Astronomy and Astrophysics, volume 505, 727-733 (2009/10-2)
CNO enrichment by rotating AGB stars in globular clusters.
DECRESSIN T., CHARBONNEL C., SIESS L., PALACIOS A., MEYNET G. and GEORGY C.
Abstract (from CDS):
AGB stars have long been held responsible for the important star-to-star variations in light elements observed in Galactic globular clusters (GCs). 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. We computed the evolution of both standard and rotating stellar models with initial masses between 2.5 and 8M☉ within the metallicity range covered by Galactic GCs. 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 M>4M☉, 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 ~-1.2), it is at odds with the constancy of C+N+O observed among GC low-mass stars. With the given input physics, our models suggest that massive (i.e., ≥4M☉) 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.
stars: AGB and post-AGB - star: rotation - star: abundances - globular clusters: general
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