Mass-losing Semiregular Variable Stars in Baade’s Windows

By cross-correlating the results of two recent large-scale surveys, the general properties of a well-defined sample of semiregular variable stars have been determined. ISOGAL mid-infrared photometry (7 and 15 μm) and MACHO V and R light curves are assembled for approximately 300 stars in the Baade's windows of low extinction toward the Galactic bulge. These stars are mainly giants of late M spectral type, evolving along the asymptotic giant branch (AGB). They are found to possess a wide and continuous distribution of pulsation periods and to obey an approximate log P-Mbol relation or set of such relations. Approximate mass-loss rates in the range of ~1 × 10-8 to 5 × 10-7 M☉ yr-1 are derived from ISOGAL mid-infrared photometry and models of stellar spectra adjusted for the presence of optically thin circumstellar silicate dust. Mass-loss rates depend on luminosity and pulsation period. Some stars lose mass as rapidly as short-period Mira variables but do not show Mira-like amplitudes. A period of 70 days or longer is a necessary but not sufficient condition for mass loss to occur. For AGB stars in the mass-loss ranges that we observe, the functional dependence of mass-loss rate on temperature and luminosity can be expressed as ∝ TαLβ, where α = -8.80 and β = +1.74, in agreement with recent theoretical predictions. If we include our mass-loss rates with a sample of extreme mass-losing AGB stars in the Large Magellanic Cloud and ignore T as a variable, we get the general result for AGB stars that ∝ L2.7, valid for AGB stars with 10-8 < < 10-4 M☉ yr-1.