1,412 research outputs found
Determining Parameters of Cool Giant Stars by Modeling Spectrophotometric and Interferometric Observations Using the SAtlas Program
Context: Optical interferometry is a powerful tool for observing the
intensity structure and angular diameter of stars. When combined with
spectroscopy and/or spectrophotometry, interferometry provides a powerful
constraint for model stellar atmospheres. Aims: The purpose of this work is to
test the robustness of the spherically symmetric version of the Atlas stellar
atmosphere program, SAtlas, using interferometric and spectrophotometric
observations. Methods: Cubes (three dimensional grids) of model stellar
atmospheres, with dimensions of luminosity, mass, and radius, are computed to
fit observations for three evolved giant stars, \psi Phoenicis, \gamma
Sagittae, and \alpha Ceti. The best-fit parameters are compared with previous
results. Results: The best-fit angular diameters and values of \chi^2 are
consistent with predictions using Phoenix and plane-parallel Atlas models. The
predicted effective temperatures, using SAtlas, are about 100 to 200 K lower,
and the predicted luminosities are also lower due to the differences in
effective temperatures. Conclusions: It is shown that the SAtlas program is a
robust tool for computing models of extended stellar atmospheres that are
consistent with observations. The best-fit parameters are consistent with
predictions using Phoenix models, and the fit to the interferometric data for
\psi Phe differs slightly, although both agree within the uncertainty of the
interferometric observations.Comment: 5 pages, 6 figures, Accepted for publication in A&A as a Research
Not
Using limb darkening to measure fundamental parameters of stars
Context. Limb darkening is an important tool for understanding stellar
atmospheres, but most observations measuring limb darkening assume various
parameterizations that yield no significant information about the structure of
stellar atmospheres. Aims. We use a specific limb-darkening relation to study
how the best-fit coefficients relate to fundamental stellar parameters from
spherically symmetric model stellar atmospheres. Methods. Using a grid of
spherically symmetric Atlas model atmospheres, we compute limb-darkening
coefficients, and develop a novel method to predict fundamental stellar
parameters. Results. We find our proposed method predicts the mass of stellar
atmosphere models given only the radius and limb-darkening coefficients,
suggesting that microlensing, interferometric, transit and eclipse observations
can constrain stellar masses. Conclusions. This novel method demonstrates that
limb-darkening parameterizations contain important information about the
structure of stellar atmospheres, with the potential to be a valuable tool for
measuring stellar masses.Comment: 8 pages, 6 figures, 2 tables, A&A accepte
Fundamental properties and atmospheric structure of the red supergiant VY CMa based on VLTI/AMBER spectro-interferometry
We investigate the atmospheric structure and fundamental properties of the
red supergiant VY CMa. We obtained near-infrared spectro-interferometric
observations of VY CMa with spectral resolutions of 35 and 1500 using the AMBER
instrument at the VLTI. The visibility data indicate the presence of molecular
layers of water vapor and CO in the extended atmosphere with an asymmetric
morphology. The uniform disk diameter in the water band around 2.0 mu is
increased by \sim20% compared to the near-continuum bandpass at 2.20-2.25 mu
and in the CO band at 2.3-2.5 mu it is increased by up to \sim50%. The closure
phases indicate relatively small deviations from point symmetry close to the
photospheric layer, and stronger deviations in the extended H2O and CO layers.
Making use of the high spatial and spectral resolution, a near-continuum
bandpass can be isolated from contamination by molecular and dusty layers, and
the Rosseland-mean photospheric angular diameter is estimated to 11.3 +/- 0.3
mas based on a PHOENIX atmosphere model. Together with recent high-precision
estimates of the distance and spectro-photometry, this estimate corresponds to
a radius of 1420 +/- 120 Rsun and an effective temperature of 3490 +/- 90 K. VY
CMa exhibits asymmetric, possibly clumpy, atmospheric layers of H2O and CO,
which are not co-spatial, within a larger elongated dusty envelope. Our revised
fundamental parameters put VY CMa close to the Hayashi limit of recent
evolutionary tracks of initial mass 25 Msun with rotation or 32 Msun without
rotation, shortly before evolving blueward in the HR-diagram.Comment: 5 pages, 5 figures, accepted for publication in Astronomy and
Astrophysics (A&A) as a Lette
Differently Shaped Hard Body Colloids in Confinement: From passive to active particles
We review recent progress in the theoretical description of anisotropic hard
colloidal particles. The shapes considered range from rods and dumbbells to
rounded cubes, polyhedra and to biaxial particles with arbitrary shape. Our
focus is on both static and dynamical density functional theory and on computer
simulations. We describe recent results for the structure, dynamics and phase
behaviour in the bulk and in various confining geometries, e.g. established by
two parallel walls which reduce the dimensionality of the system to two
dimensions. We also include recent theoretical modelling for active particles,
which are autonomously driven by some intrinsic motor, and highlight their
fascinating nonequilibrium dynamics and collective behaviour.Comment: 15 pages, 6 figures, EPJ ST (accepted
Myeloid cells which secrete S100 proteins in juvenile dermatomyositis may contribute to disease activity
ALMA observations of the variable 12CO/13CO ratio around the asymptotic giant branch star R Sculptoris
[abridged] The 12CO/13CO ratio is often used as a measure of the 12C/13C
ratio in the circumstellar environment, carrying important information about
the stellar nucleosynthesis. External processes can change the 12CO and 13CO
abundances, and spatially resolved studies of the 12CO/13CO ratio are needed to
quantify the effect of these processes on the globally determined values.
Additionally, such studies provide important information on the conditions in
the circumstellar environment. The detached-shell source R Scl, displaying CO
emission from recent mass loss, in a binary-induced spiral structure as well as
in a clumpy shell produced during a thermal pulse, provides a unique laboratory
for studying the differences in CO isotope abundances throughout its recent
evolution. We observed both the 12CO(J=3-2) and the 13CO(J=3-2) line using
ALMA. We find significant variations in the 12CO/13CO intensity ratios and
consequently in the abundance ratios. The average CO isotope abundance ratio is
at least a factor three lower in the shell (~19) than that in the present-day
(60). Additionally, variations in the ratio of more
than an order of magnitude are found in the shell itself. We attribute these
variations to the competition between selective dissociation and isotope
fractionation in the shell, of which large parts cannot be warmer than ~35 K.
However, we also find that the 12CO/13CO ratio in the present-day mass loss is
significantly higher than the 12C/13C ratio determined in the stellar
photosphere from molecular tracers (~19). The origin of this discrepancy is
still unclear, but we speculate that it is due to an embedded source of
UV-radiation that is primarily photo-dissociating 13CO. This radiation source
could be the hitherto hidden companion. Alternatively, the UV-radiation could
originate from an active chromosphere of R Scl itself....Comment: 6 pages, 5 figures, online data available at
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/A+A/556/L
The endogenous TLR-4 ligands MRP8/14 as biomarkers of inflammation in Familial Mediterranean Fever (FMF)
Ellipsoidal primary of the RS CVn binary zeta And: Investigation using high-resolution spectroscopy and optical interferometry
We have obtained high-resolution spectroscopy, optical interferometry, and
long-term broad band photometry of the ellipsoidal primary of the RS CVn-type
binary system zeta And. Based on the optical interferometry the apparent limb
darkened diameter of zeta And is 2.55 +/- 0.09 mas using a uniform disk fit.
The Hipparcos distance and the limb-darkened diameter obtained with a uniform
disk fit give stellar radius of 15.9 +/- 0.8 Rsolar, and combined with
bolometric luminosity, it implies an effective temperature of 4665 +/- 140 K.
The temperature maps obtained from high resolution spectra using Doppler
imaging show a strong belt of equatorial spots and hints of a cool polar cap.
The equatorial spots show a concentration around the phase 0.75. This spot
configuration is reminiscent of the one seen in the earlier published
temperature maps of zeta And. Investigation of the Halpha line reveals both
prominences and cool clouds in the chromosphere. Long-term photometry spanning
12 years shows hints of a spot activity cycle, which is also implied by the
Doppler images, but the cycle length cannot be reliably determined from the
current data.Comment: 9 pages, 9 figures, accepted for A&
What causes the large extensions of red-supergiant atmospheres? Comparisons of interferometric observations with 1-D hydrostatic, 3-D convection, and 1-D pulsating model atmospheres
We present the atmospheric structure and the fundamental parameters of three
red supergiants, increasing the sample of RSGs observed by near-infrared
spectro-interferometry. Additionally, we test possible mechanisms that may
explain the large observed atmospheric extensions of RSGs.
We carried out spectro-interferometric observations of 3 RSGs in the
near-infrared K-band with the VLTI/AMBER instrument at medium spectral
resolution. To comprehend the extended atmospheres, we compared our
observational results to predictions by available hydrostatic PHOENIX,
available 3-D convection, and new 1-D self-excited pulsation models of RSGs.
Our near-infrared flux spectra are well reproduced by the PHOENIX model
atmospheres. The continuum visibility values are consistent with a
limb-darkened disk as predicted by the PHOENIX models, allowing us to determine
the angular diameter and the fundamental parameters of our sources.
Nonetheless, in the case of V602 Car and HD 95686, the PHOENIX model
visibilities do not predict the large observed extensions of molecular layers,
most remarkably in the CO bands. Likewise, the 3-D convection models and the
1-D pulsation models with typical parameters of RSGs lead to compact
atmospheric structures as well, which are similar to the structure of the
hydrostatic PHOENIX models. They can also not explain the observed decreases in
the visibilities and thus the large atmospheric molecular extensions. The full
sample of our RSGs indicates increasing observed atmospheric extensions with
increasing luminosity and decreasing surface gravity, and no correlation with
effective temperature or variability amplitude, which supports a scenario of
radiative acceleration on Doppler-shifted molecular lines.Comment: Accepted for publication in A&
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