Tracing the origins of cosmic dust

Dust represents an important constituent of matter in the universe. It is known that dust nucleation within the interstellar medium (ISM) is negligible, whereas the cool outflows of stars in late phases of stellar evolution fulfill the necessary requirements. The formed dust grains - i.e. microscopic solid bodies with sizes on the order of microns - are injected into the surrounding ISM where they are subject to heavy reprocessing. At some point, they become part of dense molecular clouds and protoplanetary disks which are the starting point for the next generation of stars and planetary systems. In the long run, stardust minerals formed by this process thus provide the chemical basis for life in the universe. Although most of the interstellar dust is formed by grain growth in the ISM, the original stardust (i.e. grains with circumstellar origin) is essential as these grains act as seed particles for further condensation of refractory elements. The dense stellar winds of Asymptotic Giant Branch (AGB) stars represent the primary source of dust in the universe. The largest part of the initial mass of such evolved red giants is ejected during pronounced mass loss phases. The dusty envelopes of AGB stars can be studied by infrared (IR) spectroscopy, as the mineralogical composition is reflected in characteristic spectral features corresponding to the occurring dust species. Observational studies revealed an impressively rich circumstellar dust mineralogy. The proposed project aims at modelling the outflows of mass-losing AGB stars with the focus on a very detailed treatment of the dust formation process (collaboration with H.-P. Gail, Univ. Heidelberg). The wind models will be merged with photospheric models and serve as input for subsequent spectral synthesis. We will make use of the broad expertise in radiative transfer calculations at the applicant`s institute (collaboration with B. Aringer, Univ. Vienna and Astronomical Obs. Padova) leading to detailed realistic synthetic spectra and photometry including the contributions by all kinds of molecular and dust species. Our innovative modelling method with many advantages compared to existing approaches will be applied to generate an extensive model grid. The resulting spectra will enable a systematic and in-depth analysis of the circumstellar mineralogy of mass-losing AGB stars and its dependence on different stellar as well as wind parameters (e.g. L, Teff , M, [Fe/H], C/O, dM/dt). The model grid will be important for the interpretation of various observed spectra (feature identification) or will be used for estimating mass loss rates (fitting of spectral energy distributions, relations with photometric colours). Coupling our model spectra with synthetic AGB evolutionary models (collaboration with L. Girardi and P. Marigo, Astronomical Obs. and Univ. Padova) will allow to simulate the composition of circumstellar dust envelopes while the stars evolve during the AGB phase. In addition, we will be able to give improved estimates for dust production rates of AGB stars over their lifetimes. On the other hand, the computed synthetic spectra and photometry will provide a substantially improved input for population synthesis studies. The detailed and consistent treatment of dust effects will enable a more realistic interpretation of observational data (spectra of spatially unresolved stellar systems studied in the integrated light or colour-magnitude diagrams of populations resolved into individual stars) in the IR, where AGB stars represent the dominant contributors for intermediate-age stellar populations. In this way, the modelling will have an impact also on our understanding of the chemical evolution of galaxies. In summary, the proposed project will shed light on the important role of AGB stars within the cosmic cycle of matter and will thereby facilitate tracing the origins of cosmic dust.

Cosmic dust particles (i.e. small solid bodies with sizes on the order of micrometers) play an important role in the cosmic cycle of matter as they carry matter (esp. heavy elements), energy, and momentum. The cool outflows of red giant stars are believed to be the major source of newly formed dust grains. The project was dealing with this kind of cosmic objects, representing stars like our sun in an advanced evolutionary stage before they expell their outer layers via stellar winds and end their lives as white dwarfs. The circumstellar dust is injected into the surrounding interstellar medium and later incorporated in dense molecular clouds. After substantial reprocessing the grains end up in the next generation of stars and their planetary systems, thereby providing the raw material for earth-like planets and life in the universe (We are stardust.). Three scientific disciplines have to be combined to gather knowledge on circumstellar dust. First, spectroscopic observations of the targets and their circumstellar dust shells in the infrared (where the different dust species leave their spectroscopic footprint) obtained by space observatories (because of the intransparencies of the earths atmosphere). Second, laboratory studies investigating the properties of the corresponding dust material. And third, modelling of the dusty winds with the help of computers and sophisticated algorithms. The last point was the focus of this project. Based on detailed models for the outer layers of evolved red giant stars we calculated observable properties of such objects (as, for example, spectra containing characteristic features of molecules and dust), which can then be compared with observational data. Such a comparison - with a special emphasis on the circumstellar dust mineralogy - provide insights on the dust formation processes in stellar winds (as a function of fundamental stellar and wind parameters). In this way, studies like the ones of this project lead to a better understanding of the (dust) mass-loss phenomenon, the enrichment of the interstellar medium with heavy elements as well as the role of evolved red giants for stellar populations (galaxy evolution) and the whole cosmic cycle of matter. Not least, they shed light on the future of our own solar system. The project was carried out in collaboration with colleagues in Heidelberg, Uppsala, Padova, Brussels, and Vienna.