Circumstellar dust shells around long-period variables VII. The role of molecular opacities

The role of molecular opacities for the structure and dynamics of winds of carbon-rich AGE stars is investigated in the frame work of time-dependent h ydrodynamic models of dust forming circumstellar shells around cool pulsati ng stars. New Rosseland and Planck mean gas opacity tables have been calcul ated for T epsilon [500K, 10 000K] and n[H] epsilon [10(5)cm(-3), 10(15)cm( -3)] for solar, LMC and SMC abundances. Carbon-rich, static and time-depend ent models have been computed using either the Planck mean or the Rosseland mean for solar and LMC metalicity or a constant gas opacity (chi(g) = 2 . 10(-4)cm(2)g(-1), Bowen 1988). In the model calculations, a large gas opaci ty (Planck mean) generally causes a less dense atmosphere than a small gas opacity (Rosseland mean, constant gas opacity) which leads to smaller amoun ts of dust formed, and consequently to smaller mass loss rates [(M) over do t], lower terminal wind velocities [v(infinity)] and lower dust-to-gas rati os [rho(dust)/rho(gas)] Models with lower metalicity (LMC) form by far the smallest amount of dust and show therefore the lowest [(M) over dot], [v(in finity)], and [rho(dust)/rho(gas)]. Counteracting to the global density red uction due to strong gas absorption, the density might LOCALLY increase due to a pressure inversion. These pressure inversions are preserved even in t he hydrodynamic models where the atmosphere is disturbed by the propagation of shock waves. Due to the present determination of the temperature struct ure by grey opacities in the time-dependent models, the occurrence of press ure inversions deserves, however, further investigations by means of a more elaborate treatment of the radiative transfer in dynamic model atmospheres .