Astron. Astrophys. 363, 1026-1028 (2000)

1. Introduction

The strongly ionized stellar winds of Wolf-Rayet (WR) stars are sources of thermal radio emission. An extensive study of the radio emission of nearby WR stars in the northern sky was performed by Abbott et al.(1986) using the VLA. Recently, some southern WR stars were investigated by Leitherer et al. (1995, 1997) and Chapman et al. (1999), using the Australia Telescope. Together, the northern and southern surveys supply radio observations of, possibly, all WR stars within 3 kpc from the Sun (71 observed, 33 detected). The latter authors conclude that about 40% of the WR stars are non-thermal emitters. They speculate that interaction of the thermal WR wind with the surrounding material from a previous evolutionary stage, may be a common source of radio emission. However, they realize that in several cases the non-thermal emission is known to be related to binary interaction (van der Hucht et al. 1992; Dougherty & Williams 2000).

In all cases where the origin of the non-thermal radiation of WR stars is identified, it is due to interaction with a distant companion (years). These systems are the so-called colliding-wind binaries, notably the archetype WR 140 (Williams et al. 1990). One exception is the highly-obscured short -period binary, Cyg X-3 (WN4-7 +cc;  hrs), a high-mass X-ray binary (van Kerkwijk et al. 1992). The latter system shows radio jets, outbursts and flares and it is studied intensely since its first detection by Braes & Miley (1972, see also Fender et al. 1997 and references therein). In addition, the short-period He-rich emission-line binary system, V Sge ( hr), shows significant radio emission, possibly related to colliding winds (Lockley et al. 1997).

We present radio observations of two short-period photometrically variable WR stars. The first object, WR 46 (HD 104994, WN3p+c), is a short-period binary (7 hrs) but not a luminous X-ray source and, therefore, probably does not hold a compact companion. It is discussed extensively in three papers (Veen et al. 2000a, b, c), where it is proposed to be a close interacting binary system. The second object, WR 50 (TH 17-84; WC7+abs), has been suggested to be variable with an amplitude of about a magnitude in the blue with a possible period of 1.06 day (van Genderen et al. 1991). However, recent inspection showed this variability to be suspicious, see Sect. 3.

For both objects we tried to detect non-thermal radio-emission from the interaction with a (possible) close companion. Unfortunately, in case of a spherical homogeneous wind, the thermal radio photosphere reaches out to several hundreds of stellar radii (e.g., van der Hucht et al. 1992) and may well swallow any non-thermal radiation produced near the central binary. So, non-thermal emission may only escape from the wind if the stellar wind is inhomogeneous or non-spherical, as in the case of WR140 (Williams et al. 1990). In fact, the photometry of WR 46 indicates that its wind is distorted. We note that in case of WR 50, the companion which is visible in the spectrum could in principle also be a source of non-thermal emission. Sect. 2 describes the observations and Sect. 3 discusses the results.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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