Interstellar Depletion onto Very Small Dust Grains

We consider the depletion of elements from the interstellar gas onto apopulation of very small dust grains. Adopting a grain model in which of order4% of the cosmic C abundance is in grains with radii <= 10 Angstroms, we findthat the rate of accretion onto these grains is adequately fast to account forthe observed large depletions of elements like Ti, without invokingunreasonably high rates of mass transfer between interstellar phases or lowgrain destruction rates. If these grains are composed of arene rings, then onlya limited number of metal atoms can be locked up in them. The depletion wouldbe quenched when this limit is reached, unless there is a mechanism fortransferring the metals to larger grains and refreshing the very small grainpopulation, for example by grain coagulation and shattering in the diffuse ISM.If Fe depletes onto the very small grains, then for reasonable coagulationrates there is at least one metal atom per five C atoms in the very small grainpopulation. Furthermore, approximately 60% of the cosmic Fe is associated withthe carbonaceous grain population. It is unclear whether this scenario iscompatible with observations. However, if there is another population of verysmall grains, with a large capacity for holding Fe atoms, it might be the sinkfor the most heavily depleted elements.Comment: LaTeX (21 pages, 4 eps figures, uses aaspp4.sty); ApJ, accepte