The interstellar carbon abundance toward del Sco.

Observations are presented of a semiforbidden transition of C II at lambda 2325 toward Delta Scorpii. This line is intrinsically so weak that saturation and blending problems are completely absent, allowing the C II column density to be determined unambiguously. Furthermore, since C II is the dominant ionization state of carbon in H I regions, the total carbon abundance follows directly. While the detection of weak, semiforbidden interstellar lines requires high resolution and high photometric accuracy, observations of these lines promise substantial improvements concerning the knowledge of gas-phase abundances and, consequently, of the composition and structure of grains. The reported results represent the first direct evidence from absorption measurements that carbon is depleted from the interstellar gas by amounts sufficient to provide a substantial fraction of the mass of interstellar grains.