Evolution on the AGB and beyond: On the formation of H-deficient post-AGB stars

TL;DR

This paper reviews the evolution of post-AGB stars, emphasizing thermal pulses and overshoot modeling, to explain the formation of hydrogen-deficient Wolf-Rayet central stars through three distinct thermal pulse scenarios.

Contribution

It introduces overshoot as a crucial factor in modeling these stars and distinguishes three thermal pulse scenarios leading to hydrogen deficiency.

Findings

Overshoot modeling reproduces surface abundances of Wolf-Rayet stars.

Three thermal pulse scenarios (AFTP, LTP, VLTP) explain hydrogen deficiency.

Efficient dredge-up occurs even during the post-AGB stage.

Abstract

The evolution on the AGB and beyond is reviewed with respect to the origin of Wolf-Rayet central stars. We focus on thermal pulses due to their particular importance for the evolution of hydrogen deficient stars. It is shown that overshoot applied to all convection regions is a key ingredient to model these objects leading to intershell abundances already close to the surface abundances of Wolf-Rayet central stars. In contrast to standard evolutionary calculations, overshoot models do show dredge up for very low envelope masses and efficient dredge up was found even during the post-AGB stage. Three thermal pulse scenarios for Wolf-Rayet central stars can now be distinguished: an AGB Final Thermal Pulse (AFTP) occurring at the very end of the AGB evolution, a Late Thermal Pulse (LTP) occurring during the post-AGB evolution when hydrogen burning is still on, and a Very Late Thermal Pulse (VLTP) occurring on the cooling branch when hydrogen burning has already ceased. All scenarios lead to hydrogen-deficient post-AGB stars with carbon and oxygen abundances as observed for Wolf-Rayet stars. Hydrogen is either diluted by dredge up (AFTP, LTP) or completely burnt (VLTP).