Emission-line helium abundances in highly obscured nebulae

Citation
Mh. Armour et al., Emission-line helium abundances in highly obscured nebulae, PUB AST S P, 111(764), 1999, pp. 1251-1257
Citations number
31
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC
ISSN journal
0004-6280 → ACNP
Volume
111
Issue
764
Year of publication
1999
Pages
1251 - 1257
Database
ISI
SICI code
0004-6280(199910)111:764<1251:EHAIHO>2.0.ZU;2-P
Abstract
This paper outlines a way to determine the ionization correction factor (IC F) using only infrared data. We identify four line pairs, [Ne III] 36 mu m/ [Ne II] 12.8 mu m, [Ne III] 15.6 mu m/[Ne II] 12.8 mu m, [Ar III] 9 mu m/[A r II] 6.9 mu m, and [Ar III] 21 mu m/[Ar II] 6.9 mu m, that are sensitive t o the He ICF. This happens because the ions cover a wide range of ionizatio n, the line pairs are not sensitive to electron temperature, they have simi lar critical densities, and they are formed within the He+/H+ region of the nebula. We compute a very wide range of photoionization models appropriate for galactic H II regions. The models cover a wide range of densities, ion ization parameters, and stellar temperatures and use continua from four ver y different stellar atmospheres. The results show that each line pair has a critical intensity ratio above w hich the He ICF is always small. Below these values the ICF depends very st rongly on details of the models for three of the ratios, and so other infor mation would be needed to determine the helium abundance. The [Ar III] 9 mu m/[Ar II] 6.9 mu m ratio can indicate the ICF directly as a result of the near exact match in the critical densities of the two lines. Finally, conti nua predicted by the latest generation of stellar atmospheres are sufficien tly hard that they routinely produce significantly negative ICFs.