Chemical models of interstellar gas-grain processes .3. Molecular depletion in NGC 2024

The chemistry arising from the exchange of molecules between dust and gas in ultradense (n(H) greater than or equal to 10(7) cm(-3)) cool cores is described, Under certain assumptions concerning molecule-grain binding energies and dust temperature, slight warming of cold dust grains can induce selective desorption of mantle volatiles. It is theoretically possible for ultradense cores to become depleted in CO but to remain abundant in N-2 and so be relatively rich in nitrogen-bearing molecules. This effect may explain the apparent depletion of CO and the presence of ammonia in several cores within the NGC 2024 cloud, The theory implies that heavy metal atoms should be almost entirely absent from the gas phase of high-density cores, and that a mantle of CO, O-2, N-2 and other weakly bound molecules should cover water ice mantles, forming a 'volatile crust'. Molecular observations are suggested which will allow a test of this model.