13 research outputs found
The Chemical Composition of Red Giants, AGB Stars and Planetary Nebulae
The determinations of element abundances in red-giant stars and in particular
in AGB stars are reviewed and the resulting abundances are compared with those
obtained for planetary nebulae in the Galaxy and in nearby galaxies. The
problems, possibilities and implications of such comparisons when estimating
yields from low-mass and intermediate-mass stars are illustrated and commented
on.Comment: 8 pages, 1 figure, to appear in proceedings of IAU Symposium 234,
"Planetary Nebulae in Our Galaxy and Beyond," eds. R. Mendez & M. Barlo
Thresholds for the Dust Driven Mass Loss from C-rich AGB Stars
It is well established that mass loss from AGB stars due to dust driven winds
cannot be arbitrarily low. We model the mass loss from carbon rich AGB stars
using detailed frequency-dependent radiation hydrodynamics including dust
formation. We present a study of the thresholds for the mass loss rate as a
function of stellar parameters based on a subset of a larger grid of such
models and compare these results to previous observational and theoretical
work. Furthermore, we demonstrate the impact of the pulsation mechanism and
dust formation for the creation of a stellar wind and how it affects these
thresholds and briefly discuss the consequences for stellar evolution.Comment: 2 pages, 1 figure. To be published in the proceedings of IAU Symp.
241 on Stellar Populations as Building Blocks of Galaxies, ed. A. Vazdekis et
al. (2007). Replaced to match edited versio
Dust driven mass loss from carbon stars as a function of stellar parameters - I. A grid of Solar-metallicity wind models
[Abridged] We have computed a grid of 900 numeric dynamic model atmospheres
(DMAs) using a well-tested computer code. This grid of models covers most of
the expected combinations of stellar parameters, which are made up of the
stellar temperature, the stellar luminosity, the stellar mass, the abundance of
condensible carbon, and the velocity amplitude of the pulsation. The resultant
mass-loss rates and wind speeds are clearly affected by the choice of stellar
temperature, mass, luminosity and the abundance of available carbon. In certain
parts of the parameter space there is also an inevitable mass-loss threshold,
below which a dust-driven wind is not possible. Contrary to some previous
studies, we find a strong dependence on the abundance of free carbon, which
turns out to be a critical parameter. Furthermore, we have found that the dust
grains that form in the atmosphere may grow too large for the commonly used
small particle approximation of the dust opacity to be strictly valid. This may
have some bearing on the wind properties, although further study of this
problem is needed before quantitative conclusions can be drawn. The wind
properties show relatively simple dependences on stellar parameters above the
mass-loss threshold, while the threshold itself is of a more complicated
nature. Hence, we chose not to derive any simplistic mass-loss formula, but
rather provide a mass-loss prescription in the form of an easy-to-use FORTRAN
routine. Since this mass-loss routine is based on data coming from an
essentially self-consistent model of mass loss, it may therefore serve as a
better mass-loss prescription for stellar evolution calculations than empirical
formulae. Furthermore, we conclude that there are still some issues that need
to be investigated, such as the role of grain-sizes.Comment: 27 pages, 11 figures, to appear in Astronomy & Astrophysics.
Corrections by language editor included in this new versio
X-RED: A Satellite Mission Concept To Detect Early Universe Gamma Ray Bursts
Gamma ray bursts (GRBs) are the most energetic eruptions known in the
Universe. Instruments such as Compton-GRO/BATSE and the GRB monitor on BeppoSAX
have detected more than 2700 GRBs and, although observational confirmation is
still required, it is now generally accepted that many of these bursts are
associated with the collapse of rapidly spinning massive stars to form black
holes. Consequently, since first generation stars are expected to be very
massive, GRBs are likely to have occurred in significant numbers at early
epochs. X-red is a space mission concept designed to detect these extremely
high redshifted GRBs, in order to probe the nature of the first generation of
stars and hence the time of reionisation of the early Universe. We demonstrate
that the gamma and x-ray luminosities of typical GRBs render them detectable up
to extremely high redshifts (z~10-30), but that current missions such as HETE2
and SWIFT operate outside the observational range for detection of high
redshift GRB afterglows. Therefore, to redress this, we present a complete
mission design from the science case to the mission architecture and payload,
the latter comprising three instruments, namely wide field x-ray cameras to
detect high redshift gamma-rays, an x-ray focussing telescope to determine
accurate coordinates and extract spectra, and an infrared spectrograph to
observe the high redshift optical afterglow. The mission is expected to detect
and identify for the first time GRBs with z > 10, thereby providing constraints
on properties of the first generation of stars and the history of the early
Universe.Comment: 14 pages, 10 figures, spie.cls neede
FLUORINE ABUNDANCES IN GALACTIC ASYMPTOTIC GIANT BRANCH STARS
An analysis of the fluorine abundance in Galactic asymptotic giant branch (AGB) carbon stars (24 N-type, 5 SC-type, and 5 J-type) is presented. This study uses the state-of-the-art carbon-rich atmosphere models and improved atomic and molecular line lists in the 2.3 ÎŒm region. Significantly lower F abundances are obtained in comparison to previous studies in the literature. This difference is mainly due to molecular blends. In the case of carbon stars of SC-type, differences in the model atmospheres are also relevant. The new F enhancements are now in agreement with the most recent theoretical nucleosynthesis models in low-mass AGB stars, solving the long-standing problem of F in Galactic AGB stars. Nevertheless, some SC-type carbon stars still show larger F abundances than predicted by stellar models. The possibility that these stars are of larger mass is briefly discussed