Asterospheric Magnetic Fields and Winds of Cool Stars

Abstract

This study addresses the winds and magnetic fields in the inner asterospheres of Sun-like magnetically active stars by combining empirical relationships between rotation rate and mass loss, angular-momentum loss, and radiative losses with models of the magnetic fields at the surfaces of cool stars and in their inner asterospheres based on the solar example. Our models, for mean magnetic flux densities up to 10 times solar, suggest that the asterospheric fields of such stars are dominated by the large-scale dipole component of the surface field, as is the case for the Sun. Hence, most of the time a single current sheet is expected to separate domains of opposite magnetic polarity; the current sheets of more active stars generally have smaller latitudinal ripples. Magnetic braking requires that the total unsigned asterospheric magnetic flux increase linearly with the stellar angular velocity, which is a very much weaker increase than seen for the flux at the stellar surface. We show that this can be achieved by an increase in the radial distance at which the coronal field is forced open as surface activity increases. Combined with measured mass-loss rates and the assumption that the wind velocity is largely independent of activity, this requires the wind's Alfvén radius to be nearly constant, decreasing with surface activity with a power of only -0.16 ± 0.13. We point out that the surface flux density of energy needed to drive a cool-star wind scales linearly with the unsigned surface magnetic flux density, as does that needed to heat the corona.