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

3. Discussion

Though we expected to reach the low level thermal emission of the WR stars, we have detected neither of the WR stars down to 0.18 mJy and to 0.15 mJy at 6 cm in case of WR 46. New optical studies have appeared since the radio observations were performed, which show both stars to be weak-lined WR stars (Crowther et al. 1995; Koesterke & Hamann 1995). This class is characterised by a stellar wind weaker than the strong-lined objects. This explains why we have not detected the thermal radio emission. Naturally, there may be some unobserved non-thermal emission, but, we may conclude that neither of the objects shows strong non-thermal radio emission. For each object we will discuss what can be inferred from the observed upper limits of the radio fluxes about the distances to the objects, assuming thermal emission.

By assuming an absolute magnitude of , van der Hucht et al. (1988) derived a distance of 3.4 kpc. Niemela et al. (1995; hereafer NBS) derived a distance of 2 kpc from comparing polarimetric measurements with nearby objects on the sky. The latter analysis depends on the absence of intrinsic polarization from WR 46 itself, while the same NBS argue that the system is dominated by a circumstellar disc. Also, if the photometric variability is ascribed to a distorted atmosphere, the electron scattering may create intrinsic polarization. From interstellar absorption-line profiles at high-resolution, Crowther et al. (1995) inferred a distance of 41.5 kpc. Using the GLAZAR -space telescope, Tovmassian et al. (1996) derived from observations at 1640 Å (full bandwidth  Å) that WR 46 is a probable member of a stellar OB association at 4.0 () kpc.

In the case of WR 50 the object is suggested to be a member of a stellar cluster at a distance of 3.6 kpc (Turner 1985). However, Smith et al. (1990) developed a method based on the line emission only and therefore independent of a possible companion contributing to the contimuum. These authors derive a larger distance of 5.9 kpc.

Leitherer et al. (1997) compared their mass-loss estimates from radio observations to determinations from optical line analyses. These authors conclude that both methods lead to consistent results. Therefore, we adopt the mass-loss rate of as determined by Crowther et al. (1995) from the optical spectrum. To derive a lower limit to the distance we may rewrite the formula derived by Wright & Barlow (1975) to:

where is in yr^-1, z the average charge on each ion roughly equal to one, the terminal velocity km s^-1, the average number of electrons per ion , the Gaunt factor g at frequency is roughly 6, and is the observed flux (or upper limit). Applying this equation we derive a lower limit to the distance of WR 46 of 1.0 kpc. This limit is too small to help to decide between various available distance determinations in the literature.

As to WR 50, our lower limit to the distance of the object is 3.2 kpc, assuming ,  km s^-1, (carbon mass fraction 0.5), , , (Koesterke & Hamann 1995). This lower limit is only slightly smaller than the distance 3.6 kpc.

In addition, we have re-investigated the original photometric Walraven data as presented by van Genderen et al. (1991) and note that the bright and variable sky due to the nearby moon, and some unexplained light contribution at a fixed position relative to the telescope may explain most, if not all, variability in the data of van Genderen et al. (1991) of WR 50.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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