Condensation Anisotropy of Corundum around Agb Stars and Its Effect on Infrared Spectra

Introduction: Dust formation in mass-loss winds from asymptotic giant branch (AGB) stars is a key process for acceleration of outflows. Equilibrium condensation calculations [e.g., 1] predict that corundum (Al2O3) is one of the first condensates around oxygenrich AGB stars. The presence of presolar corundum grains originated from AGB stars in chondrites [e.g., 25] is also a strong evidence of corundum formation around AGB stars. Therefore, corundum is considered to be a possible carrier for the 13-μm feature observed in many oxygen-rich AGB stars as well as spinel (MgAl2O4) and rutile (TiO2) [6]. However, the peak position is not completely reproduced by these minerals. We have recently shown that an infrared spectrum of forsterite strongly reflects a specific combination of shape and crystallographic orientation, and the combination reflects dust-forming processes and conditions [7]. Thus, the morphology of candidate minerals for the 13-μm feature should be taken into account and it should reflect the formation condition of the grains around AGB stars. In this study, in order to understand the spectral features of corundum that reflect the dust-forming process (growth from gas), we conducted condensation experiments of corundum at high and low supersaturation ratios (S = PAlO/Peq) and investigated the effects of condensation conditions on infrared spectra.