Are Inhomogeneities of the Lighter Elements C through Al in Globular Clusters Accompanied by Variations in Heavy Neutron-Addition Elements?

Abstract

ABSTRACTAsymptotic giant branch (AGB) stars of intermediate mass have been implicated in the origin of abundance inhomogeneities within globular clusters because of their ability to synthesize elements such as nitrogen and sodium. Such stars are also thought to be sites for the synthesis of s-process elements. Abundance data from the literature are used to study the behavior of sodium relative to the s-process element barium and the r-process element europium in a sample of 14 globular clusters. These all show large intracluster scatter in [Na/Fe], however, most have minimal internal dispersion in barium and europium abundances beyond what can be attributed to observational measurement error. Upon subdividing each cluster into samples of Na-enhanced and Na-normal stars, it is found that differences in their mean [Ba/Fe] and [Eu/Fe] abundances rarely exceed 0.1 dex. It is concluded that whatever stars were responsible for enriching the high-Na red giants in globular clusters were not, by and large, substantial producers of s or r-process elements. Thermally pulsating AGB stars of mass greater than 5 M⊙ may satisfy this constraint. There are a few clusters in which the available data do provide evidence of either barium or europium inhomogeneities, such as M15, NGC 6752, M4, and NGC 3201. However, there is no consistency in regard to whether Ba or Eu enhancements correlate with sodium abundance. At least some of the stars that contributed to the element inhomogeneities within these clusters may have formed in an external environment, such that there was decoupling between the masses of the enriching stars and the time at which their ejecta became incorporated into a cluster.