Sedimentation of grains in interstellar clouds

Gravitational sedimentation of dust grains is analyzed for two idealized, spherically symmetric modesl of interstellar clouds: (i) a unifrom-density, uniform-temperature model, which permits exact analytic solution, and (ii) an isothermal, hydrostatic equilibrium model. Starting from a uniform mix of dust and gas, the ratio of dust density to gas density increases as e/sup t//tau with time constant tau of order 108 years for typical cloud parameters. For optically thin dust distributions, the ambient interstellar radiation field can increase the infall rate by an order of magnitude. An analytic solution is also found for sedimentation in a uniform magnetic field; this leads to pancake-shaped dust distribution. Finally we present a scenario for the formation of massive clouds, which would imply that interstellar clouds have highly inhomogeneous dust distributions