The Oxygen Abundance in Translucent Interstellar Clouds

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Hst Proposal Id #8273 Interstellar And Intergalactic Matter

Scientific paper

We propose to obtain high-resolution STIS echelle observations of the interstellar O I Lambda1356 absorption toward a sample of stars behind six translucent clouds characterized by large H I column densities and high extinctions {1.10 <=sssim A_V <=sssim 2.82}. Previous GHRS measurements of this weak feature in diffuse clouds {A_V <=sssim 1} have shown no evidence of density-dependent O depletion from the gas phase and indicate a total {gas + dust} interstellar O abundance that is about 2/3 of the solar value. Models of translucent clouds predict enhanced depletions of gaseous O into molecules and dust by a factor of 2 or so over those of diffuse sightlines depending on the cloud homogeneity, grain characteristics, and other factors. The primary goals of our proposed investigation are to: {1} accurately determine if the O depletion is enhanced in translucent clouds, and {2} assess the gaseous O abundance spread in terms of extinction variations. With regard to the latter objective, our targeted sightlines sample a wide variety of UV extinction properties and values for the ratio of total-to-selective extinction {3 > R > 5}. The lack of any enhanced O depletion among these translucent clouds, especially if the abundance spread is narrow, would signify that the grain characteristics are irrelevant to this issue for extinctions of A_V <=sssim 3.

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