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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 ∝ρ^{2}*T*^{α}, where ρ, *T* and α are the 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*) ∝ (*t*_{c}−*t*)^{−η/(2−α)} and *P*(*x*= 0, *t*) ∝ (*t*_{c}−*t*)^{(1−η)/(1−α)}, where *t*_{c} is the epoch at which the central density becomes infinite. For η∼ 0 the solution describes the isochoric mode, whereas for η∼ 1 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 (H i clouds and molecular clouds) by thermal instability.

*Keywords: *
hydrodynamics;
instabilities;
ISM: structure

*Journal Article.*
*4641 words.*
*Illustrated.*

*Subjects: *
Astronomy and Astrophysics

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