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

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, USA 22903-2475, USA e-mail: hliszt@nrao.edu
² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d'Hères, France
³ Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁴ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan

Received: 2 September 2008
Accepted: 20 November 2008

Abstract

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  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  ≈ 0.4 and 0.65 mag, of which 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   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 E_B-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.