Realistic ionizing fluxes for young stellar populations from 0.05 to 2 Z ⊙

We present a new grid of ionizing fluxes for O and Wolf-Rayet stars for usewith evolutionary synthesis codes and single star H II region analyses. A totalof 230 expanding, non-LTE, line-blanketed model atmospheres have beencalculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 solar) using theWM-basic code of Pauldrach et al. (2001) and the CMFGEN code of Hillier &Miller (1998). The stellar wind parameters are scaled with metallicity for bothO and W-R stars. We incorporate the new models into Starburst99 (Leitherer etal. 1999) and compare the ionizing outputs with Schaerer & Vacca (1998) andLeitherer et al. (1999). The changes in the output ionizing fluxes aredramatic, particularly below 228 A. We also find lower fluxes in the He Icontinuum for Z > 0.4 solar and ages < 7 Myr because of the increased lineblanketing. We test the accuracy of the new models by constructingphotoionization models. We show that for the dwarf O star grid, He I 5876/Hbeta decreases between Z = 1 and twice solar in a similar manner toobservations (e.g. Bresolin et al. 1999) due to the increased effect of lineblanketing. We therefore suggest that a lowering of the upper mass limit athigh abundances is not required to explain the observations. For the case of aninstantaneous burst, we plot the softness parameter "eta prime" against theabundance indicator R_23. The new models are coincident with the data ofBresolin et al. (1999), particularly during the W-R phase, unlike previousmodels which over-predict the hardness of the ionizing radiation.Comment: 21 pages, 15 postscript colour figures, includes mn2e.cls. To be published in MNRAS. Revised version containing modifications to Tables 1-