Physics
Scientific paper
Aug 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd..11e..13h&link_type=abstract
Pre-Solar Grains as Astrophysical Tools, 26th meeting of the IAU, Joint Discussion 11, 21 August 2006, Prague, Czech Republic, J
Physics
Scientific paper
Presolar grains are formed in cool stellar outflows like AGB star winds and supernova ejecta [S: :S][Author ID2: at Sat Jul 15 10:53:00 2006 ]which are mostly oxygen-rich. The details of their formation process, however, are still only poorly understood. Laboratory experiments do provide guidance for our understanding and also for theoretical approaches on dust formation, but only the link to observations does foster solid understanding of the origin of pre-solar grains. The laboratory results are most beneficially linked to observations via a model of the micro-physics of dust formation. Our model is based on the observation, that seed formation and subsequent growth are governed by considerable differences in their characteristic time-scales which, hence, justifies the separate treatment of the two processes. The creation of the first seed particles requires large supersaturation ratios or, equally, low temperatures where many solid compounds are already thermodynamically stable, and condense simultaneously on these seeds. Hence, in an oxygen-rich environment we expect the formation of dirty grain mantles which contain numerous small islands of different solid materials. We present a model where physisorbed gaseous species are transported via hopping to an adequate grain surface site where they can be chemisorbed. A unit of a particular solid is created and incorporated into the solid crystal structure. The model describes the growth and the evaporation of grains, and the respective element depletion. It predicts for instance the time-dependent material composition and the size distribution of the grain population, and the remaining abundances of the gas phase. Our model is presently applied in simulations of oxygen-rich AGB star winds and in simulations of turbulent brown dwarf atmospheres. While oxygen-rich AGB stars are thought to be the factory for the observable pre-solar grains, brown dwarf atmospheres provide, in this context, an additional test for our theoretical approach.
Helling Ch.
Woitke Peter
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