Interstellar carbon in translucent sight lines

We report interstellar C II column densities or upper limits determined from weak absorption of the 2325.4029 Å intersystem transition observed in six translucent sight lines (A_V ≳ 1) with the Space Telescope Imaging Spectrograph (STIS). The sight lines sample a wide range of interstellar characteristics, including total to selective extinction, R_V = 2.6-5.1; average hydrogen density along the sight line, 〈n(H)〉 = 3-14 cm^-3; and fraction of H in molecular form, 0-∼40%. Four of the sight lines, those toward HD 37021, HD 37061, HD 147888, and HD 207198, have interstellar gas-phase abundances that are consistent with the diffuse sight line ratio of 161 ± 17 carbon atoms in the gas per million hydrogen nuclei. We note that while it has a gas-phase carbon abundance that is consistent with the other sight lines, a large fraction of the C II toward HD 37061 is in an excited state. The sight line toward HD 152590 has a measured interstellar gas-phase carbon abundance that is well above the diffuse sight line average; the column density of C in this sight line may be overestimated because of noise structure in the data. Toward HD 27778 we find a 3 σ abundance upper limit of ≤108 C atoms in the gas per million H, a substantially enhanced depletion of C as compared to the difiuse sight line value. The interstellar characteristics toward HD 27778 are otherwise not extreme among the sample, except for an unusually large abundance of CO molecules in the gas.