Maser Emission as a Tool to Study Mass Loss from Evolved Stars

Abstract

The properties of circumstellar OH, H2O, and SiO masers which can be used to examine the mass loss process in red giant stars are discussed from an empiricist’s point of view. Two aspects of mass loss are considered: namely, the mass loss mechanism and the mass loss rate.Maser emission now provides good evidence that at radii ≳ 1000 AU the circumstellar envelopes of most oxygen-rich red giants can be modelled to first order as predominantly spherical shells whose dominant kinematic mode is radial expansion at a nearly constant velocity. This result implies that the mass loss mechanism does not produce large deviations from sphericity when averaged over the expansion timescale (~ 1000 y). More specific constraints are difficult to obtain because the observed distribution of maser emission depends not only on the distribution of gas produced by the mass loss process but also on many other factors, ranging from interaction of the outflowing gas with the ambient interstellar medium or radiation field to limitations imposed by variables of the observing program. An assessment is made of the impact of these factors on the interpretation (and associated uncertainties) of the data.The large data base provided by the many known circumstellar OH/H2O/SiO maser stars also can be used to examine possible connections between certain properties of maser emission and the stellar mass loss rate. Evidence for two such connections is presented: a) the outer radius of the OH, H2O, or SiO maser region appears to be related to the mass loss rate; b) the maser luminosity also can be influenced by the mass loss rate, but the dependence is different for each of these molecular species, providing a simple observational method to obtain a distance-independent mass loss rate by determining the flux ratio for the maser lines. These correlations support the scale of mass loss obtained from IR/CO data and can be used to derive an order-of-magnitude estimate of the mass loss rate for a given star. By combining these results with estimates of the number density of OH, H2O, and SiO maser stars in the solar neighborhood, it is concluded that long-period, oxygen-rich, Mira-type variable stars contribute at least 35% of all material recycled to the interstellar medium.KeywordsMass LossVery Long Baseline InterferometryMass Loss RateMaser EmissionStellar VelocityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.