CO and IRAS observations of the Chamaeleon molecular clouds

We present an extensive survey of the CO emission from the conspicuous complex of dark nebulae in Chamaeleon. The CO observations together with IRAS, extinction and H I data are used to study the distribution of interstellar matter in the complex. The comparison of the cloud 100μm emission with AV shows that the far-infrared brightness traces the gas over most of the surface of the complex, specifically where AV < 2mag and outside a small number of regions heated by embedded stars. For the complex as a whole and for its largest clouds,masses computed from the far infrared emission roughly agree with those derived from the CO surveys using the standard Galactic value of the CO-to-H2 conversion factor. However, among the smaller clouds the H2 masses computed from CO emission are lower than those derived from the 100μm data by as much as a factor of 10. The molecular emission is observed to be primarily associated with the 100μm emission characterized by cold IRAS colors: low 60/100μm flux ratio and no 12 and 25μm emission while the 100μm emission with warm IRAS colors is poorly traced by CO emission. The observed correlation between the spectral distribution of the dust emission with the molecular emission may be related to the effect of the far-UV extinction of dust on the formation of CO. In some regions it is clear that the warm infrared component cannot be accounted for by matter detected in the CO and H I surveys; we suggest that these regions contain molecular gas without detected CO emission.