  A few particles of presolar Al2O3 grains with sizes above 0.5 mum 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
non-dimensional quantity "Lambda_on" defined 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 ten times higher gas densities
than those presented by one-dimensional SN models. This indicates that presolar
Al2O3 grains identified as a SN origin might be formed in dense gas clumps,
allowing us to propose that the measured sizes of presolar grains can be a
powerful tool to constrain the physical conditions in which they formed. We
also briefly discuss the survival of newly formed Al2O3 grains against the
destruction in the shocked gas within the SN remnants.
