Mass-losing Semiregular Variable Stars in Baade's Windows
Genzel, R.; Alard, C.; Blommaert, J. A. D. L.; Cesarsky, C.; Epchtein, N.; Felli, M.; Fouque, P.; Ganesh, S.; Gilmore, G.; Glass, I. S.; Habing, H.; Omont, A.; Perault, M.; Price, S.; Robin, A.; Schultheis, M.; Simon, G.; van Loon, J. Th.; Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Becker, A. C.; Bennett, D. P.; Cook, K. H.; Drake, A. J.; Freeman, K. C.; Geha, M.; Griest, K.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Nelson, C.; Peterson, B. A.; Popowski, P.; Pratt, M. R.; Quinn, P. J.; Sutherland, W.; Tomaney, A. B.; Vandehei, T.; Welch, D. L.
France, Spain, Germany, Italy, Chile, India, United Kingdom, South Africa, Netherlands, United States, Australia, Canada
Abstract
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 logP-Mbol relation or set of such relations. Approximate mass-loss rates M in the range of ~1×10-8 to 5×10-7 Msolar 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 M~TαLβ, where α=-8.80+0.96-0.24 and β=+1.74+0.16-0.24, 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 M~L2.7, valid for AGB stars with 10-8<M<10-4 Msolar yr-1.