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, andCO. Aims. We interpret in greater detail the extensive observations of 12COemission from diffuse gas seen around the archetypical line of sight to Oph. Methods. The 12CO emission is imaged in position and position-velocity space, analyzed statistically, and then compared with maps of total reddening and with models of the C+-CO transition in H2-bearing diffuse clouds. Results. Around Oph, 12COemission appears in two distinct intervals of reddening centered near 0.4 and 0.65mag, of which 0.2mag is background material. Within either interval, the integrated 12CO intensity varies up to 6-12Kkms-1, compared to 1.5Kkms-1 toward Oph. Nearly80% of the individual profiles have velocity dispersions < 0.6kms-1, which are subsonic at the kinetic temperature derived from H2toward Oph, 55K. Partly as a result, 12CO emission exposes the internal, turbulent, supersonic (1-3kms-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, COabsorption line surveys toward bright stars is present in the gas seen around Oph, reflecting the extreme sensitivity of N(12CO) to ambient conditions. The two-component nature of the optical absorption toward 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)in regions of modest density n(H)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. Key words: astrochemistry - ISM: molecules - ISM: clouds
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