0 Assigned ID: 721 ----- Attempting Submission 1 (Fri Apr 10 19:43:12 GMT 2015) ----- HST Phase I Proposal 721 (archc23v1.apt) successfully submitted. Receipt: # 721-1 ----- Attempting Submission 2 (Fri Apr 10 21:17:26 GMT 2015) ----- Trace neutral interstellar species (with ionization potentials < 13.6 eV) can be used to estimate local physical conditions (n_e, n_H, T, radiation field) -- e.g., via analyses of C I fine-structure excitation and of the ionization balance. Apart from surveys of C I, however, few accurate measurements are available for other trace species with lines in the UV (Mg, Si, S, P, Cl, Zn). For the few sight lines where multiple, well-determined X I/X II ratios are available, however, the inferred n_e can differ by factors of 30; consideration of grain-assisted and dielectronic recombination has not resolved the discrepancies. Moreover, the n_H inferred from the commonly used Ca I/Ca II ratio are systematically higher, by factors of 10--100, than the values obtained from C I fine structure. Some additional factor(s) must commonly and significantly affect the ionization balance in the ISM. We propose to substantially increase the sample of accurate measurements of Mg I, Si I, S I, P I, Cl I, and Zn I, using archival STIS and GHRS echelle spectra of ~150 sight lines; these will be combined with large samples of Li I, Na I, K I, Ca I, and Fe I derived from optical spectra. The primary goal is to compare multiple diagnostics of interstellar physical conditions -- e.g., the densities inferred from various X I/X II ratios with those obtained from C I fine-structure excitation and C_2 rotational excitation -- to determine whether they are consistent with each other and with cloud models. The derived N(X I) will also be used to investigate the ionization properties of the diffuse interstellar bands and to test the generally assumed dependence of depletions on local density. ISM AND CIRCUMSTELLAR MATTER 0 Regular false true false false false SUPPORTED