ON THE GAS TEMPERATURE IN THE SHOCKED CIRCUMSTELLAR ENVELOPES OF PULSATING STARS .1. RADIATIVE HEATING AND COOLING RATES

Radiative heating and cooling in the circumstellar envelopes of cool s tars is investigated, considering gas densities of 10(4 ... 14) cm(-3) and gas temperatures of 500...20000 K. Various heating and cooling pr ocesses are included: rotational and re-vibrational transitions of pol ar molecules and of H-2, atomic line transitions, bound-free transitio ns, free-free transitions and photochemical reactions. Theoretical con cepts and computational methods are worked out, which on the one hand can account for important non-LTE effects and radiative trapping and o n the other hand allow for a fast and proper inclusion of these heatin g and cooling rates into time-dependent hydrodynamical model calculati ons. Radiative cooling timescales for a carbon-enriched gas typical fo r C-star atmospheres are calculated, and the thermal relaxation of the gas after the passage of shock waves is discussed. A gradual transiti on of the character of the propagating shock waves is expected to occu r at densities around 10(6 ... 8) cm(-3), changing from predominantly isothermal to predominantly adiabatic with decreasing gas density.