Optically Thin Stellar Winds in Early-Type Stars

The role of the radiation field in the heating, cooling, and transfer of momentum to expanding atmospheres of hot stars is examined. The general equations for radial, steady-state flow are presented, and the conditions for the existence of a transonic flow are discussed. Numerical solutions are carried out for a case in which the run of mean intensity is assumed to be that of freely streaming radiation. Limits are deduced for the ratio of the velocity of escape to the thermal speed at the sonic point. True absorptive opacity is found to be essential for the transition to supersonic flow. If this opacity is sufficiently large, the temperature stratification deviates negligibly from that in radiative equilibrium. The outward force due to radiation pressure must be only slightly less than the inward force of gravity in order to obtain sonic points reasonably close to the star. Nevertheless, ignoring the probable disruption by the interstellar medium, all stars should have stellar winds and the rate of mass loss may be as large as order (L/ ). A two-point boundaryvalue technique is presented which ensures that the atmosphere approaches the usual static behavior at the base and becomes transonic at a larger radius. Subject headings: early-type stars - stellar winds