Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles

Abstract

We aim to (1) set up simple and general analytical expressions to estimate\nmass-loss rates of evolved stars, and (2) from those calculate estimates for\nthe mass-loss rates of asymptotic giant branch (AGB), red supergiant (RSG), and\nyellow hypergiant stars in our galactic sample. Rotationally excited lines of\nCO are a very robust diagnostic in the study of circumstellar envelopes (CSEs).\nWhen sampling different layers of the CSE, observations of these molecular\nlines lead to detailed profiles of kinetic temperature, expansion velocity, and\ndensity. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame\nradiative transfer code that predicts CO line intensities in the CSEs of\nlate-type stars is used in deriving relations between stellar and\nmolecular-line parameters, on the one hand, and mass-loss rate, on the other.\nWe present analytical expressions for estimating the mass-loss rates of evolved\nstellar objects for 8 rotational transitions of the CO molecule, apply them to\nour extensive CO data set covering 47 stars, and compare our results to those\nof previous studies. Our expressions account for line saturation and resolving\nof the envelope, thereby allowing accurate determination of very high mass-loss\nrates. We argue that, for estimates based on a single rotational line, the\nCO(2-1) transition provides the most reliable mass-loss rate. The mass-loss\nrates calculated for the AGB stars range from 4x10^-8 Msun/yr up to 8x10^-5\nMsun/yr. For RSGs they reach values between 2x10^-7 Msun/yr and 3x10^-4\nMsun/yr. The estimates for the set of CO transitions allow time variability to\nbe identified in the mass-loss rate. Possible mass-loss-rate variability is\ntraced for 7 of the sample stars. We find a clear relation between the\npulsation periods of the AGB stars and their derived mass-loss rates, with a\nlevelling off at approx. 3x10^-5 Msun/yr for periods exceeding 850 days.\n