Abstract

We have studied the ratio of (CO)-C-13 and (CO)-O-18 column densities to the extinction A(J) of background field stars in the direction of three globules: B 133, B 335, L 466. Selected positions in the globules were observed in the (CO)-C-12(J = 1 - 0) line and in J = 1 - 0 and J = 2 - 1 transitions of (CO)-C-13 and (CO)-O-18 using the 15-m Swedish-ESO Submillimetre Telescope (SEST) in Chile. One of the clouds, L 466, was also mapped in the (CO)-O-18(J = 1 - 0) line. In addition to the N((CO)-C-13)/A(J) and N((CO)-O-18)/A(J) ratios also the ratios of N((CO)-C-13) and N((CO)-O-18) to A(v) and N(H-2) are given. These ratios were found to vary from cloud to cloud so that they are larger in B 335 than in B 133 and L 466. These variations are thought to primarily arise from variations of the column density ratio of CO and its isotopes to H-2. The alternative explanation in terms of the N(H-2)/A(J) ratio being larger in starforming regions (B 335) than in more quiescent regions lacking star formation (B 133 and L 466) appears less likely. We also discuss the evidence for N(CO)/A(J) variations on the basis of previous observations in star forming and non-star forming dark clouds.