An attempt to model globular cluster red giant abundance anomalies with a simulated hydrogen shell instability

It has been suggested that the anomalous abundances of Na, Mg and Al observed in globular cluster red giant stars could be the result of a thermally unstable hydrogen shell. Currently accepted reaction rates indicate that temperatures of approximately 70–75×10⁶ K are required to produce the observed enhancements in Na and Al along with depletions in Mg.

The work presented here attempts to model the H shell instability by a simple mechanism of altering the energy production in the region of the H shell. We show that even extreme cases only give rise to small intermittent temperature increases that have minimal effect on the surface abundances. Full evolutionary modelling incorporating this technique simply accelerates the evolution of the red giant branch (RGB) phase, producing the same surface abundances as other models but at an earlier time. We conclude that, unless hydrogen shell instabilities manifest themselves quite differently, they are unlikely to lead to the required temperatures and alternative explanations of the abundance anomalies are more promising.