Rotation/saturation/dynamos -- Oral Presentation

Is the efficiency of magnetic braking limited by polar spots?

Alexandre Aibeo, Centro de Astrofísica da Universidade do Porto; Escola Superior de Tecnologia de Viseu

Ferreira, M. (Centro de Astrofísica da Universidade do Porto; Universidade dos Açores - DCA); Lima, J. (Centro de Astrofísica da Universidade do Porto, Departamento de Matemática Aplicada, Faculdade Ciências Universidade do Porto)

The presence of high latitude spots on the surface of rapidly rotating cool-stars and the subsequent concentration of magnetic flux near the poles has lead to the idea that this causes a reduction in the angular momentum carried away by the stellar wind. We investigate the influence of the concentration of surface magnetic flux towards the pole on the topology of the coronal magnetic field and its efficiency in removing angular momentum from the star. If we neglect the effect of the wind, then the topology of fully open or partially open magnetic fields high in the corona is largely independent of the degree of field concentration at the surface. Therefore, a highly concentrated surface field or a dipolar field yield similar coronal fields. We apply the analytical MHD wind model of Lima et al. (2001) to determine the influence of the surface flux distribution on the efficiency of the magnetic braking. This model has the desirable properties of allowing different surface fields and, by construction, avoiding the expansion of the polar field to low latitudes. We determine the angular momentum loss rate for different surface field distributions, which have the same total magnetic flux. As the field concentration towards the pole increases, the angular momentum loss increases, contrary to what one would naively expect! This is explained by the fact that a decrease in the mass loss rate is compensated by an increase in the angular momentum loss per unit mass. Our results clearly show that the density, pressure, velocity and toroidal magnetic field variations are as important as the surface field distribution in determining the angular momentum carried by the wind. Further research is required to determine whether our results are a consequence of the particular models considered or can be regarded as a general feature.