The structure of the winds and coronae of O stars derived from H-alpha line-profile analyses

Studies of the emission-line spectra of hot stars have led to the conclusion that there are high-temperature regions or coronae in the stellar winds. To test this idea, the effects of coronal regions on H-alpha line profiles are studied by using the Sobolev escape-probability method. The temperature is assumed to rise rapidly at the base to coronal values and then, after a small radial distance, to decrease owing to radiative recombination. Such a structure gives rise to P Cygni lines that have flat emission components. The analysis is applied to Zeta Ori, O9.7 lb, and it is found that the corona must have a small thickness, less than 10% of the stellar radius. The velocity structure of the wind is deduced from a fit of the theoretical to the observed line profile. The velocity does not increase steadily outward, but reaches a plateau and then at about 2.5 stellar radii experiences a second rapid acceleration to terminal speeds. If the temperature above the coronal regions has cool values appropriate for a gas in radiative equilibrium, good agreement is obtained with mass-loss rates derived for O stars from ultraviolet line-profile and IR-excess analyses. If the temperature in the wind is of order 200,000 K, mass-loss rates larger than the single-scattering theoretical limit are required. It is argued that hard radiation from a small coronal zone can produce the high stages of ionization such as O VI observed in the winds of hot stars.