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 W_λ = 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 τCMa 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 τCMa are different from the other sight lines, the measured gas-phase C/H ratio is the same: 10⁶ C/H = 135 ± 46 for τCMa versus 10⁶ C/H = 140 ± 20 for the others (Cardelli et al.). The constant interstellar gas-phase C/H over a wide range off(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.