Dusty Giants

This project is devoted to the study of dust shells around evolved red giant stars, especially those on the Asymptotic Giant Branch (AGB). The AGB is a phase reached by all low- to intermediate-mass stars (M < 8 M sun ) towards the end of their evolution, hence, its investigation provides clues also on the future of our own solar system. On the other hand, earth-like planets could never have formed without the pre-existence of heavy elements and dust mainly produced by AGB stars of earlier stellar generations. Progress in our understanding of the structure and composition of dust surrounding AGB stars is only possible by a combination of (mainly spectroscopic) astronomical observations, calculation of synthetic model spectra and laboratory astrophysics. During the past ten years, a large amount of IR spectroscopic data on circumstellar dust shells and experimental results on cosmic dust analogs became available. Our working group was not only involved in obtaining spectra with the last major IR space telescope, ESA`s ISO satellite, but was also successful with a proposal for observations of AGB stars in a remote globular cluster with NASA`s Spitzer telescope (in orbit since 2003). Even though efforts have been taken at several specialised institutes to interpret the presently available mid-IR spectra of dust shells by means of radiative transfer calculations, the increasing precision of spectroscopic information necessitates major improvements of the modelling procedure. In parallel, further measurements of the IR properties of circumstellar dust analogs are required for an adequate interpretation of above mentioned astronomical spectra. Therefore, the proposed project aims at: a) Completion of laboratory data, especially at wavelengths and temperature regimes that have been neglected hitherto, b) computing a grid of synthetic spectra of AGB atmospheres - including surrounding dust shells - based on state- of-the-art dynamic models, c) interpretation of the Spitzer data recently observed by a combination of the methods a) and b). The main goal to be reached within the proposed project is an understanding of the composition and stratification of dusty circumstellar envelopes. Especially the influence of the dust stratification on the emerging spectra has been neglected in previous modelling procedures. This shortcoming will be surmounted by our new modelling approach (up to now applied to a very limited sub-sample of carbon-rich AGB stars only). Its confrontation with IR spectra of globular cluster AGB stars recently obtained by Spitzer - as well as a larger sample of ISO archive data - will enable us to set essential constraints on the dust formation and evolution in the respective astrophysical systems.

Cosmic dust plays a crucial role in many processes throughout the universe. To a large extent its origins are in the cool circumstellar shells of evolved red giant stars. All stars having low to intermediate masses reach this late evolutionary phase. Consequently, by studying this phase we also take a look into the future of our own solar system. On the other hand, the production of dust and heavy elements in former generations of stars is essential for the formation of earth like planets, too. As a consequence, the research on cosmic dust addresses central questions of our own human existence. In the course of the project we combined - for the first time - three, normally separated scientific approaches carried out in an international collaboration: (1) Datasets of observations on "dying" stars, mainly originating from space telescopes were analysed to get hands on the composition of the associated cosmic dust. For the near future there is a lot more to be expected from the now high inflow of Herschel imaging and spectroscopic data after the satellites successful launch in May 2009. (2) In collaboration with specialised laboratories analogue materials of cosmic dust were studied by spectroscopic means in order to improve the sometimes uncertain identifications of dusty signatures observed in stellar spectra. This successful transdisciplinary approach will be carried on in the future. (3) In the most laborious part of the project the key processes of the dusty mass loss phenomenon were tried to be modelled on computers. Crucial aspects turned out to be the interrelation with the dynamic stellar pulsation, the chemical composition of the stellar atmospheres, being the source of the dust and the complex formation of the dust itself. Only the combination of all three approaches allowed for a deeper and evidence based view on the dust mass loss phenomenon of pulsating, red giant stars and by this into the future of our own star, namely the sun.