The stellar content, metallicity and ionization structure of HII regions

Abstract

Observations of infrared ne-structure lines provide direct information on the metallicity and ionization structure of H ii regions and indirectly on the hardness of the radiation eld ionizing these nebulae. We have an- alyzed a sample of Galactic and Magellanic Cloud H ii regions observed by the Infrared Space Observatory (ISO) to examine the interplay between stellar content, metallicity and the ionization structure of H ii regions. The observed (S iv) 10.5/(S iii) 18.7 ma nd (Neiii) 15.5/(Ne ii) 12.8 m line ratios are shown to be highly correlated over more than two orders of magnitude. We have compared the observed line ratios to the results of photoion- ization models using dierent stellar energy distributions. The derived characteristics of the ionizing star depend critically on the adopted stellar model as well as the (stellar) metallicity. We have compared the stellar eective temperatures derived from these model studies for a few well-studied H ii regions with published direct spec- troscopic determinations of the spectral type of the ionizing stars. This comparison supports our interpretation that stellar and nebular metallicity influences the observed infrared ionic line ratios. We can explain the observed increase in degree of ionization, as traced by the (S iv)/(S iii )a nd (Neiii)/(Ne ii) line ratios, by the hardening of the radiation eld due to the decrease of metallicity. The implications of our results for the determination of the ages of starbursts in starburst galaxies are assessed.