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Astron. Astrophys. 354, 125-134 (2000)

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1. Introduction

Asymptotic Giant Branch (AGB) stars develop strong mass loss at rates up to [FORMULA] [FORMULA] yr-1 or more. The optical light of AGB stars with the highest [FORMULA] is almost entirely absorbed by their dusty circumstellar envelopes (CSEs), and re-emitted at longer wavelengths. These obscured AGB stars become very bright infrared (IR) objects, outshining any other star in a galaxy except for a few red supergiants (RSGs). During a brief period in their lives, they loose 50 to 80% of their initial mass. This makes them important contributors to the chemical enrichment of the interstellar medium (ISM) - they are possibly the main sources of dust particles in the Universe.

Individual galaxies differ in their metal content as a result of different star formation histories. For instance, two of our nearest neighbours, the Large and Small Magellanic Clouds (LMC & SMC) have current metallicities a factor [FORMULA] and [FORMULA] less than the average metallicity in the Milky Way ([FORMULA]solar). When pursuing a quantitative description of the history of star formation and chemical enrichment within a galaxy, it is essential to correctly take into account the mass loss from AGB stars and RSGs. This requires the dust-to-gas ratios in their CSEs to be known.

The paper is organised as follows: in Sect. 2 formulae are derived for deriving mass-loss rates and dust-to-gas ratios from measurements of optical depths and either luminosities or expansion velocities. Sect. 3 introduces samples of obscured AGB stars in the MCs and in the Milky Way and addresses the near-IR colours of their circumstellar envelopes. Relative mass-loss rates and dust-to-gas ratios are determined in Sect. 4 for the magellanic and galactic circumstellar envelopes around obscured AGB stars, and the results are discussed in Sect. 5. In Appendix A new identifications of mass-losing AGB stars with IRAS point sources in the LMC are presented and in Appendix B expansion velocities are discussed.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000