Magnetic fields in circumstellar envelopes of evolved AGB stars

TL;DR

This paper presents a time-dependent magnetohydrodynamic model explaining how large-scale magnetic fields influence the formation and symmetry of circumstellar envelopes and planetary nebulae around evolved AGB stars.

Contribution

It introduces a novel MHD model linking magnetic fields to the development of spherical and bipolar structures in circumstellar environments.

Findings

Large-scale magnetic fields are likely toroidal within envelopes.

Spherical symmetry appears during the superwind phase, with anisotropic features hidden inside.

Bipolar cavities form during the superwind phase and influence nebula shapes.

Abstract

In this paper, a time-dependent magnetohydrodynamic model is presented which aimed at understanding the superwind production by an evolved AGB star and the consecutive formation of a dense circumstellar envelope around it. We know henceforth from various observations that a large scale magnetic field, probably toroidal in shape, is duly attested within these envelopes. Where does this large scale coherent field come from ? The apparent antinomy between the quasi-round dense circumstellar envelopes and their likely descendants, i.e. the elongated or bipolar Planetary Nebulae is also questioned. How is the spherical symmetry broken ? We suggest in the present model that the nebula must effectively appear round during the superwind phase from the point of view of a distant observer. By contrast anisotropic structures are already appearing at the same time, but these ones remain hidden in the innermost regions. We predict thus the existence of a large bipolar cavity above the AGB star during the slow superwind phase. We then conjecture that the PPNe phase begins when the fast wind emitted by the core engulfs this cavity and increases the anisotropy of the distribution of gas. Thus even though paradoxically enough a beautiful evolved PNe can eventually emerge from a quasi-round dense circumstellar envelope.