Physics
Scientific paper
Nov 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........45v&link_type=abstract
PhD Thesis, Universiteit Utrecht, 2000, 177 pages, ISBN 9039325596
Physics
8
Early-Type Stars, Mass Loss, Winds, Supergiants, Stellar Evolution
Scientific paper
Mass loss from OB stars is fundamental for the evolution of massive stars and starburst galaxies. An accurate description of mass loss as a function of stellar parameters is hence necessary. However, theoretical predictions of radiation-driven winds have so far not been able to match the observed mass-loss rates. Since the discrepancy grows towards larger wind densities, it has been suggested that including the process of ''multiple-scattering'' is pivotal for describing the winds of hot stars quantitatively. In this thesis, we investigate the importance of multiple line scattering for predicting the mass-loss rates of OB stars, and study the physics of the bi-stability jump. The method is based on a Monte Carlo approach to simulate the line and continuum scatterings. First, the bi-stability jump is investigated, and it is shown that the steep decrease in the ratio between the terminal wind velocity over the escape velocity around spectral type B1 is accompanied by a jump in the mass-loss rate of a factor of five. This results from a strong increase in the line force due to iron recombination around this temperature. Consequences of the bi-stability jump for B[e] supergiants and Luminous Blue Variables are also studied. Secondly, the mass-loss predictions are extended to stars with different masses, luminosities and metallicities. Subsequently, additional bi-stability jumps are discovered, as the winds of different metallicities turn out to be driven by different ionic species. The resulting mass-loss ``recipe'' is compared with the most reliable observed mass-loss rates up to date, and it is shown that the Monte Carlo approach that simulates multiple scatterings indeed resolves the previously reported systematic discrepancy between the observations and the theory of radiation-driven winds. We therefore recommend the mass-loss recipe be used in evolutionary calculations of massive stars in our own and starburst galaxies.
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