Astron. Astrophys. 355, 979-993 (2000)

1. Introduction

Interstellar and circumstellar maser emission from the water molecule has been detected in a large number of objects (cataloged by Cesaroni et al. 1988; Brand et al. 1994). The most extensively studied H₂O masers are the transitions at  cm (22 GHz), frequently found in the circumstellar envelopes produced by mass loss from evolved stars. Early VLBI observations were done by Spencer et al. (1979); many later studies have used the Very Large Array (VLA) in A configuration (e.g Johnston et al. 1985). Other H₂O maser lines down to submillimeter wavelengths have been also detected (e.g. Menten & Young 1995).

Chapman & Cohen (1986) concluded from their study of the supergiant VX Sgr that the SiO, H₂O, and OH masers form a natural sequence at increasing radii from the star. Several authors have found that, in a few objects, the H₂O maser emission is consistent with the expanding shell models (Spencer et al. 1979; Lane et al. 1987 on IK Tau; Diamond et al. 1987). This was also reported by Reid & Menten (1990), who were able to detect as well the photosphere of W Hya, providing the stellar radius and, for the first time, could locate the circumstellar maser emission with respect to the central star. Nevertheless, in other objects there is evidence of non-spherical geometric distributions, where the standard expanding shell model fails to fit the H₂O maser data (Johnston et al. 1985; Lane et al. 1987; Bowers et al. 1993; Bowers & Johnston 1994; Yates & Cohen 1994). Careful mapping of the position and velocity of the individual maser features is necessary to illuminate this point, and new approaches of data analysis are needed particularly for studies with instruments of moderate angular resolution like the VLA.

It is also important to know whether the extent of the masing region increases with mass-loss rate, as this is predicted by model pumping calculations (Cooke & Elitzur 1985). Water maser regions have been reported to show sizes in the range  cm (or 6 AU; Spencer et al. 1979) to more than  cm ( AU; Johnston et al. 1985; Yates & Cohen 1994). Lane et al. (1987) find that H₂O masers in Mira-type stars are produced within regions of diameter 9 - 108 AU, while for supergiant stars maser shell diameters of 300 - 720 AU are observed. These results are consistent with those of Bowers et al. (1993). An empirical correlation between H₂O maser region size and mass-loss rate has been found by Lane et al. (1987) and Yates & Cohen (1994).

Most of the mentioned studies, however cannot provide information on the sizes of individual spots due to lack of angular resolution (the VLA is unable to resolve the spots, as its minimum fringe spacing is ). Spots were reported to be of the order of  cm from VLBI data (Spencer et al. 1979; Diamond et al. 1987); however recent VLBI observations estimate that compact structures almost an order of magnitude smaller (e.g  cm; Imai et al. 1997) exist.

An interesting problem that remains to be solved is to establish where the H₂O masers appear in the expanding circumstellar envelope. The existing data suggest that the distance from the star at which these masers arise is smaller than the distance at which dust is formed (measured by infrared interferometers, or by the thermal emission of SiO). This can be explained if the density is large enough inside the dust formation point, and it is compatible with the moderate velocities seen in these masers.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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