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 element of CMa, detected very recently with EUVE (Cassinelli et al. 1994), is partially explained as a consequence of transonic velocity fields of weak stellar winds. Velocity induced density changes or Doppler-shifts reduce the opacity and increase the escape probability in the resonance transitions of hydrogen and neutral helium at the depths of formation of the ground state continua. As a result the ground state occupation numbers and the corresponding absorption edges are reduced significantly, and the emergent Lyman- and He I-fluxes are enhanced. A small sequence of simple model atmospheres for B-giants including spherical extension and stellar winds is calculated to demonstrate the importance of this effect and to investigate its dependence on the mass-loss rate. It is shown that the number of hydrogen and helium ionizing 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. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman