A Model of Deep Mixing in Globular Cluster Red Giants

A long-standing problem of large star-to-star variations in the surface abundances of C, N, O, Na, and Al that is observed among red giant stars in globular clusters is discussed in the light of the recent finding by Shetrone that 24Mg is reduced among the Al-enhanced stars. A new scenario of nucleosynthesis and element mixing is proposed, whereby the surface anomalies are explained as the consequence of the inward mixing of hydrogen followed by the outward mixing of nuclear products of hydrogen burning. If hydrogen is carried down into the helium core by some extra mixing mechanism, a hydrogen-burning shell flash is ignited. The flash forces the formation of a convective shell whose outer edge extends into hydrogen-rich layers, bringing in fresh hydrogen to fuel the flash further. During a flash, the temperature at the base of the flash-driven shell can become larger than the temperature in the hydrogen-burning shell during quiescent interflash evolution, and during the decay phase of the flash the nuclear products are dredged up by surface convection, which becomes deeper in mass than during the quiescent phases.