Institute for Computational Astrophysics

Nucleosynthesis in Asymptotic Giant Branch Stars

Friday 6th February, 2004
3 pm, McNally Main (MM) 310, SMU campus

Nucleosynthesis in Asymptotic Giant Branch Stars

Dr. Amanda Karakas,
Institute for Computational Astrophysics and
Department of Astronomy & Physics,
Saint Mary's University

All stars in the mass range 1 to about 8 solar masses pass through the Asymptotic Giant Branch (AGB) phase of stellar evolution. The AGB is the last nuclear-burning phase for these stars and is very short, comprising less than 1 per cent of the main-sequence lifetime. Nevertheless, it is on the AGB that the richest nucleosynthesis occurs for low and intermediate mass stars. The nucleosynthesis is driven by thermal instabilities of the helium-burning shell, the products of which are mixed to the stellar surface by recurrent mixing episodes. Envelope burning occurs in the most massive AGB stars, which also alters the surface composition. The AGB phase is terminated when rapid, episodic mass loss expells the envelope into the interstellar medium. Thus AGB stars are important contributors to the chemical evolution of stellar systems (and galaxies), since they are responsible for producing most of the 12C, 14N and for the distribution of the main (nuclei with A > 85) component of slow-neutron capture elements in the solar system.

In this talk I will give an introduction to the structure and evolution of AGB stars. Using the latest results of stellar models I have calculated, I will discuss the sites of mixing and nucleosynthesis inside AGB stars. It is found that intermediate mass stars (over about 4 solar masses, depending on the initial composition) may be efficient at producing sodium, the heavy magnesium isotopes and 26Al. Low mass stars (less than 4 solar masses) are important for producing carbon and possibly nitrogen and sodium. I will also show an application of my results to the evolution of the heavy magnesium isotopes in the galaxy. Finally, I discuss some of the many uncertainties that affect the results, such as the modelling of convection, mass loss and nuclear reaction rates.