabola",
"Wall-Supported Parabola", "Wall-Supported Line", "Wall-Supported Asymptotic
Parabola", "Parabolic Spline of defect 1"). The variety of methods allows to
choose the "best" (statistically optimal) approximation for a given data
sample. As the criterion, we use the accuracy of determination of the extremum.
For all parameters, the statistical errors are determined. The methods are
illustrated by applications to observations of pulsating and eclipsing variable
stars, as well as to the exoplanet transits. They are used for the
international campaigns "Inter-Longitude Astronomy", "Virtual Observatory" and
"AstroInformatics". The program may be used for studies of individual objects,
also using ground-based (NSVS, ASAS, WASP, CRTS et al.) and space (GAIA,
KEPLER, HIPPARCOS/TYCHO, WISE et al.) surveys. | astro-ph_SR |
Inferring properties of small convective cores in main-sequence
solar-like pulsators: This work concerns the study of the properties of convective cores in
main-sequence models of solar-like pulsators and what information they may hold
about stellar ages. We verified that the maximum absolute frequency derivative
of particular combinations of frequencies, which we name "the slopes", provides
information on the relative size of the discontinuity in the sound-speed
profile at the border of the convectively mixed region. Since the latter is
related to the evolutionary state of stars, we show that for models with masses
above $1.3\,\rm M_\odot$, it may be possible to estimate the fraction of
stellar main-sequence evolution from the slopes. Moreover, for models with
masses below $1.2\,\rm M_\odot$ we verified that it may be possible to use the
slopes to discriminate against models with small amounts of core overshoot. | astro-ph_SR |
Primordial triples and collisions of massive stars: Massive stars are known to have a high multiplicity, with examples of higher
order multiples among the nearest and best studied objects. In this paper we
study hierarchical multiple systems (an inner binary as a component of a wider
binary) of massive stars in a clustered environment, in which a system with a
size of 100--1000 au will undergo many close encounters during the short
lifetime of a massive star. Using two types of N-body experiment we determine
the post-formation collision probabilities of these massive hierarchies. We
find that, depending on the specifics of the environment, the hierarchy, and
the amount of time that is allowed to pass, tens of percent of hierarchies will
experience a collision, typically between the two stars of the inner binary. In
addition to collisions, clusters hosting a hierarchical massive system produce
high velocity runaways at an enhanced rate. The primordial multiplicity
specifics of massive stars appear to play a key role in the generation of these
relatively small number events in cluster simulations, complicating their use
as diagnostics of a cluster's history. | astro-ph_SR |
Epicyclic frequencies of spheroidal stars with non-uniform density: We consider the gravitational potential of a rotating star with non-uniform
density to derive the orbital and epicyclic frequencies of the particles
orbiting the star. We assume that the star is composed of concentric spheroids
of constant density, with a global power-law distribution of density inside the
star. At the lowest order approximation, we recover the known result for the
Maclaurin spheroid that the maximum in the radial epicyclic frequency occurs at
$r=\sqrt{2}ae$, for eccentricities $\geq 1/\sqrt{2}$. We find that the nature
of these characteristic frequencies differs based on the geometry of the
rotating star. For an oblate spheroid, the orbits resemble retrograde-Kerr
orbits and the location of the radial epicyclic maximum approaches the stellar
surface as the density variation inside the star becomes steeper. On the
contrary, orbits around a prolate spheroid resemble prograde-Kerr orbits, but
the marginally stable orbit does not exist for prolate-shaped stars. The
orbital frequency is larger (smaller) than the Keplerian value for an oblate
(prolate) star with the equality attained as $e \rightarrow 0$ or $r
\rightarrow \infty$. The radial profiles of the angular velocity and the
angular momentum allow for a stable accreting disc around any nature of
oblate/prolate spheroid. | astro-ph_SR |
Images of unclassified and supergiant B[e] stars disks with
interferometry: B[e] stars are among the most peculiar objects in the sky. This spectral
type, characterised by allowed and forbidden emission lines, and a large
infrared excess, does not represent an homogenous class of objects, but
instead, a mix of stellar bodies seen in all evolutionary status. Among them,
one can find Herbig stars, planetary nebulae central stars, interacting
binaries, supermassive stars, and even "unclassified" B[e] stars: systems
sharing properties of several of the above. Interferometry, by resolving the
innermost regions of these stellar systems, enables us to reveal the true
nature of these peculiar stars among the peculiar B[e] stars. | astro-ph_SR |
The kinematic characteristics of magnetic O-type stars: Although magnetic fields have been discovered in ten massive O-type stars
during the last years, the origin of their magnetic fields remains unknown.
Among the magnetic O-type stars, two stars, HD36879 and HD57682, were
identified as candidate runaway stars in the past, and theta^1 Ori C was
reported to move rapidly away from its host cluster. We search for an
explanation for the occurrence of magnetic fields in O-type stars by examining
the assumption of their runaway status. We use the currently best available
astrometric, spectroscopic, and photometric data to calculate the kinematical
status of seven magnetic O-type stars with previously unknown space velocities.
The results of the calculations of space velocities suggest that five out of
the seven magnetic O-type stars can be considered as candidate runaway stars.
Only two stars, HD155806 and HD164794, with the lowest space velocities, are
likely members of Sco OB4 and NGC6530, respectively. However, the non-thermal
radio emitter HD164794 is a binary system with colliding winds, for which the
detected magnetic field has probably a different origin in comparison to other
magnetic O-type stars. | astro-ph_SR |
Dynamical investigation of the multiple star ADS 9173 AB: Star ADS 9173=WDS 14135+5147=Hip 69483 is a complex system. The B component
has a spectroscopic companion, whose orbit with a period of 4.9 years has been
known since 1986. The Gaia telescope has detected a distant faint pair over 100
arcsec away from the bright AB pair. In our article, we study the movement in a
bright pair based on long-term observations with the 26-inch refractor of the
Pulkovo Observatory. The AB pair orbit with a period of 6306 years was
calculated using the apparent motion parameters (AMP) method. The astrometric
orbit of the component B was determined on the basis of the residuals of the
homogeneous CCD observations up to 2023 with the 26-inch refractor. It is in
agreement with the spectroscopic one. The remaining secondary residuals show a
wave with a period of approximately 20 years, the reasons for which are
discussed. | astro-ph_SR |
EUV Emission and Scattered Light Diagnostics of Equatorial Coronal Holes
as Seen by Hinode/EIS: Spectroscopic diagnostics of solar coronal plasmas critically depends on the
uncertainty in the measured line intensities. One of the main sources of
uncertainty is instrumental scattered light, which is potentially most
important in low-brightness areas. In the solar corona, such areas include
polar and equatorial coronal holes, which are the source regions of the solar
wind; instrument-scattered light must thus pose a significant obstacle to
studies of the source regions of the solar wind. In this paper we investigate
the importance of instrument-scattered light on observations of equatorial
coronal holes made by the Hinode/EIS spectrometer in two different phases of
the solar cycle. We find that the instrument-scattered light is significant at
all temperatures, and in both regions it amounts to approximately 10% of the
average intensity of the neighboring quiet Sun regions. Such contribution
dominates the measured intensity for spectral lines formed at temperatures
larger than Log T = 6.15 K, and has deep implications for spectroscopic
diagnostics of equatorial coronal hole plasmas and studies of the source
regions of a large portion of the solar wind which reaches Earth. Our results
suggest that the high temperature tail of in the coronal hole plasma
distribution with temperature, however small, is an artifact due to the
presence of scattered light. | astro-ph_SR |
Achieving Fast Reconnection in Resistive MHD Models via Turbulent Means: Astrophysical fluids are generally turbulent and this preexisting turbulence
must be taken into account for the models of magnetic reconnection which are
attepmted to be applied to astrophysical, solar or heliospheric environments.
In addition, reconnection itself induces turbulence which provides an important
feedback on the reconnection process. In this paper we discuss both theoretical
model and numerical evidence that magnetic reconnection gets fast in the
approximation of resistive MHD. We consider the relation between the Lazarian &
Vishniac turbulent reconnection theory and Lapenta's numerical experiments
testifying of the spontaneous onset of turbulent reconnection in systems which
are initially laminar. | astro-ph_SR |
Characterization of solar-cycle induced frequency shift of medium- and
high-degree acoustic modes: Although it is well known that the solar acoustic mode frequency increases as
the solar activity increases, the mechanism behind it is still unknown. Mode
frequencies with 20 < l < 900 obtained by applying spherical harmonic
decomposition to MDI full-disk observations were used. First, the dependence of
solar acoustic mode frequency with solar activity was examined and evidence of
a quadratic relation was found indicating a saturation effect at high solar
activity. Then, the frequency dependence of frequency differences between the
activity minimum and maximum was analyzed. The frequency shift scaled by the
normalized mode inertia follows a simple power law where the exponent for the p
modes decreases by 37% for modes with frequency larger than 2.5 mHz. | astro-ph_SR |
Effect of latitudinal differential rotation on solar Rossby waves:
Critical layers, eigenfunctions, and momentum fluxes in the equatorial
$β$ plane: Retrograde-propagating waves of vertical vorticity with longitudinal
wavenumbers between 3 and 15 have been observed on the Sun with a dispersion
relation close to that of classical sectoral Rossby waves. The observed
vorticity eigenfunctions are symmetric in latitude, peak at the equator, switch
sign near $20^\circ$-$30^\circ$, and decrease at higher latitudes. We search
for an explanation that takes into account solar latitudinal differential
rotation. In the equatorial $\beta$ plane, we study the propagation of linear
Rossby waves (phase speed $c <0$) in a parabolic zonal shear flow, $U = -
\overline{U}\ \xi^2<0$, where $\overline{U} = 244$ m/s and $\xi$ is the sine of
latitude. In the inviscid case, the eigenvalue spectrum is real and continuous
and the velocity stream functions are singular at the critical latitudes where
$U = c$. We add eddy viscosity in the problem to account for wave attenuation.
In the viscous case, the stream functions are solution of a fourth-order
modified Orr-Sommerfeld equation. Eigenvalues are complex and discrete. For
reasonable values of the eddy viscosity corresponding to supergranular scales
and above (Reynolds number $100 \le Re \le 700$), all modes are stable. At
fixed longitudinal wavenumber, the least damped mode is a symmetric mode with a
real frequency close to that of the classical Rossby mode, which we call the R
mode. For $Re \approx 300$, the attenuation and the real part of the
eigenfunction is in qualitative agreement with the observations (unlike the
imaginary part of the eigenfunction, which has a larger amplitude in the model.
Conclusion: Each longitudinal wavenumber is associated with a latitudinally
symmetric R mode trapped at low latitudes by solar differential rotation. In
the viscous model, R modes transport significant angular momentum from the
dissipation layers towards the equator. | astro-ph_SR |
Calibration of the mixing-length parameter $α$ for the MLT and FST
models by matching with CO$^5$BOLD models: The CoRoT and Kepler missions provided a wealth of high-quality data for
solar-like oscillations. To make the best of such data for seismic inferences,
we need theoretical models with precise near-surface structure, which has
significant influence on solar-like oscillation frequencies. The mixing-length
parameter, $\alpha$, is a key factor for the near-surface structure. In the
convection formulations used in evolution codes, the $\alpha$ is a free
parameter that needs to be properly specified. We calibrated $\alpha$ values by
matching entropy profiles of 1D envelope models with those of 3D CO$^5$BOLD
models. For such calibration, previous works concentrated on the classical
mixing-length theory (MLT). Here we also analyzed the full spectrum turbulence
(FST) models. For the atmosphere part in the 1D models, we use the Eddington
grey $T(\tau)$ relation and the one with the solar-calibrated Hopf-like
function. For both the MLT and FST models with a mixing length $l=\alpha H_p$,
calibrated $\alpha$ values increase with increasing $g$ or decreasing $T_{\rm
eff}$. For the FST models, we also calibrated values of $\alpha^*$ defined as
$l=r_{\rm top}-r+\alpha^*H_{p,{\rm top}}$. $\alpha^*$ is found to increase with
$T_{\rm eff}$ and $g$. As for the correspondence to the 3D models, the solar
Hopf-like function gives a photospheric-minimum entropy closer to a 3D model
than the Eddington $T(\tau)$. The structure below the photosphere depends on
the convection model. However, not a single convection model gives the best
correspondence since the averaged 3D quantities are not necessarily related via
an EOS. Although the FST models with $l=r_{\rm top}-r+\alpha^*H_{p,{\rm top}}$
are found to give the frequencies closest to the solar observed ones, a more
appropriate treatment of the top part of the 1D convective envelope is
necessary. | astro-ph_SR |
The nonisothermal stage of magnetic star formation. II. Results: In a previous paper we formulated the problem of the formation and evolution
of fragments (or cores) in magnetically-supported, self-gravitating molecular
clouds in axisymmetric geometry, accounting for the effects of ambipolar
diffusion and Ohmic dissipation, grain chemistry and dynamics, and radiative
transfer. Here we present results of star formation simulations that accurately
track the evolution of a protostellar fragment over eleven orders of magnitude
in density (from 300 cm^-3 to \approx 10^14 cm^-3), i.e., from the early
ambipolar-diffusion--initiated fragmentation phase, through the magnetically
supercritical, dynamical-contraction phase and the subsequent magnetic
decoupling stage, to the formation of a protostellar core in near hydrostatic
equilibrium. As found by Fiedler & Mouschovias (1993), gravitationally-driven
ambipolar diffusion leads to the formation and subsequent dynamic contraction
of a magnetically supercritical core. Moreover, we find that ambipolar
diffusion, not Ohmic dissipation, is responsible for decoupling all the species
except the electrons from the magnetic field, by a density \approx 3 x 10^12
cm^-3. Magnetic decoupling precedes the formation of a central stellar object
and ultimately gives rise to a concentration of magnetic flux (a `magnetic
wall') outside the hydrostatic core --- as also found by Tassis & Mouschovias
(2005a,b) through a different approach. At approximately the same density at
which Ohmic dissipation becomes more important than ambipolar diffusion
(\gtrsim 7 x 10^12 cm^-3), the grains carry most of the electric charge as well
as the electric current. The prestellar core remains disclike down to radii ~
10 AU, inside which thermal pressure becomes important. The magnetic flux
problem of star formation is resolved for at least strongly magnetic newborn
stars by this stage of the evolution, i.e., by a central density \approx 10^14
cm^-3. The hydrostatic core has radius \approx 2 AU, density \approx 10^14
cm^-3, temperature \approx 300 K, magnetic field strength \approx 0.2 G,
magnetic flux \approx 5 x 10^18 Wb, luminosity ~ 10^-3 L_\odot, and mass ~
10^-2 M_\odot. | astro-ph_SR |
Multiscale statistical analysis of coronal solar activity: Multi-filter images from the solar corona are used to obtain temperature maps
which are analyzed using techniques based on proper orthogonal decomposition
(POD) in order to extract dynamical and structural information at various
scales. Exploring active regions before and after a solar flare and comparing
them with quiet regions we show that the multiscale behavior presents distinct
statistical properties for each case that can be used to characterize the level
of activity in a region. Information about the nature of heat transport is also
be extracted from the analysis. | astro-ph_SR |
The orbital and superhump periods of the dwarf nova SDSS
J093249.57+472523.0: We report unfiltered CCD photometry of the eclipsing dwarf nova SDSS
J093249.57+472523.0 obtained during its first confirmed outburst in 2011 March.
The outburst amplitude was at least 3.0 magnitudes above mean quiescence and it
lasted at least 11 days, although we missed the beginning of the outburst.
Superhumps having peak-to-peak amplitude up to 0.3 magnitudes were present
during the outburst, thereby establishing it to be a member of the SU UMa
family. The mean superhump period was Psh = 0.06814(11) d. Analysis of our
measurements of eclipse times of minimum, supplemented with data from other
researchers, allowed us to measure the orbital period as Porb = 0.06630354(5)
d. The superhump period excess was epsilon = 0.028(1) which is consistent with
of SU UMa systems of similar Porb. The FWHM eclipse duration varied between 6
and 13 mins and the eclipse depth was up to 1.6 magnitudes. | astro-ph_SR |
The Progress of Solar Cycle 24 at High Latitudes: The "extended" solar cycle 24 began in 1999 near 70 degrees latitude,
similarly to cycle 23 in 1989 and cycle 22 in 1979. The extended cycle is
manifested by persistent Fe XIV coronal emission appearing near 70 degrees
latitude and slowly migrating towards the equator, merging with the latitudes
of sunspots and active regions (the "butterfly diagram") after several years.
Cycle 24 began its migration at a rate 40% slower than the previous two solar
cycles, thus indicating the possibility of a peculiar cycle. However, the onset
of the "Rush to the Poles" of polar crown prominences and their associated
coronal emission, which has been a precursor to solar maximum in recent cycles
(cf. Altrock 2003), has just been identified in the northern hemisphere.
Peculiarly, this "Rush" is leisurely, at only 50% of the rate in the previous
two cycles. The properties of the current "Rush to the Poles" yields an
estimate of 2013 or 2014 for solar maximum. | astro-ph_SR |
Bipolar jets produced by a spectroscopic binary: We present evidence that the spectroscopically identified bipolar jets of the
pre-main sequence binary KH 15D are a common product of the whole binary
system, rather than being launched from either star individually. They may be
launched from the innermost part of the circumbinary disk (CBD) or may result
from the merging of two outflows driven by the individual stars. This evidence
is based on high-resolution H-alpha and [OI] 6300A line profiles obtained
during eclipse phases of this nearly edge-on system. The occultation of star A
(the only currently visible star) by the disk strongly suppresses the stellar
H-alpha and continuum emission and allows one to study the faint redshifted and
blueshifted emission components of the bipolar jets. The strongest evidence for
jet production by the whole binary system comes from the observed radial
velocity symmetry of the two jet components relative to the systemic velocity
of the binary, in combination with current accretion models from the CBD onto a
binary system. | astro-ph_SR |
The Massive Binary System 9 Sgr Revisited: New Insights into
Disentangling Methods: Disentangling techniques are often needed to obtain the spectra of the
individual components of binary or multiple systems. A thorough analysis of the
shift-and-add algorithm of Marchenko, Moffat, & Eenens (1998) reveals that in
many cases the line fluxes are poorly reproduced and spurious wings appear. The
causes of these discrepancies are discussed and a new disentangling package,
QER20, is presented which significantly reduces these errors and vastly
increases the performance. When applied to the massive binary 9 Sgr, our new
code yields line fluxes which are notably different from those previously
published and lead us to revise the spectral classification to slightly earlier
subtypes: O3V((f +)) for the primary and O5V((f)) for the secondary. We show
that with the MME98 algorithm the classification of massive stars in binaries
can be off by several subtypes whilst there are no such errors when the QER20
package is used. | astro-ph_SR |
Magnetohydrodynamic Turbulent Cascade of Coronal Loop Magnetic Fields: The Parker model for coronal heating is investigated through a high
resolution simulation. An inertial range is resolved where fluctuating magnetic
energy E_M (k_perp) \propto k_\perp^{-2.7} exceeds kinetic energy E_K (k_\perp)
\propto k_\perp^{-0.6}. Increments scale as \delta b_\ell \simeq \ell^{-0.85}
and \delta u_\ell \simeq \ell^{+0.2} with velocity increasing at small scales,
indicating that magnetic reconnection plays a prime role in this turbulent
system. We show that spectral energy transport is akin to standard
magnetohydrodynamic (MHD) turbulence even for a system of reconnecting current
sheets sustained by the boundary. In this new MHD turbulent cascade, kinetic
energy flows are negligible while cross-field flows are enhanced, and through a
series of "reflections" between the two fields, cascade more than half of the
total spectral energy flow. | astro-ph_SR |
Statistical Study and Live Catalogue of Multi-Spacecraft 3He-Rich Time
Periods over Solar Cycles 23, 24, and 25: Using ion measurements from Ultra-Low-Energy Isotope Spectrometer (ULEIS)
observations onboard Advanced Composition Explorer (ACE) and Solar Isotope
Spectrometer (SIS) observations onboard the Solar Terrestrial Observatory
(STEREO)-A and STEREO-B spacecraft, we have identified 854 3He-rich time
periods between 1997 September and 2021 March. We include all event types with
observed 3He enhancements such as corotating interaction regions (CIRs),
gradual solar energetic particle (SEP) events, interplanetary shocks, and
impulsive SEP events. We employ two different mass separation techniques to
obtain 3He, 4He, Fe, and O fluences for each event, and we determine the
3He/4He and Fe/O abundance ratios between 0.32 to 0.45 MeV/nucleon and 0.64 to
1.28 MeV/nucleon. We find a clear correlation in the 3He/4He and Fe/O abundance
ratios between both energy ranges. We find two distinct trends in the 3He/4He
vs. Fe/O relation. For low 3He/4He values, there is a positive linear
correlation between 3He/4He and Fe/O. However, at 3He/4He ~ 0.3, Fe/O appears
to reach a limit and the correlation weakens significantly. We provide a live
catalogue of 3He rich time periods that includes the robust determination of
the onset and end times of the 3He enhancements in SEP-associated periods for
different types of events observed my multiple spacecraft. This catalogue is
available for public use. New releases will follow after major additions such
as adding new periods from new missions (e.g., Parker Solar Probe and Solar
Orbiter), identifying event types (impulsive SEP events, etc.), or adding new
parameters such as remote observations detailing characteristics of the active
regions. | astro-ph_SR |
Spiral arms and instability within the AFGL 4176 mm1 disc: We present high-resolution (30 mas or 130 au at 4.2 kpc) Atacama Large
Millimeter/submillimeter Array observations at 1.2 mm of the disc around the
forming O-type star AFGL 4176 mm1. The disc (AFGL 4176 mm1-main) has a radius
of ~1000 au and contains significant structure, most notably a spiral arm on
its redshifted side. We fitted the observed spiral with logarithmic and
Archimedean spiral models. We find that both models can describe its structure,
but the Archimedean spiral with a varying pitch angle fits its morphology
marginally better. As well as signatures of rotation across the disc, we
observe gas arcs in CH$_3$CN that connect to other millimetre continuum sources
in the field, supporting the picture of interactions within a small cluster
around AFGL 4176 mm1-main. Using local thermodynamic equilibrium modelling of
the CH$_3$CN K-ladder, we determine the temperature and velocity field across
the disc, and thus produce a map of the Toomre stability parameter. Our results
indicate that the outer disc is gravitationally unstable and has already
fragmented or is likely to fragment in the future, possibly producing further
companions. These observations provide evidence that disc fragmentation is one
possible pathway towards explaining the high fraction of multiple systems
around high-mass stars. | astro-ph_SR |
Multi-Channel Three-Dimensional SOLA Inversion for Local Helioseismology: Inversions for local helioseismology are an important and necessary step for
obtaining three-dimensional maps of various physical quantities in the solar
interior. Frequently, the full inverse problems that one would like to solve
prove intractable because of computational constraints. Due to the enormous
seismic data sets that already exist and those forthcoming, this is a problem
that needs to be addressed. To this end, we present a very efficient linear
inversion algorithm for local helioseismology. It is based on a subtractive
optimally localized averaging (SOLA) scheme in the Fourier domain, utilizing
the horizontal-translation invariance of the sensitivity kernels. In Fourier
space the problem decouples into many small problems, one for each horizontal
wave vector. This multi-channel SOLA method is demonstrated for an example
problem in time-distance helioseismology that is small enough to be solved both
in real and Fourier space. We find that both approaches are successful in
solving the inverse problem. However, the multi-channel SOLA algorithm is much
faster and can easily be parallelized. | astro-ph_SR |
GW Librae: Still Hot Eight Years Post-Outburst: We report continued Hubble Space Telescope (HST) ultraviolet spectra and
ground-based optical photometry and spectroscopy of GW Librae eight years after
its largest known dwarf nova outburst in 2007. This represents the longest
cooling timescale measured for any dwarf nova. The spectra reveal that the
white dwarf still remains about 3000 K hotter than its quiescent value. Both
ultraviolet and optical light curves show a short period of 364-373 s, similar
to one of the non-radial pulsation periods present for years prior to the
outburst, and with a similar large UV/optical amplitude ratio. A large
modulation at a period of 2 h (also similar to that observed prior to outburst)
is present in the optical data preceding and during the HST observations, but
the satellite observation intervals did not cover the peaks of the optical
modulation so it is not possible to determine its corresponding UV amplitude.
The similarity of the short and long periods to quiescent values implies the
pulsating, fast spinning white dwarf in GW Lib may finally be nearing its
quiescent configuration. | astro-ph_SR |
Analytical expressions for the envelope binding energy of giants as a
function of basic stellar parameters: The common-envelope (CE) phase is an important stage in the evolution of
binary stellar populations. The most common way to compute the change in
orbital period during a CE is to relate the binding energy of the envelope of
the Roche-lobe filling giant to the change in orbital energy. Especially in
population-synthesis codes, where the evolution of millions of stars must be
computed and detailed evolutionary models are too expensive computationally,
simple approximations are made for the envelope binding energy. In this study,
we present accurate analytic prescriptions based on detailed stellar-evolution
models that provide the envelope binding energy for giants with metallicities
between Z = 10-4 and Z = 0.03 and masses between 0.8 Msun and 100 Msun, as a
function of the metallicity, mass, radius and evolutionary phase of the star.
Our results are also presented in the form of electronic data tables and
Fortran routines that use them. We find that the accuracy of our fits is better
than 15% for 90% of our model data points in all cases, and better than 10% for
90% of our data points in all cases except the asymptotic giant branches for
three of the six metallicities we consider. For very massive stars (M > 50
Msun), when stars lose more than ~20% of their initial mass due to stellar
winds, our fits do not describe the models as accurately. Our results are more
widely applicable - covering wider ranges of metallicity and mass - and are of
higher accuracy than those of previous studies. | astro-ph_SR |
Predicting convective blueshift and radial-velocity dispersion due to
granulation for FGK stars: To detect Earth-mass planets using the Doppler method, a major obstacle is to
differentiate the planetary signal from intrinsic stellar variability (e.g.,
pulsations, granulation, spots and plages). Convective blueshift, which results
from small-scale convection at the surface of Sun-like stars, is relevant for
Earth-twin detections as it exhibits Doppler noise on the order of 1 m/s. Here,
we present a simple model for convective blueshift based on fundamental
equations of stellar structure. Our model successfully matches observations of
convective blueshift for FGK stars. Based on our model, we also compute the
intrinsic noise floor for stellar granulation in the radial velocity
observations. We find that for a given mass range, stars with higher
metallicities display lower radial-velocity dispersion due to granulation, in
agreement with MHD simulations. We also provide a set of formulae to predict
the amplitude of radial-velocity dispersion due to granulation as a function of
stellar parameters. Our work is vital in identifying the most amenable stellar
targets for EPRV surveys and radial velocity follow-up programmes for TESS,
CHEOPS, and the upcoming PLATO mission. | astro-ph_SR |
The dependence of the evolution of SN type Ia progenitors on the C
burning rate uncertainty and parameters of convective boundary mixing: Evolution of a supernova type Ia progenitor requires formation of a CO white
dwarf, which implies a dependence on the C burning rate (CBR). It can also be
affected by the recently identified possibility of C flame quenching by
convective boundary mixing. We present first results of our study of the
combined effect of these two potential sources of uncertainty on the SN Ia
progenitor evolution. We consider the possibility that the CBR is higher than
its currently recommended value by as much as a factor of 1000 if unidentified
resonances are important, or that it is significantly lower because of the
hindrance effect. For stellar models that assume the Schwarzschild boundary for
convection, the maximum initial mass for the formation of CO WDs increases from
M_i ~ 5.5 Msun for the CBR factor of 1000 to M_i > 7.0 Msun for the CBR factor
of 0.01. For C-flame quenching models, hybrid C-O-Ne WDs form for a range of
initial mass of Delta M_i ~ 1 Msun, which increases a fraction of stars that
form WDs capable of igniting C in a thermonuclear runaway. The most extreme
case is found for the CBR factor of 0.1 that is supported by the hindrance
model. This nuclear physics assumption, combined with C flame quenching, leads
to the formation of a hybrid C-O-Ne WD with a mass of 1.3 Msun. Such WDs do not
need to accrete much mass to reach the Chandrasekhar limit. | astro-ph_SR |
A resolved debris disk around the candidate planet-hosting star HD95086: Recently, a new planet candidate was discovered on direct images around the
young (10-17 Myr) A-type star HD95086. The strong infrared excess of the system
indicates that, similarly to HR8799, {\ss} Pic, and Fomalhaut, the star harbors
a circumstellar disk. Aiming to study the structure and gas content of the
HD95086 disk, and to investigate its possible interaction with the newly
discovered planet, here we present new optical, infrared and millimeter
observations. We detected no CO emission, excluding the possibility of an
evolved gaseous primordial disk. Simple blackbody modeling of the spectral
energy distribution suggests the presence of two spatially separate dust belts
at radial distances of 6 and 64 AU. Our resolved images obtained with the
Herschel Space Observatory reveal a characteristic disk size of ~6.0x5.4 arcsec
(540x490 AU) and disk inclination of ~25 degree. Assuming the same inclination
for the planet candidate's orbit, its re-projected radial distance from the
star is 62 AU, very close to the blackbody radius of the outer cold dust ring.
The structure of the planetary system at HD95086 resembles the one around
HR8799. Both systems harbor a warm inner dust belt and a broad colder outer
disk and giant planet(s) between the two dusty regions. Modelling implies that
the candidate planet can dynamically excite the motion of planetesimals even
out to 270 AU via their secular perturbation if its orbital eccentricity is
larger than about 0.4. Our analysis adds a new example to the three known
systems where directly imaged planet(s) and debris disks co-exist. | astro-ph_SR |
Exploring the RCrA environment with SPHERE: Discovery of a new stellar
companion: Aims. R Coronae Australis (R CrA) is the brightest star of the Coronet nebula
of the Corona Australis (CrA) star forming region. It has very red colors,
probably due to dust absorption and it is strongly variable. High contrast
instruments allow for an unprecedented direct exploration of the immediate
circumstellar environment of this star. Methods. We observed R CrA with the
near-IR channels (IFS and IRDIS) of SPHERE at VLT. In this paper, we used four
different epochs, three of them from open time observations while one is from
the SPHERE guaranteed time. The data were reduced using the DRH pipeline and
the SPHERE Data Center. On the reduced data we implemented custom IDL routines
with the aim to subtract the speckle halo.We have also obtained pupil-tracking
H-band (1.45-1.85 micron) observations with the VLT/SINFONI near-infrared
medium-resolution (R~3000) spectrograph. Results. A companion was found at a
separation of 0.156" from the star in the first epoch and increasing to 0.18400
in the final one. Furthermore, several extended structures were found around
the star, the most noteworthy of which is a very bright jet-like structure
North-East from the star. The astrometric measurements of the companion in the
four epochs confirm that it is gravitationally bound to the star. The SPHERE
photometry and the SINFONI spectrum, once corrected for extinction, point
toward an early M spectral type object with a mass between 0.3 and 0.55 M?. The
astrometric analyis provides constraints on the orbit paramenters: e~0.4,
semi-major axis at 27-28 au, inclination of ~ 70{\deg} and a period larger than
30 years. We were also able to put constraints of few MJup on the mass of
possible other companions down to separations of few tens of au. | astro-ph_SR |
Intense bipolar structures from stratified helical dynamos: We perform direct numerical simulations of the equations of
magnetohydrodynamics with external random forcing and in the presence of
gravity. The domain is divided into two parts: a lower layer where the forcing
is helical and an upper layer where the helicity of the forcing is zero with a
smooth transition in between. At early times, a large-scale helical dynamo
develops in the bottom layer. At later times the dynamo saturates, but the
vertical magnetic field continues to develop and rises to form dynamic bipolar
structures at the top, which later disappear and reappear. Some of the
structures look similar to $\delta$ spots observed in the Sun. This is the
first example of magnetic flux concentrations, owing to strong density
stratification, from self-consistent dynamo simulations that generate bipolar,
super-equipartition strength, magnetic structures whose energy density can
exceeds the turbulent kinetic energy by even a factor of ten. | astro-ph_SR |
Molecules with ALMA at Planet-forming Scales (MAPS) XIII: HCO$^+$ and
disk ionization structure: We observed HCO$^+$ $J=1-0$ and H$^{13}$CO$^+$ $J=1-0$ emission towards the
five protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480
as part of the MAPS project. HCO$^+$ is detected and mapped at
0.3\arcsec\,resolution in all five disks, while H$^{13}$CO$^+$ is detected
(SNR$>6 \sigma$) towards GM Aur and HD 163296 and tentatively detected (SNR$>3
\sigma$) towards the other disks by a matched filter analysis. Inside a radius
of $R\sim 100$ au, the HCO$^+$ column density is flat or shows a central dip.
At outer radii ($\gtrsim 100$ au), the HCO$^+$ column density decreases
outwards, while the column density ratio of HCO$^+$/CO is mostly in the range
of $\sim 10^{-5}-10^{-4}$. We derived the HCO$^+$ abundance in the warm CO-rich
layer, where HCO$^+$ is expected to be the dominant molecular ion. At $R\gtrsim
100$ au, the HCO$^+$ abundance is $\sim 3 \times 10^{-11} - 3\times 10^{-10}$,
which is consistent with a template disk model with X-ray ionization. At the
smaller radii, the abundance decreases inwards, which indicates that the
ionization degree is lower in denser gas, especially inside the CO snow line,
where the CO-rich layer is in the midplane. Comparison of template disk models
with the column densities of HCO$^+$, N$_2$H$^+$, and N$_2$D$^+$ indicates that
the midplane ionization rate is $\gtrsim 10^{-18}$ s$^{-1}$ for the disks
around IM Lup, AS 209, and HD 163296. We also find hints of an increased
HCO$^+$ abundance around the location of dust continuum gaps in AS 209, HD
163296, and MWC 480. This paper is part of the MAPS special issue of the
Astrophysical Journal Supplement. | astro-ph_SR |
The diversity of light curve variations of Blazhko stars: The ground-based Konkoly Blazhko Survey I and II aim to collect accurate,
extended, multicolour light curves of galactic field RRab stars in order to
determine the incidence rate of the modulation in the sample, to study the
modulation in details, to study long-term changes in the modulation properties
and to find changes in the mean global physical parameters of the stars with
Blazhko phase. Here the diversity of light curve variations of Blazhko stars is
demonstrated through a sub-sample of the modulated RRab stars found by the
first part of this survey. | astro-ph_SR |
The single-sided pulsator CO~Camelopardalis: CO~Cam (TIC 160268882) is the second ``single-sided pulsator'' to be
discovered. These are stars where one hemisphere pulsates with a significantly
higher amplitude than the other side of the star. CO~Cam is a binary star
comprised of an Am $\delta$~Sct primary star with $T_{\rm eff} = 7070 \pm
150$\,K, and a spectroscopically undetected G main-sequence secondary star. The
dominant pulsating side of the primary star is centred on the L$_1$ point. We
have modelled the spectral energy distribution combined with radial velocities,
and independently the {\em TESS} light curve combined with radial velocities.
Both of these give excellent agreement and robust system parameters for both
stars. The $\delta$~Sct star is an oblique pulsator with at least four low
radial overtone (probably) f~modes with the pulsation axis coinciding with the
tidal axis of the star, the line of apsides. Preliminary theoretical modelling
indicates that the modes must produce much larger flux perturbations near the
L$_1$ point, although this is difficult to understand because the pulsating
star does not come near to filling its Roche lobe. More detailed models of
distorted pulsating stars should be developed. These newly discovered
single-sided pulsators offer new opportunities for astrophysical inference from
stars that are oblique pulsators in close binary stars. | astro-ph_SR |
Temporal and Spatial Evolutions of a Large Sunspot Group and Great
Auroral Storms around the Carrington Event in 1859: The Carrington event is considered to be one of the most extreme space
weather events in observational history within a series of magnetic storms
caused by extreme interplanetary coronal mass ejections (ICMEs) from a large
and complex active region (AR) emerged on the solar disk. In this article, we
study the temporal and spatial evolutions of the source sunspot active region
and visual aurorae, and compare this storm with other extreme space weather
events on the basis of their spatial evolution. Sunspot drawings by Schwabe,
Secchi, and Carrington describe the position and morphology of the source AR at
that time. Visual auroral reports from the Russian Empire, Iberia, Ireland,
Oceania, and Japan fill the spatial gap of auroral visibility and revise the
time series of auroral visibility in mid to low magnetic latitudes (MLATs). The
reconstructed time series is compared with magnetic measurements and shows the
correspondence between low to mid latitude aurorae and the phase of magnetic
storms. The spatial evolution of the auroral oval is compared with those of
other extreme space weather events in 1872, 1909, 1921, and 1989 as well as
their storm intensity, and contextualizes the Carrington event, as one of the
most extreme space weather events, but likely not unique. | astro-ph_SR |
Gaps in the Main-Sequence of Star Cluster Hertzsprung Russell Diagrams: The presence of gaps or regions of small numbers of stars in the main
sequence of the Hertzsprung Russell Diagram (HRD) of star clusters has been
reported in literature. This is interesting and significant as it could be
related to star formation and/or rapid evolution or instabilities. In this
paper, using Gaia DR3 photometry and confirmed membership data, we explore the
HRD of nine open clusters with reported gaps, identify them and assess their
importance and spectral types. | astro-ph_SR |
NIKA2 observations around LBV stars: emission from stars and
circumstellar material: Luminous Blue Variable (LBV) stars are evolved massive objects, previous to
core-collapse supernova. LBVs are characterized by photometric and
spectroscopic variability, produced by strong and dense winds, mass-loss events
and very intense UV radiation. LBVs strongly disturb their surroundings by
heating and shocking, and produce important amounts of dust. The study of the
circumstellar material is therefore crucial to understand how these massive
stars evolve, and also to characterize their effects onto the interstellar
medium. The versatility of NIKA2 is a key in providing simultaneous
observations of both the stellar continuum and the extended, circumstellar
contribution. The NIKA2 frequencies (150 and 260 GHz) are in the range where
thermal dust and free-free emission compete, and hence NIKA2 has the capacity
to provide key information about the spatial distribution of circumstellar
ionized gas, warm dust and nearby dark clouds; non-thermal emission is also
possible even at these high frequencies. We show the results of the first NIKA2
survey towards five LBVs. We detected emission from four stars, three of them
immersed in tenuous circumstellar material. The spectral indices show a complex
distribution and allowed us to separate and characterize different components.
We also found nearby dark clouds, with spectral indices typical of thermal
emission from dust. Spectral indices of the detected stars are negative and
hard to be explained only by free-free processes. In one of the sources,
G79.29+0.46, we also found a strong correlation of the 1mm and 2mm continuum
emission with respect to nested molecular shells at 0.1 pc from the LBV. The
spectral index in this region clearly separates four components: the LBV star,
a bubble characterized by free-free emission, and a shell interacting with a
nearby infrared dark cloud. | astro-ph_SR |
On WD-WD Mergers in Triple Systems: The Role of Kozai Resonance with
Tidal Friction: White dwarf-white dwarf (WD-WD) mergers may lead to type Ia supernovae
events. Thompson (2011) suggested that many such binaries are produced in
hierarchical triple systems. The tertiary induces eccentricity oscillations in
the inner binary via the Kozai-Lidov mechanism, driving the binary to high
eccentricities, and significantly reducing the gravitational wave merger
timescale (T_GW) over a broad range of parameter space. Here, we investigate
the role of tidal forces in these systems. We show that tidal effects are
important in the regime of moderately high initial relative inclination between
the inner binary and the outer tertiary. For 85 < i_0 < 90 degrees (prograde)
and 97 < i_0 < 102 degrees (retrograde), tides combine with GW radiation to
dramatically decrease T_GW. In the regime of high inclinations between 91 < i_0
< 96 degrees, the inner binary likely suffers a direct collision, as in the
work of Katz & Dong (2012) and tidal effects do not play an important role. | astro-ph_SR |
Variable stars in Palomar 13; an evaporating globular cluster: We present new CCD $VI$ photometry of the distant globular cluster Pal 13.
Fourier decomposition of the light curves of the three cluster member RRab
stars lead to estimations of [Fe/H]=-1.65, and a distance of 23.67$\pm$0.57
kpc. Light and colour near minimum phases for RRab stars leads to an estimate
of $E(B-V)$=0.104 $\pm$ 0.001. A $V/(V-I)$ colour-magnitude diagram, built
exclusively with likely star members, shows consistency with the above
parameters and an age of 12 Gyrs. A search of variable stars in the field of
view of our images revealed the variability of a red giant cluster member and
of three probably non-member stars; two RRab stars and one W Virginis star or
CW. The GAIA proper motions of member stars in Pal 13 show a significant
scatter, consistent with the scenario of the cluster being tidally stripped. | astro-ph_SR |
The Outburst of the Young Star Gaia19bey: We report photometry and spectroscopy of the outburst of the young stellar
object Gaia19bey. We have established the outburst light curve with archival
Gaia G, ATLAS Orange, ZTF r-band and Pan-STARRS rizy-filter photometry, showing
an outburst of approximately 4 years duration, longer than typical EXors but
shorter than FUors. Its pre-outburst SED shows a flat far-infrared spectrum,
confirming the early evolutionary state of Gaia19bey and its similarity to
other deeply embedded young stars experiencing outbursts. A lower limit to the
peak outburst luminosity is approximately 182 L_sun at an assumed distance of
1.4 kpc, the minimum plausible distance. Infrared and optical spectroscopy near
maximum light showed an emission line spectrum, including HI lines, strong red
CaII emission, other metal emission lines, infrared CO bandhead emission, and a
strong infrared continuum. Towards the end of the outburst, the emission lines
have all but disappeared and the spectrum has changed into an almost pure
continuum spectrum. This indicates a cessation of magnetospheric accretion
activity. The near-infrared colors have become redder as Gaia19bey has faded,
indicating a cooling of the continuum component. Near the end of the outburst,
the only remaining strong emission lines are forbidden shock-excited emission
lines. Adaptive optics integral field spectroscopy shows the H_2 1--0 S(1)
emission with the morphology of an outflow cavity and the extended emission in
the [FeII] line at 1644 nm with the morphology of an edge-on disk. However, we
do not detect any large-scale jet from Gaia19bey. | astro-ph_SR |
The Sun Remains Relatively Refractory Depleted: Elemental Abundances for
17,412 Gaia RVS Solar Analogs and 50 Planet Hosts: The elemental abundances of stars, particularly the refractory elements
(e.g., Fe, Si, Mg), play an important role in connecting stars to their
planets. Most Sun-like stars do not have refractory abundance measurements
since obtaining a large sample of high-resolution spectra is difficult with
oversubscribed observing resources. In this work we infer abundances for C, N,
O, Na, Mn, Cr, Si, Fe, Ni, Mg, V, Ca, Ti, Al, and Y for solar analogs with Gaia
RVS spectra (R=11,200) using the Cannon, a data-driven method. We train a
linear model on a reference set of 34 stars observed by Gaia RVS with precise
abundances measured from previous high resolution spectroscopic efforts (R >
30,000--110,000). We then apply this model to several thousand Gaia RVS solar
analogs. This yields abundances with average upper limit precisions of
0.04--0.1 dex for 17,412 stars, 50 of which are identified planet (candidate)
hosts. We subsequently test the relative refractory depletion of these stars
with increasing element condensation temperature compared to the Sun. The Sun
remains refractory depleted compared to other Sun-like stars regardless of our
current knowledge of the planets they host. This is inconsistent with theories
of various types of planets locking up or sequestering refractories.
Furthermore, we find no significant abundance differences between identified
close-in giant planet hosts, giant planet hosts, and terrestrial/small planet
hosts and the rest of the sample within our precision limits. This work
demonstrates the utility of data-driven learning for future exoplanet
composition and demographics studies. | astro-ph_SR |
Origin of the CEMP-no Group Morphology in the Milky Way: The elemental-abundance signatures of the very first stars are imprinted on
the atmospheres of CEMP-no stars, as various evidence suggests they are
bona-fide second-generation stars. It has recently been recognized that the
CEMP-no stars can be sub-divided into at least two groups, based on their
distinct morphology in the $A$(C)-[Fe/H] space, indicating the likely existence
of multiple pathways for their formation. In this work, we compare the halo
CEMP-no group morphology with that of stars found in satellite dwarf galaxies
of the Milky Way -- a very similar $A$(C)-[Fe/H] pattern is found, providing
clear evidence that halo CEMP-no stars were indeed accreted from their host
mini-halos, similar in nature to those that formed in presently observed
ultra-faint dwarfs (UFDs) and dwarf spheroidal (dSph) galaxies. We also infer
that the previously noted "anomalous" CEMP-no halo stars (with high $A$(C) and
low [Ba/Fe] ratios) that otherwise would be associated with Group I may have
the same origin as the Group III CEMP-no halo stars, by analogy with the
location of several Group III CEMP-no stars in the UFDs and dSphs and their
distinct separation from that of the CEMP-$s$ stars in the $A$(Ba)-$A$(C)
space. Interestingly, CEMP-no stars associated with UFDs include both Group II
and Group III stars, while the more massive dSphs appear to have only Group II
stars. We conclude that understanding the origin of the CEMP-no halo stars
requires knowledge of the masses of their parent mini-halos, which is related
to the amount of carbon dilution prior to star formation, in addition to the
nature of their nucleosynthetic origin. | astro-ph_SR |
Chemical Compositions of a sample of candidate post-AGB stars: We have derived elemental abundances for a sample of nine IRAS sources with
colours similar to those of post-AGB stars. For IRAS 01259+6823, IRAS
05208-2035, IRAS 04535+3747 and IRAS 08187-1905 this is the first detailed
abundance analysis based upon high resolution spectra. Mild indication of
s-processing for IRAS 01259+6823, IRAS 05208-2035 and IRAS 08187-1905 have been
found and a more comprehensive study of s-process enhanced objects IRAS
17279-1119 and IRAS 22223+4327 have been carried out. We have also made a
contemporary abundance analysis of the high galactic latitude supergiants BD+39
4926 and HD 107369. The former is heavily depleted in refractories and
estimated [Zn/H] of -0.7 dex most likely gives initial metallicity of the star.
For HD 107369 the abundances of alpha and Fe-peak elements are similar to those
of halo objects and moderate deficiency of s-process elements is seen. IRAS
07140-2321 despite being a short period binary with circumstellar shell does
not exhibit selective depletion of refractory elements. We have compiled the
stellar parameters and abundances for post-AGB stars with s-process
enhancement, those showing significant depletion of condensable elements and
those showing neither. The compilation shows that the s-process enhanced group
contains very small number of binaries, and observed [alpha/Fe] are generally
similar to thick disc values. It is likely that they represent AGB evolution of
single stars.The compilation of depleted group contains larger fraction of
binaries and generally supports the hypothesis of dusty discs surrounding
binary post-AGB stars inferred via the shape of their SED and mid IR
interferometry. IRAS 07140-2321 and BD+39 4926 are difficult to explain with
this scenario and indicate the existence of additional parameter/condition
needed to explain the depletion phenomenon. | astro-ph_SR |
Ensemble Learning for CME Arrival Time Prediction: The Sun constantly releases radiation and plasma into the heliosphere.
Sporadically, the Sun launches solar eruptions such as flares and coronal mass
ejections (CMEs). CMEs carry away a huge amount of mass and magnetic flux with
them. An Earth-directed CME can cause serious consequences to the human system.
It can destroy power grids/pipelines, satellites, and communications.
Therefore, accurately monitoring and predicting CMEs is important to minimize
damages to the human system. In this study we propose an ensemble learning
approach, named CMETNet, for predicting the arrival time of CMEs from the Sun
to the Earth. We collect and integrate eruptive events from two solar cycles,
#23 and #24, from 1996 to 2021 with a total of 363 geoeffective CMEs. The data
used for making predictions include CME features, solar wind parameters and CME
images obtained from the SOHO/LASCO C2 coronagraph. Our ensemble learning
framework comprises regression algorithms for numerical data analysis and a
convolutional neural network for image processing. Experimental results show
that CMETNet performs better than existing machine learning methods reported in
the literature, with a Pearson product-moment correlation coefficient of 0.83
and a mean absolute error of 9.75 hours. | astro-ph_SR |
Shape-based Feature Engineering for Solar Flare Prediction: Solar flares are caused by magnetic eruptions in active regions (ARs) on the
surface of the sun. These events can have significant impacts on human
activity, many of which can be mitigated with enough advance warning from good
forecasts. To date, machine learning-based flare-prediction methods have
employed physics-based attributes of the AR images as features; more recently,
there has been some work that uses features deduced automatically by deep
learning methods (such as convolutional neural networks). We describe a suite
of novel shape-based features extracted from magnetogram images of the Sun
using the tools of computational topology and computational geometry. We
evaluate these features in the context of a multi-layer perceptron (MLP) neural
network and compare their performance against the traditional physics-based
attributes. We show that these abstract shape-based features outperform the
features chosen by the human experts, and that a combination of the two feature
sets improves the forecasting capability even further. | astro-ph_SR |
A new way to infer variations of the seismic solar radius: We show that the mean phase of waves propagating all the way from the far
side of the Sun to the front side, as measured by seismic holography, varies
with time. The change is highly anticorrelated with solar cycle activity and is
consistent with other recent results on the variation of the seismic radius of
the Sun. The phase change that we observe corresponds to a few kilometers
difference in the seismic solar radius from solar maximum to solar minimum in
agreement with inferrences from global helioseismology studies. | astro-ph_SR |
Multiple Star Systems in the Orion Nebula: This work presents an interferometric study of the massive-binary fraction in
the Orion Trapezium Cluster with the recently comissioned GRAVITY instrument.
We observe a total of 16 stars of mainly OB spectral type. We find three
previously unknown companions for $\theta ^1$ Ori B, $\theta ^2$ Ori B, and
$\theta ^2$ Ori C. We determine a separation for the previously suspected
companion of NU Ori. We confirm four companions for $\theta ^1$ Ori A, $\theta
^1$ Ori C, $\theta ^1$ Ori D, and $\theta ^2$ Ori A, all with substantially
improved astrometry and photometric mass estimates. We refine the orbit of the
eccentric high-mass binary $\theta ^1$ Ori C and we are able to derive a new
orbit for $\theta ^1$ Ori D. We find a system mass of 21.7 $M_{\odot}$ and a
period of $53$ days. Together with other previously detected companions seen in
spectroscopy or direct imaging, eleven of the 16 high-mass stars are multiple
systems. We obtain a total number of 22 companions with separations up to 600
AU. The companion fraction of the early B and O stars in our sample is about 2,
significantly higher than in earlier studies of mostly OB associations. The
separation distribution hints towards a bimodality. Such a bimodality has been
previously found in A stars, but rarely in OB binaries, which up to this point
have been assumed to be mostly compact with a tail of wider companions. We also
do not find a substantial population of equal-mass binaries. The observed
distribution of mass ratios declines steeply with mass, and like the direct
star counts, indicates that our companions follow a standard power law initial
mass function. Again, this is in contrast to earlier findings of flat mass
ratio distributions in OB associations. We exclude collision as a dominant
formation mechanism but find no clear preference for core accretion or
competitive accretion. | astro-ph_SR |
Measuring stellar rotation periods with Kepler: We measure rotation periods for 12151 stars in the Kepler field, based on the
photometric variability caused by stellar activity. Our analysis returns stable
rotation periods over at least six out of eight quarters of Kepler data. This
large sample of stars enables us to study the rotation periods as a function of
spectral type. We find good agreement with previous studies and vsini
measurements for F, G and K stars. Combining rotation periods, B-V color, and
gyrochronology relations, we find that the cool stars in our sample are
predominantly younger than ~1Gyr. | astro-ph_SR |
Rotational Splitting of Pulsational Modes: Mode splittings produced by uniform rotation and a particular form of
differential rotation are computed for two-dimensional rotating 10 Mo ZAMS
stellar models. The change in the character of the mode splitting is traced as
a function of uniform rotation rate, and it is found that only relatively slow
rotation rates are required before the mode splitting becomes asymmetric about
the azimuthally symmetric (m=0) mode. Increased rotation produces a
progressively altered pattern of the individual modes with respect to each
other. Large mode splittings begin to overlap with the mode splittings produced
by different radial and latitudinal modes at relatively low rotation rates. The
mode splitting pattern for the differentially rotating stars we model is
different than that for uniformly rotating stars, making the mode splitting a
possible discriminant of the internal angular momentum distribution if one
assumes the formidable challenge of mode identification can be overcome. | astro-ph_SR |
The HST Large Programme on NGC6752. II. Multiple populations at the
bottom of the main sequence probed in NIR: Historically, multiple populations in Globular Clusters (GCs) have been
mostly studied from ultraviolet and optical filters down to stars that are more
massive than ~0.6 solar masses. Here we exploit deep near-infrared (NIR)
photometry from the Hubble Space Telescope to investigate multiple populations
among M-dwarfs in the GC NGC6752. We discovered that the three main populations
(A, B and C), previously observed in the brightest part of the color-magnitude
diagram, define three distinct sequences that run from the main-sequence (MS)
knee towards the bottom of the MS (~0.15 solar masses). These results, together
with similar findings on NGC2808, M4, and omega Centauri, demonstrate that
multiple sequences of M-dwarfs are common features of the color-magnitude
diagrams of GCs. The three sequences of low-mass stars in NGC6752 are
consistent with stellar populations with different oxygen abundances. The range
of [O/Fe] needed to reproduce the NIR CMD of NGC6752 is similar to the oxygen
spread inferred from high-resolution spectroscopy of red-giant branch (RGB)
stars. The relative numbers of stars in the three populations of M-dwarfs are
similar to those derived among RGB and MS stars more massive than ~0.6 solar
masses. As a consequence, the evidence that the properties of multiple
populations do not depend on stellar mass is a constraint for the formation
scenarios. | astro-ph_SR |
2M17091769+3127589: a mass-transfer binary with an extreme mass ratio: We present the orbital solution of a peculiar double-lined spectroscopic and
eclipsing binary system, 2M17091769+3127589. This solution was obtained by a
simultaneous fit of both APOGEE radial velocities and TESS and ASAS-SN light
curves to determine masses and radii. This system consists of an
$M=0.256^{+0.010}_{-0.006}$ $M_\odot$, $R=3.961^{+0.049}_{-0.032}$ $R_{\odot}$
red giant and a hotter $M=1.518 ^{+0.057}_{-0.031}$ $M_\odot$,
$R=2.608^{+0.034}_{-0.321}$ $R_{\odot}$ subgiant. Modelling with the MESA
evolutionary codes indicates that the system likely formed 5.26 Gyrs ago, with
a $M=1.2$ $M_\odot$ primary that is now the system's red giant and a $M=1.11$
$M_\odot$ secondary that is now a more massive subgiant. Due to Roche-lobe
overflow as the primary ascends the red giant branch, the more evolved
"primary" (i.e., originally the more massive star of the pair) is now only
one-sixth as massive as the "secondary". Such a difference between the initial
and the current mass ratio is one of the most extreme detected so far.
Evolutionary modelling suggests the system is still engaged in mass transfer,
at a rate of $\dot{M} \sim 10^{-9}$ $M_\odot$ yr$^{-1}$, and it provides an
example of a less evolved precursor to some of the systems that consist of
white dwarfs and blue stragglers. | astro-ph_SR |
Physical conditions around 6.7 GHz methanol masers-I: Ammonia: Methanol masers at 6.7 GHz are known to be tracers of high-mass star
formation in our Galaxy. In this paper, we study the large scale physical
conditions in the star forming clumps/cores associated with 6.7 GHz methanol
masers using observations of the (1,1), (2,2) and (3,3) inversion transitions
of ammonia with the Effelsberg telescope. The gas kinetic temperature is found
to be higher than in infrared dark clouds, highlighting the relatively evolved
nature of the maser sources. Other than a weak correlation between maser
luminosity and the ammonia line width, we do not find any differences between
low and high luminosity methanol masers. | astro-ph_SR |
A Comprehensive Radiative Magnetohydrodynamics Simulation of Active
Region Scale Flux Emergence from the Convection Zone to the Corona: We present a comprehensive radiative magnetohydrodynamic simulation of the
quiet Sun and large solar active regions. The 197 Mm wide simulation domain
spans from 18 (10) Mm beneath the photosphere to 113 Mm in the solar corona.
Radiative transfer assuming local thermal equilibrium, optically-thin radiative
losses, and anisotropic conduction transport provide the necessary realism for
synthesizing observables to compare with remote sensing observations of the
photosphere and corona. This model self-consistently reproduces observed
features of the quiet Sun, emerging and developed active regions, and solar
flares up to M class. Here, we report an overview of the first results. The
surface magnetoconvection yields an upward Poynting flux that is dissipated in
the corona and heats the plasma to over one million K. The quiescent corona
also presents ubiquitous propagating waves, jets, and bright points with sizes
down to 2 Mm. Magnetic flux bundles emerge into the photosphere and give rise
to strong and complex active regions with over $10^{23}$ Mx magnetic flux. The
coronal free magnetic energy, which is approximately 18\% of the total magnetic
energy, accumulates to approximately $10^{33}$ erg. The coronal magnetic field
is clearly non-force-free, as the Lorentz force needs to balance the pressure
force and viscous stress as well as drive magnetic field evolution. The
emission measure from $\log_{10}T{=}4.5$ to $\log_{10}T{>}7$ provides a
comprehensive view of the active region corona, such as coronal loops of
various lengths and temperatures, mass circulation by evaporation and
condensation, and eruptions from jets to large-scale mass ejections. | astro-ph_SR |
The Joint Milli-Arcsecond Pathfinder Survey (JMAPS): Mission Overview
and Attitude Sensing Applications: The Joint Milliarcsecond Pathfinder Survey (JMAPS) is a Department of Navy
bright star astrometric all-sky survey scheduled for launch in the 2012
timeframe. Mission objectives include a complete update of star positions for
the 2015 epoch to accuracy levels of 1 milliarcsecond (5 nano-radians) for
bright stars, as well as demonstration of 10 milliarcsecond attitude
determination capability and 50 milli-arcsecond attitude control on-orbit. In
the following paper, we describe the general instrument design and expected
performance. We also discuss the new mission capabilities enabled by the
unprecedented attitude determination accuracy of such an instrument, and focus
specifically on the application to long distance (50,000-100,00 km) formation
flying and solar system navigation. | astro-ph_SR |
Differential photometry of delta Scorpii during 2011 periastron: Hundred observations of Delta Scorpii over 200 days, from April 2 to October
16, 2011, have been made for AAVSO visually and digitally from Rio de Janeiro,
Rome and Paris. The three most luminous pixels either of the target star and
the two reference stars are used to evaluate the magnitude through differential
photometry. The main sources of errors are outlined. The system of Delta
Scorpii, a spectroscopic double star, has experienced a close periastron in
July 2011 within the outer atmospheres of the two giant components. The whole
luminosity of Delta Scorpii system increased from about Mv=1.8 to 1.65 peaking
around 5 to 15 July 2011, but there are significant rapid fluctuations of 0.2 -
0.3 magnitudes occurring in 20 days that seem to be real, rather than a
consequence of systematic errors due to the changes of reference stars and
observing conditions. This method is promising for being applied to other
bright variable stars like Betelgeuse and Antares. After August the magnitude
remained constant at Mv=1.8 until the last observation on October 16 made in
twilight from Rome. | astro-ph_SR |
Optical flare activity in the low-mass eclipsing binary GJ~3236: We present our observations of the low-mass eclipsing binary GJ~3236. We have
analyzed a phased $R_C$ light-curve and confirmed previously determined
fundamental parameters of the components. We detected evolution of the spot(s)
and found that there exists a large spot near a polar region of the primary
component and another spot either on the primary or the secondary component. We
also observed 7 flare events and determined a flare rate of about 0.1 flares
per hour. We observed two high energy, long-term flares with a complex light
curve and possibly four weak short-term flaring events. A majority of the
flares was detected in the $R_C$ filter, which indicate their high energy. | astro-ph_SR |
A first eROSITA view of ultracool dwarfs: We present the first X-ray detections of ultracool dwarfs (UCDs) from the
first all-sky survey of the extended ROentgen Survey with an Imaging Telescope
Array (eROSITA) onboard the Russian Spektrum-Roentgen-Gamma (SRG) mission. We
use three publicly available input catalogs of spectroscopically confirmed UCDs
and Gaia-selected UCD candidates that together comprise nearly 20000 objects.
We first extracted all X-ray sources from the catalog of the first eROSITA
survey, eRASS1, that have a UCD or candidate within three times their
positional uncertainty. Then we examined all Gaia objects in the vicinity of
these 96 X-ray sources and we associated them to the most plausible counterpart
on the basis of their spatial separation to the X-ray position and their
multiwavelength properties. This way we find 40 UCDs that have a secure
identification with an X-ray source and 18 plausible UCD X-ray emitters.
Twenty-one of these X-ray emitting UCDs have a spectroscopic confirmation,
while the others have been selected based on Gaia photometry and we computed
spectral types from the G-J color. The spectral types of the X-ray emitting
UCDs and candidates range between M5 and M9, and the distances range from 3.5
to 190 pc. The majority of the UCDs from the eRASS1 sample show a ratio of
X-ray to bolometric luminosity well above the canonical saturation limit of log
(Lx/Lbol) ~ -3. For the two most extreme outliers, we showed through an
analysis of the eRASS1 light curve that these high values are due to flaring
activity. The X-ray spectra of the two brightest objects both reveal an
emission-measure weighted plasma temperature of kT ~ 0.75 keV. These
observations demonstrate the potential of eROSITA for advancing our knowledge
on the faint coronal X-ray emission from UCDs by building statistical samples
for which the average X-ray brightness, flares, and coronal temperatures can be
derived. | astro-ph_SR |
Temporal and latitudinal variations of Ca-K plage and network area: An
implication to meridional flows: The Ca-K spectroheliograms obtained at the Kodaikanal observatory (KO) are
used to generate a uniform time series using the equal contrast technique (ECT)
to study the long and short-term variation in the solar chromosphere. The
percentage of plage, Enhanced network (EN), Active network (AN), and Quiet
network (QN) area at various latitudes is compared with the activity at
35$^{\circ}$ latitude and also with the sunspot number for the period of 1907
-- 1984. The values of phase differences indicate that the activity begins at
$\sim$45$^{\circ}$ latitude and shift progressively to the lower latitude at a
speed of $\sim$~9.4~m~sec$^{-1}$ . The shift speed slows down gradually and
reaches $\sim$~3~m~sec$^{-1}$ at $\sim$5$^{\circ}$ latitude. No phase
difference between the variations of Ca-K activity at 55$^{\circ}$,
65$^{\circ}$, and 75$^{\circ}$ latitude belts implies that changes in the
activity are happening simultaneously. The analysis shows that the activity at
polar latitude belts is anti-correlated with the sunspot number. This study
indicates that a multi-cell meridional flow pattern could exist in the solar
convection zone. One type of cell could transport the magnetic elements from
mid-latitude to low-latitude belts through meridional flows, and the other one
could be operating in the polar region. | astro-ph_SR |
Discovery of possible molecular counterparts to the infrared Double
Helix Nebula in the Galactic center: We have discovered two molecular features at radial velocities of -35 km/s
and 0 km/s toward the infrared Double Helix Nebula (DHN) in the Galactic center
with NANTEN2. The two features show good spatial correspondence with the DHN.
We have also found two elongated molecular ridges at these two velocities
distributed vertically to the Galactic plane over 0.8 degree. The two ridges
are linked by broad features in velocity and are likely connected physically
with each other. The ratio between the 12CO J=2-1 and J=1-0 transitions is 0.8
in the ridges which is larger than the average value 0.5 in the foreground gas,
suggesting the two ridges are in the Galactic center. An examination of the K
band extinction reveals a good coincidence with the CO 0 km/s ridge and is
consistent with a distance of 8 +/-2 kpc. We discuss the possibility that the
DHN was created by a magnetic phenomenon incorporating torsional Alfv\'en waves
launched from the circumnuclear disk (Morris, Uchida & Do 2006) and present a
first estimate of the mass and energy involved in the DHN. | astro-ph_SR |
Transients Obscured by Dusty Disks: Dust absorption is invoked in a number of contexts for hiding a star that has
survived some sort of transient event from view. Dust formed in a transient is
expanding away from the star and, in spherical models, the mass and energy
budgets implied by a high optical depth at late times make such models
untenable. Concentrating the dust in a disk or torus can in principle hide a
source from an equatorial observer using less mass and so delay this problem.
However, using axisymmetric dust radiation transfer models with a range of
equatorial dust concentrations, we find that this is quite difficult to achieve
in practice. The polar optical depth must be either low or high to avoid
scattering optical photons to equatorial observers. Most of the emission
remains at wavelengths easily observed by JWST, and the equatorial brightness
is reduced by at most a factor of ~2 compared to isotropic emission even for
equatorial (visual) optical depths of 1000. It is particularly difficult to
hide a source with silicate dusts because the absorption feature near 10\
microns frequently leads to the emission being concentrated just bluewards of
the feature, near 8 microns. | astro-ph_SR |
Principle of Minimum Energy in Magnetic Reconnection in a Self-Organized
Critical Model for Solar Flares: Solar flares are an abrupt release of magnetic energy in the Sun's atmosphere
due to reconnection of the coronal magnetic field. This occurs in response to
turbulent flows at the photosphere which twist the coronal field. Similar to
earthquakes, solar flares represent the behavior of a complex system, and
expectedly their energy distribution follows a power law. We present a
statistical model based on the principle of minimum energy in a coronal loop
undergoing magnetic reconnection, which is described as an avalanche process.
We show the distribution of peaks for the flaring events in this self-organized
critical system is scale-free. The obtained power law index 1.84 $ \pm $ 0.02
for the peaks is in good agreement with satellite observations of soft X-ray
flares. The principle of minimum energy can be applied for general avalanche
models to describe many other phenomena. | astro-ph_SR |
The 1600 Angstrom Emission Bump in Protoplanetary Disks: A Spectral
Signature of H$_{2}$O Dissociation: The FUV continuum spectrum of many accreting pre-main sequence stars,
Classical T Tauri Stars (CTTSs), does not continue smoothly from the
well-studied Balmer continuum emission in the NUV, suggesting that additional
processes contribute to the short-wavelength emission in these objects. The
most notable spectral feature in the FUV continuum of some CTTSs is a broad
emission approximately centered at 1600~\AA, which has been referred to as the
"1600 A Bump". The origin of this feature remains unclear. We have assembled
archival FUV spectra of 37 disk-hosting systems observed by the {\it Hubble
Space Telescope}. Clear 1600 A Bump emission is observed above the smooth,
underlying 1100-1800 A continuum spectrum in 19/37 Classical T Tauri disks in
the HST sample, with the detection rate in transition disks (8/8) being much
higher than in primordial or non-transition sources (11/29). We describe a
spectral deconvolution analysis to separate the Bump (spanning 1490~--~1690 A)
from the underlying FUV continuum, finding an average Bump luminosity,
$L$(Bump7 x 10^{29} erg s^{-1}. We find that the 1600 A Bump is characterized
by a peak wavelength of 1598.6 +/- 3.3 A.
Contrary to previous studies, we find that this feature is inconsistent with
models of H2 excited by electron-impact. We show that this Bump emits roughly
10-80% of the total fluorescent H2 luminosity for stars with well-defined Bump
features. Energetically, this suggests that the carrier of the 1600 A Bump
emission is powered by Ly-a photons. We argue that the most likely mechanism is
Ly-a-driven dissociation of H2O in the inner disk, r < 2 AU. We demonstrate
that non-thermally populated H2O fragments can qualitatively account for the
observed emission (discrete and continuum), and find that the average
Ly-a-driven H2O dissociation rate is 1.7x 10^{42} water molecules s^{-1}. | astro-ph_SR |
Discovery of periodic class II methanol masers associated with
G339.986-0.425 region: Ten new class II methanol masers from the 6.7-GHz Methanol Multibeam survey
catalogues III and IV were selected for a monitoring programme at both 6.7 and
12.2 GHz with the 26m Hartebeesthoek Radio Astronomy Observatory (HartRAO)
radio telescope for two years and nine months, from August 2012 to May 2015. In
the sample, only masers associated with G339.986-0.425 were found to show
periodic variability at both 6.7 and 12.2 GHz. The existence of periodic
variation was tested with four independent methods. The analytical method gave
the best estimation of the period, which was 246 $\pm$ 1 days. The time series
of G339.986-0.425 show strong correlations across velocity channels and between
the 6.7 and 12.2 GHz masers. The time delay was also measured across channels
and shows structure across the spectrum which is continuous between different
maser components. | astro-ph_SR |
The Chromospheric Response to the Sunquake generated by the X9.3 Flare
of NOAA 12673: Active region NOAA 12673 was extremely volatile in 2017 September, producing
many solar flares, including the largest of solar cycle 24, an X9.3 flare of 06
September 2017. It has been reported that this flare produced a number of
sunquakes along the flare ribbon (Sharykin & Kosovichev 2018; Zhao & Chen
2018). We have used co-temporal and co-spatial Helioseismic and Magnetic Imager
(HMI) line-of-sight (LOS) and Swedish 1-m Solar Telescope observations to show
evidence of the chromospheric response to these sunquakes. Analysis of the Ca
II 8542 \AA\space line profiles of the wavefronts revealed that the crests
produced a strong blue asymmetry, whereas the troughs produced at most a very
slight red asymmetry. We used the combined HMI, SST datasets to create
time-distance diagrams and derive the apparent transverse velocity and
acceleration of the response. These velocities ranged from 4.5 km s$^{-1}$ to
29.5 km s$^{-1}$ with a constant acceleration of 8.6 x 10$^{-3}$ km s$^{-2}$.
We employed NICOLE inversions, in addition to the Center-of-Gravity (COG)
method to derive LOS velocities ranging 2.4 km s$^{-1}$ to 3.2 km s$^{-1}$.
Both techniques show that the crests are created by upflows. We believe that
this is the first chromospheric signature of a flare induced sunquake. | astro-ph_SR |
Chemical Yields from Supernovae and Hypernovae: We review the final stages of stellar evolution, supernova properties, and
chemical yields as a function of the progenitor's mass M. (1) 8 - 10 Ms stars
are super-AGB stars when the O+Ne+Mg core collapses due to electron capture.
These AGB-supernovae may constitute an SN 2008S-like sub-class of Type IIn
supernovae. These stars produce little alpha-elements and Fe-peak elements, but
are important sources of Zn and light p-nuclei. (2) 10 - 90 Ms stars undergo
Fe-core collapse. Nucleosynthesis in aspherical explosions is important, as it
can well reproduce the abundance patterns observed in extremely metal-poor
stars. (3) 90 - 140 Ms stars undergo pulsational nuclear instabilities at
various nuclear burning stages, including O and Si-burning. (4) 140 - 300 Ms
stars become pair-instability supernovae, if the mass loss is small enough. (5)
Stars more massive than 300 Ms undergo core-collapse to form intermediate mass
black holes. | astro-ph_SR |
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