How to kill a cool giant: Exploring wind-driving dust species

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Date/Time:Friday, 06 Apr 2012 from 4:10 pm to 5:00 pm
Location:Room 3
Contact:Lee Anne Willson
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Talk by Suzanne Höfner, Uppsala University (Sweden)

The cool massive outflows observed in evolved AGB stars are usually attributed to a two-stage process: atmospheric levitation by pulsation-induced shock waves followed by radiative acceleration of dust grains which transfer momentum to the surrounding gas through collisions. There is, however, no general consensus at present which types of dust grains are triggering the outflows.

To identify critical properties of potential wind-driving dust species we use detailed radiation-hydrodynamical models which include a parameterized dust description, complemented by simple analytical estimates to help with the physical interpretation of the numerical results. The adopted simplified dust description is constructed to mimic different chemical and optical grain properties in order to systematically study the effects of a realistic radiation field on the second stage of the mass loss mechanism. Our models confirm that metallic iron and Fe-bearing silicates are not viable as wind-drivers due to their NIR optical properties and resulting large condensation distances. A promising candidate is Mg2SiO4 with grain sizes of 0.1-1 micron, where scattering contributes significantly to the radiative acceleration, as suggested by our earlier theoretical work. Models based on a detailed description of the formation of such grains show photometric colors in good agreement with observations.