1,282 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
VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), sigma Oph, BM Sco, and HD 206859
We add four warmer late-type supergiants to our previous
spectro-interferometric studies of red giants and supergiants.
V766 Cen (=HR 5171 A) is found to be a high-luminosity log(L/L_sun)=5.8+-0.4
source of Teff 4290+-760 K and radius 1490+-540 Rsun located close to both the
Hayashi and Eddington limits; this source is consistent with a 40 Msun
evolutionary track without rotation and current mass 27-36 Msun. It exhibits
NaI in emission arising from a shell of radius 1.5 Rphot and a photocenter
displacement of about 0.1 Rphot. V766 Cen shows strong extended molecular (CO)
layers and a dusty circumstellar background component. This suggest an
optically thick pseudo-photosphere at about 1.5 Rphot at the onset of the wind.
V766 Cen is a red supergiant located close to the Hayashi limit instead of a
yellow hypergiant already evolving back toward warmer Teff as previously
discussed.
The stars sigma Oph, BM Sco, and HD 206859 are found to have lower
luminosities of about log(L/Lsun)=3.4-3.5 and Teff of 3900-5300 K,
corresponding to 5-9 Msun tracks. They do not show extended molecular layers as
observed for higher luminosity red supergiants of our sample. BM Sco shows an
unusually strong contribution by an over-resolved circumstellar dust component.
These stars are more likely high-mass red giants instead of red supergiants.
This leaves us with an unsampled locus in the HR diagram corresponding to
luminosities log(L/Lsun)~3.8-4.8 or masses 10-13 Msun, possibly corresponding
to the mass region where stars explode as type II-P supernovae during the RSG
stage.
Our previously found relation of increasing strength of extended molecular
layers with increasing luminosities is now confirmed to extend to double our
previous luminosities and up to the Eddington limit. This might further point
to steadily increasing radiative winds with increasing luminosity.
[Abridged]Comment: 16 pages, 14 figures, accepted for publication in Astronomy and
Astrophysics (A&A
VLTI/AMBER spectro-interferometric imaging of VX Sgr's inhomogenous outer atmosphere
Aims. We aim to explore the photosphere of the very cool late-type star VX
Sgr and in particular the existence and characterization of molecular layers
above the continuum forming photosphere. Methods. We obtained interferometric
observations with the VLTI/AMBER interferometer using the fringe tracker FINITO
in the spectral domain 1.45-2.50 micron with a spectral resolution of about 35
and baselines ranging from 15 to 88 meters.We perform independent image
reconstruction for different wavelength bins and fit the interferometric data
with a geometrical toy model.We also compare the data to 1D dynamical models of
Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG)
and asymptotic giant branch (AGB) stars. Results. Reconstructed images and
visibilities show a strong wavelength dependence. The H-band images display two
bright spots whose positions are confirmed by the geometrical toy model. The
inhomogeneities are qualitatively predicted by 3D simulations. At about 2,00
micron and in the region 2,35 - 2,50 micron, the photosphere appears extended
and the radius is larger than in the H band. In this spectral region, the
geometrical toy model locates a third bright spot outside the photosphere that
can be a feature of the molecular layers. The wavelength dependence of the
visibility can be qualitatively explained by 1D dynamical models of Mira
atmospheres. The best-fitting photospheric models show a good match with the
observed visibilities and give a photospheric diameter of theta = 8,82+-0,50
mas. The H2O molecule seems to be the dominant absorber in the molecular
layers. Conclusions. We show that the atmosphere of VX Sgr rather resembles
Mira/AGB star model atmospheres than RSG model atmospheres. In particular, we
see molecular (water) layers that are typical for Mira stars.Comment: 9 Pages, Accepted for publication on Astronomy & Astrophysics, two
references update
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&
From the Circumnuclear Disk in the Galactic Center to thick, obscuring tori of AGNs
We compare three different models of clumpy gas disk and show that the
Circumnuclear Disk (CND) in the Galactic Center and a putative, geometrically
thick, obscuring torus are best explained by a collisional model consisting of
quasi-stable, self-gravitating clouds. Kinetic energy of clouds is gained by
mass inflow and dissipated in cloud collisions. The collisions give rise to a
viscosity in a spatially averaged gas dynamical picture, which connects them to
angular momentum transport and mass inflow. It is found that CND and torus
share the same gas physics in our description, where the mass of clouds is 20 -
50 M_sun and their density is close to the limit of disruption by tidal shear.
We show that the difference between a transparent CND and an obscuring torus is
the gas mass and the velocity dispersion of the clouds. A change in gas supply
and the dissipation of kinetic energy can turn a torus into a CND-like
structure and vice versa. Any massive torus will naturally lead to sufficiently
high mass accretion rates to feed a luminous AGN. For a geometrically thick
torus to obscure the view to the center even super-Eddington accretions rates
with respect to the central black hole are required.Comment: 9 pages, no figures. Accepted for publication in A&
The dynamic atmospheres of Mira stars: comparing the CODEX models to PTI time series of TU And
Our comprehension of stellar evolution on the AGB still faces many
difficulties. To improve on this, a quantified understanding of large-amplitude
pulsator atmospheres and interpretation in terms of their fundamental stellar
parameters are essential. We wish to evaluate the effectiveness of the recently
released CODEX dynamical model atmospheres in representing M-type Mira
variables through a confrontation with the time-resolved spectro-photometric
and interferometric PTI data set of TU And. We calibrated the interferometric
K-band time series to high precision. This results in 50 nights of
observations, covering 8 subsequent pulsation cycles. At each phase, the flux
at 2.2m is obtained, along with the spectral shape and visibility points
in 5 channels across the K-band. We compared the data set to the relevant
dynamical, self-excited CODEX models. Both spectrum and visibilities are
consistently reproduced at visual minimum phases. Near maximum, our
observations show that the current models predict a photosphere that is too
compact and hot, and we find that the extended atmosphere lacks H2O opacity.
Since coverage in model parameter space is currently poor, more models are
needed to make firm conclusions on the cause of the discrepancies. We argue
that for TU And, the discrepancy could be lifted by adopting a lower value of
the mixing length parameter combined with an increase in the stellar mass
and/or a decrease in metallicity, but this requires the release of an extended
model grid.Comment: 5 pages, 5 figures, accepted for publication in A&A Letter
J, H, K spectro-interferometry of the Mira variable S Orionis
Aims: We present J, H, K interferometry with a spectral resolution of 35 for
the Mira variable S Orionis. We aim at measuring the diameter variation as a
function of wavelength that is expected due to molecular layers lying above the
continuum-forming photosphere.
Methods: Visibility data of S Ori were obtained at phase 0.78 with the
VLTI/AMBER instrument using the fringe tracker FINITO at 29 spectral channels
between 1.29 and 2.32 mu. Apparent uniform disk (UD) diameters were computed
for each spectral channel. In addition, the visibility data were directly
compared to predictions by recent self-excited dynamic model atmospheres.
Results: S Ori shows significant variations in the visibility values as a
function of spectral channel that can only be described by a clear variation in
the apparent angular size with wavelength. The closure phase values are close
to zero at all spectral channels, indicating the absence of asymmetric
intensity features. The apparent UD angular diameter is smallest at about 1.3
and 1.7 mu and increases by a factor of ~1.4 around 2.0 mu. The minimum UD
angular diameter is 8.1 pm 0.5 mas, corresponding to ~420 R_sun. The S Ori
visibility data and the apparent UD variations can be explained reasonably well
by a dynamic atmosphere model that includes molecular layers.
Conclusions: The measured visibility and UD diameter variations with
wavelength resemble and generally confirm the predictions by recent dynamic
model atmospheres. [abridged]Comment: 4 pages including 1 table and 4 color figures. Accepted for
publication as a Letter in Astronomy and Astrophysic
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