Cool Stars 14 - Submitted Abstract # 43
This version created on 05 October 2006

Theoretical mass loss rates of cool main-sequence stars


Volkmar Holzwarth, Max-Planck-Institut for Solar System Research
Moira Jardine, University of St Andrews

In recent years, it became possible to infer the mass loss rates of
cool stars from astrospheric absorption features in their Ly alpha
emission line profiles.  These data have been used to predict the
impact of stellar winds on planetary atmospheres and the detectability
of planetary radio emission.  Guided by the empirical constraints, we
have modeled wind ram pressures, mass fluxes, and terminal wind
velocities of main sequence stars in the framework of a polytropic
magnetised wind model.  Assuming power laws for the dependence of the
wind parameters on the stellar rotation rate, models based on moderate
rates of increase yield wind ram pressures in agreement with most
empirical constraints.  Yet they cannot account for some moderately
rotating targets, whose high apparent mass loss rates are inconsistent
with their observed coronal and magnetic properties.  For fast
magnetic rotators, the magneto-centrifugal driving of the outflows
produces terminal velocities in excess of the surface escape velocity,
which can lead to overestimations of their mass loss rates.  The
predicted mass loss rates of cool main-sequence stars do not exceed
about ten times the solar value.  Since our results indicate a weaker
dependence of the mass loss rate on the coronal X-ray flux than
previously suggested, the impact on planetary atmospheres is less
severe and the detectability of magnetospheric radio emission is lower
then expected by some investigations.  Considering the rotational
evolution of a solar-type star, we find that the mass loss rates and
wind ram pressures are highest during its pre-main sequence phase.

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