Dynamics of cool fronts in optically thin hot plasmas

Steady cool fronts supported by nonsaturated thermal conduction in opticall y thin plasmas are analyzed. Explicit relations for the pressure (p(2)), nu mber density (n(2)), temperature (T-2), and flow velocity (v(2)) behind the front are found as functions of the temperature T-1 and the intake Mach nu mber M-1 ahead of the cool front. Weak cool fronts may exist only for intak e velocities less than the isothermal sound speed. Behind these kinds of fr onts, the flow retards, cools down, and is compressed. Strong cool fronts e xist for intake Mach numbers greater than a threshold value depending on T- 1. In these fronts, two compression branches appear, a high-compression bra nch well below the isotherm for which n(1)/n(2) < (gamma - 1)/(gamma + 1) a nd a weak-compression branch, close to (but below) the isotherm. In particu lar, in the high-compression branch for low values of M. quasi-isobaric coo l fronts occur when the ratio n(1)/n(2) close to the maximum value, where t his branch just emerges, beyond which the pressure of the flow behind the f ront increases when n(1)/n(2) decreases. The implications of these results on the formation of cool condensations in the hot interstellar medium gas a nd in the solar atmosphere are outlined.