The Dust-Phase Abundances of SI, fe, and MG Towards HD93521 in the Galactic Halo

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Hst Proposal Id #5878 Interstellar Medium

Scientific paper

Three orbits are requested to obtain high-resolution, high S/Nobservations with the GHRS echelle gratings of the interstellar Mg IILambdaLambda1239.9, 1240.4; Si II Lambda1808.0; Fe II Lambda2260.8; andMg II LambdaLambda2796.4, 2803.5 absorption lines towards the starHD93521 in the Galactic halo. The goal of the observations is tomeasure precise column densities of Si II, Fe II, and Mg II in the nineinterstellar clouds along this line of sight. Previous observationshave suggested that interstellar silicate grains have a core - mantlestructure, with the core consisting of Fe_2SiO_4 and the mantle ofMg_2SiO_4. This simple picture has been challenged by the suggestionthat the theoretical f-values currently adopted for the Mg IILambdaLambda1239.9, 1240.4 lines upon which most Mg II results arebased --- are a factor of ~5 too small. The proposed program aims toresolve the f-value issue and provide a detailed description of thecomposition and structure of interstellar silicate grains from the Si,Fe, and Mg depletions and dust-phase abundances derived from themeasured column densities.

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