A model for globular cluster extreme anomalies

In spite of the efforts made in recent years, there is still no comprehensive explanation for the chemical anomalies of globular cluster (GC) stars. Among these anomalies, the most striking is oxygen depletion, which reaches values down to O/Fe∼−0.4 in most clusters, but in M13 it goes down to less than O/Fe∼−1. In this work we suggest that the anomalies are due to the superposition of two different events, as follows. (i) Primordial self-enrichment; this is required to explain the oxygen depletion down to a minimum value O/Fe∼−0.4. (ii) Extra mixing in a fraction of the stars already born with anomalous composition; these objects, starting with already low O/Fe, will reduce the oxygen abundance down to the most extreme values. Contrary to other models that invoke extra mixing to explain the chemical anomalies, we suggest that this mixing is active only if there is a fraction of the stars in which the primordial composition is not only oxygen-depleted, but also extremely helium-rich (Y∼ 0.4), as found in a few GCs from their main-sequence multiplicity. We propose that the rotational evolution (and an associated extra mixing) of extremely helium-rich stars may be affected by the fact that they develop a very small or non-existent molecular weight barrier during the evolution. We show that extra mixing in these stars, having initial chemistry that has already been CNO processed, affects mainly the oxygen abundance, as well as (to a much smaller extent) the sodium abundance. The model also predicts a large fluorine depletion concomitant with the oxygen depletion, and a further enhancement of the surface helium abundance, which reaches values close to Y= 0.5 in the computed models. We stress that, in this tentative explanation, those stars that are primordially oxygen-depleted, but are not extremely helium-rich, do not suffer deep extra mixing.