Coupled MHD 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 which (if either) of the two dominates the outflow power. Recent observations lead us to consider how such coupled MHD winds may be operating in planetary nebulae (PN). In this context, 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-AGB 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 we discuss the morphologies which may emerge. The coupled winds might help explain the shapes of a number of remarkable multi-shell or multi-polar nebulae. Magnetic activity such as X-ray flares may be associated with the both central star and the disk and would be a valuable diagnostic for the dynamical role of MHD processes in PNe.

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