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Kitsikis, Agis (2008): Theoretical AGB and post-AGB Models for Synthetic Population Studies. Dissertation, LMU München: Faculty of Physics
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Abstract

Near the end of their evolution, moderate-mass stars (approximatively between 1 and 8 solar masses) go through the asymptotic giant branch (AGB) phase. The AGB and the transition to the ensuing post-AGB are very time (both computational and human) consuming to simulate. Hence, large grids of models originating from detailed calculations are rare; only few stars are generally computed. Some stellar libraries have nonetheless been constituted in the past. These are very sought-after by population synthesis modelers, as they try to reproduce observed properties of larger stellar systems by assembling together many individual models. However such AGB and post-AGB grids can rapidly become outdated because of constant input-physics ameliorations. So-called synthetic AGB models are a solution as they are computationally less time demanding: nonetheless their major drawback is that they remain approximations. In this context, new detailed models for stars of 1 to 6 solar masses, and compositions with a total heavy element content that corresponds to values between a fortieth and a double of the solar one (for two different relative scalings), have been computed. They evolve through all stellar phases from the main sequence to the white dwarf stage; here focus is on the AGB and post-AGB parts. The parameter space coverage is comparable only with the latest synthetic grids, as none of the previous detailed stellar libraries extend over such wide mass/metallicity intervals. Furthermore, the most recent physics, relevant for the AGB and post-AGB, are included. These updates, at the level of sophistication brought by this work, are present neither in synthetic nor in any detailed computations. Because they are of paramount importance, their inclusion is the major asset of this work. Specifically, three updates have been implemented: a) new chemistry-dependent low-temperature opacities b) up-to-date chemistry-dependent mass-loss prescriptions c) convective overshooting. With overshooting, envelope mixing will be more efficient and dredge-up additional material from deeper layers. The stellar surface will thus be converted from an oxygen- to a carbon-rich one, and changes will directly affect the opacities and in turn the mass-loss rates. While ignored in previous models, these transformations are now taken into account. We show, how such amendments were a necessity by comparing the results with older grids, and how models are in agreement, without any prior calibration, with observations. The new stellar library contains 100 AGB and 60 post-AGB sequences. The AGB tracks cover the entire parameter space. Post-AGB models are also available for the full metallicity range but, because of convergence issues, only for star between 1 and 2 solar masses. To our knowledge this work provides the scientific community with the most up-to-date, self-consistent and extended grid, originating from detailed AGB and post-AGB models. It is expected to be a valuable input-tool for population synthesis work and aims to supersede the more than 10-year old outdated tracks that are currently still being used