Initial submission. Subject to revision. -1820043832 Assigned ID: 1231 ----- Attempting Submission 1 (Thu Feb 28 16:24:44 GMT 2013) ----- Silicon is a relatively abundant metal that is critical in interstellar chemistry. However, there are significant questions about its gas- and dust-phase abundances in the interstellar medium (ISM). We will use a method of stellar modeling and absorption line analysis that has recently been demonstrated to precisely determine gas-phase column densities (of carbon) even using saturated lines. We have identified 39 targets (using 107 datasets) that will allow us to carry out this analysis. This will allow us to more precisely determine the poorly known Si II gas-phase abundance in lines of sight with A_V>1 magnitude and see how the Si gas-phase abundance depends on physical properties of the line of sight. With this large sample we will specifically test a hypothesis about the role of silicon in small interstellar dust grains, and complete a survey of the six most abundant metals dust-relevant metals (C, N, O, Mg, Si, and Fe) in lines of sight with A_V>1. Our proposed analysis represents an upgrade over previous studies in the following ways: (1) we will use high S/N, high-resolution STIS UV spectra, a major improvement over older Copernicus and IUE studies; (2) we will use more than seven times the number of interstellar lines of sight with measured abundances and complete extinction parameters than the only comparable STIS survey of Si II in the ISM; and (3) we will use detailed stellar modeling and all available Si II absorption lines in order to reduce systematic and statistical uncertainties, compared to studies based on only the weak line at 2335.12 Angstroms (which is typically used to study Si II and has a much less certain f-value). ISM AND CIRCUMSTELLAR MATTER 12 Small false false 0.0 0.0 false false false SUPPORTED