CO column density and extinction in the Chamaeleon II-III dark-cloud complex

We carried out (CO)-C-13 (J = 1-0) and (CO)-O-18 (J = 1-0) observations of the Chamaeleon II-III dark-cloud complex with the NANTEN radio telescope. The column densities of both molecular isotopes were derived assuming LTE. The A(V) values were obtained by scaling the A(J) values that were derived using an adaptive-grid star-count method applied to the DENIS J-band data. We established the A(V)-CO isotope column-density relations in Cha 11 and 111, and compared them with those in Cha I. The slopes of the A(V)-(CO)-C-13 relations for Cha 11 and III are steeper than that for Cha I. Those of the A(V)-(CO)-O-18 relations are similar among the three clouds. The total column density ratio, N((CO)-C-13)/N((CO)-O-18), in Cha I tends to be small compared with those in Cha 11 or Cha 111; the ratios range from similar to 5 to similar to 25 at low extinction in Cha It and 111, but at most similar to 10 in Cha I. We suggest that the increase of N(13 CO) due to the (CO)-C-13 formation process causes cloud-to-cloud variations in the A(V)-N((CO)-C-13) correlation.