Abundances and Physical Conditions in the Interstellar Gas toward HD 185418

Details Abundances and Physical Conditions in the Interstellar Gas toward HD 185418 Abundances and Physical Conditions in the Interstellar Gas toward HD 185418

Oct 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...596..350s&link_type=abstract

The Astrophysical Journal, Volume 596, Issue 1, pp. 350-361.

Astronomy and Astrophysics

Astronomy

33

Ism: Abundances, Ism: Clouds, Ism: Structure, Stars: Individual: Henry Draper Number: Hd 185418, Ultraviolet: Ism

Scientific paper

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)=(53deg,-2.2d) 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 Å. 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 H2 are present in the FUSE spectra, with a kinetic temperature for the lowest rotational levels T01=100+/-15 K. Analysis of the C I fine-structure excitation at that temperature implies an average local density of hydrogen nH=6.3+/-2.5 cm-3. Chemical arguments based on analyses of CH, CH+, and C2 indicate that a fraction of the CH (>=20%) is formed as a by-product of nonequilibrium CH+ synthesis. The electron density ne, 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(H2)=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.
Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The observations are associated with proposal 8241. Based in part on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium.

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