The abundance of sulfur in extragalactic H II regions

Sulfur abundances have been derived from CCD observations of S III λλ9069, 9532 and S II λλ6717, 6731 in 13 extragalactic H II regions for which 7. 2 < 12 + log (O/H) < 8. 6. Complementary photoionization model calculations are used to estimate the fraction of sulfur in S^+3^, which can be important in nebulae where the O^++^ fraction is large. The data show no evidence for an increase in S/O in regions with low O/H, in contradiction with earlier claims. Simple chemical evolution models were computed to predict the evolution of S/O for conjectured cases in which either the slope of the IMF or the upper mass limit for star formation varies with metallicity. In both cases the models predict that S/O increases with O/H. However, there is great uncertainty in the predicted yield for sulfur from hydrostatic burning calculations for massive stars, as well as in the effect of processing of silicon-shell elements in supernova explosions. Moreover, the scatter in S/O at a given O/H, possibly due mainly to observational uncertainties, is too large at the present time to constrain the predicted variations in S/O. The use of the ratio η = (O^+^/O^++^) / (S^+^/S^++^) proposed by Vilchez and Pagel as a measure of Tₑff_ for the ionizing source of a nebula is investigated via photoionization modeling. It is found that O^+^/O versus S^+^/S^++^ appears to uniquely determine both Tₑff_ and ionization parameter, for a given metallicity. LTE and non-LTE model atmospheres give somewhat different results, mainly because of differences in the depth of the O^+^ ionization edge in different atmosphere models. However, for high-ionization H II regions such as NGC 4861 and NGC 2363, the observed O^+^/O and S^+^/S^++^ correspond to Tₑff_ ~ 100, 000 K, which is inconsistent with other observed spectral features in these objects. It is suggested that the discrepancy may be due to poorly known atomic data or to deviations from simple nebular geometries in giant H II regions. Thus, while the relation between O^+^/O^++^ and S^+^/S^++^ may be used to study relative variations of Tₑff_, it does not appear possible at present to infer correct absolute values of Tₑff_ for individual nebulae.