Radiation Driven Atmospheres of O-type Stars — Synthetic UV-spectra of Consistent Atmospheric Models as a Spectroscopic Tool

Abstract

Spectroscopic analyses with the purpose of interpreting the UV‐spectra of O stars provide a powerful tool for the determination of the stellar parameters, the chemical composition and the ionizing fluxes of O‐type stars. To realize this objective, realistic atmospheric models are required, which fully reproduce the observed high resolution spectra of hot stars via consistently calculated synthetic spectra, and which thus provide energy distributions in the EUV and ionizing fluxes in a detailed and correct way. Examples of already initiated work with the new generation of tools for 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.Since the physics of the atmospheres of hot stars are strongly affected by velocity gradients from the expansion of the atmosphere that dominate the spectra at all wavelength ranges, hydrodynamic model atmospheres for O‐type stars are necessary as basis for the synthesis and analysis of the spectra. It is shown that recent advances in the modeling of stellar winds driven by radiation pressure have made it possible to fit the wind‐sensitive features in the UV‐spectra of hot stars almost completely, opening the way for a hydrodynamically consistent determination of stellar radii, masses, luminosities and abundances from the UV spectrum alone. In detail we discuss applications of the diagnostic techniques by example of two of the most luminous O supergiants in the Galaxy.Furthermore, as stellar winds are a common feature of hot luminous stars, not only young stars with masses above 20 M⊙ are affected by the corresponding physics, but also low mass central stars of planetary nebulae in the advanced stages of post‐AGB evolution. Hence, predictions from post‐AGB evolutionary calculations can be tested quantitatively as well. In this context, surprising consequences of using the new generation of state‐of‐the‐art model atmospheres for the analysis of a sample of Central Stars of Planetary Nebulae are finally presented.