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Paper: Atmospheric Dynamics of Luminous Late-Type Stars
Volume: 154, Cool Stars, Stellar Systems and the Sun: Tenth Cambridge Workshop
Page: 1569
Authors: Airapetian, V. S.; Ofman, L.; Robinson, R. D.; Carpenter, K.; Davila, J.
Abstract: We present first results of magnetohydrodynamic (MHD) calculations of winds from luminous late-type stars using an existing, 2.5D, non-linear MHD code recently developed by Ofman & Davila (e.g., Ofman & Davila 1997). We assume that the wind is initiated in a hydrostatic atmosphere with an isothermal pressure scale height of 0.072 R* and a ``chromospheric hole'' modeled by a transverse density structure and a radial magnetic field. To ensure that we are accurately assessing the terminal velocity of the wind, we carried out the calculations to a height of 20 stellar radii. We find that in the higher density (low Alfven velocity) regions outside of the ``chromospheric hole'' the Alfven waves are freely propagating. Ponderomotive forces associated with these waves drive radial, compressive motions and contribute to stellar wind acceleration. The compressive motions then excite slow magnetosonic waves which non-linearly steepen into solitary waves that propagate on top of a background flow. This situation is similar to solar coronal hole models. In the lower density ``chromospheric hole'' region the Alfven wave are strongly reflected, and produce a substantial outflow, with both radial and azimuthal velocities approaching the local Alfven speed. Our results are in qualitative agreement with observational signatures of winds in cool, luminous late-type stars.
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