1999A&A...342..257K

We have used the IRAM 30m telescope to map the C¹⁸O(1->0) and (2->1) transitions in a region of 0.5x0.5pc² (200''x200'') in the nearby (460pc distance) molecular cloud IC5146 at resolutions of down to 0.025pc. Additionally, we have observed the corresponding C¹⁷O transitions towards 24 positions in the central region of the cloud as a check on optical depth effects. On the basis of these observations, we conclude that the C¹⁸O emission observed by us is optically thin and that the observed C¹⁸O(2->1)/(1->0) ratio is consistent with gas at 10 K and density in the range 10⁴ to 10⁵ cm^–3. We have estimated C¹⁸O column densities over the region mapped, using an LVG program to correct for excitation effects, and have compared them, at the same spatial resolution of 30'', with the dust extinction derived from the NIR images of Lada et al. (1998ApJ...495..871L) From this, we find a roughly constant, though with considerable dispersion, ratio of C¹⁸O column density to visual extinction (A_V) in directions with A_V less than ∼10 magnitudes. The ratio is compatible with previous determinations of the [C¹⁸O]/[H₂] abundance ratio, e.g. 1.7*10^–7 as obtained by Frerking et al. (1982) At extinctions higher than ∼10 magnitudes, however, there is a systematic fall-off in the ratio N(C¹⁸O)/A_V which we attribute to depletion of C¹⁸O onto dust ice mantles within the dense, n>10⁴cm^–3, and cold, T_dust≲15K, T_gas∼10K, core interior. This fall-off is also seen, though with less statistical significance, in our observed dependence of N(C¹⁷O)/A_V with A_V. We thus think it unlikely that optical depth effects are influencing our conclusions. We have additionally made pointed C¹⁸O(1->0) and (2->1) observations towards 94 background stars with extinction greater than 10 magnitudes from the study of Lada et al. (1994ApJ...429..694L) These in general confirm our mapping results.