Ba and Eu abundances in M 15 giant stars^⋆,⋆⋆

¹ Laboratoire Lagrange (UMR 7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 04, France
e-mail: ccworley@ast.cam.ac.uk
² Institute of Astronomy, University of Cambridge, Madingley Rd, Cambridge, CB3 0HA, UK
³ JINA: Joint Institute for Nuclear Astrophysics and the Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
⁴ INAF Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 13 January 2013
Accepted: 21 February 2013

Abstract

Aims. We investigate the Ba and Eu abundances for a sample of 63 giant stars in the globular cluster M 15. This is the largest sample of M 15 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.

Methods. 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.

Results. 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.

Conclusions. 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 M 15.