Dynamos in Asymptotic-Giant-Branch Stars As the Origin of Magnetic Fields Shaping Planetary Nebulale

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Abstract
Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage$^{1}$. A shock forms near the boundary between the winds, which creates a relatively dense shell that provides the characteristic appearance of a planetary 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$^{2}$. While a magnetic field could launch and collimate a bipolar outflow, the origin of such a field has hitherto been unclear, as previous work suggested that a field could not be generated${^3}$. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, in a dynamo at the interface between a rapidly rotating core and the more slowly rotating envelope of the star. The field is strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly$^{4}$ slow rotation of most white dwarf stars.
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