Flash-assisted deep mixing in globular cluster red giants

We discuss a new model of deep mixing in red giants caused by shell flashes triggered by the inward penetration of hydrogen into the helium core. The objective is to explain the large star-to-star variations of proton-capture elements that are observed among globular cluster red giants (Fujimoto, Aikawa, & Kato). The characteristics of this mixing model are explored by computing the products of nucleosynthesis during hydrogen shell flashes in red giants. It is shown that, during sufficiently strong flashes, both Mg-24 and Na-23 are depleted by burning. For moderately and very metal-poor clusters, the burned Mg-24 is converted mostly into Al-27. Thus, the model not only resolves the difficulties arising from the observed underabundance of Mg-24, but also removes the problem of sodium overproduction, which would otherwise accompany the aluminum enrichment observed. For moderately metal-poor cluster giants, Mg-25 and Mg-26 survive with nearly scaled-solar abundances, while for very metal-poor cluster giants, all the magnesium isotopes are depleted and aluminum also burns appreciably into Si-28. For metal-rich cluster giants, shell flashes are quenched before Mg-25 and Mg-26 can burn to synthesize aluminum. These properties agree well with the manner in which observed abundance anomalies vary with cluster metallicity, including the absence of large Al-enrichments for red giants in metal-rich clusters. From a detailed comparison with observed abundances in M13 giants, we deduce constraints on the strength of the mixing mechanism responsible for the inward penetration of hydrogen into the helium core. We discuss further implications of the present results, in particular the importance of star-star interactions for the proper understanding of the evolution of stars in globular clusters.