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Astron. Astrophys. 337, 141-144 (1998)

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1. Introduction

Maser emission from evolved stars is associated with dusty, oxygen rich circumstellar envelopes (CSEs), mainly around red supergiants (RSGs) and stars at the tip of the Asymptotic Giant Branch (AGB) (Elitzur 1992; Habing 1996). The strongest maser lines originate from OH (at 1612 MHz), H2O (at 22 GHz), and SiO (at 43 and 86 GHz), located progressively deeper into the CSE. Hence these lines are used to trace the mass-loss history and velocity structure of the CSE, through their photon fluxes and line profiles (Chapman & Cohen 1986; Lewis 1989, 1990). Maser lines are also excellent tracers of the stellar space velocity and have been used to map the kinematics and mass distribution in our Galaxy (e.g. Lindqvist et al. 1992; Sevenster et al. 1995).

Circumstellar OH and SiO maser emission has recently been detected in the Large Magellanic Cloud (LMC) (Wood et al. 1986, 1992; van Loon et al. 1996, 1998a). Wood et al. (1992) argue that the outflow velocities in CSEs of OH/IR stars in the LMC are a factor two lower than in corresponding Milky Way stars, a difference they attribute to less efficient driving of the outflow caused by the lower metallicity in the LMC. However, Zijlstra et al. (1996) find the difference to be only 20 to 30%, from their analysis of the same data. Moreover, the SiO maser emission from the LMC RSG IRAS04553-6825 (van Loon et al. 1996) showed that in this case, the outflow velocity as derived from the OH maser profile was underestimated by more than a factor two. The relation between outflow velocity and metallicity is therefore still uncertain.

In the LMC, SiO maser emission is generally too faint to be detected, with IRAS04553-6825 the only successful detection to date. Stellar H2O maser emission has not been found outside the Galaxy before.

Here we present the discovery of the first extra-galactic source of circumstellar H2O maser emission - viz. the RSG IRAS04553-6825 in the LMC. We also present spectroscopic results that confirm the LMC metallicity of its stellar photosphere. We then discuss the implications for the structure of the CSE by comparing with the galactic red supergiant NML Cyg.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998