103 research outputs found
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 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
Hubble Space Telescope Observations of SV Cam: II. First Derivative Lightcurve Modelling using PHOENIX and ATLAS Model Atmospheres
The variation of the specific intensity across the stellar disc is essential
input parameter in surface brightness reconstruction techniques such as Doppler
imaging, where the relative intensity contributions of different surface
elements are important in detecting starspots. We use PHOENIX and ATLAS model
atmospheres to model lightcurves derived from high precision (S/N ~ 5000) HST
data of the eclipsing binary SV Cam (F9V + K4V), where the variation of
specific intensity across the stellar disc will determine the contact points of
the binary system lightcurve. For the first time we use chi^2 comparison fits
to the first derivative profiles to determine the best-fitting model
atmosphere. We show the wavelength dependence of the limb darkening and that
the first derivative profile is sensitive to the limb-darkening profile very
close to the limb of the primary star. It is concluded that there is only a
marginal difference (< 1sigma) between the chi^2 comparison fits of the two
model atmospheres to the HST lightcurve at all wavelengths. The usefulness of
the second derivative of the light-curve for measuring the sharpness of the
primary's limb is investigated, but we find that the data are too noisy to
permit a quantitative analysis.Comment: 10 pages, 20 figures, accepted by MNRA
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&
Extended envelopes around Galactic Cepheids III. Y Oph and alpha Per from near-infrared interferometry with CHARA/FLUOR
Unbiased angular diameter measurements are required for accurate distances to
Cepheids using the interferometric Baade Wesselink method (IBWM). The precision
of this technique is currently limited by interferometric measurements at the
1.5% level. At this level, the center-to-limb darkening (CLD) and the presence
of circumstellar envelopes (CSE) seem to be the two main sources of bias. The
observations we performed aim at improving our knowledge of the interferometric
visibility profile of Cepheids. In particular, we assess the systematic
presence of CSE around Cepheids in order determine accurate distances with the
IBWM free from CSE biased angular diameters. We observed a Cepheid (Y Oph) for
which the pulsation is well resolved and a non-pulsating yellow supergiant
(alpha Per) using long-baseline near-infrared interferometry. We interpreted
these data using a simple CSE model we previously developed. We found that our
observations of alpha Per do not provide evidence for a CSE. The measured CLD
is explained by an hydrostatic photospheric model. Our observations of Y Oph,
when compared to smaller baseline measurements, suggest that it is surrounded
by a CSE with similar characteristics to CSE found previously around other
Cepheids. We have determined the distance to Y Oph to be d=491+/-18 pc.
Additional evidence points toward the conclusion that most Cepheids are
surrounded by faint CSE, detected by near infrared interferometry: after
observing four Cepheids, all show evidence for a CSE. Our CSE non-detection
around a non-pulsating supergiant in the instability strip, alpha Per, provides
confidence in the detection technique and suggests a pulsation driven mass-loss
mechanism for the Cepheids.Comment: accepted for publication in Ap
Resolving Vega and the inclination controversy with CHARA/MIRC
Optical and infrared interferometers definitively established that the
photometric standard Vega (alpha Lyrae) is a rapidly rotating star viewed
nearly pole-on. Recent independent spectroscopic analyses could not reconcile
the inferred inclination angle with the observed line profiles, preferring a
larger inclination. In order to resolve this controversy, we observed Vega
using the six-beam Michigan Infrared Combiner on the Center for High Angular
Resolution Astronomy Array. With our greater angular resolution and dense
(u,v)-coverage, we find Vega is rotating less rapidly and with a smaller
gravity darkening coefficient than previous interferometric results. Our models
are compatible with low photospheric macroturbulence and also consistent with
the possible rotational period of ~0.71 days recently reported based on
magnetic field observations. Our updated evolutionary analysis explicitly
incorporates rapid rotation, finding Vega to have a mass of 2.15+0.10_-0.15
Msun and an age 700-75+150 Myrs, substantially older than previous estimates
with errors dominated by lingering metallicity uncertainties
(Z=0.006+0.003-0.002).Comment: Accepted for publication in ApJ Letter
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