Effective temperature of ionizing stars of extragalactic HII regions

DORS, O. L.; HÄGELE, G. F.; CARDACI, M. V.; KRABBE, A. C.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2017 vol. 466

0035-8711

The effective temperature (Teff) of the radiation field of the ionizing star(s) of a large sample of extragalactic HII regions was estimated using the R= log([OII](3727)/[OIII]5007) index. We used a grid of photoionization models to calibrate the Teff-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 HII 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 Teff in 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 Teff dependent S2=[S II](6717+31)/Ha 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 HII regions, whose emission-line intensities were compiled from the literature, do not show any Teff-Z correlation. On the other hand it seems to be hints of the presence of an anti-correlation between Teff-U. We found that the majority of the studied HII regions (87%) present Teff values in the range between 37 and 40 kK, with an average value of 38.5 kK. We also studied the variation of Teff as a function of the galactocentric distance for 14 spiral galaxies. Our results are in agreement with the idea of the existence of positive Teff gradients along the disk of spiral galaxies.