Stellar evolution of low and intermediate-mass stars. Pt. 1 Mass loss on the AGB and its consequences for stellar evolution

We have performed extensive stellar evolution calculations for initial masses between 1 and 7 M_sun all the way from the main sequence through the AGB towards the stage of white dwarfs. Mass loss has been taken into account with different descriptions for the RGB, AGB and post-AGB phase. On the AGB we considered mass loss with a formula based on dynamical calculations of the atmospheres of Mira-like stars. Our results are consistent with empirical initial-final mass relations. It is shown that hot bottom burning and mass loss are closely connected. The overluminosity as well as the nucleosynthesis of hot bottom burning models can depend sensitively on mass loss. Furthermore, we discuss the influence of mass loss on the internal structure reached at the tip of the AGB, and emphasize that post-AGB timescales will depend strongly on the AGB history, i.e. on the initial mass and the applied mass-loss law. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Stellar evolution of low and intermediate-mass stars. Pt. 2 Post-AGB evolution

We present a set of evolutionary tracks for central stars of planetary nebulae in the range from 0.53 to 0.94 M_sun. These models are based on extensive stellar evolution calculations for initial masses between 1 and 7 M_sun which have been carried out all the way from the main sequence through the AGB towards the stage of white dwarfs. Concerning mass losses during the post-AGB evolution we smoothly reduced the high AGB mass-loss rates and then applied rates adapted from the radiation driven wind theory. The transition time from the AGB to the centralstar region depends strongly on the treatment of mass-loss beyond the AGB. Furthermore, our calculations indicate that massive central stars can fade much more slowly than hitherto assumed. It is shown that the fading time scales and the aGB history are closely connected, and that therefore the preceding mass loss on the AGB plays an important role for the post-AGB evolution. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Stellar evolution of low and intermediate-mass stars III An application of evolutionary post-AGB models: the variable central star FG Sagittae

Based on a set of evolutionary calculations for thermally pulsating post-AGB models we introduce a robust method to measure FG Sge's mass which was found to be 0.61 M_sun. The corresponding evolutionary timescale is consistent with the expansion age of the planetary nebula. Assuming that FG Sge's surface was already enriched with heavy elements on the AGB we propose that during the central-star evolution these elements were removed from the superficial radiative layers due to, e.g., dust/gas separation, leaving the deeper convective layers unchanged. During the flash the outward moving envelope convection mixed the stored heavy elements back to the surface. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Evolutionary models of white dwarfs with helium cores

We present seven evolutionary tracks for low-mass white dwarfs with helium cores, ranging in mass from 0.179 to 0.414 M_sun. We generated the pre-white dwarf models from a 1 M_sun sequence extending up to the tip of its red-giant branch by applying high mass-loss rates at appropriate positions, and we followed their evolution across the Hertzsprung-Russell diagram and down the cooling path. We discuss the internal structures and cooling properties of these new models and compare them with those of recently published models for low-mass white dwarfs which are based on simplified initial configurations. We also demonstrate that our new models seem to remove the apparent discrepancies between the characteristic ages of millisecond pulsars and the cooling ages of their white dwarf companions. (orig.)Proceedings of the 11. European workshop on white dwarfs, 19 refs.Available from TIB Hannover: RR 7310(98-25) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

On the validity of the core-mass luminosity relation for TP-AGB stars with efficient dredge-up

We investigate the validity of the core mass - luminosity relation (CMLR), orginally described by Paczynski (1970), for asymptotic giant branch stars under the presence of third dredge-up events. We find, that models with efficient third dredge-up with less massive cores than those associated with hot bottom burning (Bloecker and Schoenberner 1991) do not obey the linear CMLR. Complete evolutionary calculations of thermal pulse stellar models which consider overshoot according to an exponential diffusive algorithm show systematically larger third dredge-up for lower core masses (0.55 M_sun<or#approx#M_H<or#approx#0.8 M_sun) than any other existing models. We present and discuss the luminosity evolution of these models. (orig.)Available from TIB Hannover: RR 7310(98-29) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The role of convective boundaries

We investigate the influence of convective overshoot on stellar evolution models of the thermal pulse AGB phase with M_Z_A_M_S = 3M_sun. An exponential diffusive overshoot algorithm is applied to all convective boundaries during all evolutionary stages. We demonstrate that overshooting at the bottom of the pulse-driven convective zone, which forms in the intershell during the He-shell flash, leads to more efficient third dredge-up. Some overshoot at the bottom of the convective envelope removes the He-H discontinuity, which would otherwise prohibit the occurrence of the third dredge-up for this stellar mass. However, no correlation between the amount of envelope overshoot and the efficiency of the third dredge-up has been found. Increasingly efficient third dredge-up eventually leads to a carbon star model. Due to the partial mixing efficiency in the overshoot region a "1"3C-pocket can form after the third dredge-up event which may be crucial for n-capture nucleosynthesis. (orig.)15 refs.Available from TIB Hannover: RR 7310(98-38) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The evolution of helium white dwarfs. Pt. 2 Thermal instabilities

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 M_sun), 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 10"5_sun. 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. (orig.)Available from TIB Hannover: RR 7310(99-41) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Thermodynamical properties of stellar matter. Pt. 1 Equation of state for stellar interiors

Considering the Helmholtz free energy which provides detailed knowledge of the charged many-particle physics, we present an analytical formulation of the equation of state (EOS) for fully ionized matter. The nonideal effects due to exchange and correlation interactions of charged particles are described by Pade approximants, which cover a wide range of densities and temperatures. The formulae are valid at any electron degeneracy, for a very broad region of Coulomb coupling, and are applicable to any chemical mixture. Relativistic generalizations are taken into account for the most predominant astrophysical conditions. Finally we present results for H, He, C and O plasmas and give comparisons with other EOS approaches. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

From the tip of the AGB towards a planetary: a hydrodynamical simulation

We present a first exploratory investigation of the dynamical evolution of a dusty stellar wind envelope along the upper AGB and its transformation into a planetary nebula. We find the existence of AGB stars with detached shells to be a natural consequence of the mass loss variations during a thermal pulse. It is also demonstrated that due to the large dynamical effects caused by the ionizing radiation field and the fast wind of the central star, it is impossible to deduce the AGB mass loss history from the planetary's density and velocity distribution. The structure of the halo, however, is still determined by the AGB mass loss history. The rapid decline of mass loss expected in the aftermath of thermal pulses leads to extended shells of low densities and explains halos with sharp boundaries. (orig.)15 refs.Available from TIB Hannover: RR 7310(96-11) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Through the upper AGB towards a planetary: A hydrodynamical simulation. -Time-dependent hydrodynamical models of circumstellar dust shells around carbon-and oxygen-rich AGB stars

Available from TIB Hannover: RR 7310(96-17)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDeutsche Agentur fuer Raumfahrtangelegenheiten (DARA) GmbH, Bonn (Germany)DEGerman