Volume 487, Number 2, August IV 2008
|Page(s)||645 - 657|
|Section||Stellar structure and evolution|
|Published online||16 June 2008|
On the reliability of mass-loss-rate estimates for AGB stars*
Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden e-mail: email@example.com
2 Onsala Space Observatory, 439 92 Onsala, Sweden
3 European Southern Observatory, Casilla 19001, Santiago 19, Chile
Accepted: 30 May 2008
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.
Key words: stars: AGB and post-AGB / stars: carbon / stars: late-type / stars: mass-loss
© ESO, 2008
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