The track of AGB stars in pre-solar dust grains

Presolar dust grains allow deducing the kind of stars that contributed material to the molecular cloud of which our solar system was formed. These grains can be identified in meteoritic material based on their deviating isotopic ratios of some key elements. Due to nucleosynthesis processes depending on stellar mass and evolutionary stage, each potential kind of dust producing stellar source has its characteristic pattern of isotopic ratios, and can, thus, be identified as the source of a given presolar dust grain. For this, however, a correct description of these individual fingerprints is needed. One of the main sources of dust grains in the universe are stars of low and intermediate mass during their so called Asymptotic Giant Branch (AGB) phase, an evolutionary stage of low temperature, high luminosity, and high mass loss, that is preceding the final phase of the star as a white dwarf. These stars seem to be responsible for most of the oxide and silicate grains among the presolar dust particles. Accordingly, a key indicator for characterizing the fingerprint of low- and intermediate mass stars in these grains are the abundances of the three oxygen isotopes 16O, 17O and 18O. Interpretations of presolar grains are mostly done using stellar evolution and nucleosynthesis models. In particular during the past decade this kind of comparison revealed severe differences between standard models and the isotopic ratios found in the dust grains. Several modifications in our understanding of stellar nucleosynthesis and mixing have been suggested. However, what is lacking are accurate isotopic ratios measured directly for evolved stars to constrain the various model approaches. Of paramount importance is the study of multiple constraining indicators to avoid ambiguity and low accuracy of the results. Providing such a set of measurements is the aim of this project. Within the three years of the project, a large sample of stars including AGB stars in the field, in clusters of the LMC and in the LMC field shall be studied. High resolution near-infrared spectra will be obtained for this purpose or exist already in our data archives. We will use synthetic spectra based on state of the art atmospheric models to derive abundances and isotopic ratios by spectral fitting. The results will be confronted with model predictions and measurements from presolar grains, thus directly contributing to our understanding of mixing and nucleosynthesis processes during the decisive late stages of the evolution of low and intermediate mass stars.