Effect of gas drag on the dynamics of protostellar clumps in molecular clouds

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Abstract
Molecular clouds are observed to have substructure within them, and to consist of dense condensations or clumps which are surrounded by a less-dense interclump medium. The interclump gas could retard the motion of the clumps in the cloud through a drag force caused by dynamical friction. The effect of such a drag force on the dynamics of protostellar clumps in a molecular cloud is numerically analysed in this paper. As the dynamical drag acting on a clump is dependent on its mass, clumps of different masses have, in general, different retardations. It is found that the most massive objects suffer the maximum decelerating effects and settle to the centre in a shorter time than do the lower mass clumps. Thus there is a clustering of the clumps towards the centre of the cloud, and a radial segregation of mass is established in the cloud. The time-scale of the process is governed by the cloud parameters, and is shorter for denser clouds. The most massive protostellar clumps in the cloud are thus expected to lie at the centre of the cloud. The mass segregation observed in many young clusters of pre-main-sequence stars could be a result of dynamical drag.

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