Carbon in the diffuse interstellar medium

We have obtained spectra of the interstellar intersystem C II] lambda 2325 line toward the star tau 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-lambda = 0.21 +/- 0.07 m Angstrom, which corresponds to a column density of 7.6 +/- 2.5 x 10(16) cm(-2). Of the six interstellar lines of sight that have reliably measured (>2 sigma) carbon abundances, the tau CMa sight line has the lowest fractional H-2 abundance, f(H-2), by over 3 orders of magnitude. Although this suggests that the physical conditions in the interstellar gas toward 7 CMa are different from the other sight lines, the measured gas-phase C/H ratio is the same: 10(6) C/H = 135 +/- 46 for tau CMa versus 10(6) C/H = 140 +/- 20 for the others (Cardelli et al.). The constant interstellar gas-phase C/H over a wide range off(H,) 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.