The evolution of refractory interstellar grains in the solar neighborhood

Abstract

The abundance of thermally condensed refractory grains and corresponding interstellar metal depletions are investigated using models for the evolution of the solar neighborhood. The calculations include a self-consistent treatment of red giant winds; planetary nebulae; protostellar nebulae and supernovae as sources of grains; star formation and encounters with supernova blast waves as sinks, for a variety of birthrate histories; chemical evolution models; and mass-loss parameters. We find that the maximum possible fraction of an element that can be locked up in thermally condensed cores is about 60% for the highly refractory elements like Ca, Al, and Ti, and about 30% for the more abundant refractory elements like Si, Mg, Mn, and Fe, independent of model parameters. Typically observed interstellar depletions (approx.10--10/sup 3/) and their elemental variations can therefore not be interpreted in terms of condensation efficiencies in the sources, and must instead be attributed to the selective growth of mantles in interstellar clouds. The strong upper limit on the abundance of thermally condensed grains yields constraints on the nature and composition of interstellar grains and the filling factor of the interstellar coronal gas. The relative importance of various condensation sites is also discussed, and we find that any interstellar grainmore » model that requires the depletion of any element in thermally condensed grains to be larger than approx.1.2 must require the formation of grains in supernovae.« less