Formation of secondary critical points in thermally conducting interstellar gases

•This paper presents phase space aspects of nonlinear wave propagation in astrophysical gases.•The existence of secondary critical points in thermally conducting gases is established under some very general conditions.•The role of secondary critical points in structure formation in the ISM is discussed.

The interstellar medium (ISM) is a thermally conducting gaseous medium consisting of neutral ions, and in some cases free electrons. Considering the ISM as a continuous medium, phase space analysis is applied here to investigate the nonlinear problem of heating and cooling wave front evolution in thermally conducting homogeneous interstellar gases. With an arbitrary net heating function a general criterion for determining the critical points is established which shows that for a given value of the thermal conductivity exponent αα, secondary critical points emerge as a necessary feature of the gaseous medium. For a thermally conducting electron gas and for a neutral ion gas, we explicitly state the conditions for the occurrence of the secondary critical points. These critical points can either be unstable nodes, saddle points or degenerate nodes. The existence of secondary critical points is of significance in the formation of globule-like stable structures in the ISM, and in cloud formation phenomena in the interstellar gases.