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 \rho ^ { 2 } T ^ { \alpha } , where \rho , T and \alpha are density , temperature and a free parameter , respectively .
Given \alpha , a family of self-similar solutions with one parameter \eta is found in which the central density and pressure evolve as follows : \rho ( x = 0 ,t ) \propto ( t _ { \mathrm { c } } - t ) ^ { - \eta / ( 2 - \alpha ) } and P ( x = 0 ,t ) \propto ( t _ { \mathrm { c } } - t ) ^ { ( 1 - \eta ) / ( 1 - \alpha ) } , where t _ { \mathrm { c } } is an epoch when the central density becomes infinite .
For \eta \sim 0 , the solution describes the isochoric mode , whereas for \eta \sim 1 , the solution describes the isobaric mode .
The self-similar solutions exist in the range between the two limits ; that is , for 0 < \eta < 1 .
No self-similar solution is found for \alpha > 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 .