On the crystallization of small silica particles in circumstellar environments

M. John, E. Müller, A. B. C. Patzer, M. Lüttke, E. Sedlmayr

AG Tagung München 2001

Abstract:

Infrared Space Observatory (ISO) observations have revealed the presence of crystalline silicate dust in circumstellar environments of some evolved stars (e.g. Waters et al. 1996, A&A, 315, L361). Molster et al. (2001, A&A, 366, 923), for instance, reported the discovery of a carbon-rich AGB star surrounded by a highly crystalline silicate dust shell as indicated by the high resolution ISO-SWS spectrum. However, it is still a matter of debate, whether the presence of crystalline dust is restricted only to certain phases of the stellar evolution (e.g. Kemper et al. 2000, A&A, 369, 132).

In order to understand the process, which leads to the formation of crystalline structures in circumstellar environments, the microphysical rearrangement of small silica particles is investigated by means of molecular dynamic calculations. The results indicate a dynamic coexistence of amorphous-like and symmetrical, crystalline structures. Such rearrangement processes depend on the energy transfer to the grain governed by the physical conditions of the astrophysical environment. Therefore, the effect of the degree of crystallinity on optical properties is additionally exemplified by Mie calculations.

Some implications regarding the condensation and the mineralogy of silicate dust particles in the circumstellar environments of AGB and post-AGB objects are discussed.