Abundances and physical conditions in the interstellar gas toward HD 1854181

We present a study of the abundances and physical conditions in the interstellar gas toward the moderately reddened B0.5 V star HD 185418. This star is located at (l,b)=(53degrees,-2.degrees2) and has an estimated distance of 790 pc and a reddening E(B-V)=0.50. This work is based on the analysis of Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) spectra covering the range from 912 to 1361 Angstrom. We used both sets of far-ultraviolet data, together with high-resolution optical data, to derive accurate gas-phase column densities for important interstellar species such as C I, C I*, C I**, N I, O I, O I*, Mg II, S I, S III, Cl I, Mn II, Fe II, Ni II, Cu II, Ge II, Kr I, and CO. Numerous lines of H-2 are present in the FUSE spectra, with a kinetic temperature for the lowest rotational levels T-01=100+/-15 K. Analysis of the C I fine-structure excitation at that temperature implies an average local density of hydrogen n(H)=6.3+/-2.5 cm(-3). Chemical arguments based on analyses of CH, CH+, and C-2 indicate that a fraction of the CH (greater than or equal to20%) is formed as a by-product of nonequilibrium CH+ synthesis. The electron density n(e), derived under the assumption of photoionization equilibrium, ranges from 0.03 to 0.32 cm(-3) for the six different neutral/first-ion ratios considered. The relatively complex component structure seen in high-resolution spectra of K I, Na I, and Ca II, the relatively low average volume densities, the modest molecular fraction f(H-2)=0.44, and the measured depletions all indicate no evidence for the presence of translucent components in the light path. The line of sight rather contains a mixture of cold and warm neutral diffuse gas. The detection of Si III and S III indicates the presence of some ionized gas (at a 1% level) along the line of sight. We could not determine unambiguously whether this ionized gas was physically related to the neutral components.