A new derivation of the extinction-to-CO column density ratio in the Chamaeleon I dark cloud

We have analyzed new datasets of (CO)-C-13 (J = 1-0)and (CO)-O-18 (J = 1-0) emission obtained with NANTEN and visual extinction, Av, derived from DENIS J-band observations in the Chamaeleon I dark cloud. The column densities of both molecular isotopes, N((CO)-C-13) and N((CO)-O-18), have been derived assuming LTE. Both N((CO)-C-13) and IV((CO)-O-18) are linearly correlated with Av in the range Av less than or similar to 10 mag. WE established the relationships N((CO)-C-13)(cm(-2)) = (1.2 +/- 0.1) x 10(15)Av(mag) + (0.7 +/- 0.3) x 10(15) and N((CO)-O-18)(cm(-2)) = (3.5 +/- 0.3) x 10(14)Av(mag) - (5.7 +/- 13) x 10(14). In a region of clustered T Tauri stars, however, both the AV-N(13CO) and Av-N((CO)-O-18) relationships are by a factor of similar to 2 steeper than the above. This may be mainly due to underestimates of the A(V) values, and the J-band star count might deserve a more careful examination for possible underestimates of associated young stars. Compared with the relationships for the other clouds, the slope of the A(V)-N((CO)-C-13) relationship is relatively shallower and that of the A(V)-N((CO)-O-18) relationship in a non-cluster forming region is slightly steeper. These facts suggest that cloud-to-cloud variation is significant in the A(V)-N(CO) relationships.