1,628 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
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
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
557 GHz Observations of Water Vapor Outflows from VY CMa and W Hydrae
We report the first detection of thermal water vapor emission in the 557 GHz,
ground state transition of ortho-HO toward VY Canis
Majoris. In observations obtained with the Submillimeter Wave Astronomy
Satellite (SWAS), we measured a flux of Jy, in a spectrally resolved
line centered on a velocity km s with a full width half
maximum of km s, somewhat dependent on the assumed line shape.
We analyze the line shape in the context of three different radial outflow
models for which we provide analytical expressions. We also detected a weaker
557 GHz emission line from W Hydrae. We find that these and other HO
emission line strengths scale as suggested by Zubko and Elitzur (2000).Comment: Astrophysical Journal Letters, accepte
Movement disorders and syndromic autism: a systematic review
Movement disorders are reported in idiopathic autism but the extent to which comparable movement disorders are found in syndromic/co-morbid autism is unknown. A systematic search of Medline, Embase, PsychINFO and CINAHL on the prevalence of specific movement disorder in syndromic autism associated with specific genetic syndromes identified 16 papers, all relating to Angelman syndrome or Rett syndrome. Prevalence rates of 72.7–100% and 25.0–27.3% were reported for ataxia and tremor, respectively, in Angelman syndrome. In Rett syndrome, prevalence rates of 43.6–50% were reported for ataxia and 27.3–48.3% for tremor with additional reports of dystonia, rigidity and pyramidal signs. However, reliable assessment measures were rarely used and recruitment was often not described in sufficient detail
Tests of stellar model atmospheres by optical interferometry III: NPOI and VINCI interferometry of the M0 giant gamma Sge covering 0.5 - 2.2 microns
Aims: We present a comparison of the visual and NIR intensity profile of the
M0 giant gamma Sagittae to plane-parallel ATLAS 9 as well as to plane-parallel
& spherical PHOENIX model atmospheres. Methods: We use previously described
visual interferometric data obtained with the NPOI in July 2000. We apply the
recently developed technique of coherent integration, and thereby obtain
visibility data of more spectral channels and with higher precision than
before. In addition, we employ new measurements of the K-band diameter of gamma
Sagittae obtained with the instrument VINCI at the VLTI in 2002. Results: The
spherical PHOENIX model leads to a precise definition of the Rosseland angular
diameter and a consistent high-precision diameter value for our NPOI and
VLTI/VINCI data sets of Theta_Ross=6.06 pm 0.02 mas, with the Hipparcos
parallax corresponding to R_Ross=55 pm 4 R_sun, and with the bolometric flux
corresponding to an effective temperature T_eff=3805 pm 55 K. Our visual
visibility data close to the first minimum and in the second lobe constrain the
limb-darkening effect and are generally consistent with the model atmosphere
predictions. The visual closure phases exhibit a smooth transition between 0
and pi. Conclusions: The agreement between the NPOI and VINCI diameter values
increases the confidence in the model atmosphere predictions from optical to
NIR wavelengths as well as in the calibration and accuracy of both
interferometric facilities. The consistent night-by-night diameter values of
VINCI give additional confidence in the given uncertainties. The closure phases
suggest a slight deviation from circular symmetry, which may be due to surface
features, an asymmetric extended layer, or a faint unknown companion.Comment: 12 pages, 9 figures, accepted by A&A. Also available from
http://www.aanda.org/articles/aa/pdf/forth/aa5853_06.pd
Elastic properties of thin h-BN films investigated by Brillouin light scattering
Hexagonal BN films have been deposited by rf-magnetron sputtering with simultaneous ion plating. The elastic properties of the films grown on silicon substrates under identical coating conditions have been de-termined by Brillouin light scattering from thermally excited surface phonons. Four of the five independent elastic constants of the deposited material are found to be c11 = 65 GPa, c13 = 7 GPa, c33 = 92 GPa and c44 = 53 GPa exhibiting an elastic anisotropy c11/c33 of 0.7. The Young's modulus determined with load indenta-tion is distinctly larger than the corresponding value taken from Brillouin light scattering. This discrepancy is attributed to the specific morphology of the material with nanocrystallites embedded in an amorphous matrix
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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