Flash‐assisted Deep Mixing in Globular Cluster Red Giants

Abstract

We discuss a new model of deep mixing in red giants caused by shell Ñashes 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 Ñashes in red giants. It is shown that, during sufficiently strong Ñashes, both 24Mg and 23Na are depleted by burning. For moderately and very metal-poor clusters, the burned 24Mg is converted mostly into 27Al. Thus, the model not only resolves the difficulties arising from the observed underabundance of 24Mg, but also removes the problem of sodium overproduction, which would otherwise accompany the aluminum enrichment observed. For moderately metal-poor cluster giants, 25Mg and 26Mg 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 28Si. For metal-rich cluster giants, shell Ñashes are quenched before 25Mg and 26Mg 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.

Thermal Stability of Rotating Low‐Mass Subgiants and Red Giants

“…To check out this hypothesis, which could potentially provide an explanation for the phenomenon of Li-rich red giants (Palacios et al 2001) and the C-N, O-Na, and Mg-Al abundance correlations in globular cluster giants (see, e.g., Aikawa et al 2001, Messenger & Lattanzio 2002, we have prepared computer programs that (1) treat the effects of rotation on the internal structure and evolution of low-mass stars and (2) perform a linear analysis of the thermal stability of these stars. The code designed for the latter purpose was found to give results for a nonrotating 0.8 M stellar model having Y ¼ 0:20 and Z ¼ 0:0001 (a test case) essentially identical to those obtained using an improved version of the original Von Rudloff et al code, which was also prepared as part of this investigation.…”

“…Following Kippenhahn & Thomas (1970) and Meynet & Maeder (1997), we use the Roche potential, which leads 1 Aikawa et al (2001) have proposed that a thermal instability of the H-burning shell may be caused by the ingestion of protons into the He core just below the H shell by rotationally induced extra mixing. Similarly, Palacios et al (2001) to the following equation for the isobaric surfaces (giving them axial and equatorial symmetry):…”

“…The anticorrelations of C-N, O-Na, and Mg-Al indicate the simultaneous operation of the CNO, NeNa, and MgAl cycles of H burning. If the H-burning shells in lowmass metal-poor RGB stars were to experience sufficiently powerful thermonuclear flashes (such as those associated with the He shell in AGB stars), it is possible that they could produce chemical abundance anomalies resembling those observed in GCs (Aikawa, Fujimoto, & Kato 2001). Recently, Messenger & Lattanzio (2002) tried to similate an instability in the H-burning shell, by artificially altering the quantity nucl , to follow up on the suggestion made by Von Rudloff et al (1988) that rotation may cause a thermal instability of the H-burning shell in low-mass RGB stars.…”

The s-Process Nucleosynthesis in Extremely Metal-Poor Stars as the Generating Mechanism of Carbon Enhanced Metal-Poor Stars

“…According to the prescription in [6], we followed the time evolution of the temperature of the nucleosynthesis site during the helium shell flashes. Table I shows the list of models computed in this study.…”

Various Modes of Helium Mixing in Globular Cluster Giants and Their Possible Effects on the Horizontal Branch Morphology

“…In the second scenario, on the other hand, the mixing has to be intermittent. Although both types of mixing are conceivable in principle, it is only the latter mechanism that has been demonstrated to be able to reproduce the abundance anomalies of Al and Mg and the relationships between Na and Al and between Mg and Al observed in globular cluster giants (Aikawa et al 2001(Aikawa et al , 2004. However, it is also true that the latter mechanism with high temperatures alone cannot explain the observed abundance variations of C and O that are much larger than that of Mg; there should be a supplementary mechanism for mixing matter processed in a lowtemperature range in which C and O can burn but 24 Mg cannot (Cavallo et al 1998), which can be due either to intermittent mixing with weak flashes of lower temperatures or to continuous mixing from quiescent hydrogen shell burning, or to both ( Fujimoto et al 1999).…”