2006A&A...447..597K

We investigate the relationship between the C¹⁸O column density and the visual extinction in Chamaeleon I and in a part of the Chamaeleon III molecular cloud. The C¹⁸O column densities, N(C¹⁸O), are calculated from J=1-0 rotational line data observed with the SEST telescope. The visual extinctions, A_V, are derived using JHK photometry from the 2MASS survey and the NICER color excess technique. In contrast with the previous results of Hayakawa et al. (2001PASJ...53.1109H), we find that the average N(C¹⁸O)/A_V ratios are similar in Cha I and Cha III, and lie close to values derived for other clouds, i.e. N(C¹⁸O)≃2x10¹⁴cm^–2(A_V-2). We find, however, clear deviations from this average relationship towards individual clumps. Larger than average N(C¹⁸O)/A_V ratios can be found in clumps associated with the active star forming region in the northern part of Cha I. On the other hand, some regions in the relatively quiescent southern part of Cha I show smaller than average N(C¹⁸O)/A_V ratios and also very shallow proportionality between N(C¹⁸O) and A_V. The shallow proportionality suggests that C¹⁸O is heavily depleted in these regions. As the degree of depletion is proportional to the gas density, these regions probably contain very dense, cold cores, which do not stand out in CO mappings. A comparison with the dust temperature map derived from the ISO data shows that the most prominent of the potentially depleted cores indeed coincides with a dust temperature minimum. It seems therefore feasible to use N(C¹⁸O) and A_V data together for identifying cold, dense cores in large scale mappings.