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

Context. Most diffuse clouds are only known as kinematic features in absorption spectra, but those with appreciable H2 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 12CO emission from diffuse gas seen around the archetypical line of sight to ζ Oph. Methods. The 12CO emission is imaged in position and position-velocity space, analyzed statistically, and then compared with maps of total reddening $E^\infty_{B-V}$ and with models of the C+ – CO transition in H2-bearing diffuse clouds. Results. Around ζ Oph, 12CO emission appears in two distinct intervals of reddening centered near $E^\infty_{B-V}$ ≈ 0.4 and 0.65 mag, of which $\la$0.2 mag is background material. Within either interval, the integrated 12CO intensity varies up to 6–12 K km s-1, compared to 1.5 K km s-1 toward ζ 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 H2 toward ζ Oph, 55 K. Partly as a result, 12CO 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 EB-V in global, CO absorption line surveys toward bright stars is present in the gas seen around ζ Oph, reflecting the extreme sensitivity of N(12CO) to ambient conditions. The two-component nature of the optical absorption toward ζ 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 12CO lines in diffuse gas arise at N(H2) ≈ 1021 cm-2in regions of modest density n(H) ≈ 200-500 cm-3and 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.