On the Growth and Destruction of Ice Mantles on Interstellar Graphite Grains

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Abstract
Graphite particles ejected from stars tend to grow ice mantles in interstellar clouds. It is shown that cloud collisions following close upon encounters with O and B stars have the effect of destroying all ice grains and mantles present in the colliding clouds by a low energy sputtering process. Destruction of mantles by evaporation during cloud encounters with O and B stars is also considered; but this process is found to be less efficient except for the case of very bright stars, with bolometric magnitude less than −5. Graphite grains and cores are not destroyed by either of these processes. It is also possible that ice grains and mantles are removed by mutual grain impacts during cloud collisions as proposed by Oort and van de Hulst (9). It is shown, however, that this process is less important that the sputtering mechanism proposed in the present paper. A steady-state size distribution of ice mantles has been obtained. It is shown that at any instant after a steady state is reached, the number of grains with small ice mantles, of about 3 × 10 −6 cm or less in thickness, is of the same order as the total number of grains with ice mantles of all sizes. It is therefore concluded that the optical properties of grains are determined mainly by the graphite component and not by the ice mantles as recently suggested by van de Hulst (7). A further restriction to mantle growth arises fiom the running out of available O atoms. If most of the interstellar carbon is locked in grains, a core of radius 3 × 10 −6 cm can grow an ice mantle only up to a radius 6 × 10 −6 cm before all the interstellar O atoms are used up.

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