2003MNRAS.343..259S

We have observed several cloud cores in the Orion B (L1630) molecular cloud in the 2 ⟶ 1 transitions of C¹⁸ O, C¹⁷ O and ¹³ C¹⁸ O. We use these data to show that a model where the cores consist of very optically thick C¹⁸ O clumps cannot explain their relative intensities. There is strong evidence that the C¹⁸ O is not very optically thick. The CO emission is compared with previous observations of dust continuum emission to deduce apparent molecular abundances. The abundance values depend somewhat on the temperature but, relative to `normal abundance' values, the CO appears to be depleted by about a factor of 10 at the core positions. CO condensation on dust grains provides a natural explanation for the apparent depletion both through gas-phase depletion of CO, and through a possible increase in dust emissivity in the cores. The high brightness of HCO<sup>+</sup> relative to CO is then naturally accounted for by time-dependent interstellar chemistry starting from `evolved' initial conditions. Theoretical work has shown that condensation of H₂O, which destroys HCO⁺, would allow the HCO⁺ abundance to increase while that of CO is falling.