Abundance trends among neutron capture elements in giants of globular clusters M5, M3, M13, M92, and M15

Abstract

Abundances of Y, Ba, Ce, and Nd are presented for giants in the globular clusters M5, M3, M13, M92, and M15. The spectra for these analysis are the same Lick Hamilton echelle spectra that have been used previously to demonstrate the existence of large, anticorrelated abundance variations of oxygen and sodium among giants in each of these clusters. The four heavy elements studied here display no such variations. They correlate neither with Na/Fe nor O/Na abundance ratios, and the observed star-to-star scatters within each cluster are not in excess of observational uncertainties. The abundance ratios among these elements and with respect to the Fe-peak elements agree well with field stars of the same metallicity range. The elements Y, Ba, Ce, and Nd are formed exclusively by neutron capture nucleosynthesis events; thus their near constancy as Na/Fe varies by 0.5-0.8 dex within a cluster, and the constancy of Sc (whose abundance is also sensitive to neutron capture events), places rather severe constraints on possible neutron capture scenarios for the production of Na. This (negative) result strengthens the case for globular cluster Na enhancements via proton capture syntheses.