Fig. A.1
Sketch of the adopted spherical configuration. Left: the initial state, with uniform density and magnetic field; there are no separate ionized and dust shells. Right: present-day state with a cavity inside 
, an ionized shell between and 
, and a dust shell between 
and 
. The present-day radii , , and correspond to initial radii 0, 
, and 
, respectively.
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