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 mum) 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-M-bol relation or set of such relations. Approximate mass-loss rates (M) over dot in the range of similar to 1x10(-8) to 5 x 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 AGE stars in the mass-loss ranges that we observe, the functional dependence of mass-loss rate on temperature and luminosity can be expressed as (M) over dot proportional to (TLbeta)-L-alpha, where a = -8.80(-0.24)(+0.96) and beta = + 1.74(-0.24)(+0.16), in agreement with recent theoretical predictions. If we include our mass-loss rates with a sample of extreme mass-losing AGE stars in the Large Magellanic Cloud and ignore T as a variable, we get the general result for AGE stars that (M) over dot proportional to L-2.7, valid for AGB stars with 10(-8) < (M) over dot ( 10(-4) M-. yr(-1).