Theory of charged dust clouds: Equilibrium

Gravitational equilibria of low density dust clouds imbedded in plasma background are studied. These are a new class of astrophysical objects, in which the electric field in quasineutral plasma balances the self-gravity of the dust. Heuristic arguments based on the total potential energy of a uniform dust ball are given for the existence of stable equilibrium and the mass limit for the maximum dust mass $MAS$⁠, which can be supported against its self-gravity. The gravitational collapse is initiated by the destabilization of the fundamental radial mode as the mass of the dust cloud $MD→MAS$⁠. The physics of the mass limit is similar to the Chandrasekhar’s mass limit for compact objects. One parameter family of spherically symmetric, static solutions of radius $R(≈0.1AU)$ and the total mass of the dust cloud $MD(≈1019g)$⁠, for given central dust density, are constructed. A finite flux of photoelectrons is shown to decrease $R$ and $MD$⁠. Two-dimensional rotating dust disk equilibrium, characterized by a sharp and diffuse boundary, is also constructed. These dust equilibria (spherical clouds, rotating disks), which are predicted to be within the interstellar clouds, could be relevant for planet formation, as they provide locations where a significant quantity of dust can be stably trapped. The accretion and coagulation interaction between these trapped dust grains may lead to planet formation.