207 research outputs found
The theory of stellar winds
We present a brief overview of the theory of stellar winds with a strong
emphasis on the radiation-driven outflows from massive stars. The resulting
implications for the evolution and fate of massive stars are also discussed.
Furthermore, we relate the effects of mass loss to the angular momentum
evolution, which is particularly relevant for the production of long and soft
gamma-ray bursts. Mass-loss rates are not only a function of the metallicity,
but are also found to depend on temperature, particularly in the region of the
bi-stability jump at 21 000 Kelvin. We highlight the role of the bi-stability
jump for Luminous Blue Variable (LBV) stars, and discuss suggestions that LBVs
might be direct progenitors of supernovae. We emphasize that radiation-driven
wind studies rely heavily on the input opacity data and linelists, and that
these are thus of fundamental importance to both the mass-loss predictions
themselves, as well as to our overall understanding of the lives and deaths of
massive stars.Comment: 6 pages, invited review Astrophysics and Space Science, Vol 336,
Issue 1, pp. 163-167 (special HEDLA 2010 Issue
NLTE wind models of hot subdwarf stars
We calculate NLTE models of stellar winds of hot compact stars (central stars
of planetary nebulae and subdwarf stars). The studied range of subdwarf
parameters is selected to cover a large part of these stars. The models predict
the wind hydrodynamical structure and provide mass-loss rates for different
abundances. Our models show that CNO elements are important drivers of subdwarf
winds, especially for low-luminosity stars. We study the effect of X-rays and
instabilities on these winds. Due to the line-driven wind instability, a
significant part of the wind could be very hot.Comment: 7 pages, to appear in Astrophysics and Space Science. The final
publication will be available at springerlink.com
The Eddington factor as the key to understand the winds of the most massive stars. Evidence for a Γ-dependence of Wolf-Rayet type mass loss
First Stars. II. Evolution with mass loss
The first stars are assumed to be predominantly massive. Although, due to the
low initial abundances of heavy elements the line-driven stellar winds are
supposed to be inefficient in the first stars, these stars may loose a
significant amount of their initial mass by other mechanisms.
In this work, we study the evolution with a prescribed mass loss rate of very
massive, galactic and pregalactic, Population III stars, with initial
metallicities and , respectively, and initial masses
100, 120, 150, 200, and 250 during the hydrogen and helium burning
phases.
The evolution of these stars depends on their initial mass, metallicity and
the mass loss rate. Low metallicity stars are hotter, compact and luminous, and
they are shifted to the blue upper part in the Hertzprung-Russell diagram. With
mass loss these stars provide an efficient mixing of nucleosynthetic products,
and depending on the He-core mass their final fate could be either
pair-instability supernovae or energetic hypernovae. These stars contributed to
the reionization of the universe and its enrichment with heavy elements, which
influences the subsequent star formation properties.Comment: Accepted for publication in Astrophysics & Space Science. 15 pages,
18 figure
The VLT-FLAMES survey of massive stars: Wind properties and evolution of hot massive stars in the LMC
[Abridged] We have studied the optical spectra of 28 O- and early B-type
stars in the Large Magellanic Cloud, 22 of which are associated with the young
star-forming region N11. Stellar parameters are determined using an automated
fitting method, combining the stellar atmosphere code FASTWIND with the
genetic-algorithm optimisation routine PIKAIA. Results for stars in the LH9 and
LH10 associations of N11 are consistent with a sequential star formation
scenario, in which activity in LH9 triggered the formation of LH10. Our sample
contains four stars of spectral type O2, of which the hottest is found to be
~49-54 kK (cf. ~45-46 kK for O3 stars). The masses of helium-enriched dwarfs
and giants are systematically lower than those implied by non-rotating
evolutionary tracks. We interpret this as evidence for efficient
rotationally-enhanced mixing, leading to the surfacing of primary helium and to
an increase of the stellar luminosity. This result is consistent with findings
for SMC stars by Mokiem et al. For bright giants and supergiants no such
mass-discrepancy is found, implying that these stars follow tracks of modestly
(or non-)rotating objects. Stellar mass-loss properties were found to be
intermediate to those found in massive stars in the Galaxy and the SMC, and
comparisons with theoretical predictions at LMC metallicity yielded good
agreement over the luminosity range of our targets, i.e. 5.0 < log L/L(sun) <
6.1
Galactic Plane H Surveys: IPHAS & VPHAS+
The optical Galactic Plane H surveys IPHAS and VPHAS+ are
dramatically improving our understanding of Galactic stellar populations and
stellar evolution by providing large samples of stars in short lived, but
important, evolutionary phases, and high quality homogeneous photometry and
images over the entire Galactic Plane. Here I summarise some of the
contributions these surveys have already made to our understanding of a number
of key areas of stellar and Galactic astronomy.Comment: 5 pages, 2 figures, refereed proceeding of the "The Universe of
Digital Sky Surveys" conference, November 2014, to be published in the
Astrophysics and Space Science Proceeding
Thermal radiation processes
We discuss the different physical processes that are important to understand
the thermal X-ray emission and absorption spectra of the diffuse gas in
clusters of galaxies and the warm-hot intergalactic medium. The ionisation
balance, line and continuum emission and absorption properties are reviewed and
several practical examples are given that illustrate the most important
diagnostic features in the X-ray spectra.Comment: 37 pages, 16 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 9; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
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