Evaporation and Condensation of HI clouds in thermally bistable interstellar media: semi-analytic description of isobaric dynamics of curved interfaces

We analyze the evaporation and condensation of spherical and cylindrical HI clouds of the cold neutral medium surrounded by the warm neutral medium. Because the interstellar medium including those two phases is well described as a thermally bistable fluid, it is useful to apply pattern formation theories to the dynamics of the interface between the two phases. Assuming isobaric evolution of fluids and a simple cubic form of the heat-loss function, we show the curvature effects of the interface. We find that approximate solutions for spherical clouds are in good agreement with numerically obtained solutions. We extend our analysis to general curved fronts taking into account the curvature effects explicitly. We find that the curvature effects always stabilise curved interfaces under assumptions such as isobaric evolution we adopt in this Letter.