A plum-pudding model for interstellar grains

Abstract

THERE seem to be two basic mechanisms by which interstellar grains can grow; the condensation of gaseous molecules (mostly of the CNO type) on to their surfaces and the coagulation on collision of individual grains. In general, as has been shown by Simons and Williams¹, condensation leads to a more rapid growth rate than coagulation resulting from the thermal motion of the grains. If, however, relative motions can be set up within the grain population, then this increases the collision rate and so also increases the rate of growth by coagulation. Now, for small grains, the force on them from radiation pressure depends on a different function of the grain radius, a, than does the viscous drag generated by the ambient gas. (For simple models these functions are respectively a³ and a².) Consequently, grains of differing sizes will be caused to move with different velocities through the ambient gas by radiation, the larger ones moving faster. This causes these grains to capture further grains by coagulation which in turn enhances their relative velocity even further. The purpose of the present communication is to show that this growth caused by radiation pressure coagulation is probably of the same order as that from gas atom condensation, and to point out an important consequence of this.