Theoretical Spectra of Circumstellar Dust Shells around Carbon-rich Asymptotic Giant Branch Stars

J.M. Winters, C. Dominik and E. Sedlmayr

Astronomy & Astrophysics, Vol. 288, 255-272 (1994)

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Abstract:

Realistic modelling of circumstellar dust shells around evolved stars has to include a physical treatment of the interaction among hydrodynamics, 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 determined by the three fundamental stellar parameters stellar mass M_*, stellar luminosity L_* and effective temperature T_{eff} and by the element abundances epsilon_i.

A detailed calculation of the transport coefficients 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 discuss the dependence of the resulting spectra on the stellar parameters in terms of infrared two colour diagrams. Application of these model calculations to the prominent infrared object IRC+10216 yields best agreement with the observed spectrum and the visibility data at maximum light for the stellar parameters M_*=0.7M_sun, L_*=2.4*10^4 L_sun, T_*=2010K and a carbon to oxygen ratio of epsilon_C/epsilon_O = 1.40, which corresponds to a mass-loss rate of dot{M}=8*10^{-5} M_sun 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 deduced. The total mass contained in the circumstellar dust shell implies an initial main sequence mass of M_{ZAMS}>=1.3M_sun for IRC+10216.

Keywords:

Stars: carbon - circumstellar matter - Stars: individual: IRC +10216 - Stars: mass-loss - Stars: AGB