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Astron. Astrophys. 322, 291-295 (1997)

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1. Introduction

OH 39.7+1.5 (AFGL 2290) is an OH/IR star discovered as a bright infrared source by the AFGL-Infrared-Sky-Survey, and subsequently detected as a strong 1612 MHz OH maser by Allen et al. (1977). The star is a long-period variable with a period [FORMULA] days at a distance of [FORMULA] kpc (Van Langevelde et al. 1990), and has a stellar radial velocity inferred from the OH maser spectrum of 20 km s-1 (Slootmaker et al. 1985).

The water maser in OH 39.7+1.5 was discovered by Marques Dos Santos et al. (1979) and later remeasured by Nyman et al. (1986) and Engels et al. (1986). The latter found that the water maser profile varied in a unique manner, exhibiting a double-peaked profile similar to the 1612 MHz OH maser profiles during the bright phase of the star and a single line close to the stellar radial velocity while the star was dim. Lewis & Engels (1991) suggested that competitive gain between maser gain paths explains this behaviour. Consequently, OH 39.7+1.5 was considered as an important source for a new monitoring campaign, which we started in 1990 to study the H2 O maser variability in various types of stars (Engels et al. 1993). Fortunately, the maser spectrum showed the same kind of change again, enabling a closer study of this phenomenon with the aid of interferometric maps.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998