Magnetic Fields in Paradigms of Planetary Nebulae and Related MHD Frontiers

TL;DR

This paper explores how magnetic fields and magnetohydrodynamic processes influence the shaping and launching of planetary nebulae, highlighting observational and theoretical challenges in understanding these phenomena.

Contribution

It discusses potential MHD mechanisms, including dynamos and accretion processes, that could explain the aspherical shapes of planetary nebulae and their precursors.

Findings

Magnetic fields are likely dynamically important in evolved stars.

Magnetohydrodynamic outflows may explain nebula shaping.

Observational signatures of MHD engines are still being identified.

Abstract

Many, if not all, post AGB stellar systems swiftly transition from a spherical to a powerful aspherical pre-planetary nebula (pPNE) outflow phase before waning into a PNe. The pPNe outflows require engine rotational energy and a mechanism to extract this energy into collimated outflows. Just radiation and rotation are insufficient but a symbiosis between rotation, differential rotation and large scale magnetic fields remains promising. Present observational evidence for magnetic fields in evolved stars is suggestive of dynamically important magnetic fields, but both theory and observation are rife with research opportunity. I discuss how magnetohydrodynamic outflows might arise in pPNe and PNe and distinguish different between approaches that address shaping vs. those that address both launch and shaping. Scenarios involving dynamos in single stars, binary driven dynamos, or accretion engines cannot be ruled out. One appealing paradigm involves accretion onto the primary post-AGB white dwarf core from a low mass companion whose decaying accretion supply rate owers first the pPNe and then the lower luminosity PNe. Determining observational signatures of different MHD engines is a work in progress. Accretion disk theory and large scale dynamos pose many of their own fundamental challenges, some of which I discuss in a broader context.