Mass Loss from T Tauri Stars.

The broad emission lines of H and Ca II in the spectra of T Tauri stars have been interpreted in terms of a simplified model for the mass4oss phenomenon assumed to be taking place. The expanding envelope is taken to be spherically symmetric, with the ejected atoms all having the same initial velocity and subjected only to gravitational forces. The emitting volume, essentially an H ii region, is surrounded by an absorbing shell moving radially outward that produces the violet-displaced absorption components. A fit of computed line profiles to observed contours then gives the particle density at the surface of the star. This value combined with estimates of the stellar temperature and radius gives the present rate of mass loss. For six representative stars with varying emission4ine strengths these rates range from 0 3 to 5.8 X 1O me , with an average of 3.7 X 10-8 me year '. No definite mechanism is proposed for the mass ejection, although it seems reasonable that a flux of high-energy protons postulated to account for the large extent of the ionixed envelope may also be indirectly responsible for the ejection. From the statistics of T Tauri stars, it is estimated that at any one time approximately 40 per cent of the contracting stars may be losing mass. A preliminary analysis of the effect of mass ejection on the time scale of the contraction, making use of Iben's theoretical tracks, indicates that mass ejection may be of some significance. Although it produces only about a 10 per cent decrease in the contraction time, the energy involved may at times be comparable to the radiated luminosity. Furthermore, a 1.0-me star may altogether lose as much as 0 4 me, a non-negligible amount. It would seem that the effect of mass ejection should be considered in future computations of evolutionary tracks.