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

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1. Introduction

The Lyman ionizing energy distributions of hot, massive stars are important in the study of young starburst regions and galaxies, via population synthesis codes (e.g. Leitherer et al. 1999). However, the interstellar medium (ISM) conspires to prevent this energy from reaching the Earth's atmosphere. Interstellar dust severely degrades transmitted fluxes at ultraviolet and optical wavelengths, while the Lyman continuum of hot stars is completely absorbed by intervening hydrogen atoms for all but a few nearby B stars (e.g. Cassinelli et al. 1995). Therefore, indirect measurements of the extreme UV flux are necessary.

Until recently, the standard method of obtaining the ionizing distributions of hot stars was to rely on line blanketed, plane parallel, LTE model predictions from Kurucz (1991). However, the need to consider non-LTE effects, spherical geometry and line blanketing effects has recently led to the development of complex model atmospheres (e.g. Hubeny & Lanz 1995; de Koter et al. 1997; Pauldrach et al. 1998), with Lyman continuum ionizing flux distributions obtained via synthesis of the accessible UV and optical stellar spectrum. However, these different codes predict quite different ionizing spectra for early-type stars of identical temperatures. While attempts at testing theoretical stellar O-type models via observations of stellar clusters have been made (Stasinska & Schaerer 1998; Oey & Kennicutt 1998), definitive results require studies of individual stars and their associated H II regions.

In the case of Wolf-Rayet (WR) stars, evolutionary synthesis models for young starbursts known to contain these stars, called as `WR galaxies', still rely on unblanketed, non-LTE model atmospheres of Schmutz et al. (1992). What effect does line blanketing have on the predicted Lyman continua of WR stars? Recent theoretical calculations by Crowther (1999) suggest that the effect can be substantial, such that unblanketed WR models overestimate the hardness of their ionizing spectra, which is supported by observations of extra-galactic giant H II regions by Bresolin et al. (1999).

In principle, H II regions associated with their central star are ideal tracers of the Lyman ionizing flux distributions. However, suitable nebulae are rare, since their properties are often poorly known. The principal studies for WR stars are those of Rosa & Mathis (1990) and Esteban et al. (1993) who combined WR model fluxes from Schmutz et al. (1989, 1992) with observed properties of WR ring nebulae, to investigate the properties of the central stars. Esteban et al. varied the temperatures of unblanketed WR models until agreement was reached between the observed nebular properties and those predicted by photo-ionization modelling. In general, comparisons with (independent) stellar analyses of the central stars was found to be reasonable, except that lower temperatures were required from the photo-ionization models for late-type WN (WNL) stars, especially the Galactic WN8 star WR124 and its associated H II region, M1-67, for which no agreement was achieved.

In this work, we shall depart from the technique of Esteban et al. in that stellar properties of WR124 are fixed from a spectroscopic study, which are then tested against the nebular properties of M1-67 via photo-ionization modelling using CLOUDY (Ferland 1996). Sirianni et al. (1998) showed M1-67 to be very clumpy. Hubble Space Telescope (HST) imaging of M1-67 by Grosdidier et al. (1998) revealed an astonishingly complex nebula, allowing measurements of the nebular radial density distribution and integrated H[FORMULA] flux. Our work therefore represents the first attempt to test the reliability of predicted WR Lyman continuum distributions using both unblanketed (Hillier 1987) and blanketed models (following Hillier & Miller 1998, 1999) plus robust nebular properties. In our study we compare these predictions with the code of de Koter et al. (1997), augmented to allow for line blanketing following Schmutz (1994, 1997). In this way, we will be able to quantitatively assess the effects of different codes and line blanketing techniques on the ionizing energy produced by WR stars.

In Sect. 2 we will discuss observations of WR124 and M1-67. In Sect. 3 the model atmospheres codes are introduced and discussed, with stellar results compared in Sect. 4. The photo-ionization modelling technique is discussed and results are presented in Sect. 5. Finally, conclusions are drawn in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999