ON THE GAS TEMPERATURE IN THE SHOCKED CIRCUMSTELLAR ENVELOPES OF PULSATING STARS .2. SHOCK-INDUCED CONDENSATION AROUND R-CORONAE-BOREALIS STARS

A physical mechanism is presented, which may be essential for the occa sional onset of dust formation in the circumstellar envelopes of pulsa ting R CrB stars. We study the thermal energy balance, the chemistry a nd the nucleation in fixed fluid elements of the circumstellar envelop es around R CrB stars, which are periodically hit by strong shock wave s caused by the stellar pulsation. Non-LTE radiative heating and cooli ng via free-free, bound-free and atomic line transitions and via rotat ional and re-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 intern al energy, and then reexpands 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 densi ties n(<He>) = 10(7...10) cm(-3), this finally causes substantial lowe r gas temperatures than in radiative equilibrium. Thus, the preconditi ons for effective carbon nucleation (high densities and low gas temper atures for a sufficiently long time) may be temporarily present quite near to the photosphere of a pulsating R CrB star. The presented mecha nism leads to gas temperatures as low as 1500 K already outside of a r adial distance of only 1.5 - 3R, despite of the high effective temper atures of R CrB stars.