A deep insight into the Mg–Al–Si nucleosynthesis in massive asymptotic giant branch and super-asymptotic giant branch stars

The stars in globular clusters are known to differ in their surface chemistry: spectroscopic investigations in recent decades outlined the presence of star-to-star differences in the abundances of the light elements, up to aluminium (and possibly silicon), suggesting that some stars were contaminated by an advanced proton-capture nucleosynthesis. The asymptotic giant branch (AGB) stars are one of the most promising candidates in producing the pollution of the intracluster medium, via the ejection of gas processed by hot bottom burning, from which new stellar generations are formed. This work is focused on the degree of nucleosynthesis involving magnesium, aluminium and silicon that these sources may experience. The key ingredient in determining the degree of magnesium depletion, and the amount of aluminium that can be produced, is the rate of proton capture on 25Mg, forming 26Al; an increase in this cross-section by a factor of 2 with respect to the highest value allowed by the NACRE compilation allows the reproduction of the extent of the Mg depletion observed, and is in qualitative agreement with the positive Al–Si correlation observed in a few clusters. The main uncertainties associated with the macro- and microphysics input are discussed and commented upon, and a comparison with recent spectroscopic results for globular clusters showing some degree of Mg–Al anticorrelation and Al–Si correlation is presented.