Magnetohydrodynamic Stellar and Disk Winds: Application to Planetary Nebulae

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Abstract
MHD winds can emanate from both stars and surrounding disks. When the two systems are coupled by accretion, it is of interest to know how much wind power is available and which (if either) of the two rotators dominates that power. We investigate this in the context of multipolar planetary nebulae (PNs) and protoplanetary nebulae (PPNs), for which recent observations have revealed the need for a wind power source in excess of that available from radiation driving and a possible need for magnetic shaping. We calculate the MHD wind power from a coupled disk and star, where the former results from binary disruption. The resulting wind powers depend only on the accretion rate and stellar properties. We find that if the stellar envelope were initially slowly rotating, the disk wind would dominate throughout the evolution. If the envelope of the star were rapidly rotating, the stellar wind could initially be of comparable power to the disk wind until the stellar wind carries away the star's angular momentum. Since an initially rapidly rotating star can have its spin and magnetic axes misaligned to the disk, multipolar outflows can result from this disk wind system. For times greater than a spin-down time, the post-asymptotic giant branch stellar wind is slaved to the disk for both slow and rapid initial spin cases, and the disk wind luminosity dominates. We find a reasonably large parameter space where a hybrid star+disk MHD-driven wind is plausible and where both or either can account for PPN and PN powers. We also speculate on the morphologies which may emerge from the coupled system. The coupled winds might help explain the shapes of a number of remarkable multishell or multipolar nebulae. Magnetic activity such as X-ray flares may be associated with both the central star and the disk and would be a valuable diagnostic for the dynamical role of MHD processes in PNs.

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