AAS 198th Meeting, June 2001
Session 41. The Milky Way Galaxy
Display, Tuesday, June 5, 2001, 10:00am-6:30pm, Exhibit Hall

[41.06] Stalking the Cosmic 3-He Abundance: The 3-Helium Plateau

T. M. Bania (BU-IAR), R. T. Rood (UVa), D. S. Balser (NRAO-GB)

3-Helium is one of the isotopes produced in the Big Bang. Models predict that it is also produced, perhaps in prodigious amounts, by Solar-type stars. Thus measurements of the {}3{\rm He} abundance are of interest both for cosmology and the chemical evolution of the Galaxy.

We have now detected the hyperfine line of {}3{\rm He}+ in more than 30 {H\,{\sc ii}} regions which are located throughout the Milky Way's gaseous disk, from the Galactic Center to the far reaches of the outer Galaxy. We have thus detected {}3{\rm He} over a larger fraction of the Galaxy than any other light isotope.

In a subset of simple'' {H\,{\sc ii}} regions which should have the most reliable abundances we find that Galactic {H\,{\sc ii}} regions are not {}3{\rm He} enriched and, furthermore, that there is no trend in the {}3{\rm He}/{\rm H} abundance ratio with either position in the Galaxy or {H\,{\sc ii}} region metallicity. From these results we suggest that stars neither create nor destroy {}3{\rm He} in large amounts. Given this we argue that the mean abundance of our simple {H\,{\sc ii}} region sample, {}3{\rm He}/{\rm H} = 1.79 ±0.65 \times 10-5, is a reasonable approximation of the primordial value. Despite the fact that the {H\,{\sc ii}} regions suggest no stellar production, we do find high {}3{\rm He} abundances in some planetary nebulae. The scenario proposed by Charbonnel in which most (but not all) low mass stars undergo rotationally driven mixing on the red giant branch above the luminousity function bump seems consistent with our observations and chemical evolution models.

This research was supported by the National Science Foundation (AST 97-31484).