Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...510..759b&link_type=abstract
The Astrophysical Journal, Volume 510, Issue 2, pp. 759-783.
Astronomy and Astrophysics
Astronomy
36
Ism: H Ii Regions, Ism: Abundances, Nuclear Reactions, Nucleosynthesis, Abundances, Radio Lines: Ism
Scientific paper
We are making precise determinations of the abundance of the light isotope of helium, ^3He. The ^3He abundance in Milky Way sources provides strong constraints for theories of stellar evolution, chemical evolution, and cosmology. Here we report on new ^3He^+/H^+ abundance determinations for a sample of 21 Galactic H II regions observed for the past 14 yr with the NRAO 140 Foot telescope. The abundances are based on improved ^3He^+ observations, as well as new models for source density structure. The models are constrained by the observed ^3He^+ emission, the strength of the H and ^4He recombination lines, and the continuum emission at 3.5 cm wavelength. Density structure serves only to increase the actual ^3He^+/H^+ abundance ratio over that derived using a uniform density sphere assumption. The density structure models allow one to identify sources that are ``simple'' in that their structure corrections are negligible. These objects provide a pool of H II regions that can in principle yield accurate ^3He^+/H^+ abundances. For the seven simple H II regions in our sample there is no evidence for any systematic trend of the ^3He^+/H^+ abundance with any other parameter. For these sources, we derive an average abundance ratio by number of ^3He^+/H^+=(1.6+/-0.5)x10^-5 (statistical error).
Balser Dana S.
Bania Thomas M.
Rood Robert T.
Wilson Thomas L.
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