PHASE-TRANSITIONS IN GRAVITATING SYSTEMS AND THE FORMATION OF CONDENSED OBJECTS

The thermal equilibrium of a self-gravitating classical fluid with loc al equation of state corresponding to a system of hard spheres is stud ied numerically. When the volume effectively occupied by the particles is much smaller than the accessible volume, a phase transition occurs at which the system Can shuttle between a quasi-uniform state and one in which a highly condensed nucleus is immersed in a dilute atmospher e. Under isothermal contact conditions, the two states have different energies, under isoenergetic conditions, they have different temperatu res. The isothermal transition bridges a region of negative specific h eat in the family of isoenergetic systems. The phase transitions mark nonlinear fluid stability thresholds. These can differ by orders of ma gnitude from the traditional linear ones, i.e. the gravothermal catast rophe and Jeans instability, which only mark the stability limits of t hermally metastable regions. It is discussed how phase transitions may give the proper onset criteria for the formation of condensed objects from the size of planetoids up to stars.