On the Origin of Crystalline Silicate in Circumstellar Envelopesof Oxygen-rich Asymptotic Giant Branch Stars

Abstract

Formation of crystalline silicate is explored within the framework of dust formation in steady state dust-driven winds around oxygen-rich asymptotic giant branch (AGB) stars by applying a kinetic theory of crystallization. In the calculations, formation of not only homogeneous corundum (Al2O3) and silicate grains but also heterogeneous grains consisting of a corundum core and a silicate mantle is taken into account for the stellar parameters M*=1.0 M☉, L*=2×104 L☉, and T*=2500 K. Crystallization calculations based on the thermal history of dust grains after their formation lead to the result that only silicate in the mantles of heterogeneous grains can be completely crystallized through the thermal processing caused by the accretion of silicate on precondensed corundum grains when the mass-loss rate is greater than 3×10−5 M☉ yr-1. On the other hand, the homogeneous silicate grains remain amorphous. The radiative transfer calculations taking olivine as a representative of crystalline silicate reproduce the behavior of silicate features revealed by the Infrared Space Observatory-Short-Wavelength Spectrometer observations. The crystalline silicate observed around oxygen-rich AGB stars reflects the formation process of dust grains prevailing in the circumstellar envelopes and is a diagnostic of high mass-loss rates at the late stage of AGB star evolution.