Other
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998baas...30..760a&link_type=abstract
American Astronomical Society Meeting, 191, #125.05
Other
Scientific paper
We present the results of a magnetohydrodynamic (MHD) simulation of winds from luminous late-type stars using a 2.5D, non-linear MHD computer code. In this simulation we assume that the wind is generated within a hydrostatic atmosphere with an initial isothermal pressure scale height of 0.072 Rstar and a radial magnetic field. We also assume a transverse density gradient which we we refer to as a ``chromospheric hole''. Tortional Alfven waves are generated at the stellar surface by a forcing function having a single frequency, which is comparable to the turn-over frequency of convective cells in giant stars. 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 and a time period of more than 180 Alfven transit times, which ensures that a steady state has been reached. 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 similar to the case of solar coronal holes. This produces a fast (40-80 km/s) and relatively dense component of the wind. In the lower density ``chromospheric hole'' region the Alfven waves are strongly reflected and produce an outflow with both radial and azimuthal velocities which are ~ 10% of the local Alfven speed. This component of the wind is slow ( ~ 10-30 km/s) and less dense than the wind initiated outside of the hole. Depending on the magnetic topology in the atmosphere of a luminous late - type star, we may therefore expect either one (fast) or two components to the wind. Our results are consistent with recent observations of two discrete components to the wind in the K5 III hybrid star gamma Dra. These components were detected in the Mg II h and k resonance lines and had velocities of 67 and 30 km/s, with the higher velocity component having a mass loss rate which is 10 times that of the slower speed wind.
Airapetian Vladimir S.
Carpenter Ken
Davila Javier
Offman L.
Robinson Rebecca
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