The primordial helium abundance: Toward understanding and removing the cosmic scatter in the dY/dZ relation

Citation
Dr. Ballantyne et al., The primordial helium abundance: Toward understanding and removing the cosmic scatter in the dY/dZ relation, ASTROPHYS J, 536(2), 2000, pp. 773-777
Citations number
32
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTROPHYSICAL JOURNAL
ISSN journal
0004-637X → ACNP
Volume
536
Issue
2
Year of publication
2000
Part
1
Pages
773 - 777
Database
ISI
SICI code
0004-637X(20000620)536:2<773:TPHATU>2.0.ZU;2-8
Abstract
We present results from photoionization models of low-metallicity H II regi ons. These nebulae form the basis for measuring the primordial helium abund ance. Our models show that the helium ionization correction factor (ICF) ca n be nonnegligible for nebulae excited by stars with effective temperatures larger than 40,000 K. Furthermore, we find that when the effective tempera ture rises to above 45,000 K, the ICF can be significantly negative. This r esult is independent of the choice of stellar atmosphere. However, if an H II region has an [O III] lambda 5007/[O I] lambda 6300 ratio greater than 3 00, then our models show that, regardless of its metallicity, it will have a negligibly small ICF. A similar, but metallicity-dependent, result was fo und using the [O III] lambda 5007/H beta ratio. These two results can be us ed as selection criteria to remove nebulae with potentially nonnegligible I CFs. Use of our metallicity-independent criterion on the data of Izotov & T huan results in a 20% reduction of the rms scatter about the best-fit Y-Z l ine. A fit to the selected data results in a slight increase of the value o f the primordial helium abundance.