Astron. Astrophys. 347, 556-564 (1999)

1. Introduction

Supersoft X-ray binaries, highly luminous ( ergs s^-1) and low-temperature (20-40 eV) X-ray sources, were established as a new class of objects by ROSAT (Trümper et al. 1991, Greiner et al. 1991, van den Heuvel et al. 1992). The archetype supersoft binary, CAL 83, exhibits a 25 pc ionization nebula (Pakull et al. 1989, Remillard et al. 1995). Aimed at identifying supersoft X-ray sources in the Andromeda Galaxy via an ionization nebula we have searched for extended optical emission within the X-ray error boxes of the 15 supersoft sources within M 31 (Greiner et al. 1996). One optical nebula was found (Fig. 1), and we obtained detailed optical data - spectroscopy as well as narrow-band imaging - to determine its relation to the positionally correlated supersoft X-ray source RX J0045.5+4206.

Fig. 1. A 3333 H image of the field around the Wolf-Rayet star MLA 1159 (marked by two dashes) and the clearly visible nebula. The position of the X-ray source RX J0045.5+4206 with its 3 error of 24" is shown as the overlayed circle. Based on the observed optical and X-ray properties, this ROSAT source seems not to be associated with the Wolf-Rayet star or the nebula.

Anticipating the identification of the central star as a Wolf-Rayet star, we will briefly introduce this stellar type and its relation to nebulae before proceeding with the observations. Wolf-Rayet (WR) stars were first identified by their broad emission lines (Wolf & Rayet 1867). These broad emission lines are thought to be indicative of a thick expanding stellar atmosphere. The line widths imply terminal velocities of 1000-3000 km s^-1, suggesting that the outflowing stellar material is gravitationally unbound. WR stars have typical mass loss rates of a few times 10^-5 /yr and are the most powerful sources of stellar winds among massive early-type stars (Barlow 1982, Abbott & Conti 1987, Maeder & Conti 1994).

A considerable fraction of WR stars, though not all, are surrounded by ring nebula (e.g. Miller & Chu 1993, Marston 1997) which are thought to be leftovers of the evolutionary phase when the massive WR-progenitors stripped off their outer envelopes. Theoretically one would expect two shells around a WR star according to the evolutionary stages, i.e. a large fossil bubble of ISM swept up during the main-sequence stage, and a small bubble of circumstellar material blown away from the WR wind. Follow-up studies of the WR nebulae have shown, however, that not all ring nebulae are wind-blown bubbles consisting of ISM (Weaver et al. 1977). In some cases the nebulae contain mostly stellar matter with which the WR wind is interacting, while in other cases the nebulae are ionized by the UV flux of the central WR star.

The population of Wolf-Rayet stars in the Milky Way and the Local Group has been the subject of ample studies (e.g. Massey 1998, Massey & Johnson 1998). It is important not only for the study of each particular star, but also because of their role in stellar evolution as well as their contribution to and interaction with the interstellar matter. It provides also evolutionary information about the galaxies they belong to. There have been systematic searches of WR stars in our Galaxy, the Magellanic Clouds, M 31 and M 33 as well as several other nearby galaxies (e.g. Massey 1999, Massey & Johnson 1998).

M 31 has been surveyed several times with different goals. In an early attempt to establish the existence of gaseous nebulae, Baade & Arp (1964) compiled a list of 688 emission nebula. One of these nebulae, called BA 1-642, is the nebula which we found during our cross-correlation with supersoft X-ray sources. The same object is also listed as PAV78 915 in the catalog of Pellet et al. (1978). A systematic search for WR stars in M 31 was conducted largely by Moffat & Shara (1983, 1987). The central object of this nebula has been classified as a WR star candidate (object MLA 1159; Meyssonier et al. 1993) based on an objective-prism survey of M 31 with a dispersion of 2000 Å/mm in the 4350-5300 Å range.

Here we report the confirmation of this WR star classification and present results of a photometric and spectroscopic study of the WR star as well as the ring-like nebulae surrounding it.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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