Institute for Computational Astrophysics

Globular Cluster Archaeology: Nucleosynthesis and Extra Mixing in Extinct Stars

Wednesday 19th May, 2004
10.30 am, McNally Main (MM) 310, SMU campus  

Globular Cluster Archaeology: Nucleosynthesis and Extra Mixing in Extinct Stars

Dr. Pavel Denissenkov
University of Victoria

I have used the latest observational data on the evolutionary variations of the surface chemical composition in low-mass metal-poor stars to constrain the basic properties of extra mixing in the majority of upper red giant branch (RGB) stars. A possible mechanism of this "canonical" extra mixing is turbulent diffusion driven by rotation. I have also put forward the hypothesis that some of the upper RGB stars may be experiencing "enhanced" extra mixing, which is much faster and somewhat deeper than canonical extra mixing. This could explain the phenomenon of Li-rich giants. I propose that enhanced extra mixing may be triggered by the spinning up of these giants as a result of their swallowing of jovian-type planets or due to the tidal synchronization of their spin and orbital rotation in close binary systems.

I have also computed the evolution of the surface abundances of C, N and O in two types of stars that are thought to have contributed to the star-to-star abundance variations in globular clusters: intermediate-mass asymptotic giant branch (IM-AGB) stars and low-mass upper RGB stars. I will show that the variation of the C abundance by a factor of 10 accompanied by apparent signatures of O burning, such as a very high N abundance or a deficit in O, as observed in some of the globular cluster subgiants and red giants, could not originate from the hot-bottom burning in the IM-AGB stars. At the same time, such the peculiar CNO abundances could come from the vicinity of the hydrogen burning shell in the upper RGB stars, provided that some of them had experienced enhanced extra mixing in the past. This is a robust nucleosynthesis prediction. It is confirmed by a simple formula that allows to "measure" temperatures in the hydrogen burning shells in extinct stars. This new finding makes the problem of the origin of the abundance inhomogeneities in globular clusters more intriguing.