ABUNDANCE OF INTERSTELLAR CARBON TOWARD ZETA-OPHIUCHI

We report on the detection of the weak intersystem transition of C II] lambda2325 angstrom in the sight line toward zeta Oph using the Ech-B mode (3.5 km s-1 resolution) of the Goddard High-Resolution Spectrograph. The high-quality spectrum is characterized by an empirically measured signal-to-noise of 450, in excellent agreement with that expected from photon statistics. The measured equivalent width of the C II] line is W(lambda) = 0.52 +/- 0.12 mangstrom, implying a C II column density of 1.80(+/- 0.42) x 10(17) atoms cm-2 and (C/H)zeta Oph = 1.32(+/- 0.32) x 10(-4). This C/H is 1.9 +/- 0.7 smaller than that measured toward xi Per by Cardelli et al. (1991a). We find that some of this difference may be attributable to inhomogeneities in the cosmic abundance of carbon. However, a comparison between specific aspects of the observed extinction curves (e.g., integrated 2175 angstrom bump and far-UV extinction optical depths per H atom) for the zeta Oph and xi Per sight lines, indicates that at least part of this difference may also be consistent with some conversion of carbon grains and/or PAHs to gas phase C II.