Infrared photometry and evolution of mass-losing AGB stars. III. Mass loss rates of MS and S stars

摘要

Context. The asymptotic giant branch (AGB) phase marks the end of theevolution for low- and intermediate-mass stars, which are fundamentalcontributors to the mass return to the interstellar medium and to the chemicalevolution of galaxies. The detailed understanding of mass loss processes ishampered by the poor knowledge of the luminosities and distances of AGB stars.Aims. In a series of papers we are trying to establish criteria permitting amore quantitative determination of luminosities for the various types of AGBstars, using the infrared (IR) fluxes as a basis. An updated compilation of themass loss rates is also required, as it is crucial in our studies of theevolutionary properties of these stars. In this paper we concentrate ouranalysis on the study of the mass loss rates for a sample of galactic S stars.Methods. We reanalyze the properties of the stellar winds for a sample ofgalactic MS, S, SC stars with reliable estimates of the distance on the basisof criteria previously determined. We then compare the resulting mass lossrates with those previously obtained for a sample of C-rich AGB stars. Results.Stellar winds in S stars are on average less efficient than those of C-rich AGBstars of the same luminosity. Near-to-mid infrared colors appear to be crucialin our analysis. They show a good correlation with mass loss rates inparticular for the Mira stars. We suggest that the relations between the ratesof the stellar winds and both the near-to-mid infrared colors and the periodsof variability improve the understanding of the late evolutionary stages of lowmass stars and could be the origin of the relation between the rates of thestellar winds and the bolometric magnitudes.