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Abstract: Context: Over the past thirty years a wealth of observations of CO and othermolecules in optical-uv absorption in diffuse clouds has accumulated for whichno comparable CO emission line data exist. Aims: To acquire mm-wave J=1-0 COemission line profiles toward a substantial sample of commonly-studiedoptical-uv absorption line targets and to compare with the properties of theabsorbing gas, especially the predicted emission line strengths. Methods: Usingthe ARO 12m telescope we observed mm-wavelength J=1-0 CO emission with spectralresolution R ~ 3x10^6 and spatial resolution 1- toward a sample of 110 lines ofsight previously studied in optical-uv absorption lines of CO, \HH, CH, etc.Results: Interstellar CO emission was detected along 65 of the 110 lines ofsight surveyed and there is a general superabundance of CO emission given thedistribution of galactic latitudes in the survey sample. Much of the emissionis optically thick or very intense and must emanate from dark clouds or warmdense gas near HII regions. Conclusions: Judging from the statisticalsuperabundance of CO emission, seen also in the total line of sight reddening,the OB star optical-uv absorption line targets must be physically associatedwith the large quantities of neutral gas whose CO emission was detected, inwhich case they are probably influencing the absorbing gas by heating and-orphotoionizing it. This explains why CO-H2 and 12CO-13CO ratios differ somewhatbetween $uv$ and mm-wave absorption line studies. Because the lines of sighthave been preselected to have AV < 1 mag, relatively little of the associatedmaterial actually occults the targets, making it difficult for CO emission lineobservations to isolate the foreground gas contribution.



Author: H. S. Liszt

Source: https://arxiv.org/







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