Thermal instability: A statistical approach

A new statistical approach is presented to study the process of the thermal instability of optically thin unmagnetized plasma. In this approach, the time evolution of the mass distribution function over temperature, phi(T), is calculated. The function phi(T) characterizes the statistical properties of the multiphase medium of an arbitrarily spaced three-dimensional structure of arbitrary (small or large) temperature perturbations. We construct our theory using the isobarical condition (P = const over space). The developed theory is illustrated in the ease of a thermal instability of a slowly expanding interstellar cloud. Numerical solutions of the equations of the statistical theory are constructed. The proposed theory can be relevant to the formation of the two-phase medium in the similar to 1 pc region around quasars. Then small warm (T similar or equal to 10(4)K) clouds are formed as a result of the thermal instability in an expanded gas fragment, which is a product of either a star-star or a star-accretion disk collision.