Consistent Models of the Expanding Atmospheres of Hot Star

Adalbert Pauldrach
Ludwig-Maximilian-University, Munich

The quantitative spectroscopic analysis of the UV spectra of hot stars provides a powerful tool for the determination of the stellar parameters, the chemical composition, and the ionizing fluxes of O-type stars, but it requires the construction of detailed atmospheric models along with consistently calculated synthetic spectra that fully reproduce the observed high-resolution spectra. As will be shown, such a tool is becoming more and more relevant to current astronomical research as the spectral analysis of hot, luminous stars is of growing astrophysical interest. Examples of already-initiated work with a new generation of tools for the quantitative UV spectroscopy of hot stars that have been developed during the last two decades are presented, and the status of the continuing effort to construct realistic models for hot star atmospheres is reviewed.The models presented are based on the concept of homogeneous, stationary, and spherically symmetric radiation-driven winds, where the expansion of the atmosphere is due to the scattering and absorption of photons by Doppler-shifted metal lines. Despite the simplifying assumptions of homogeneity, stationarity, and spherical symmetry, developing such a method is nevertheless not straightforward, since modeling the atmospheres of hot stars involves replicating a tightly interwoven mesh of physical processes: the equations of radiation hydrodynamics including the energy equation, the equations of statistical equilibrium for all important ions based on detailed atomic physics, and the radiative transfer equation at all transition frequencies have to be solved simultaneously.

Essential steps which form the basis of the theoretical framework developed will be described, and special emphasis will be given to the effect complicating the system the most: the overlap of spectral lines induced by the velocity field of the expanding atmosphere, which shifts, at different radii, up to 1000 spectral lines of different ions into resonance at each frequency. Since the behavior of most of the UV spectral lines depends critically on a detailed and consistent description of this effect, particular attention will be given to the correct treatment of the Doppler-shifted line radiation transport of the metal lines, the corresponding coupling to the conservation of energy, and the back-reaction on the radiative rates in the statistical equilibrium.

Finally, after almost two decades of work, the potential of the now available improved method will be tested via an application to two of the most luminous O-stars in the Galaxy, and it will be investigated whether the UV diagnostic techniques applied in this analysis lead to an agreement of observed and synthetic spectra that can be regarded as a measure of excellent quality.

Presentation (PDF File)

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