Internal Structure and Optical Appearance of Circumstellar Dust Shells around Cool Carbon Giants

J.M. Winters

Dissertation, Technische Universität Berlin, 1994

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Abstract:

Cool late-type giant stars represent important stages in the evolution of low and intermediate mass stars. These objects generally are surrounded by extended circumstellar dust shells accompanied by considerable mass loss.

In this thesis, detailed physical models of the dust forming circumstellar shells around late-type carbon-rich giants are presented. These models include a consistent treatment of hydrodynamics, thermodynamics, chemistry, dust formation and growth and of the radiative transfer problem. The complete atmospheric structure as well as the infrared appearance of the models is a result of the calculations.

This work is divided into two major parts.

In the first part, a selfconsistent solution of the stationary, spherical symmetric dust driven wind problem is presented. The resulting shell structure and the mass loss rate are completely determined by the four fundamental stellar parameters stellar mass M_*, stellar luminosity L_*, stellar temperature T_* and the element abundances epsilon_i. The results imply, that in a stationary dust driven wind the common assumption of the small particle approximation for calculating the transport coefficients of the dust component is not justified. In particular in the inner shell region, the radiative force acting on the grains is essentially reduced by the presence of large particles. The dependence of the models on the stellar parameters is discussed and the resulting spectra are compared to the observations in the frame of infrared two colour diagrams. Application of these model calculations to the prominent infrared object IRC +10216 yields best agreement with the observed energy
distribution and the interferometric data obtained at light maximum 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 yields a mass loss rate of dot M=8*10^{-5} M_sun yr^{-1}. 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.

In the second part, the results of detailed frequency and angle-dependent radiative transfer calculations are presented, which are based on time-dependent models for the pulsating atmospheres around carbon-rich Long-period variable stars (LPVs). These models are specified by the four fundamental stellar parameters and two additional quantities, which describe the interior pulsation of the star. In these calculations, the transport coefficients of the dust component are described in the small particle limit of Mie theory, which is justified in the case of a pulsating atmosphere. Due to the periodic formation of dust layers, an inhomogeneous radial dust distribution is produced, which decisively influences the optical appearance of the models. It is shown that the synthetic lightcurves exhibit intermediate maxima and secondary periods, which similarly occur in the observed lightcurves of real LPVs. The spectra of the dynamic models are compared with observations in the frame of a
near infrared two colour diagram. For a physically consistent modelling of the near infrared spectra, it turns out to be necessary to take the effects of the stellar pulsation into account. The discrete shell-like distribution of dust also causes significant structures in the surface brightness profiles and in the synthetic visibility functions, which are in qualitative agreement with the respective observations.

The discrete structure of the circumstellar dust shell for the first time provides an explanation for both, the significant characteristics present in the observed lightcurves of carbon-rich LPVs as well as for the small-scale structures present in the observed spatial spectra and in the brightness profiles.