Beryllium Abundances as a Test of Mixing in Red Giants

Details Beryllium Abundances as a Test of Mixing in Red Giants Beryllium Abundances as a Test of Mixing in Red Giants

Feb 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999noao.prop..157w&link_type=abstract

NOAO Proposal ID #1999A-0157

Physics

Scientific paper

Evolved low mass stars (i.e., with initial masses below ~ 2 M_&sun;) exhibit chemical anomalies on their surfaces which are not predicted by standard stellar evolution theory. In particular, stars with M_V <=0 show ^12C/^13C ratios between 4 and 10 rather than the predicted ratio of 20-25. These are seen in globular clusters and in the field, and have important implications for a wide variety of subjects ranging from galactic chemical evolution to cosmology. Many "extra mixing" explanations have been invoked, usually based on schematic physical assumptions. If it is correct, the identification by Charbonnel et al. (1998) that the specific physical property controlling extra mixing is the molecular weight (or (mu)) - gradient in a giant would mark a major breakthrough in our understanding. We can test the (mu) mechanism by observing the Beryllium abundance in red giants above and below the so-called luminosity function bump on the red giant branch, since (mu)-gradients greatly differ above and below it. The gradients also show that extra- mixing is more effective for stars with very low metallicities, so observing stars of different metallicity provides a further test. The test involves observing Be, which is destroyed at temperatures reached in the mixing regions.

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