Light nuclei in galactic globular clusters: constraints on the self-enrichment scenario from nucleosynthesis

¹ Institut d'Astrophysique de Paris, UMR7095 CNRS, Univ. P. & M. Curie, 98bis Bd. Arago, 75104 Paris, France e-mail: prantzos@iap.fr
² Geneva Observatory, University of Geneva, chemin des Maillettes 51, 1290 Sauverny, Switzerland e-mail: Corinne.Charbonnel@obs.unige.ch
³ Laboratoire d'Astrophysique de Toulouse et Tarbes, CNRS UMR 5572, OMP, Université Paul Sabatier 3, 14 Av. E. Belin, 31400 Toulouse, France
⁴ Dpt. of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 2759-3255, USA e-mail: iliadis@unc.edu

Received: 31 January 2007
Accepted: 26 April 2007

Abstract

Aims.Hydrogen-burning is the root cause of the star-to-star abundance variations of light nuclei in Galactic globular clusters (GC). In the present work we constrain the physical conditions that gave rise to the observed abundance patterns of Li, C, N, O, Na, Mg, Al, as well as Mg isotopes in the typical case of NGC 6752.

Methods.We perform nucleosynthesis calculations at constant temperature, adopting realistic initial abundances for the proto-cluster gas. We use a detailed nuclear reaction network and state-of-the-art nuclear reaction rates.

Results.Although simplistic, our analysis provides original results and new constraints on the self-enrichment scenario for GCs. Our parametric calculations allow us to determine a narrow range of temperature where the observed extreme abundances of all light elements and isotopes in NGC 6752 are nicely reproduced simultaneously. This agreement is obtained after mixing of the H-processed material with ~ of unprocessed gas. 
We show that the observed C-N, O-Na, Mg-Al, Li-Na and F-Na anticorrelations as well as the behaviour of the Mg isotopes can be recovered by assuming mixing with various dilution factors. Li production by the stars that build up the other abundance anomalies is not mandatory in the case of NGC 6752.

Conclusions.Observations of O, Na, Mg and Al constrain the temperature range for H-burning; such temperatures are encountered in the two main candidate “polluters” proposed for GCs, namely massive AGBs and the most massive main-sequence stars. Furthermore, observations require dilution of H-burning processed material with pristine one. They provide no clue, however, as to the nature of the unprocessed material required for mixing. The complementary observations of the fragile Li and F clearly point to ISM origin for the mixed material.