Infrared excess sources in the Canadian Galactic Plane Survey

Using data sets from the Canadian Galactic Plane Survey, we have conducted a multiwavelength study of interstellar components, covering the region 102.5 degrees < l < 141.5 degrees and -3.03 degrees < b < 5.41 degrees. By comparing column density tracers of dust, atomic hydrogen, and molecular gas (traced by CO emission), we have found regions where the dust optical depth shows evidence of more gas than is predicted by the H i and CO observations. Within this population of infrared excess sources, it is possible to discriminate between sources associated with low and high dust temperatures. We interpret the colder temperature sources as molecular clouds/clumps not traced by the CO J = 1-0 transition. Possible reasons include the depletion of CO onto dust grains in the coldest, densest regions of molecular clouds, or photodissociation of CO on the outskirts of molecular clouds.