1996A&A...305..920H

We have investigated the ratio of carbon monoxide column density to colour excess E(J-K) of background field stars in the direction of three different local clouds: the Coalsack, Chamaeleon I (Cha I) and R Coronae Australis (R CrA). For these dark clouds a uniform set of colour excess values towards highly reddened background stars is available from the literature based on near-infrared photometry data. Using the 15-m Swedish-ESO Submillimeter Telescope (SEST), we have observed towards these background stars the ¹²CO, ¹³CO and C¹⁸O J=1-0 emission lines and, in a few selected directions, the J=1-0 transition of C¹⁷O. We have also derived the N(CO)/A_V ratio and, based on a range of assumed gas-to-dust-ratios, the N(CO)/N(H₂) ratio. We find that the N(CO) to E(J-K) ratio varies from cloud to cloud: it is a factor of ∼2 larger in Cha I and R CrA than in the Coalsack. Our results can be interpreted in two alternative ways: firstly that the N(CO)/N(H₂) ratio is higher in active star forming regions (Cha I, R CrA, and L 1641) than in more quiescent regions without star formation (Coalsack) and secondly that the ratio N(H₂)/E(J-K) changes from cloud to cloud and is higher in active star forming regions than in quiescent clouds.