Role of Molecular Opacities in Circumstellar Dust Shells

Ch. Helling

Dissertation, Technische Universität Berlin (1999)

Gzipped PostScript version (1.5 MB)

Abstract:

This thesis investigates the influence of molecular opacities on the structure and the dynamics of circumstellar dust shells around carbon stars on the Asymptotic Giant Branch (AGB). The investigations are performed in the framework of time-dependent spherical symmetric model computations, where a coupled equation system is solved describing the hydrodynamics, the chemical composition of the gas, the radiative transfer in grey approximation, and the dust formation, growth and evaporation in a self-consistent way. The role of the molecular opacities concerns the following: (i) molecular opacities have a decisive influence on the structure of the atmosphere as function of optical depth, and (ii) molecules can directly exert a considerable force on the gas by radiation pressure. The evaluation of both effects requires the calculation of the flux-mean opacity which in this work is either replaced by the Planck mean opacity (upper limit) or by the Rosseland mean opacity (lower
limit).

New Planck mean and Rosseland mean gas opacities have been calculated and tabulated for different element abundances, based on several millions of spectral lines of CO, TiO, SiO, H2O, CH, CN, C2, C3, HCN, and C2H2 beside several continuum sources. Both mean opacities show a similar behavior at high temperatures, but can differ by as much as three orders of magnitude at low temperatures due to the strong frequency-dependence of the molecular opacities. The larger Planck mean gas opacities reach about 10% and 30% of the dust opacity in case of a fully condensed carbon-rich and oxygen-rich gas, respectively.

In the model calculations, a large gas opacity (e.g. Planck means) generally causes a less dense atmosphere, which consequently leads to smaller amounts of dust, smaller mass loss rates, lower terminal wind velocities and lower dust-to-gas ratios.

When using the Planck means, both the initial static models and the dynamical models for carbon stars exhibit local pressure inversions in the inner regions, i.e. a local increase of pressure density with increasing distance from the central star. These inversions are caused by strong radiation pressure on molecules. A closer inspection in the (T,
ho) plane reveals that the inversions coincide with the maximum particle concentration of C3, suggesting a causal connection. The occurrence of pressure inversions illustrates the important influence molecular opacities can have especially on the inner dust-free regions of a circumstellar shell.

Thus, strong molecular opacities cause two contrary effects: a general lowering of the density, but also local density enhancements caused by the pressure inversions. In conjunction with the dust formation, which strongly depends on density and temperature, new interrelations and additional effects occur in the models, which are not present in case of small gas opacities. For instance, radiation pressure on molecules can introduce perturbations into the velocity field, which may be amplified and can steepen into shocks by dust formation at larger distances. Shock waves generated by the interior pulsation of the star can be damped and partly reflected when they encounter a finite region of increasing density.

An analysis of the chemical composition of the gas in the time-dependent models reveals a multi-layered structure of the molecules situated behind shock waves. The levitation of the outer atmospheres due to pulsation-induced shocks and/or radiation pressure on molecules leads to much larger molecular densities than expected from standard (static) model atmospheres, in particular regarding the polyatomic molecules like H2O, CO2, SO2, HCN and C2H2. These molecules reach their highest particle densities at radial distance as large as 1.5 to 3 stellar radii. A comparison of the calculated column densities of these molecules with recent Infrared Space Observatory (ISO)-observations shows that the levitation provides a promising approach to explain the formation of the so-called "warm molecular layers1. Dust formation at the outer edge of such a
density-enhanced region amplifies the levitation and causes an additional backwarming of the molecular layer.

1 This term has been invented by observational astronomers in order to describe the strong absorption and/or emission features attributed to these molecules.