Dust and wind formation in low-metallicity AGB-stars

Ch. Helling, T. Arndt, E. Sedlmayr

IAU Colloquium 185: Radial and Nonradial Pulsations as Probes of Stellar Physics eds. Aerts, Bedding, Christensen-Dalsgaard

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Abstract:

Combined investigations of IRAS and ISO observations have revealed new constrains on the dust forming circumstellar envelopes of AGB stars in the Magellanic Clouds (e.g. van Loon et al. 1999). So far, radiation pressure on newly formed dust grains has been studied as promising candidate for driving massive outflows by various authors mainly in the frame work of circumstellar envelopes of solar metallicity stars: dust is an important dynamical component in such luminous objects due to its large opacity resulting in e.g. the appearance of multi-periodic dust shells (Fleischer et al. 1995) or the onset of a dust-induced super-wind phase (Schröder et al. 1999).

The amount of dust formed depends on the metal content of the gas which, in turn, influences the local temperature and density by radiative transfer effects providing the conditions for the gas-solid phase transition. The gas opacity naturally decreases with decreasing metallicity (e.g. Helling et al. 2000), and therefore, its influence on the thermodynamic condition in low metallicity stars is smaller than in a solar composition gas.

First results will be presented in which the role of the metal content of the gas for the dust and the wind formation in carbon-rich AGB-stars is investigated in the framework of one-dimensional numerical simulations solving the equations of time-dependent hydrodynamics, (grey) radiative transfer, equilibrium chemistry, and dust formation (nucleation, growth, evaporation; Gail & Sedlmayr 1988). The gas opacity is approximated by its upper limit, the Planck mean opacity. The pulsation of the star is simulated by a sinusoidal variation of the inner boundary. Our first model results suggest that:

• The decrease of the metal abundance significantly lowers the amount of dust formed and thereby causes a considerable reduction of the mean mass loss rate and mean outflow velocity compared to solar metallicity stars. In contrast, the mean dust-to-gas ratio decreases only slightly.
• Radiation pressure on dust grains alone is too small to exceed the gravitational deceleration of the star. Also the radiation pressure on molecules is not strong enough to drive an outflow. Only both components together, radiation pressure on dust and radiation pressure on molecules, provide an radiative acceleration just large enough to overcome the gravity of the star.
• Present low-metallicity models develop a rather quasi-static structure (i.e. long hydrodynamical time scales) where the dust is present solely in one unique shell. New dust is formed continously at the inner edge of this shell which slowly extents as the outer parts move outward.

The results suggest that the wind of metal poor stars cannot be driven by radiation pressure on dust grains alone. An additional driving mechanism different from those already included in the models (pulsation, radiation pressure on dust and/or molecules) possibly exists.

Fleischer, A.J., Gauger, A., Sedlmayr E., 1995, A&A 297, 543

Gail H.-P., Sedlmayr E., 1988, A&A 206, 153

Helling Ch., Winters J. M., Sedlmayr E., 2000, A&A 358, 651

van Loon J., Groenewegen M., de Koter A., Trams N., Waters L., et al., 1999, A&A 351, 559

Schröder K.-P., Winters J. M., Sedlmayr E., 1999, A&A 349, 898

Keywords:

low metallicities, LMC