3. The AGB starting model
We start with an AGB model with which has been evolved over 22 thermal pulses, including convective overshoot at all convective boundaries. The treatment and efficiency (f=0.016) of overshoot is the same as in Herwig et al. (1997). In comparison to models without overshoot the intershell region is much stronger enriched in carbon and oxygen (mass fractions [He/C/O]=[0.35/0.43/0.19]), which causes a stronger third dredge-up (Herwig et al., 1999). At the thermal pulse (TP) the hydrogen-free core has a mass of and dredge-up starts to operate, leading to a carbon star model at the last computed TP. At this stage the model star has a total mass of and . We then artificially increase the mass loss () in order to force the model to leave the AGB at the right phase to develop a very late thermal pulse. This procedure is justified for this exploratory work because it does not affect the nucleosynthesis and mixing during the very late TP.
© European Southern Observatory (ESO) 1999
Online publication: August 25, 1999