2,882 research outputs found
Revised element abundances for WC-type central stars
According to previous spectral analyses of Wolf-Rayet type central stars,
late [WC] subtypes show systematically higher carbon-to-helium abundance ratios
than early [WC] subtypes. If this were true, it would rule out that these stars
form an evolutionary sequence. However, due to the different parameter domains
and diagnostic lines, one might suspect systematic errors being the source of
this discrepancy. In an ongoing project we are therefore checking the [WC]
analyses by means of the last generation of non-LTE models for expanding
stellar atmospheres which account for line-blanketing and wind clumping. So
far, the abundance discrepancy is not resolved. Further element abundances (H,
N, Fe) are determined and compared with evolutionary predictions.Comment: 4 pages, 5 figures, in conference proceedings of "Planetary Nebulae
in our Galaxy and Beyond" IAU Symposion 234, 2006, editors: Michael J.
Barlow, Roberto H. M\'ende
Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates
Context: The mass-loss rate is a key parameter of massive stars. Adequate
stellar atmosphere models are required for spectral analyses and mass-loss
determinations. Present models can only account for the inhomogeneity of
stellar winds in the approximation of small-scale structures that are optically
thin. This treatment of ``microclumping'' has led to reducing empirical
mass-loss rates by factors of two and more. Aims: Stellar wind clumps can be
optically thick in spectral lines. We investigate how this ``macroclumping''
impacts on empirical mass-loss rates. Methods: The Potsdam Wolf-Rayet (PoWR)
model atmosphere code is generalized in the ``formal integral'' to account for
clumps that are not necessarily optically thin. Results: Optically thick clumps
reduce the effective opacity. This has a pronounced effect on the emergent
spectrum. Our modeling for the O-type supergiant zeta Puppis reveals that the
optically thin H-alpha line is not affected by wind porosity, but that the PV
resonance doublet becomes significantly weaker when macroclumping is taken into
account. The reported discrepancies between resonance-line and
recombination-line diagnostics can be resolved entirely with the macroclumping
modeling without downward revision of the mass-loss rate. Conclusions:
Mass-loss rates inferred from optically thin emission, such as the H-alpha line
in O stars, are not influenced by macroclumping. The strength of optically
thick lines, however, is reduced because of the porosity effects. Therefore,
neglecting the porosity in stellar wind modeling can lead to underestimating
empirical mass-loss rates.Comment: A&A (in press), see full abstract in the tex
Wind-Interaction Models for the Early Afterglows of Gamma-Ray Bursts: The Case of GRB 021004
Wind-interaction models for gamma-ray burst afterglows predict that the
optical emission from the reverse shock drops below that from the forward shock
within 100s of seconds of the burst. The typical frequency of the
synchrotron emission from the forward shock passes through the optical band
typically on a timescale of minutes to hours. Before the passage of ,
the optical flux evolves as and after the passage, the decay
steepens to , where is the exponent for the assumed
power-law energy distribution of nonthermal electrons and is typically . The steepening in the slope of temporal decay should be readily
identifiable in the early afterglow light curves. We propose that such a
steepening was observed in the R-band light curve of GRB 021004 around day 0.1.
Available data at several radio frequencies are consistent with this
interpretation, as are the X-ray observations around day~1. The early evolution
of GRB 021004 contrasts with that of GRB 990123, which can be described by
emission from interaction with a constant density medium.Comment: 16 pages, 1 figure, submitted to ApJ
The metallicity dependence of WR winds
Wolf-Rayet (WR) stars are the most advanced stage in the evolution of the
most massive stars. The strong feedback provided by these objects and their
subsequent supernova (SN) explosions are decisive for a variety of
astrophysical topics such as the cosmic matter cycle. Consequently,
understanding the properties of WR stars and their evolution is indispensable.
A crucial but still not well known quantity determining the evolution of WR
stars is their mass-loss rate. Since the mass loss is predicted to increase
with metallicity, the feedback provided by these objects and their spectral
appearance are expected to be a function of the metal content of their host
galaxy. This has severe implications for the role of massive stars in general
and the exploration of low metallicity environments in particular. Hitherto,
the metallicity dependence of WR star winds was not well studied. In this
contribution, we review the results from our comprehensive spectral analyses of
WR stars in environments of different metallicities, ranging from slightly
super-solar to SMC-like metallicities. Based on these studies, we derived
empirical relations for the dependence of the WN mass-loss rates on the
metallicity and iron abundance, respectively.Comment: 5 pages, 4 figures, to be published in the Proceedings of the IAU
Symposium No. 329 "The lives and death-throes of massive stars
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