The puzzling MgAl anticorrelation in globular-cluster red giants: primordial plus deep mixing scenario?

TL;DR

This paper investigates the origin of Mg-Al abundance variations in globular-cluster red giants, evaluating primordial and deep mixing models to explain observed anticorrelations and proposing observational tests for these scenarios.

Contribution

It introduces a combined primordial plus deep mixing model to explain Mg-Al anticorrelations, highlighting the need for specific nuclear reaction conditions and stellar processes.

Findings

Deep mixing alone cannot explain Mg-Al anticorrelation without ad hoc assumptions.

Primordial enrichment by intermediate-mass AGB stars can account for initial Mg-Al variations.

Proposes observational tests to distinguish between models.

Abstract

Star-to-star abundance variations of C, N, O, Na and Al in globular-cluster red giants have been recently supplemented by the finding that [Mg/Fe] is depleted in stars with extremely large [Al/Fe] (Shetrone 1996a). This and other new spectroscopic results allow us to test current models of stellar evolution and nucleosynthesis, as well as those of the formation and chemical enrichment of globular clusters. In an effort to explain self-consistently these observations we have considered two possibilities: (1) a deep mixing scenario which assumes that in red giants some kind of (extra)mixing transports products of nuclear reactions from the hydrogen burning shell (HBS) to the base of the convective envelope; and (2) a combination of primordial and deep mixing scenarios. It is shown that (1) cannot account for the anticorrelation of [Mg/Fe] vs. [Al/Fe] without additional ad hoc assumptions, among which we identify a strong but still undetected low energy resonance in the reaction 24Mg(p,gamma)25Al, and episodical increases of the HBS temperature up to the value T approx. 74x10^6 K. In (2) intermediate mass AGB stars are assumed to produce the decreased 24Mg and increased 25Mg initial abundances in some globular-cluster low mass stars and Al is synthesized at the expense of 25Mg in the HBS and transported to the surface of the red giant by extramixing. We discuss advantages and deficiencies of both scenarios and propose some observational tests.