19 research outputs found
Fundamental Parameters of He-Weak and He-Strong Stars
We carried out low resolution spectroscopic observations in the wavelength
range 3400-4700 A of 20 He-weak and 8 He-strong stars to determine their
fundamental parameters by means of the Divan-Chalonge-Barbier (BCD)
spectrophotometric system. For a few He-weak stars we also estimate the
effective temperatures and the angular diameters by integrating absolute fluxes
observed over a wide spectral range. Non-LTE model calculations are carried out
to study the influence of the He/H abundance ratio on the emergent radiation of
He-strong stars and on their Teff determination. We find that the effective
temperatures, surface gravities and bolometric absolute magnitudes of He-weak
stars estimated with the BCD system and the integrated flux method are in good
agreement between each other, and they also agree with previous determinations
based on several different methods. The mean discrepancy between the visual
absolute magnitudes derived using the Hipparcos parallaxes and the BCD values
is on average 0.3 mag for He-weak stars, while it is 0.5 mag for He-strong
stars. For He-strong stars, we note that the BCD calibration, based on stars in
the solar environment, leads to overestimated values of Teff. By means of model
atmosphere calculations with enhanced He/H abundance ratios we show that larger
He/H ratios produce smaller BD which naturally explains the Teff
overestimation. We take advantage of these calculations to introduce a method
to estimate the He/H abundance ratio in He-strong stars. The BD of HD 37479
suggests that the Teff of this star remains fairly constant as the star
spectrum undergoes changes in the intensity of H and He absorption lines. Data
for the He-strong star HD 66765 are reported for the first time.Comment: Accepted for publication in A&
Calibration Observations of Fomalhaut with the VLTI
An investigation of the stability of the transfer function of the European
Southern Observatory's Very Large Telescope Interferometer has been carried out
through observations of Fomalhaut over a wide range in hour angle. No
significant variation in the transfer function was found for the zenith angle
range 5-70 degrees. The projected baseline varied between 139.7 m and 49.8 m
during the observations and, as an integral part of the determination of the
transfer function, a new accurate limb-darkened angular diameter for Fomalhaut
has been established. This has led to improved values for the emergent flux,
effective temperature, radius and luminosity.Comment: 6 pages, 4 figures, uses an.cls, accepted for publication in Astron.
Nach
Infrared Imaging of SDSS Quasars: Implications for the Quasar K correction
We have imaged 45 quasars from the Sloan Digital Sky Survey (SDSS) with
redshifts 1.85 < z < 4.26 in JHKs with the KPNO SQIID imager. By combining
these data with optical magnitudes from the SDSS we have computed the restframe
optical spectral indices of this sample and investigate their relation to
quasar redshift. We find a mean spectral index of = -0.55+/-0.42 with
a large spread in values. We also find possible evolution of the form alpha_o =
(0.148+/-0.068)z - (0.964+/-0.200)$ in the luminosity range -28.0 < M_i <
-26.5. Such evolution suggests changes in the accretion process in quasars with
time and is shown to have an effect on computed quasar luminosity functions.Comment: 20 pages, 10 figures, 3 tables, accepted to the Astronomical Journa
New limb-darkening coefficients and synthetic photometry for model-atmosphere grids at Galactic, LMC, and SMC abundances
New grids of Atlas9 models have been calculated using revised convection
parameters and updated opacity-distribution functions, for chemical
compositions intended to be representative of solar, [M/H] = +0.3, +0.5, Large
Magellanic Cloud (LMC), and Small Magellanic Cloud (SMC) abundances. The grids
cover T(eff) = 3.5-50kK, from log(g) = 5.0 to the effective Eddington limit.
Limb-darkening coefficients and synthetic photometry are presented in the
UBVRIJHKLM, uvby, ugriz, WFCAM, Hipparcos/Tycho, and Kepler passbands for these
models, and for Castelli's comparable `new-ODF' grids. Flux distributions are
given for the new models. The sensitivity of limb-darkening coefficients to the
adopted physics is illustrated
Circumstellar material in the Vega inner system revealed by CHARA/FLUOR
Only a handful of debris disks have been imaged up to now. Due to the need
for high dynamic range and high angular resolution, very little is known about
the inner planetary region, where small amounts of warm dust are expected to be
found. We investigate the close neighbourhood of Vega with the help of infrared
stellar interferometry and estimate the integrated K-band flux originating from
the central 8 AU of the debris disk. We performed precise visibility
measurements at both short (~30 m) and long (~150 m) baselines with the FLUOR
beam-combiner installed at the CHARA Array (Mt Wilson, California) in order to
separately resolve the emissions from the extended debris disk (short
baselines) and from the stellar photosphere (long baselines). After revising
Vega's K-band angular diameter (3.202+/-0.005 mas), we show that a significant
deficit in squared visibility (1.88+/-0.34%) is detected at short baselines
with respect to the best-fit uniform disk stellar model. This deficit can be
either attributed to the presence of a low-mass stellar companion around Vega,
or as the signature of the thermal and scattered emissions from the debris
disk. We show that the presence of a close companion is highly unlikely, as
well as other possible perturbations (stellar morphology, calibration), and
deduce that we have most probably detected the presence of dust in the close
neighbourhood of Vega. The resulting flux ratio between the stellar photosphere
and the debris disk amounts to 1.29+/-0.19% within the FLUOR field-of-view
(~7.8 AU). Finally, we complement our K-band study with archival photometric
and interferometric data in order to evaluate the main physical properties of
the inner dust disk. The inferred properties suggest that the Vega system could
be currently undergoing major dynamical perturbations.Comment: A&A, accepted -- Press release available at
http://www2.cnrs.fr/presse/communique/848.ht
Radiative diffusion in stellar atmospheres: diffusion velocities
The present paper addresses some of the problems in the buildup of element
stratification in stellar magnetic atmospheres due to microscopic diffusion, in
particular the redistribution of momentum among the various ionisation stages
of a given element and the calculation of diffusion velocities in the presence
of inclined magnetic fields.
We have considerably modified and extended our CARAT code to provide
radiative accelerations, not only from bound-bound but also from bound-free
transitions. In addition, our code now computes ionisation and recombination
rates, both radiative and collisional. These rates are used in calculating the
redistribution of momentum among the various ionisation stages of the chemical
elements. A careful comparison shows that the two different theoretical
approaches to redistribution that are presently available lead to widely
discrepant results for some chemical elements, especially in the magnetic case.
In the absence of a fully satisfactory theory of redistribution, we propose to
use the geometrical mean of the radiative accelerations from both methods.
Diffusion velocities have been calculated for 28 chemical elements in a T_eff
= 12000K, log g = 4.00 stellar magnetic atmosphere with solar abundances.
Velocities and resulting element fluxes in magnetic fields are discussed; rates
of abundance changes are analysed for systematic trends with field strength and
field direction. Special consideration is given to the Si case and our results
are confronted in detail with well-known results derived more than two decades
ago.Comment: To be published in Astronomy & Astrophysics (accepted 02/03/2006