AAS 203rd Meeting, January 2004
Session 110 Interstellar Medium I
Poster, Thursday, January 8, 2004, 9:20am-4:00pm, Grand Hall

## [110.04] Interstellar Carbon Abundances Along Translucent Sightlines

U.J. Sofia (Whitman Coll.), J.T. Lauroesch, D.M. Meyer (Northwestern Univ.), S.I.B. Cartledge (LSU)

We report interstellar C {\sc ii} column densities or upper limits determined from weak absorption of the \lambda2325.403Å\ intersystem transition in 6 translucent sightlines (AV > ~1) observed with STIS. Carbon abundances are essential for understanding the ISM because, depending on whether it is in the gas or dust, interstellar C may play substantial roles in heating or cooling, molecular chemistry and extinction. Singly ionized carbon is an important species to measure because it represents the dominant ion state of this abundant element in neutral interstellar regions. The eight previous absorption measurements of interstellar C {\sc ii}, seven in diffuse sightlines and one in a translucent sightline, have been consistent with a single gas-phase carbon-to-hydrogen ratio. Carbon, however, should be more readily incorporated into dust in denser interstellar regions. The targets observed for this study were chosen to maximize the chance of finding variation in the interstellar gas-phase C/H; enhanced oxygen depletions (compared to diffuse sightlines) have previously been found toward each of the stars. We find that carbon in forms other than C {\sc ii} is usually insignificant, although one sightline may show a substantial abundance of excited C {\sc ii}. This work is supported by grants from the Space Telescope Science Institute to Whitman College and Northwestern University.

Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.