138 research outputs found
MESAS: Measuring the Emission of Stellar Atmospheres at Submm/mm wavelengths
In the early stages of planet formation, small dust grains grow to become mm
sized particles in debris disks around stars. These disks can in principle be
characterized by their emission at submillimeter and millimeter wavelengths.
Determining both the occurrence and abundance of debris in unresolved
circumstellar disks of A-type main-sequence stars requires that the stellar
photospheric emission be accurately modeled. To better constrain the
photospheric emission for such systems, we present observations of Sirius A, an
A-type star with no known debris, from the JCMT, SMA, and VLA at 0.45, 0.85,
0.88, 1.3, 6.7, and 9.0 mm. We use these observations to inform a PHOENIX model
of Sirius A's atmosphere. We find the model provides a good match to these data
and can be used as a template for the submm/mm emission of other early A-type
stars where unresolved debris may be present. The observations are part of an
ongoing observational campaign entitled Measuring the Emission of Stellar
Atmospheres at Submm/mm wavelengths (MESAS)Comment: 17 pages, 1 figure, Accepted to AJ on April 25th 201
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
The Mass of the Black Hole in Cygnus X-1
Cygnus X-1 is a binary star system that is comprised of a black hole and a
massive giant companion star in a tight orbit. Building on our accurate
distance measurement reported in the preceding paper, we first determine the
radius of the companion star, thereby constraining the scale of the binary
system. To obtain a full dynamical model of the binary, we use an extensive
collection of optical photometric and spectroscopic data taken from the
literature. By using all of the available observational constraints, we show
that the orbit is slightly eccentric (both the radial velocity and photometric
data independently confirm this result) and that the companion star rotates
roughly 1.4 times its pseudosynchronous value. We find a black hole mass of M
=14.8\pm1.0 M_{\sun}, a companion mass of M_{opt}=19.2\pm1.9 M_{\sun}, and the
angle of inclination of the orbital plane to our line of sight of i=27.1\pm0.8
deg.Comment: Paper II of three papers on Cygnus X-1; 27 pages including 5 figures
and 3 tables, ApJ in pres
Tests of stellar model atmospheres by optical interferometry: VLTI/VINCI limb-darkening measurements of the M4 giant psi phe
We present K-band interferometric measurements of the limb-darkened intensity
profile of the M4 giant star psi Phoenicis obtained with VLTI/VINCI.
High-precision squared visibility amplitudes in the second lobe of the
visibility function were obtained employing two 8.2 m UTs. This succeeded one
month after light from UTs was first combined for interferometric fringes. In
addition, we sampled the visibility function at small spatial frequencies using
the 40cm test siderostats. Our measurement constrains the diameter of the star
as well as its CLV. We construct a spherical hydrostatic PHOENIX model
atmosphere based on spectrophotometric data from the literature and confront
its CLV prediction with our interferometric measurement. We compare as well CLV
predictions by plane-parallel hydrostatic PHOENIX, ATLAS9, and ATLAS12 models.
We find that the Rosseland angular diameter as predicted by comparison of the
spherical PHOENIX model with spectrophotometry is in good agreement with our
interferometric diameter measurement. The shape of our measured visibility
function in the second lobe is consistent with all considered PHOENIX and ATLAS
model predictions, and significantly different from UD and FDD models. We
derive high-precision fundamental parameters for psi Phe, namely a Rosseland
angular diameter of 8.13 +/- 0.2 mas, with the Hipparcos parallax corresponding
to a Rosseland linear radius R of 86 +/- 3 Rsun and an effective temperature of
3550 +/- 50 K, with R corresponding to a luminosity of log (L/Lsun)=3.02 +/-
0.06. Together with evolutionary models, these values are consistent with a
mass of 1.3 +/- 0.2 Msun, and a surface gravity of log g = 0.68 +/- 0.11.Comment: 13 pages, 6 figures, accepted for publication in A&
- …