2008A&A...492..743L

Comparison of optical/uv absorption line data with high-resolution profiles of mm-wave CO emission provides complementary information on the absorbing gas, as toward ζ Oph. Over the past thirty years a wealth of observations of CO and other molecules in optical/uv absorption in diffuse clouds has accumulated for which no comparable CO emission line data exist. To acquire mm-wave J=1-0 CO emission line profiles toward a substantial sample of commonly-studied optical/uv absorption line targets and to compare with the properties of the absorbing gas, especially the predicted emission line strengths. Using the ARO 12 m telescope, we observed mm-wavelength J=1-0 CO emission with spectral resolution R≃3x10⁶ and spatial resolution 1' toward a sample of 110 lines of sight previously studied in optical/uv absorption lines of CO, H₂, CH, etc. Interstellar CO emission was detected along 65 of the 110 lines of sight surveyed and there is a general superabundance of CO emission given the distribution of galactic latitudes in the survey sample. Much of the emission is optically thick or very intense and must emanate from dark clouds or warm dense gas near HII regions. Judging from the statistical superabundance of CO emission, seen also in the total line of sight reddening, the OB star optical/uv absorption line targets must be physically associated with the large quantities of neutral gas whose CO emission was detected, in which case they are probably influencing the absorbing gas by heating and/or photoionizing it. This explains why CO/H₂ and ¹²CO/¹³CO ratios differ somewhat between uv and mm-wave absorption line studies. Because the lines of sight have been preselected to have A_V≲1mag, relatively little of the associated material actually occults the targets, making it difficult for CO emission line observations to isolate the foreground gas contribution.