The destruction and growth of dust grains in interstellar space - III. Surface recombination, heavy element depletion and mantle growth

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Abstract
It is shown that metallic elements in interstellar clouds will be trapped on the surfaces of graphite and iron grains, leading to their differential gas-phase depletion, whereas non-metal elements will be ejected as saturated hydrides from such grains (and as monohydrides from other grains), with no consequent depletion. A consideration of surface reaction pathways leads to the prediction that Na, K and Zn should undergo less depletion than other metals, in accordance with observations. The factors governing the return of depleted metallic species to the gas phase are discussed. In diffuse clouds the growth of mantles is prevented by photo-desorption and by the ejection of non-metal atoms during surface recombination, but in dense clouds which have sufficient ultraviolet shielding, and where the non-metals are in saturated molecules, mantle growth will take place. Upon exposure to the unshielded interstellar ultraviolet radiation field such mantles will rapidly be destroyed. Thus mantles should be found only in dense, shielded regions of interstellar space.

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