Magnetically Controlled Accretion Flows and Winds from Planets
September 26, 2013
Abstract: This talk considers two related problems involving magnetically controlled flows in young stellar objects and in exoplanets. First we revisit the problem of magnetically controlled accretion onto young and forming stars (with a focus on analytic results). The accretion flow depends on the field geometry, and we generalize previous work to include magnetic fields with multiple components, including dipole, octupole, and split monopole contributions. Observations indicate that accreting young stars have substantial dipole and octupole components, and that accretion flow is transonic. The inclusion of octupole field components produces higher densities at the stellar surface and smaller hot spots, which occur at higher latitudes; the magnetic truncation radius is smaller (larger) for octupole components that are aligned (anti-aligned) with the stellar dipole. Next we consider the effects of magnetic fields on exoplanet outflows, which have now been observed. For typical parameters, the magnetic field pressure near the planetary surface dominates the ram pressure of the outflow by several orders of magnitude. Magnetically controlled outflows differ significantly from previous spherical models: The outflow rates are somewhat smaller, and the flow is launched primarily from the polar regions of the planet.