On the gas temperature in the shocked circumstellar envelopes of pulsating stars. II. Shock induced condensation around R Coronae Borealis stars.

A physical mechanism is presented, which may be essential for the occasional onset of dust formation in the circumstellar envelopes of pulsating RCrB stars. We study the thermal energy balance, the chemistry and the nucleation in fixed fluid elements of the circumstellar envelopes around RCrB stars, which are periodically hit by strong shock waves caused by the stellar pulsation. Non-LTE radiative heating and cooling via free-free, bound-free and atomic line transitions and via rotational and ro-vibrational transitions of polar molecules is taken into account. After the heating and compression due to an outrunning shock, the considered fluid element first radiates away its excess of internal energy, and then re-expands according to the periodicity, which is a typical feature in such pulsating envelopes. This reexpansion causes adiabatic cooling. Within a particular range of the gas particle densities n__= 10^7...10^cm^-3^, this finally causes substantial lower gas temperatures than in radiative equilibrium. Thus, the preconditions for effective carbon nucleation (high densities and low gas temperatures for a sufficiently long time) may be temporarily present quite near to the photosphere of a pulsating RCrB star. The presented mechanism leads to gas temperatures as low as 1500K already outside of a radial distance of only 1.5-3R_*_, despite of the high effective temperatures of RCrB stars.