A Comprehensive Study of High-metallicity Giant Extragalactic H Ii Regions: Ionizing Populations

Theoretically, the evolution of a young stellar population depends on metallicity at least through two very important effects: the increasing opacity of the stellar material and the dependence of mass loss on metal content in high mass stars. As a consequence of the first, the effective temperature of ionizing stars should be lower in regions of higher metallicity (see for example McGaugh 1991) but, on the other hand, as a consequence of the second, that is if the strength of stellar winds increases with metallicity, the loss of the outer envelopes of the most massive stars can increase their surface temperature to very high values. These highly evolved massive O stars are identified with the Wolf-Rayet population. It must be stressed the importance of observing WR stars in high metallicity H II regions in order to verify both the assumed mass loss rates and the predicted characteristics of the WR features in these environments. This work must rely on the calculation of accurate oxygen abundances in these regions (see Castellanos, Dı́az & Terlevich 2002, these proceedings). Wolf-Rayet features have been detected in the four observed GEHRs, and are specially prominent in region H13 (NGC 628). In this region, when comparing both the emission line spectrum and the inferred abundances with those derived in the nearby region H3, it is confirmed that they close resemble each other, despite the presence of Wolf-Rayet stars in region H13. From this result, it can be concluded that the presence of WR stars in this region does not alter its ionization structure. This fact can be understood in terms of the low derived value for the WR/O star ratio. Another fact that supports the previous conclusion is the isothermal behavior observed in H13.