We present a new grid of ionizing fluxes for O and Wolf-Rayet 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 _ { \odot } ) using the WM-basic code of Pauldrach et al .
( 2001 ) for O stars and the cmfgen code of Hillier & Miller ( 1998 ) 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 ( 1997 ) and the pure helium W-R models of Schmutz , Leitherer & Gruenwald ( 1992 ) .
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 ( Leitherer et al .
1999 ) and compare the ionizing outputs for an instantaneous burst and continuous star formation with the work of Schaerer & Vacca ( 1998 ; SV98 ) and Leitherer et al .
( 1999 ) .
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 \lambda 4686 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 \geq 0.4 Z _ { \odot } and ages \leq 7 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 \lambda 5786 /H \beta decreases between 1 and 2 Z _ { \odot } in a similar manner to observations ( e.g .
Bresolin et al .
1999 ) .
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 \lambda 5786 /H \beta , as has been suggested in the past ( e.g .
Shields & Tinsley 1976 ; Bresolin et al .
1999 ) .
For an H ii region where the ionizing flux is provided by an instantaneous burst of total mass 10 ^ { 6 } M _ { \odot } , we plot the softness parameter \eta ^ { \prime } 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 .
( 1999 ) , 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 .