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.

This project lead to a much better understanding of the origin of a considerable fraction of so-called pre-solar grains, tiny solids that are found in meteorites representing the composition of dust particles formed in the circumstellar environment of evolved stars. For this, the isotopic compositions of carbon and oxygen found in highly evolved red giant stars was measured and analysed. It is a basic message from stellar and galactic evolution research that all life on earth and this planet itself are in some sense made of stardust, because all the elements heavier than hydrogen, helium and lithium required processing of light elements in the deep interiors of previous generations of stars. But sometimes we even find real pieces of stardust: tiny particles that survived unchanged the formation of the solar system. They represent material once formed in the circumstellar shells of dying stars or in heavy stellar explosions. We can say that this material is indeed of pre-solar origin since it comes with a very special isotopic composition of some key elements. Model computations allow attributing an individual dust grain to a type of star in a particular evolutionary status. However, conclusive observational evidence for these attributions has not been provided yet. Developments on the observational and the modelling side allowed accessing this problem over the last few years, and this has also been the topic of this research project. The formation of dust grains around stars occurs during the late stages of stellar evolution as a part of a very efficient mass loss process that finally ends the life of most stars. Therefore, the focus of this project was set to a study of objects in these very late evolutionary phases, namely red giants. Of particular interest are objects shortly before ejecting their gas and dust envelope, which are difficult to model due to their large extensions, their low surface temperatures, and the pulsational instability of their atmospheres. In the course of this project we obtained high-resolution spectra in the near infrared range for a large sample of highly evolved stars and modelled them with synthetic spectra computed in the course of this project. Together with some very important data sets from observatory archives we secured a dataset of unprecedented size, coverage, and quality to investigate the link between the various types of evolved stars and the detected pre-solar grains. Our analysis of the oxygen and carbon isotopic compositions not only allowed a direct comparison with these grains but also made it possible to determine the stellar mass in a large fraction of the stars in our sample. Mass, however, is the key to all evolutionary effects in stars.