CNO enrichment by rotating AGB stars in globular clusters

¹ Geneva Observatory, University of Geneva, 51 ch. des Maillettes, 1290 Versoix, Switzerland e-mail: decressin@astro.uni-bonn.de
² Argenlader Institut für Astronomie (AIfA), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS UMR 5572, Université de Toulouse, 14 Av. E. Belin, 31400 Toulouse, France
⁴ Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, ULB – CP 226, 1050 Brussels, Belgium
⁵ Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia
⁶ Groupe de Recherche en Astronomie et Astrophysique du Languedoc, UMR 5024, Université Montpellier II, CNRS, place Eugène Bataillon, 34095 Montpellier, France

Received: 10 February 2009
Accepted: 14 July 2009

Abstract

Context. AGB stars have long been held responsible for the important star-to-star variations in light elements observed in Galactic globular clusters (GCs).

Aims. We analyse the main impacts of a first generation of rotating intermediate-mass stars on the chemical properties of second-generation GC stars. The rotating models were computed without magnetic fields and without the effects of internal gravity waves. They account for the transports by meridional currents and turbulence.

Methods. We computed the evolution of both standard and rotating stellar models with initial masses between 2.5 and 8  within the metallicity range covered by Galactic GCs.

Results. During central He-burning, rotational mixing transports fresh CO-rich material from the core towards the hydrogen-burning shell, leading to the production of primary ¹⁴N. In stars more massive than  , the convective envelope reaches this reservoir during the second dredge-up episode, resulting in a large increase in the total C+N+O content at the stellar surface and in the stellar wind. The corresponding pollution depends on the initial metallicity. At low- and intermediate-metallicity (i.e., [Fe/H] lower than or equal to ), it is at odds with the constancy of C+N+O observed among GC low-mass stars.

Conclusions. With the given input physics, our models suggest that massive (i.e., ≥4 ) rotating AGB stars have not shaped the abundance patterns observed in low- and intermediate-metallicity GCs. Our non-rotating models, on the other hand, do not predict surface C+N+O enhancements, hence are in a better position as sources of the chemical anomalies in GCs showing the constancy of the C+N+O. However at the moment, there is no reason to think that intermediate-mass stars were not rotating. On the contrary there is observational evidence that stars in clusters have higher rotational velocities than in the field.