Disk Formation by Asymptotic Giant Branch Winds in Dipole Magnetic Fields

We present a simple, robust mechanism by which an isolated star can producean equatorial disk. The mechanism requires that the star have a simple dipolemagnetic field on the surface and an isotropic wind acceleration mechanism. Thewind couples to the field, stretching it until the field lines become mostlyradial and oppositely directed above and below the magnetic equator, as occursin the solar wind. The interaction between the wind plasma and magnetic fieldnear the star produces a steady outflow in which magnetic forces direct plasmatoward the equator, constructing a disk. In the context of a slow (10 km/s)outflow (10^{-5} M_sun/yr) from an AGB star, MHD simulations demonstrate that adense equatorial disk will be produced for dipole field strengths of only a fewGauss on the surface of the star. A disk formed by this model can bedynamically important for the shaping of Planetary Nebulae.Comment: 14 pages, 8 figures, 1 table, accepted by Ap