COMPLETE STELLAR MODELS: SPECTRAL AND INTERIOR EVOLUTION OF MASSIVE STARS

Abstract

ABSTRACT This thesis work presents the first "complete stellar models" for massive stars, which consistently treat the stellar interior, the atmosphere, and the stellar winds. This approach allows to simultaneously predict basic stellar parameters (luminosity, radii, temperatures), nucleosynthesis (abundances), as well as the detailed emergent spectrum through the relevant evolutionary phases (corresponding to OB, LBV and Wolf--Rayet stars). On the other hand, our modelling including the stellar winds also allows to study the influence of the outer layers on the stellar structure and evolution. Conceptually the thesis is divided in two main parts. In the first part we construct the first non-LTE line blanketed hydrodynamic models of spherically expanding atmospheres of hot stars. The entire domain from the optically thick photosphere out to the terminal velocity of the wind is treated. We discuss in detail the effects of line blanketing on the atmospheric structure and on the predicted spectrum. We study the influence of the hydrodynamic structure on the profiles of both photospheric and wind lines. Our results also show that for precise determinations of stellar parameters and abundances of hot luminous stars, the use of plane parallel models may lead to systematic errors. In the second part we develop the "complete stellar models" (CoStar). As a first application we study the main sequence (MS) interior and spectral evolution of massive stars at solar metallicity. The evolutionary tracks and the interior evolution are found to be basically unchanged by the realistic treatment of the outer layers. The main CoStar predictions presented and discussed for the MS are the following: 1. Ejected mass of the most important elements. Deposition of wind momentum and mechanical energy 2. Estimates of mass loss rates due to radiation pressure including multiple scattering and line overlap 3. Continuous spectral energy distribution (EUV to IR) and ionising fluxes 4. UBVRIJHKLMN photometric evolution 5. UV colours 6. Detailed metal line blanketed UV spectra 7. Non-LTE hydrogen and helium line spectra in the visible and IR, including theoretical K band spectra 8. Fit formulae to derive mass loss rates from H-alpha, P-alpha and B-alpha equivalent width measurements As a second application we present Wolf-Rayet (WR) models with spherically outflowing envelopes. We introduce a simple analytical method to describe the structure of a spherically expanding envelope with strong mass outflow. Independently of the wind parameters (wind density, opacity, velocity law) the interior structure and evolution of WR stars is found to be unaffected by the outer layers. On the other hand, the stellar parameters (radii, effective temperatures) depend on the wind structure. We derive subphotospheric radii (taucont ~10-20) for WNE and WC/WO stars with strong mass loss rates. With respect to wind-free stellar models the predicted radius increase may reach up to a factor of ~4 for the most luminous WNE or WC stars, improving the comparisons with observations. The stellar parameters obtained with the new treatment allow a better assignment of theoretical spectra to evolutionary tracks of evolved WR stars (WNE, WC). This provides the base for future studies of the spectral evolution of post main-sequence massive stars and their descendants. We also point out the possible importance of the iron opacity peak for the acceleration of WR winds in the optically thick part, which may be essential for the understanding of the dynamics of WR winds.