On the reliability of mass-loss-rate estimates for AGB stars^*

¹ Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden e-mail: sofia@astro.su.se
² Onsala Space Observatory, 439 92 Onsala, Sweden
³ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 19 October 2007
Accepted: 30 May 2008

Abstract

Context. In the recent literature there has been some doubt as to the reliability of CO multi-transitional line observations as a mass-loss-rate estimator for AGB stars.

Aims. Using new well-calibrated CO radio line observations, the main aim of the work presented here is to carefully evaluate the reliability of CO mass-loss-rate estimates for intermediate- to high-mass-loss-rate AGB stars with different photospheric chemistries.

Methods. Mass-loss rates for 10 intermediate- to high-mass-loss-rate AGB stars are derived using a detailed non-LTE, non-local radiative transfer code based on the Monte-Carlo method to model the CO radio line intensities. The circumstellar envelopes are assumed to be spherically symmetric and formed by constant mass-loss rates. The energy balance is solved self-consistently and the effects of dust on the radiation field and thermal balance included. An independent estimate of the mass-loss rate is also obtained from the combination of dust radiative transfer modelling with a dynamical model of the gas and dust particles.

Results. We find that the CO radio line intensities and shapes are successfully reproduced for the majority of our objects when assuming a constant mass-loss rate. Moreover, the CO line intensities are only weakly dependent on the adopted micro-turbulent velocity, in contrast to recent claims in the literature. The two methods used in the present work to derive mass-loss rates are consistent within a factor of ~3 for intermediate- to high-mass-loss-rate objects, indicating that this is a lower limit to the uncertainty in present mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss rates from the dust/dynamical model are systematically higher than those from the CO model for the carbon stars and vice versa for the M-type stars. This could be ascribed to a discrepancy in the adopted CO/H₂-abundance ratio, but we caution that the sample is small and systematic errors cannot be excluded.