INTERSTELLAR DEPLETIONS UPDATED - WHERE ALL THE ATOMS WENT

Measures of the depletion of interstellar elements from the gas phase are usually derived by assuming that the general composition of the in terstellar medium is identical to that of the Sun. A compilation of st ellar composition data, including B stars as well as field F and G sta rs, however, calls this assumption into question. In this Letter we co nsider the impact on derived depletions if the reference abundances ar e derived from stars in the solar neighborhood rather than from the Su n, and we discuss the implications for current models of the interstel lar dust. Using recent, accurate gas-phase column densities for zeta O phiuchi, we show that the systematically lower depletions resulting fr om our revised cosmic abundances are in conflict with most dust models because insufficient quantities of raw materials are available to exp lain the observed extinction. A Kramers-Kronig analysis for the zeta O ph line of sight shows that the revised depletions are consistent with the required opacity of interstellar dust only if the density of the grain material is near 1 g cm(-3) suggesting that the grain structure must be open (i.e., the grains must by fluffy, porous, or fractal in s tructure).