Imaging galactic diffuse clouds: CO emission, reddening and turbulent flow in the gas around ζ Ophiuchi

Context. Most diffuse. use clouds are only known as kinematic features in absorption spectra, but those with appreciable H(2) content may be visible in the emission of such small molecules as CH, OH, and CO. Aims. We interpret in greater detail the extensive observations of (12)CO emission from diffuse gas seen around the archetypical line of sight to zeta Oph. Methods. The (12)CO emission is imaged in position and position-velocity space, analyzed statistically, and then compared with maps of total reddening E(B-V)(infinity) and with models of the C+ -CO transition in H(2)-bearing diffuse clouds. Results. Around zeta Oph, (12)CO emission appears in two distinct intervals of reddening centered near E(B-V)(infinity) approximate to 0.4 and 0.65 mag, of which less than or similar to 0.2 mag is background material. Within either interval, the integrated (12)CO intensity varies up to 6-12 K km s(-1), compared to 1.5 K km s(-1) toward zeta Oph. Nearly 80% of the individual profiles have velocity dispersions sigma(v) < 0.6 km s(-1), which are subsonic at the kinetic temperature derived from H(2) toward zeta Oph, 55 K. Partly as a result, (12)CO emission exposes the internal, turbulent, supersonic (1-3 km s(-1)) gas flows with especial clarity in the cores of strong lines. The flows are manifested as resolved velocity gradients in narrow, subsonically-broadened line cores. Conclusions. The scatter between N(CO) and E(B-V) in global, CO absorption line surveys toward bright stars is present in the gas seen around zeta Oph, reflecting the extreme sensitivity of N((12)CO) to ambient conditions. The two-component nature of the optical absorption toward zeta Oph is coincidental and the star is occulted by a single body of gas with a complex internal structure, not by two distinct clouds. The very bright (12)CO lines in diffuse gas arise at N(H(2)) approximate to 1021 cm- 2 in regions of modest density n(H) approximate to 200-500 cm(-3) and somewhat more complete C+ -CO conversion. Given the variety of structure in the foreground gas, it is apparent that only large surveys of absorption sightlines can hope to capture the intrinsic behavior of diffuse gas.