0 Assigned ID: 1132 ----- Attempting Submission 1 (Fri Apr 07 23:35:50 GMT 2017) ----- In theoretical models of the interstellar medium, thermal instability plays a major role in segregating the neutral gas into two distinct phases in pressure equilibrium: a warm neutral medium and a cold neutral medium. Temperatures intermediate between the warm and cold phases are forbidden because at constant thermal pressure gas at such temperatures would be thermally unstable. While existing observational results on the physical conditions in interstellar clouds generally support these ideas, studies tend to disagree as to the extent of the thermally unstable component. We have recently developed a technique that allows us to simultaneously determine the densities and temperatures in interstellar clouds through the analysis of O I fine-structure excitations. Here, we propose an archival survey of the O I fine-structure lines seen in high-resolution STIS spectra of approximately 40 sight lines. We will derive the densities and temperatures that correspond to the observed O I excitation ratios at discrete velocities along each line of sight. We then use this information to construct a phase diagram for the interstellar medium for comparison with the results of theoretical models of a two-phase medium. These comparisons will serve as a basic test of the validity of such models, and will allow us to determine the extent to which the gas is (or is not) in thermal equilibrium, yielding important inputs for future modeling efforts. Galaxies and the IGM 0 Regular false true false SUPPORTED