Observations of (C-12)O and (C-13)O J = 9-8 in Galactic molecular clouds

We have observed the 12CO and 13CO J = 9-8 transitions at a spectral resolution of 1.5 km s-1 in four Galactic molecular clouds: W3, DR 21, W51, and M17. The line shapes are generally similar to those of low-J CO lines for each species, with the exception that line reversals produced by less excited foreground gas are not evident. The ratio of 12CO and 13CO intensities shows that for all sources the J = 9-8 12CO line is optically thick with τ ∼ 5-20. This high optical depth requires a significant amount of dense (nH2 > 105 cm-3) gas. The peak brightness temperature of the 12CO J = 9-8 line is similar to or less than the dust temperature for all sources, and most of the detected J = 9-8 radiation appears to originate in the cool (T ∼ 50 K) molecular gas which comprises the bulk of the clouds. Non-LTE modeling suggests that the gas giving rise to the J = 9-8 radiation is clumped on a scale smaller than our 80" beam, and that part of the radiation arises from a warmer (T ≥ 100 K) component which is very beam diluted. We do not detect the large amount of hot (T ∼ 200-500 K), spatially extended gas that has been inferred from previous observations of the 12CO J = 7-6 line. In addition to a narrow component, the 12CO J = 9-8 line shows evidence for a broad emission component (ΔV ∼ 18 km s-1) in W51 and possibly DR 21. The temperatures and densities of the regions emitting the broader emission appear similar to those producing the dominant quiescent line component in both sources. This suggests that the phenomenon producing the greater velocity dispersion does not significantly heat the molecular material.