PHASE-TRANSITION TIME SCALES FOR COOLING AND ISOTHERMAL MEDIA

It has been demonstrated that the collapse of a sub-Jeans mass cloud may proceed in two stages: first, a phase transition takes place and drives the cloud to high enough densities for gravity to take over; then, gravity completes the process and generates a "stellar" state. The cooling regime of the interstellar medium (where the effective adiabatic exponent 0 < Γ < 1) has turned out to be very important in this mechanism, as it allows a wide spectrum of masses to collapse, independently of the choice of Γ. It is imperative that we know the characteristic time τpt within which the phase transition of a gas cloud can occur. This time depends on the adopted Γ, the mass of the cloud, and the amplitude of acting disturbances. We have used mild disturbances and, for Γ < 1, have found that τpt is comparable to the dynamical time τff of the initial state, or, in the worst cases, only a few orders of magnitude larger than τff. When the same disturbances act in an isothermal cloud, on the other hand, we have found that τpt Gt; τff. Slightly stronger perturbations seem to be able to induce collapse of a limited spectrum of cloud masses in the isothermal regime in times less than one Hubble time. This comparison between the cooling and the isothermal regimes also indicates that the Γ < 1 regime can be very important in problems related to star formation and the structure and evolution of the interstellar medium. Any phase transition is, however, a stochastic process, so it generally requires several dynamical times to develop in the interstellar medium of a galaxy.