Methods for determining AGB mass loss rates based on radio data

Abstract

In the radio regime the mass-loss rate of AGB stars is best probed using molecular (and atomic) line emission arising in the CSE formed by the stellar wind. The numerical modelling of the circumstellar emission where intricate interplays between physical and chemical processes take place, is a challenge. The derived mass-loss rates depend crucially on the assumptions in the circumstellar model, of which some can be constrained if enough observational data exist. Therefore, a reliable mass-loss-rate determination for an individual star requires, in addition to a detailed radiative transfer analysis, good observational constraints in the form of multi-line observations and radial brightness distributions. Of the methods used to estimate mass-loss rates from galactic AGB stars those based on radiative transfer modelling of CO line emission are most commonly used and possibly also the most accurate. Typically, CO multi-transitional observations can constrain the mass-loss rate to better than 50%, within the adopted circumstellar model. Comparison with complementary methods, such as estimates based on dust radiative transfer modelling coupled with a dynamical model, are consistent within a factor of three.