STELLAR WINDS AND THE EUV CONTINUUM EXCESS OF EARLY B-GIANTS

The strong excess of radiation in the Lyman- and He I-continuum of the B-giant epsilon CMa, detected very recently with EWE (Cassinelli et a l. 1994), is partially explained as a consequence of transonic velocit y fields of weak stellar winds. Velocity induced density changes or Do ppler-shifts reduce the opacity and increase the escape probability in the resonance transitions of hydrogen and neutral helium at the depth s of formation of the ground state continua. As a result the ground st ate occupation numbers and the corresponding absorption edges are redu ced significantly, and the emergent Lyman- and He I- fluxes are enhanc ed. A small sequence of simple model atmospheres for B-giants includin g spherical extension and stellar winds is calculated to demonstrate t he importance of this effect and to investigate its dependence on the mass-loss rate. It is shown that the number of hydrogen and helium ion izing photons of early B-giants is a strong function of the mass-loss rate. This might have consequences for the diffuse ionizing radiation field in galaxies.