Dynamos in asymptotic-giant-branch stars as the origin of magnetic fields shaping planetary nebulae

Planetary nebulae are thought to be formed when a slow wind from the progen itor giant star is overtaken by a subsequent fast wind generated as the sta r enters its white dwarf stage(1). A shock forms near the boundary between the winds, creating the relatively dense shell characteristic of a planetar y nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape(2). A magnetic field could launch and collimate a bipolar outflow, but the origin of such a field has hithert o been unclear, and some previous work has even suggested that a field coul d not be generated(3). Here we show that an asymptotic-giant-branch (AGB) s tar can indeed generate a strong magnetic field, having as its origin a dyn amo at the interface between the rapidly rotating core and the more slowly rotating envelope of the star. The fields are strong enough to shape the bi polar outflows that produce the observed bipolar planetary nebulae. Magneti c braking of the stellar core during this process may also explain the puzz lingly(4) slow rotation of most white dwarf stars.