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 (W-R) stars for use with evolutionary synthesis codes and single-star H II region analyses. A total of 230 expanding, non-LTE, line-blanketed model atmospheres have been calculated for five metallicities (0.05, 0.2, 0.4, 1 and 2 Z.) using the WM-BASIC code of Pauldrach, Hoffmann & Lennon for O stars and the CMFGEN code of Hillier & Miller for W-R stars. The stellar wind parameters are scaled with metallicity for both O and W-R stars. We compare the ionizing fluxes of the new models with the CoStar models of Schaerer & de Koter and the pure helium W-R models of Schmutz, Leitherer & Gruenwald. We find significant differences, particularly above 54 eV, where the emergent flux is determined by the wind density as a function of metallicity. The new models have lower ionizing fluxes in the He I continuum with important implications for nebular line ratios. We incorporate the new models into the evolutionary synthesis code STARBURST99 and compare the ionizing outputs for an instantaneous burst and continuous star formation with the work of Schaerer & Vacca (SV98), and Leitherer et al. The changes in the output ionizing fluxes as a function of age are dramatic. We find that, in contrast to previous studies, nebular He II lambda4686 will be at, or just below, the detection limit in low metallicity starbursts during the W-R phase. The new models have lower fluxes in the He I continuum for Z greater than or equal to 0.4 Z. and ages less than or equal to7 Myr because of the increased line blanketing. We test the accuracy of the new model atmosphere grid by constructing photoionization models for simple H II regions, and assessing the impact of the new ionizing fluxes on important nebular diagnostic line ratios. For the case of an H II region where the ionizing flux is given by the WM-basic dwarf O star grid, we show that He I lambda5786/Hbeta decreases between 1 and 2 Z. in a similar manner to observations. We find that this decline is caused by the increased effect of line blanketing above solar metallicity. We therefore suggest that a lowering of the upper mass limit at high abundances is not required to explain the diminishing strength of He I lambda5786/Hbeta, as has been suggested in the past. For an H II region where the ionizing flux is provided by an instantaneous burst of total mass 10(6) M., we plot the softness parameter eta' against the abundance indicator R(23) for ages of 1-5 Myr. The new models are coincident with the observational data of Bresolin et al., particularly during the W-R phase, unlike the previous models of SV98 which generally over-predict the hardness of the ionizing radiation. The new model grid and updated STARBURST99 code can be downloaded from http://www.star.ucl.ac.uk/starburst.