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Session 82 - Globular Clusters.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
One of the principal arguments in favor of a primordial origin for the abundance anomalies in globular cluster red giant branch (RGB) stars comes from the observed enhancements of Na and Al and from the anti-correlation between these enhancements and the O abundance. Recently Denisenkov and Denisenkova (1990) and Langer, Hoffman and Sneden (1993) have suggested that Na and Al might be produced around the top of the hydrogen shell in low mass RGB stars due to proton capture on Ne and Mg. In this case the observed Na and Al enhancements might not be primordial at all but rather might arise from interior mixing during the course of the RGB evolution.
We have investigated this possibility by computing a number of RGB evolutionary sequences for metallicities in the range from [Fe/H] = -2.3 to solar. The distribution of the various isotropic abundances around the hydrogen shell in these models was determined with a nuclear reaction network code kindly provided by Arnett (1995).
In regard to the CNO elements we find that C and O are depleted just above the hydrogen shell due to CN and ON cycling and that the extent of these C and O depleted regions increases substantially at low metallicities, in agreement with the earlier results of Sweigart and Mengel (1979). In regard to Na and Al we find a significant production of ^23Na in many of our models via Ne-Na cycling starting from ^20Ne. In some models a significant amount of ^23Na is siphoned into ^24Mg which together with ^25Mg and ^26Mg leads to the production of ^27Al.
We are presently testing the dependence of our results on the uncertainties in the various reaction rates. These results will then be combined with appropriate mixing scenarios to predict surface abundances along the RGB.
