CNO enrichment by rotating AGB stars in globular clusters

TL;DR

This study investigates how rotating AGB stars influence the chemical composition of second-generation stars in globular clusters, finding that rotation affects element enrichment and challenges previous assumptions about their role in observed abundance patterns.

Contribution

It provides the first detailed models of rotating intermediate-mass AGB stars and assesses their impact on globular cluster chemical anomalies, highlighting limitations of rotation in explaining observations.

Findings

Rotating models produce primary 14N during He-burning.

Massive rotating AGB stars increase C+N+O in stellar winds.

Non-rotating models better match observed C+N+O constancy.

Abstract

AGB stars have long been held responsible for the important star-to-star variations in light elements observed in Galactic globular clusters. We analyse the main impacts of a first generation of rotating intermediate-mass stars on the chemical properties of second-generation globular cluster 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 8 Msun within the metallicity range covered by Galactic globular clusters. 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 > 4 Msun, 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, it is at odds with the constancy of C+N+O observed among globular cluster low-mass stars. With the given input physics, our models suggest that massive rotating AGB stars have not shaped the abundance patterns observed in low- and intermediate-metallicity globular clusters. Our non-rotating models, on the other hands, do not predict surface C+N+O enhancements, hence are in a better position as sources of the chemical anomalies in globular clusters 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.