Magnetic fields in circumstellar envelopes of evolved AGB stars. (arXiv:1911.06617v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Pascoli_G/0/1/0/all/0/1">Gianni Pascoli</a>
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.
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.
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