PHASE SEGREGATION IN A 2-PHASE SELF-GRAVITATING GAS-MIXTURE

We discuss the stability of a self-gravitating two-phase gas mixture b y linear analysis. The mixture is composed of diffuse phase and immers ed dense clouds in pressure equilibrium. Earlier we have shown that in a two-phase self-gravitating mixture, two fundamental wave modes are relevant: (1) the gravitationally modified sound mode with the intrins ic Jeans length lambda(J1) and (2) the modified void (or pattern forma tion) mode with the Jeans length lambda(J2). In this paper, we describ e the effect of stabilization of perturbations in a two-phase self-gra vitating mixture: when the diffuse phase dominates in volume (the volu me filling factor alpha similar or equal to 1), the pattern formation mode is gravitationally unstable for lambda> 1(cr), while the fast aco ustic mode remains gravitationally stable for arbitrary lambda; when t he dense phase dominates (alpha much less than 1), the slow acoustic m ode is gravitationally unstable, while the fast pattern formation mode is stable. The stabilization is caused by displacement of the light c omponent by agglomerating dense clouds: gravitationally unstable dense phase displaces light diffuse phase and thus excites in it wave motio ns. We argue that gravitational instability of the pattern formation m ode results in the segregation of phases. The critical length lambda(c r) depends on parameters of phases and their volume filling factors an d can vary from 8 pc for molecular clouds to 0.1-0.5 kpc for spiral ga laxies and protogalaxies. We briefly discuss possible consequences of the stabilization effect for the two-component model of protogalaxies.