Cool Stars 14 - Submitted Abstract # 231
This version created on 05 October 2006

On magnetic braking of young cool stars


Joao Miguel Ferreira, University of Azores
Alexandre Aibeo, Centro de Astrofisica da Universidade do Porto
Joao Lima, Centro de Astrofisica da Universidade do Porto

The observation of significant magnetic flux at high latitudes on
rapidly rotating cool stars is likely to lead to a coronal field
topology radically different from the solar case.  In addition, the
angular momentum loss induced by the wind and magnetic field on these
stars can also be significantly altered with important implications to
the star's rotational evolution.  Here, we investigate whether
magnetic flux concentration towards the pole leads to a decrease in
angular momentum loss rate.  We solve the wind equations on a
prescribed totally open field as in the classical Weber & Davis model
(1967) but with a surface magnetic flux distribution concentrated
towards high latitudes.  However, we do not assume the poloidal field
to be radial and expand as a split monopole while maintaining the
surface latitudinal flux distribution, as this completely ignores
force balance across the field.  Instead, we obtain analytical
potential field solutions with an equatorial current sheet.  We find
that coronal field lines are pushed from high to low latitudes and
rapidly approach the split monopole field with no memory of the
surface flux distribution.  The braking rate of the highly
concentrated field is very similar to the uniform field.  We conclude
that the concentration of magnetic flux towards the poles does not
directly imply a reduced angular momentum loss rate.  We suggest that
this effect may still contribute to a reduced braking if it leads to a
reduction in the amount of open magnetic flux (cf.  Mestel & Spruit
1987).

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