The evolution of refractory interstellar grains in the solar neighborhood

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. It is found 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 (1.0 to 0.000) 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.