ANISOTROPIC EVAPORATION OF FORSTERITE AND ITS IMPLICATION FOR DUST FORMATION CONDITIONS IN CIRCUMSTELLAR ENVIRONMENTS

Crystalline silicates are observed in many protoplanetary disks and some dust shells around evolved stars. The peak positions of infrared (IR) spectra of forsterite, which is the most abundant circumstellar silicate, vary with dust temperature, composition, size, and crystallinity. However, there is another important factor that affects IR spectra, which is the shape with a specific crystallographic orientation called the 'crystallographically anisotropic shape'. We focused on anisotropic evaporation of crystalline forsterite as one of the possible processes that change the crystallographically anisotropic shape of forsterite grains, and carried out evaporation experiments of single crystals of forsterite in hydrogen gas (0.01-10 Pa) and at temperatures of 1150-1660 deg. C. Forsterite evaporated anisotropically in all experimental conditions, and the anisotropy depended on temperature and hydrogen gas pressure. The results enabled us to calculate crystallographically anisotropic shapes of heated forsterite as a function of temperature and hydrogen pressure, and their corresponding IR spectra. Distinctly, different sets of peak positions were seen in IR spectra of grains with different combination of shapes and their orientation reflecting the heating conditions. The results were applied to the IR spectrum of a protoplanetary disk, HD100546, which suggests that forsterite dust particles that experienced evaporation exist with dominant secondarily fragmented forsterite formed by small-body collisions. We propose that detailed IR spectroscopy of forsterite, and probably other anisotropic crystals, is a new tool to estimate temperature and pressure conditions of circumstellar environments where dust formed.