A study of element depletions in interstellar gas

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Abstract
High-resolution ultraviolet observations of seven lightly reddened stars at intermediate galactic latitudes have been obtained with the IUE satellite. The lines-of-sight to the seven stars are in approximately the same direction but are of varying length. The observed interstellar absorption lines have been analysed to estimate abundances in the interstellar gas in each line-of-sight. The pattern of depletions from element to element for a given line-of-sight has been compared with condensation temperatures appropriate for grain-formation in cool stellar atmospheres or pre-stellar nebulae, and with the first ionization potential of the elements. In addition the levels of depletion of a given element toward different stars has been compared with estimates of the average density of the interstellar medium in the different lines-of-sight. For certain elements the level of depletion is found to depend on present densities in the interstellar medium, suggesting that accretion of gas atoms on to grains and possibly sputtering of grain atoms back to the gas phase subsequently modify the pattern of depletions produced by the original grain-formation process. The degree of correlation between depletion and density is found to vary from element to element; the extreme cases being iron whose depletion is strongly density- dependent and silicon whose depletion appears to be insensitive to density, at least for the range of densities sampled in these observations. These results have been compared with previous observations and discussed in the light of current models for the depletion process.

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