Context : Most diffuse 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 ^ { \infty } _ { B - V } 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 ^ { \infty } _ { B - V } \approx 0.4 and 0.65 mag , of which \la 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 } ) \approx 10 ^ { 21 } ~ { } { cm } ^ { -2 } in regions of modest density n ( H ) \approx 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 .