Ba & Eu Abundances in M15 giant stars

TL;DR

This study analyzes Ba and Eu abundances in 63 giant stars of M15, revealing bimodal distributions linked to r-process nucleosynthesis and complex cluster evolution.

Contribution

It provides the largest Ba abundance dataset for M15 giants and identifies bimodal heavy element distributions with implications for nucleosynthesis and cluster formation.

Findings

Bimodal distribution of Ba, Eu, La abundances in M15 giants

Evidence of r-process dominated pollution in the cluster

Possible Na-O anticorrelations indicating complex evolution

Abstract

To investigate the Ba and Eu abundances for a sample of 63 giant stars in the globular cluster M15. This is the largest sample of M15 giants stars for which Ba abundances have been determined and, due to the target selection of the original research programme, the Ba abundances are complete along the red giant branch. Stellar parameters were taken from the previous key study and a microturbulence-surface gravity relation was determined for precise measurement of the Ba line at 6496.898 Angstroms, which has a high sensitivity to microturbulence. Element abundances for Ba, La, Eu, Ca, Ni and Fe were calculated using spectrum synthesis and equivalent widths techniques. A bimodal distribution in Ba, Eu and La abundances was found within the sample. The low Ba,Eu,La mode had mean abundances of <[Ba/H]>=-2.41+/-0.16, <[Eu/H]>=-1.80+/-0.08 and <[La/H]>=-2.19+/-0.13 while the high Ba,Eu,La mode had mean abundances of <[Ba/H]>=-2.00+/-0.16, <[Eu/H]>=-1.65+/-0.13 and <[La/H]>=-1.95+/-0.11. Both modes are indicative of a pollution scenario dominated by the r-process, hence contributions from explosive nucleosynthesis of massive stars. There may be evidence of further enhancement by another heavy element process and of potential anticorrelations in Na-O for both modes indicating a complex formation and evolution history for M15.