STEADY EVAPORATION AND CONDENSATION OF ISOLATED CLOUDS IN HOT PLASMA

Clouds embedded in a thermally stable, hot intercloud medium can undergo steady evaporation or condensation. Numerical and approximate analytical results are presented for clouds of arbitrary size; magnetic effects are ignored. Thermal conduction dominates radiative cooling over distances smaller than a critical length, the "Field length." Clouds smaller than the Field length always evaporate. Clouds much larger than the Field length are effectively planar and evaporate only if the pressure is less than a critical value, psat. At higher pressure, the hot gas condenses onto the cloud; psat, is close to the maximum pressure at which the hot gas can exist in equilibrium, so condensation is generally unsteady. By contrast, the domain of classical evaporation, in which the cloud evaporates and the conductivity has its classical value, is quite broad. Results are presented for the case in which the intercloud medium is heated by Compton scattering of X-rays and cooled by inverse Compton scattering and by bremsstrahlung, which may be relevant in active galactic nuclei and accretion disk coronae of galactic X-ray sources. In particular, if the intensity of the X-rays irradiating the disk is comparable to that in the corona above the disk, then thermal conduction drives the corona close to thermal instability, and it is likely to be cloudy.