Self-similar solutions for the dynamical condensation of a radiative gas layer

Abstract

A new self-similar solution describing the dynamical condensation of a radiative gas is investigated under a plane-parallel geometry. The dynamical condensation is caused by thermal instability. The solution is applicable to generic flow with a net cooling rate per unit volume and time $\propto ρ^2 T^α$, where $ρ$, $T$ and $α$ are density, temperature and a free parameter, respectively. Given $α$, a family of self-similar solutions with one parameter $η$ is found in which the central density and pressure evolve as follows: $ρ(x=0,t)\propto (t_\mathrm{c}-t)^{-η/(2-α)}$ and $P(x=0,t)\propto (t_\mathrm{c}-t)^{(1-η)/(1-α)}$, where $t_\mathrm{c}$ is an epoch when the central density becomes infinite. For $η\sim 0$, the solution describes the isochoric mode, whereas for $η\sim1$, the solution describes the isobaric mode. The self-similar solutions exist in the range between the two limits; that is, for $0<η<1$. No self-similar solution is found for $α>1$. We compare the obtained self-similar solutions with the results of one-dimensional hydrodynamical simulations. In a converging flow, the results of the numerical simulations agree well with the self-similar solutions in the high-density limit. Our self-similar solutions are applicable to the formation of interstellar clouds (HI cloud and molecular cloud) by thermal instability.