PROBING THE PHYSICAL CONDITIONS OF SUPERNOVA EJECTA WITH THE MEASURED SIZES OF PRESOLAR Al2O3 GRAINS

A few particles of presolar Al2O3 grains with sizes above 0.5 μm are believed to have been produced in the ejecta of core-collapse supernovae (SNe). In order to clarify the formation condition of such large Al2O3 grains, we investigate the condensation of Al2O3 grains for wide ranges of the gas density and cooling rate. We first show that the average radius and condensation efficiency of newly formed Al2O3 grains are successfully described by a nondimensional quantity on L ,d efined as the ratio of the timescale with which the supersaturation ratio increases to the collision timescale of reactant gas species at dust formation. Then we find that the formation of submicron-sized Al2O3 grains requires at least 10 times higher gas densities than those presented by one-dimensional SN models. This indicates that presolar Al2O3 grains identified as having their origin in SNe might be formed in dense gas clumps, allowing us to propose that the measured sizes of presolar grains can be a powerful tool for constraining the physical conditions in which they formed. We also briefly discuss the survival of newly formed Al2O3 grains against destruction in the shocked gas within SN remnants.