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F. Najarro, R.P. Kudritzki, J.P. Cassinelli, O. Stahl, and D.J. Hillier
The strong excess of radiation in the Lyman- and Hei-continuum of the B-giant {epsilon} CMa , detected very recently with EUVE (Cassinelli et al.), 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 Hei- 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.
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