3. Discussion and conclusion
With the parameters derived above for the luminosity of the WC6 star and the size, density and emission line strength of the nebula it seems reasonable to assume that both are physically connected. The geometrical appearance with the nebula being centered around the WR star supports this conclusion. The nebula itself seems to form a shell and based on the kinematic age and mass is thought to be interstellar matter swept up by the wind of the WR star MLA 1159.
An important question to be addressed is on the mechanism which produces the nebular emission. The lack of filamentary [OIII] structures together with a diffuse [OIII] emission in seemingly shell geometry and the identical extent of the nebula at different emission lines suggests that the nebula is not shock excited, but rather photoionized. Also, the emission line width and the low density which is similar to that in typical HII regions suggest photoionization as the main origin. Finally, the satisfactory fits obtained with the code Cloudy to explain the relative line strengths implies that photoionization is a reasonable assumption.
Our successful model is characterized by an ionization parameter and it is interesting to note that previous investigations of Wolf-Rayet nebulae (and giant HII regions) found values for the ionization parameter close to -3 as well, with a rather narrow range in U (e.g., Esteban et al. 1993, Bresolin et al. 1999) which led to the suggestion that the stellar winds cause a close scaling of ionizing flux and shell gas density (Shields 1986).
We finally note that photoionization modelling employing blackbody continua for the ionizing flux instead of real WR star atmospheres are expected to derive lower temperatures. This is due to the fact that the (He- and) C-rich stellar atmosphere absorbs most of the extreme UV ionizing photons ( 228 Å), thus emitting many fewer UV photons than a blackbody of the same temperature (Koesterke & Hamann 1997).
Because of the original goal of identifying a supersoft X-ray source we mention here that the X-ray source RX J0045.5+4206 is very likely not related to either the WC6 star or the nebula. If RX J0045.5+4206 were within the nebula, it would be the dominant ionization source and produce very strong [OIII] emission, i.e. [OIII] /H10-20 which is not observed. With a bolometric (blackbody approximation) luminosity of RX J0045.5+4206 of the order of 1037 erg s-1 the X-ray emission is also very unlikely produced by the WC6 star which typically have X-ray luminosities around a few times 1032 erg s-1. Deeper imaging is necessary to identify the counterpart of RX J0045.5+4206 which we now suspect North-East of the nebula BA 1-642.
© European Southern Observatory (ESO) 1999
Online publication: June 30, 1999