Other
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........22c&link_type=abstract
PhD Dissertation, Delaware Univ. Wilmington, DE United States
Other
22
X Rays, Hot Stars, Stellar Winds, Stellar Models, Shock Wave Propagation, Thermal Emission, Absorbers (Materials), Attenuation, Emission, Energy Budgets, Energy Dissipation, Luminosity, Main Sequence Stars, Massive Stars
Scientific paper
The radiatively-driven winds of massive stars have been known for fifteen years to emit large fluxes of X-rays, but the mechanisms and even the locations of X-ray emission remain uncertain. Because these winds are known to be unstable on both theoretical and observational grounds, the most likely source of X-rays is wind material heated by the shocks that result from this instability. However, testing this hypothesis requires a model of the amount of hot material in the wind, of emission from that material, and of the attenuation by overlying wind material. To date, models of X-ray emission have assumed the existence of shocks with specified characteristics, while models of the wind driving force and wind instabilities have treated the wind as isothermal. I have developed a more comprehensive model that treats both the development of shocks and the resulting X-ray emission within a single framework. This model extends the time-dependent dynamical models of Owocki, Castor, and Rybicki (1988) to include energy balance, in order to study the hot, X-ray emitting regions. It incorporates the current version of the thermal emission code of J. Raymond (Raymond and Smith 1977) to calculate radiative energy losses, and to calculate the X-ray spectrum emitted by the hot regions shown in the wind model. Finally, the model makes simple estimates of X-ray attenuation within the wind, using X-ray cross sections from Morrison and McCammon (1983) I have modeled X-ray emission in a high density wind from a massive star such as that of zeta Pup (O4(n)f) and in a low density wind typical of a main sequence star such as tau Sco (BO V) or mu Col (09.5 V). In general, the instability of the wind line force generates many shocks, but only a small fraction of these are strong enough to heat shocked material to X-ray emitting temperatures. The modeled X-ray luminosity Lx can easily match, or even exceed, the observed value for the high density wind, since the emission measure inferred from observation is a small fraction of the entire wind. However, the observed Lx for the less dense winds of later-type main sequence stars is so high that a substantial fraction of the entire wind must be hot; this does not occur in the models unless the base of the wind undergoes large (25%) fluctuations in density or another parameter. In this dissertation, I describe the development and testing of the model; in particular, I discuss difficulties that arise in trying to resolve the hot postshock cooling regions well, and approaches I have chosen to get around this difficulty. I give results for models of low density winds and of high density winds, and compare model spectra to the observed spectra of tau Sco, mu Col, and zeta Pup.
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