Synthetic Spectra for Type Ia Supernovae at Early Epochs

We present the current status of our construction of synthetic spectra for type Ia supernovae. These properly take into account the effects of NLTE and an adequate representation of line blocking and blanketing. The models are based on a sophisticated atomic database. We show that the synthetic spectrum reproduces the observed spectrum of 'normal' SN-Ia near maximum light from the UV to the near-IR. However, further improvements are necessary before truly quantitative analyses of observed SN-Ia spectra can be performed. In particular, the inner boundary condition has to be fundamentally modified. This is due to the dominance of electron scattering over true absorption processes coupled with the flat density structure in these objectsComment: To appear in "Proceedings of the IAU Colloquium 192 - Supernovae (10 Years of SN1993J)", eds. J.M. Marcaide and K.W. Weile

Simulation of stellar instabilities with vastly different timescales using domain decomposition

Strange mode instabilities in the envelopes of massive stars lead to shock waves, which can oscillate on a much shorter timescale than that associated with the primary instability. The phenomenon is studied by direct numerical simulation using a, with respect to time, implicit Lagrangian scheme, which allows for the variation by several orders of magnitude of the dependent variables. The timestep for the simulation of the system is reduced appreciably by the shock oscillations and prevents its long term study. A procedure based on domain decomposition is proposed to surmount the difficulty of vastly different timescales in various regions of the stellar envelope and thus to enable the desired long term simulations. Criteria for domain decomposition are derived and the proper treatment of the resulting inner boundaries is discussed. Tests of the approach are presented and its viability is demonstrated by application to a model for the star P Cygni. In this investigation primarily the feasibility of domain decomposition for the problem considered is studied. We intend to use the results as the basis of an extension to two dimensional simulations.Comment: 15 pages, 10 figures, published in MNRA

The VLT-FLAMES survey of massive stars: observations in the Galactic clusters NGC3293, NGC4755 and NGC6611

We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi-Element Spectrograph (FLAMES) instrument at the Very Large Telescope (VLT). Here we present observations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph (R ~ 25,000), in fields centered on the open clusters NGC 3293, NGC 4755 and NGC 6611. These data are supplemented by a further 50 targets observed with the Fibre-Fed Extended Range Optical Spectrograph (FEROS, R = 48,000). Following a description of our scientific motivations and target selection criteria, the data reduction methods are described; of critical importance the FLAMES reduction pipeline is found to yield spectra that are in excellent agreement with less automated methods. Spectral classifications and radial velocity measurements are presented for each star, with particular attention paid to morphological peculiarities and evidence of binarity. These observations represent a significant increase in the known spectral content of NGC 3293 and NGC 4755, and will serve as standards against which our subsequent FLAMES observations in the Magellanic Clouds will be compared.Comment: 26 pages, 9 figures (reduced size). Accepted for publication in A&A. A copy with full res. figures is available from http://www.ing.iac.es/~cje/flames_mw.ps.gz. Minor changes following correction of proof

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 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

Mass-loss rates of Very Massive Stars

We discuss the basic physics of hot-star winds and we provide mass-loss rates for (very) massive stars. Whilst the emphasis is on theoretical concepts and line-force modelling, we also discuss the current state of observations and empirical modelling, and address the issue of wind clumping.Comment: 36 pages, 15 figures, Book Chapter in "Very Massive Stars in the Local Universe", Springer, Ed. Jorick S. Vin

The physics of the massive stars in our galaxy and parent galaxies of different metallicity Final report

Available from TIB Hannover: F01B1609 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany); DLR Deutsches Zentrum fuer Luft- und Raumfahrt e.V., Bonn (Germany)DEGerman

Hot luminous stars in nearby galaxies

We discuss a new method for the determination of stellar distances using the dependence of the stellar mass-loss rate upon luminosity, the so-called wind momentum - luminosity relationship. We review some recent results on wind momentum rates and metallicities of O-stars and B-supergiants in the Magellanic Clouds and include some new results derived using HST data. Clear evidence for a metallicity effect in the Small Magellanic Cloud is found, the effect for the Large Magellanic Cloud being only moderate. We discuss the need to calibrate the relationship using supergiants in these galaxies so that the method may be applied to supergiants in more distant galaxies. The potential of this method with regard to both distance and metallicity determinations is illustrated by results for two supergiants in each of the galaxies M31 and M33. We also show new HST/WFPC2 images of massive O-stars in the Magellanic Clouds and, in combination with previous optical and HST/FOS spectra, suggest that these data constitute evidence that one of these stars, Sk-67 211, may have a mass of close to 200 M_sun. (orig.)31 refs.Available from TIB Hannover: RR 4697(925) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The X-ray emission from shock cooling zones in O star winds

A semi-empirical model is developed for the X-ray emission from O star winds, and used to analyze recent ROSAT PSPC spectra. The X-rays are assumed to originate from cooling zones behind shock fronts, where the cooling is primarily radiative at small radii in the wind, and due to expansion at large radii. The shocks are dispersed in a cold background wind whose X-ray opacity is provided by detailed NLTE calculations. This model is a natural extension of the Hillier et al. (1993) model of isothermal wind shocks. By assuming spatially constant shock temperatures, these authors achieved good fits to the data only by postulating two intermixed shock families of independent temperature and filling factor - i.e., by adjusting in parallel four parameters. By applying the present model to the analysis of high S/N PSPC spectra of three O-stars (#zeta# Pup, #iota# Ori, #zeta# Ori), we achieve fits of almost the same quality with only two parameters. This supports the idea that the two- or multi-component X-ray spectra are indeed due to stratified cooling layers. (orig.)36 refs.Available from TIB Hannover: RR 4697(985) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman