Dust shells around oxygen-rich Miras and long-period variables

K.S. Jeong

Dissertation, Institut für Astronomie & Astrophysik, TU Berlin (2000)

PDF (4.2 MB)

edocs-Server der TUB: Abstract, PDF.

Abstract:

Circumstellar dust shells around cool pulsating late type stars are the major sources for the replenishment of the interstellar medium with processed material, in particular in the form of complex molecules and of dust particles. In contrast to the carbon-rich situation, where mainly homogeneous amorphous carbon grains and also PAHs are formed, almost all carbon is locked up in the chemically inert CO molecule in an oxygen-rich gas. Therefore, grains can only be formed from molecules containing the remaining oxygen and those less abundant elements (Fe, Si, Mg, S, Al, and Ti). Hence, a heterogeneouscomposition of the emerging dust component has to be expected in this case. Nucleation, the onset of the dust formation, requires the information about the cluster size distribution, which can be calculated from the Gibbs free energy of formation. Often it is the case that the data needed are not avaiable for the condensates. In order to calculate the thermochemical properties, e.g.
Gibbs free energy of formation for TiO2 clusters, the geometric optimisation of the electronic structures has been performed in the quantum mechanical calculations based on the density functional theory. The Gibbs free energies of (TiO2)n clusters resulting from the quantum mechanical density functional theory are in good agreement with those values derived by approximation in terms of the surface energy of the solid bulk, which is considered in the nucleation theory. Therefore, it is appropriate to further use the modified nucleation theory for calculating the homogeneous nucleation rates of the high temperature condensates. Among the considered primaey condensates Fe, SiO, TiO and TiO2, only the titanium oxides show efficient nucleation rates at temperatures above 1000 K, where TiO2 nucleation is by far more efficient than TiO due to its higher gas phase concentration. The
TiO2 nuclei grow by accretion of those molecular species which can form stable condensates under the thermodynamic conditions prevailing in the circumstellar environment. The result is a heterogeneous grain mantle composition. The consistent description of a dust forming circumstellar shell constitutes a non-linearly coupled problem comprising hydrodynamics, thermodynamics, chemistry, dust formation, growth and evaporation, and the radiative transfer. Therefore, a simultaneous solution of the complete system, including the dust formation, is indispensable for the reliable description of a circumstellar dust shell. The first consistent model calculations for oxygen-rich circumstellar shells around Miras and long-period variables including dust formation have been performed. The solution of the equation system is completely determined by the 4 fundamental stellar parameters, stellar luminosity, stellar mass, stellar temperature, and abundances of the elements
and the additional 2 pulsation parameters, the pulsation amplitude and period. The model studied in this work with 20000 solar luminosity, 1 solar mass, 2000 K of stellar temperature, pulsation amplitude of 5 km/s and period of 300 days, and solar metallicity produces a mass loss rate of 0.00006 solar masses per year and an outflow velocity of 16 km/s which show a good agreement with the observation. Dust plays the key role in the determination of the structure of also the oxygen-rich circumstellar shells by driving the wind through the radiation pressure on dust as it has been shown in case of the carbon-rich circumstellar shells. Due to the periodic formation of the dust layers, the radial dust distribution reveals inhomegeneous characteristics. The non-linear effect due to the strong coupling of the whole systems of hydrodynamics,thermodynamics, radiative transfer, chemistry, and the dust fromation reflects the multiperiodicity such that the radial structure of circumstellar dust
shells repeats in the time scale of nP with the pulsationa period P and integer n. In the time variation of the mass loss rates, this multiperiodicity is also reflected, involving with the different timescales, e.g. the duration time of the successive two layers a the shell.