Onset of convection near a point source of heat and gravity.

A paradoxical symmetry breaking--development of a unidirectional flow via thermal instability--is predicted by an analytical solution for compressible convection near a point source of both heat and gravity. Such a flow can propel a cosmic hot body (e.g., a protostar) in a molecular cloud. This convection emerges at the critical Rayleigh number Ra = l(l+1) [2l(l+1)+1+3 mu(V)/2]gamma/(gamma-1), where l is the number of flow cells, mu(V) is the second-to-first viscosity ratio, and gamma is the specific heat ratio.