The ionizing stars of extragalactic H II regions

Medium-resolution spectra from 3650 to 10000 Angstrom are presented for 96 giant H II regions distributed in 20 spiral galaxies. In order to interpret the data, we have calculated two separate grids of photoionization models, adopting single-star atmospheres (Kurucz) and star clusters synthesized wi th different initial mass functions (IMFs) as ionizing sources. Additional models were computed with more recent non-LTE stellar atmospheres, in order to check the effects of different stellar ionizing fluxes. We use the radi ation softness parameter eta' = ([O II]/[O III])/([S II]/[S III]) of Vilche z & Pagel to test for a metallicity dependence of the effective temperature s of the ionizing stars. Our results are consistent with a significant decr ease in mean stellar temperatures of the ionizing stars with increasing met allicity. The magnitude of the effect, combined with the behavior of the He I lambda 5876/H beta ratio, suggest a smaller upper mass limit for star fo rmation at abundances higher than solar, even when considering the effects of metallicity on stellar evolution and atmospheric line blanketing. Howeve r, the exact magnitudes of the stellar temperature and IMF variations are d ependent on the choice of stellar atmosphere and evolution models used, as well as on uncertainties in the nebular abundance scale at high metalliciti es. Our results also constrain the systematic behavior of the ionization pa rameter and the N/O ratio in extragalactic H II regions. The observed spect ral sequences are inconsistent with current stellar evolution models, which predict a luminous, hot W-R stellar population in evolved H rr regions old er than 2-3 Myr. This suggests either that the hardness of the emitted Lyma n continuum spectrum has been overestimated in the models or that some mech anism disrupts the H II regions before the W-R phases become important.