On the Injection of Grains into Interstellar Clouds

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Abstract
Dust grains expelled from cool stars are injected into interstellar clouds with typical velocities of several thousand km s–1. The interaction of such high speed grains with interstellar gas is discussed. Grains are shown to be preferentially stopped within gas clouds of number density |$n_\text{H}\,\approx \,10-30\,\text{cm}^{-3}$|⁠, thus leading to the observed positive correlation between gas and dust. Impinging gas atoms, which have initial relative energies |$\sim {10}^{2}\,\text{KeV},$| produce several interesting effects. Grains are reduced in radius by a small factor due to sputtering, and the slowing of impinging atoms within grains leads to an initial heating of grains to temperatures ~ 200 K and to the accumulation of solid-state defects. The displacement defect concentration saturates at a value of ~ 1 per cent. The dissipation of the kinetic energy of grains within interstellar clouds could provide a major source of heating and ionization of such clouds.

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