Takeru K. Suzuki, Shinsuke Imada, Ryuho Kataoka, Yoshiaki Kato, Takuma Matsumoto, Hiroko Miyahara, Saku Tsuneta
(Abridged)We investigate mass losses via stellar winds from sun-like main sequence stars with a wide range of activity levels. We perform forward-type magnetohydrodynamical numerical experiments for Alfven wave-driven stellar winds with a wide range of the input Poynting flux from the photosphere. Increasing the magnetic field strength and the turbulent velocity at the stellar photosphere from the current solar level, the mass loss rate rapidly increases at first owing to the suppression of the reflection of the Alfven waves. The surface materials are lifted up by the magnetic pressure associated with the Alfven waves, and the cool dense chromosphere is intermittently extended to 10-20% of the stellar radius. The densities of the corona and transition region above the chromosphere is also high, which leads to efficient radiative losses. Eventually most of the input Poynting energy from the stellar surface escapes by the radiation. As a result, there is no more sufficient energy remained for the kinetic energy of the wind; the stellar wind saturates in very active stars, as observed in Wood et al. The saturation level is positively correlated with B_{r,0}f_0, where B_{r,0} and f_0 are the magnetic field strength and the filling factor of open flux tubes at the photosphere. If B_{r,0}f_0 is relatively large >~ 5 G, the mass loss rate could be as high as 1000 times. If such a strong mass loss lasts for ~1 billion years, the stellar mass itself is affected, which could be a solution to the faint young sun paradox. Our simulations show that 0.1-10% of the input Poynting flux is transferred to the final kinetic energy of the stellar winds. We derive Reimers-type scaling relations that estimate the mass loss rate and the wind kinetic energy of sun-like stars from this energetics consideration.
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http://arxiv.org/abs/1212.6713
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