Effective temperature of ionizing stars of extragalactic H II regions

The effective temperature (T-eff) of the radiation field of the ionizing star(s) of a large sample of extragalactic H II regions was estimated using the R = log([O II] (lambda lambda 3726 + 29)/[O III] l5007) index. We used a grid of photoionization models to calibrate the T-eff-R relation finding that it has a strong dependence with the ionizing parameter, while it shows a weak direct dependence with the metallicity (variations in Z imply variations in U) of both the stellar atmosphere of the ionizing star and the gas phase of the H II region. Since the R index varies slightly with the Teff for values larger than 40 kK, the R index can be used to derive the T-eff in the 30-40 kK range. A large fraction of the ionization parameter variation is due to differences in the temperature of the ionizing stars and then the use of the (relatively) low T-eff dependent S2 = [S II] (lambda lambda 6717 + 31)/H alpha emission-line ratio to derive the ionization parameter is preferable over others in the literature. We propose linear metallicity dependent relationships between S2 and U. Teff and metallicity estimations for a sample of 865 H II regions, whose emission-line intensities were compiled from the literature, do not show any T-eff-Z correlation. On the other hand, it seems to be hints of the presence of an anticorrelation between T-eff-U. We found that the majority of the studied H II regions (similar to 87 per cent) present Teff values in the range between 37 and 40 kK, with an average value of 38.5(+/- 1) kK. We also studied the variation of T-eff as a function of the galactocentric distance for 14 spiral galaxies. Our results are in agreement with the idea of the existence of positive T-eff gradients along the disc of spiral galaxies.