Post-Asymptotic Giant Branch Stellar Evolution and Nucleosynthesis

01.01.2017 bis 31.12.2018

The understanding of stars and stellar evolution is of central importance to astrophysics. In particular, asymptotic giant branch (AGB) stars play an important role in the chemical enrichment of the galaxies, and in the creation of heavy elements through slow-neutron-capture-processes (s-processes) (Herwig 2005).
The transition between the AGB and white dwarf phases, from now on the post-AGB phase, is the least understood phase of the evolution of low-mass single stars. Studies of the Central Stars of Planetary Nebulae (CSPNe) and post-AGB stars point to significant discrepancies with the theoretical predictions (e.g. Brown et al. 2008, Gesicki et al 2014). In particular, in spite of all theoretical evidence to the contrary, the Planetary Nebulae Luminosity Function (PNLF) is a precise extragalactic standard candle (Ciardullo 2012).
About a fifth of the stars departing from the AGB are expected to undergo a final thermal pulse during their post-AGB evolution (“Born Again AGB” scenario, Iben 1984). During these events the core of the former AGB star is exposed to the surface either through burning or dilution of the remaining H-rich envelope. Post-AGB H-deficient stars, such as [WC]-CSPNe and PG1159 (see Werner & Herwig 2006) are presently understood to be the bare core of AGB stars which were exposed after a born again event. While in AGB stars the s-processed material can only be observed once it is dredged up to the much more massive H-rich envelope, in PG1159 stars the regions where s-processes took place can be directly observed at the photosphere. Consequently, spectroscopic studies of these stars offer unique information to test our current understanding of the formation of s-process elements.
In this context, the objectives of the present plan are: a) to improve our present understanding of s-process nucleosynthesis on the AGB by studying H-deficient post-AGB stars, and b) to improve our understanding of the post-AGB evolution by studying the PNLF and post-AGB in nearby populations.