Depletion of CO in a cold dense cloud core of IC 5146

We have used the IRAM 30m telescope to map the (CO)-O-18(1-->0) and (2-->1) transitions in a region of 0.5 x 0.5 pc(2) (200 " x 200 ") in the nearby ( 460 pc distance) molecular cloud IC 5146 at resolutions of down to 0.025 pc . Additionally, we have observed the corresponding (CO)-O-17 transitions to wards 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 (C O)-O-18 emission observed by us is optically thin and that the observed (CO )-O-18(2-->1)/(1-->0) ratio is consistent with gas at 10 K and density in t he range 10(4) to 10(5) cm(-3). We have estimated (CO)-O-18 column densitie s over the region mapped, using an LVG program to correct for excitation ef fects; and have compared them, at the same spatial resolution of 30 ", with the dust extinction derived from the NIR images of Lada et al. (1998). Fro m this, we find a roughly constant, though with considerable dispersion, ra tio of (CO)-O-18 column density to visual extinction (Av) in directions wit h Av less than similar to 10 magnitudes. The ratio is compatible with previ ous determinations of the [(CO)-O-18]/[H-2] abundance ratio, e.g. 1.7 10(-7 ) as obtained by Frerking et al. (1982). At extinctions higher than similar to 10 magnitudes, however, there is a systematic fall-off in the ratio N(( CO)-O-18)/A(V) which we attribute to depletion of (CO)-O-18 onto dust ice m antles within the dense, n greater than or similar to 10(4) cm(-3), and col d, T-dust less than or similar to 15 K, T-gas similar to 10 K, core interio r. This fall-off is also seen, though with less statistical significance, i n our observed dependence of N((CO)-O-17)/A(V) with A(V). We thus think it unlikely that optical depth effects are influencing our conclusions. We hav e additionally made pointed (CO)-O-18(1-->0)) and (2-->1) observations towa rds 94 background stars with extinction greater than 10 magnitudes from the study of Lada et al. (1994). These in general confirm our mapping results.