TY - JOUR A1 - Langer, Norbert A1 - Heger, Alexander A1 - Wellenstein, Stephan A1 - Herwig, Falk T1 - Mixing and nucleosynthesis in rotating TP-AGB stars Y1 - 1999 ER - TY - JOUR A1 - Herwig, Falk A1 - Schönberner, Detlef A1 - Blöcker, Thomas T1 - On the validity of the core-mass luminosity relation for TP-AGB stars with efficient dredge-up Y1 - 1998 ER - TY - JOUR A1 - Herwig, Falk A1 - Schönberner, Detlef A1 - Blöcker, Thomas T1 - Violation of the Core Mass - Luminosity relation for AGB models wich experience the thord dredge-up Y1 - 1999 ER - TY - JOUR A1 - Herwig, Falk A1 - Blöcker, Thomas A1 - Schönberner, Detlef T1 - The role of convective boundaries Y1 - 1999 ER - TY - JOUR A1 - Herwig, Falk A1 - Blöcker, Thomas A1 - Langer, Norbert A1 - Driebe, Thomas T1 - On the formation of hydrogen-deficient post-AGB stars N2 - We present an evolutionary sequence of a low mass star from the Asymtotic Giant Branch (AGB) through its post- AGB stage, during which its surface chemical composition changes from hydrogen-rich to strongly hydrogen-deficient as consequence of a very late thermal pulse, following the so-called born-again scenario. The internal structure and abundance changes during this pulse are computed with a %newly developed numerical method which allows the physically consistent calculation of stellar layers where thermonuclear and mixing time scale are comparable --- a situation which occurs when the helium flash driven convection zone extends to the hydrogen-rich surface layers during the pulse peak. The final surface mass fractions are [He/C/O]=[0.38/0.36/0.22], where the high oxygen abundance is due to diffusive overshoot employed during the AGB evolution. These models are the first to achieve general agreement with the surface abundance pattern observed in hydrogen-deficient post-AGB stars --- e.g. the PG 1159 stars or the WR-type central stars of planetary nebulae ---, confirming the born-again scenario with a physically consistent calculation and supporting the occurrence of convective overshooting in thermally pulsing AGB stars. Y1 - 1999 ER - TY - JOUR A1 - Herwig, Falk A1 - Blöcker, Thomas T1 - Overshoot in giant stars Y1 - 1999 ER - TY - JOUR A1 - Herwig, Falk T1 - Internal mixing and surface abundance of [WC]-CSPN N2 - Recent advances in constructing stellar evolution models of hydrogen-deficient post-asymptotic giant branch (AGB) stars are presented. Hydrogen-deficiency can originate from mixing and subsequent convective burning of protons in the deeper layers during a thermal pulse on the post-AGB (VLTP). Dredge-up alone may also be responsible for hydrogen- deficiency of post-AGB stars. Models of the last thermal pulse on the AGB with very small envelope masses have shown efficient third dredge-up. The hydrogen content of the envelope is diluted sufficiently to produce H-deficient post-AGB stars (AFTP). Moreover, dredge-up alone may also cause H-deficiency during the Born-again phase (LTP). During the second AGB phase a convective envelope develops. A previously unknown lithium enrichment at the surface of Born-again stellar models may be used to distinguish between objects with different post-AGB evolution. The observed abundance ratios of C, O and He can be reproduced by all scenarios if an AGB starting model with inclusion of overshoot is used for the post- AGB model sequence. An appendix is devoted to the numerical methods for models of proton capture nucleosynthesis in the He-flash convection zone during a thermal pulse. Y1 - 2001 ER - TY - JOUR A1 - Herwig, Falk T1 - The evolution of AGB stars with convection overshoot Y1 - 2000 ER - TY - JOUR A1 - Driebe, Thomas A1 - Schönberner, Detlef A1 - Blöcker, Thomas A1 - Herwig, Falk T1 - The evolution of helium white dwarfs : I. the companion of the millisecond pular PSR J1012+5307 Y1 - 1998 ER - TY - JOUR A1 - Driebe, Thomas A1 - Blöcker, Thomas A1 - Schönberner, Detlef A1 - Herwig, Falk T1 - The evolution of helium white dwarfs : II. Thermal instabilities N2 - We calculated a grid of evolutionary models for white dwarfs with helium cores (He-WDs) and investigated the occurrence of hydrogen-shell flashes due to unstable hydrogen burning via CNO cycling. Our calculations show that such thermal instabilities are restricted to a certain mass range (M approx 0.21 ... 0.30 Msun), consistent with earlier studies. Models within this mass range undergo the more hydrogen shell flashes the less massive they are. This is caused by the strong dependence of the envelope mass on the white dwarf core mass. The maximum luminosities from hydrogen burning during the flashes are of the order of 105 Lsun. Because of the development of a pulse-driven convection zone whose upper boundary temporarily reaches the surface layers, the envelope's hydrogen content decreases by Delta X approx 0.06 per flash. Our study further shows that an additional high mass-loss episode during a flash-driven Roche lobe overflow to the white dwarf's companion does not affect the final cooling behaviour of the models. Independent of hydrogen shell flashes the evolution along the final white dwarf cooling branch is determined by hydrogen burning via pp-reactions down to effective temperatures as low as approx 8000 K. Y1 - 1999 ER - TY - JOUR A1 - Driebe, Thomas A1 - Blöcker, Thomas A1 - Herwig, Falk A1 - Schönberner, Detlef T1 - Diffusive overshooting in hot bottom burning AGB models Y1 - 1998 ER - TY - JOUR A1 - Blöcker, Thomas A1 - Driebe, Thomas A1 - Herwig, Falk T1 - AGB evolution with overshoot : hot bootom burning vs. dredge-up Y1 - 1999 ER -