Astron. Astrophys. 350, 1007-1017 (1999)

6. Conclusions

We have used WR124 and its associated nebula M1-67 to assess the reliability of Lyman ionizing flux distributions from current stellar models. For WR124, we find that stellar properties obtained with two line blanketed, non-LTE extended model atmospheres, ISA -wind (de Koter et al. 1997) and CMFGEN (Hillier & Miller 1998) are in excellent agreement. However, comparisons with the observed properties of M1-67 favour the ionizing energy distribution obtained by CMFGEN rather than ISA -wind, reconciling previous inconsistencies identified by Esteban et al. (1993). Certainly, more general conclusions await comparisons for a variety of stars.

In the future we propose to extend our study to WR stars of different spectral type. One well known example is HD 192163 (WR136) and its ring nebula NGC 6888, for which Esteban et al. (1993) also identified a major inconsistency. However, in contrast with M1-67, there is evidence that nebular emission in NGC 6888 may be produced by shocks rather than photo-ionization. Further, the properties of other Galactic WR nebulae are often poorly known, such that different stellar ionizing distributions can reproduce the nebular properties by selecting, for example, alternative electron densities or shell thickness. WR nebulae in the LMC may provide more suitable candidates (Dopita et al. 1994).

In general, study of young stellar populations in external galaxies via associated H II regions relies heavily on suitable stellar ionizing flux distributions (e.g. Leitherer et al. 1999). Currently, the only WR models that are generally available at this time are unblanketed, pure helium energy distributions (Schmutz et al. 1992). The calculation of a large multi-parameter grid of line-blanketed models remains a formidable computational challenge, unless codes such as ISA -wind can be relied upon. Crowther (1999) finds that, in general, the H I and He I ionizing fluxes of early-type WR stars are not greatly affected by line blanketing. However, the additional blanketing from C and O in early WC stars produces a softer ionizing spectrum than for early WN stars of identical temperatures, with negligible flux emitted 300 Å. Indeed, unblanketed models are probably to be preferred for the study of young starburst regions in very metal deficient regions. However, the (uncertain) role that metallicity may play on mass-loss rates of Wolf-Rayet stars does need to be quantitatively investigated (e.g. Crowther et al. 1999).

© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999
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