Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........7b&link_type=abstract
PhD Dissertation, Boston Univ. Boston, MA United States
Astronomy and Astrophysics
Astronomy
7
Abundance, Big Bang Cosmology, H Ii Regions, Helium Isotopes, Milky Way Galaxy, Planetary Nebulae, Cosmochemistry, Radio Astronomy, Astronomical Models, Galactic Evolution, Line Spectra, Radio Observation, Stellar Evolution, Radio Spectra
Scientific paper
The abundance of the helium isotope He-3 in the Galaxy is determined using radio observations and theoretical models. The He-3 abundance is important because it can be used to test theories of stellar evolution, galactic evolution, and cosmology. The radio spectral line of He-3(+) at 8.7 GHz is measured in 12 Galactic ionized nebulae (H II regions) and 6 planetary nebulae. Using these observations, the helium abundance ratio, He-3/H, is derived with a computer program that calculates the He-3(+) emission together with the radio recombination line and continuum emission. Hydrogen and helium radio recombination lines are observed to constrain these theoretical models. Additional constraints come from high angular resolution radio continuum images. The He-3/H ratio measured for planetary nebulae is roughly 100 times that for Galactic H II regions. Since low-mass stars are the progenitors of planetary nebulae, this confirms that low-mass stars are the major source of He-3(+) in the Galaxy. This is consistent with theoretical models of stellar evolution. The H II region abundances provide a robust constraint for models of galactic chemical evolution. The ionized gas in H II regions is processed by many generations of stars so H II region He-3 abundances represent a combination of primordial abundances and subsequent processing by stars. The H II regions with the smaller amounts of ionized gas have the higher He-3 abundances. This suggests that the smaller H II regions can be contaminated by local conditions. Overall, the He-3 abundances in H II regions represent a slight enhancement over the primordial He-3 abundance. The standard Big Bang theory for the origin of the universe predicts that during the first 100 seconds of time copious amounts of the light elements are produced. Big Bang theory is constrained by the primordial He-3 abundance and other light element abundances. Because stars enrich the Galaxy with He-3, the lowest He-3/H abundance ratio measured in the Galaxy is an upper limit to the primordial He-3/H. Although the two lowest abundances in the sample contradict the Big Bang theory, the most reliable He-3 abundance, for the H II region W43, is consistent with the Big Bang theory and the observations of other light elements.
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