Introduction

H ii regions are used as the most reliable targets from which to derive actual abundances in the interstellar medium (ISM). Kunth & Sargent (1986) discussed the problem of determining the heavy-element abundances of very-metal-poor blue compact dwarf galaxies from emission lines of H ii regions in the light of local self-enrichment by massive stars but placed more stress on the effect of material ejected by Type-II supernovae (SNe II). However, already during the Wolf-Rayet (WR) phase of massive-stellar evolution the stellar wind peels off the outermost model: 605 time: 3.22E+06 yr model: 461 time: 3.22E+06 yr model: 605 time: 3.22E+06 yr model: 461 time: 3.22E+06 yr

Figure 34.1. Left panel: the 12C distribution within the stellar-wind bubble and the H ii region for comparison with the temperature distribution (right panel). Both figures are snapshots at the end of the lifetime of a star of mass 85M0. All plots cover the whole computational domain of 60 pc x 60 pc.

Figure 34.1. Left panel: the 12C distribution within the stellar-wind bubble and the H ii region for comparison with the temperature distribution (right panel). Both figures are snapshots at the end of the lifetime of a star of mass 85M0. All plots cover the whole computational domain of 60 pc x 60 pc.

stellar layers, so that elements from shell-burning regimes are released into the surrounding ISM already at later stages of their normal lifetimes. The stellar-wind energetics let one presume that this gas is deposited into the hot phase only, but it has not yet reliably been investigated in detail how and to what extent the complex structure of the stellar-wind bubble (SWB) could facilitate the cooling of wind material, thereby making observations of it as H ii gas attainable.

That WR stars should play an important role in C enrichment of the ISM at Solar metallicity was advocated by Dray et al. (2003). Their models predict that the C enrichment by WR stars is at least comparable to that by AGB stars, while the enrichment by N is dominated by AGB stars and the O enrichment is dominated by SNe II. In their investigation, however, they summed over all gas phases and did not evaluate the abundances in specific gas phases with detailed diagnostics, such as in the warm H ii gas.

In a series of models of radiation-driven and wind-blown bubbles produced by massive stars we investigated the effects of structuring and energizing the surrounding ISM for stars of masses 15M0 (Kroeger etal. 2007), 35MQ (Freyer etal. 2006), 60Mq (Freyer et al. 2003), and 85MQ (Kroeger et al. 2006b). From these, we could conclude that differently strong but significant structures are formed by the dynamical and radiative processes involving both the SWB and the enveloping H ii region (Freyer et al. 2003) where hot gas mixes with warm gas and cools further to a "warm" phase. The mixing occurs mainly at the back of the SWB shell with photo-evaporated material and through turbulence in an interface between the SWB and the shell (see e.g. Figure 34.1).

Nevertheless, the WR stage is metal-dependent in the sense that, first, the lower the metallicity the more massive a star has to be in order for it to evolve through the WR stages and, secondly, the lower the metallicity the shorter the WR lifetimes, with the consequence that not all WR stages are reached. The first point means that the number of WR stars decreases with decreasing metallicity. Schaller et al. (1992) found that for a metallicity of Z = 0.001 the minimal zero-age main-sequence (ZAMS) mass for a WR star is >80M0, while at Solar Z = 0.02 it is >25MQ as discussed by Chiosi & Maeder (1986).

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