2013A&A...553A..47W

We 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 giant 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 Å, 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.