ALMA reveals the coherence of the magnetic field geometry in OH
  231.8+4.2
  In a continuing effort to investigate the role of magnetic fields in evolved
low and intermediate mass stars (principally regarding the shaping of their
envelopes), we present new ALMA high resolution polarization data obtained for
the nebula OH 231.8+4.2. We found that the polarized emission likely arises
from aligned grains in the presence of magnetic fields rather than radiative
alignment and self scattering. The ALMA data show well organized electric field
orientations in most of the nebula and the inferred magnetic field vectors
(rotated by 90 degrees) trace an hourglass morphology centred on the central
system of the nebula. One region in the southern part of OH 231.8+4.2 shows a
less organized distribution probably due to the shocked environment. These
findings, in conjunction with earlier investigations (maser studies and dust
emission analysis at other scales and wavelengths) suggest an overall magnetic
hourglass located inside a toroidal field. We propose the idea that the
magnetic field structure is closely related to the architecture of a magnetic
tower and that the outflows were therefore magnetically launched. While the
current dynamical effect of the fields might be weak in the equatorial plane
principally due to the evolution of the envelope, it would still be affecting
the outflows. In that regard, the measurement of the magnetic field at the
stellar surface, which is still missing, combined with a full MHD treatment are
required to better understand and constrain the events occurring in OH
231.8+4.2.
