MASS-LOSING M SUPERGIANTS IN THE SOLAR NEIGHBORHOOD

We compile a list of the 21 mass-losing red supergiants (20 M type, one G type, L > 105 L⊙) within 2.5 kpc of the Sun. These supergiants are highly evolved descendants of main-sequence stars with initial masses larger than 20 M⊙. The surface density is between about 1 and 2 kpc-2. As found by previous authors, these stars are much less concentrated toward the Galactic center than W-R stars which are also highly evolved massive stars. Although with considerable uncertainty, we estimate that the mass return by the M supergiants is somewhere between 1×10-5 and 3×10-5 M⊙ kpc-2 yr-1. In the hemisphere facing the Galactic center there is much less mass loss from M supergiants than from W-R stars, but in the anticenter direction the M supergiants return more mass than do the W-R stars. The duration of the M supergiant phase appears to be between 2×105 and 4×105 yr. During this phase a star of initially at least 20 M⊙ returns perhaps 3-10 M⊙ into the interstellar medium.