THEORETICAL SPECTRA OF CIRCUMSTELLAR DUST SHELLS AROUND CARBON-RICH ASYMPTOTIC GIANT BRANCH STARS

Realistic modelling of circumstellar dust shells around evolved stars has to include a physical treatment of the interaction among hydrodyna mics, thermodynamics, radiative transfer, chemistry and dust formation and -growth. A self-consistent solution of this problem is presented in the case of stationary, spherical symmetric dust-driven winds. The resulting shell structure and the mass-loss rate are completely determ ined by the three fundamental stellar parameters stellar mass M, stel lar luminosity L and effective temperature T(eff) and by the element abundances epsilon(i). A detailed calculation of the transport coeffic ients of the dust component by means of the particle size distribution function and the solution of the non-grey radiative transfer problem provide realistic synthetic spectra of the dust shell models. We discu ss the dependence of the resulting spectra on the stellar parameters i n terms of infrared two colour diagrams. Application of these model ca lculations to the prominent infrared object IRC +10216 yields best agr eement with the observed spectrum and the visibility data at maximum l ight for the stellar parameters M = 0.7M., L* = 2.4 . 10(4) L., T* = 2010K and a carbon to oxygen ratio of epsilon(C)/epsilon(O) = 1.40, wh ich corresponds to a mass-loss rate of M = 8 . 10(-5) M.yr-1. In this model only amorphous carbon grains are considered as the main opacity source. From this model a distance to IRC + 10216 of d = 170pc is dedu ced. The total mass contained in the circumstellar dust shell implies an initial main sequence mass Of M(ZAMS) greater-than-or-equal-to 1.3M . for IRC+10216.