Nucleosynthesis and energy budget in massive star cores
The core helium flash in low-mass stars of very low or zero metallicity
The luminosity function of globular clusters: theoretical predictions and comparison to real star counts
Problem: the usual mixing-length theory (MLT) of convection is time-independent and local. One needs an improved theory, which is still simple enough to be included in standard stellar evolution codes. Dynamical effects must be excluded as the timescales of stellar evolution are much too long.
Idea: implement a time-dependent, non-local theory similar to MLT into the Garchng stellar evolution code, test its properties, fix free parameters (comparison with observations or dynamical simulations) and then run standard evolution cases. The "Kuhfuss-theory" is one and the first possibility, but others may also be interesting. For this theory, a previous thesis has alread prepared the ground and code, and should be followed first.
Requirements: physical understanding, programming and numerical skills, general overview of stellar evolution
Problem: Significant abundance variations in globular cluster stars are sometimes explained by an earlier phase of pollution by intermediate-mass stars on the AGB. Salaris, Weiss, et al (2006) have shown that this would lead to a separation of normal and anomalous stars on the horizontal branch. Looking at these stars can help to solve the mystery of the origin of the abundance variations.
Idea: Taking spectra of cool and hot HB stars in clusters like M3 and M13 and others one can determine abundances of O, Na, Mg, Al, and C and N. Comparing the spread of abundances as function of Teff along the HB with that predicted by stellar evolution calculations one can verify or falsify ideas about pollution by AGB or RGB stars.
Requirements: active participation in observations of HB stars and in spectral analysis (up to doing it completely alone).Stellar evolution calculations to compare with data.
Note: this project is subject to the availability of observing time or spectra
Problem: ALL population synthesis programs rest on the so-called simple stellar populations (SSP). Observationally these would be globular clusters in the case of old, metal-poor populations. However, a comparison of results from CMDs with integrated spectra of GCs shows often very controversal results, such that one cannot really believe in population synthesis models of non-resolved galaxies.
Idea: One has first to make sure that the SSPs are reliably reproduced by stellar evolution plus spectral synthesis. For this one has to calibrate the stellar evolution models such that they are able to match the main-sequence and red-giant branch (Teff) of globular clusters. The same is true for the luminosity function. This will include a variable mixing-length parameter. Then, a SSP is created and compared with observations (will be available). The aim is to present a set of isochrones which is tested and verified against a broad range of clusters (globular and open).
Requirements: use of observational data, stellar evolution calculations, use of synthetic spectra (with coworkers), analysis of integrated data of clusters
Asteroseismology will be the most important development and active field in stellar astrophysics in the near future. Space missions will allow to probe the interior of stars in all evolutionary phases. For solar models the calibration of models in terms of a well-defined set of input physics data has been shown to be a necessary prerequisite to obtain meaningful physical information from seismology. A similar calibration for general stellar models is completely missing. The aim of the thesis is to provide a set of well-defined input physics modules and to select test-cases (e.g. ZAMS at given composition; evolution to tip of RGB for fixed mass) to be distributed in the community, to obtain/calculate solutions for the test-cases, to compare them and to investigate the remaining residual differences. Since this requires access to different stellar evolution codes (including of course the Garching code) and the cooperation of colleagues at different institutions, this thesis will be done within the COROT/ESTA network.
The matching of star clusters (open or globular) by stellar evolution models in terms of CMDs or LFs is far from being satisfying, since there are too many unknown physical details such as overshooting, rotation, and diffusion. The project aims at investigating in more detail than ever and with a greater variety of physical input stellar clusters in this respect. The clusters should be program clusters for the Eddington or any similar mission. From the quality of the reproduction of cluster properties (CMD, LF, Li-abundance, element variations) a subset of promising stellar models should be selected and investigated further for their oscillation properties. It is to be seen whether these are robust or still uncertain within the range of physical assumptions plausible from the first step. The thesis includes use of the Garching stellar evolution code and the implementation of a stellar oscillation program available in the community. Observational data will have to be assessed, too.
Mixing length theory is still the only practical approach to treat convection in stellar models. Usually, a constant value for the free parameter of the theory (αMLT ), obtained from fitting the solar radius, is applied for all models in all evolutionary phases, while hydrodynamical calculations indicate a clear variation with effective temperature. A varying αMLT will influence in particular effective temperatures on the red giant branch, which in turns has a strong effect on the analysis of elliptical galaxies. In this thesis stellar models with varying αMLT are to be computed such that effective temperatures of red giants in clusters are matched. This has never been attempted before. From this secondary calibration a new set of models for population synthesis should then be provided. As a second aspect of the thesis the models should be matched with up-to-date effective temperatures of stars along cluster branches. There are many individual results from spectroscopic analyses of cluster giants, which have, however, never been used before for checking stellar models.
The dense cores of red giants, as found in globular clusters, are an ideal laboratory for elementary particle physics. In particular, any emission of weakly interacting particles, such as neutrinos or axions, influences the evolution in this phase and leads to measurable effects in the colour-magnitude-diagrams of clusters. In the past Raffelt and Weiss have shown that this puts strong constraints on such particle properties as the electro-magnetic moment of neutrinos or the mass of axions. These limits are in part better than any terrestrial laboratory limit obtained so far. In the last years large progress has been made both in the physics of the stellar models and the observational data. This includes new equations of state and opacities for the models, and detailed star counts along the RGB for the clusters. The thesis is to revisit this problem with all new data and models and investigate updated particle property limits. In particular, the effect of additional energy losses not only on the brightness at the red giant branch tip, but also on the evolutionary speed along the giant branch, the number of stars at certain brightness or in various evolutionary phases (RGB, HB, at the so-called bump) can now be investigated. The thesis includes the calculation of stellar models with an existing program, which will have to be modified only slightly, and the collection and analysis of cluster colour-magnitude diagrams. Some statistical analysis might also be useful.
Extragalactic system very often show super-solar metal abundances. For purpose of population synthesis, appropriate stellar evolution tracks and isochrones have to be provided. However, at such high metallicities, the variation of helium with metallicity becomes a crucial dimension in composition parameter space, contrary to metal-poor systems. The project (master thesis or shorter) is to investigate with up-to-date models the consequences of different dY/dZ-gradients on colours and integrated properties of metal-rich stars and population.