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Session 12 - Stellar Evolution - Theory.
Display session, Wednesday, January 07
Exhibit Hall,
We model the full multi-wavelength spectrum, including the extreme ultraviolet (EUV) continuum of three early B giants, \epsilon CMa, \beta CMa, and \alpha Vir with spherical, non-LTE, fully line-blanketed, model atmospheres. Comparisons of these models to the spectrophotometric data, along with the HIPPARCOS parallax measurements, provide fundamental stellar parameters for these stars. We find close agreement between the model spectra and the measured EUV fluxes from \epsilon CMa and \beta CMa. The realistic treatment of early B giant atmospheres, with models including spherical geometry and NLTE metal line blanketing, results in the prediction of larger Lyman continuum fluxes, by up to a factor of two, than existing plane-parallel models for these stars. We present a grid of \tt PHOENIX NLTE model atmospheres of late O and early B stars and provide predictions for the hydrogen and helium ionizing fluxes as a function of effective temperature.
The increased EUV flux predicted for B stars has significant implications for our understanding of the ionization of the material surrounding these stars. We present photoionization models of the H II region around \alpha Vir, calculated using CLOUDY, with our model atmosphere as the input continuum. The distance to this star and its radius are known and so it is a well constrained system. We find that our model can reproduce the measured extent of the H II region, the H\alpha surface brightness, the [S II]\lambda6716/H\alpha ratio and the temperature of the region given by Reynolds (1988). We also estimate the amount of the ionizing radiation escaping from the surroundings and therefore available to ionize the diffuse ISM.
