Carbon in the Diffuse Interstellar Medium

We have obtained spectra of the interstellar intersystem C II] λ2325 line toward the star τ Canis Majoris. The absorption spectra were obtained with the echelle mode (3.5 km s-1 resolution) of the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope and have a co-added signal-to-noise ratio of approximately 850. The C II] line has a measured equivalent width Wλ = 0.21 ± 0.07 mÅ, which corresponds to a column density of 7.6 ± 2.5 × 1016 cm-2. Of the six interstellar lines of sight that have reliably measured (>2 σ) carbon abundances, the τ CMa sight line has the lowest fractional H2 abundance, f(H2), by over 3 orders of magnitude. Although this suggests that the physical conditions in the interstellar gas toward τ CMa are different from the other sight lines, the measured gas-phase C/H ratio is the same: 106 C/H = 135 ± 46 for τ CMa versus 106 C/H = 140 ± 20 for the others (Cardelli et al.). The constant interstellar gas-phase C/H over a wide range of f(H2) suggests that even under very different conditions, no carbon is being exchanged between the gas and dust phases of the interstellar medium. It also supports, and extends to a larger distance, the suggestion by Cardelli et al. that the intrinsic (gas-phase plus dust-phase) interstellar C/H ratio in the vicinity of the Sun is constant and below the solar C/H value.