Interferometric observations of magnetic fields in forming stars
  The magnetic field is a key ingredient in the recipe of star formation. Over
the past two decades, millimeter and submillimeter interferometers have made
major strides in unveiling the role of the magnetic field in star formation at
progressively smaller spatial scales. From the kiloparsec scale of molecular
clouds down to the inner few hundred au immediately surrounding forming stars,
the polarization at millimeter and submillimeter wavelengths is dominated by
polarized thermal dust emission, where the dust grains are aligned relative to
the magnetic field. Interferometric studies have focused on this dust
polarization and occasionally on the polarization of spectral-line emission. We
review the current state of the field of magnetized star formation in the
context of several questions that continue to motivate the studies of high- and
low-mass star formation. By aggregating and analyzing the results from
individual studies, we come to several conclusions: (1) Magnetic fields and
outflows from low-mass protostellar cores are randomly aligned, suggesting that
the magnetic field at ~1000 au scales is not the dominant factor in setting the
angular momentum of embedded disks and outflows. (2) Recent measurements of the
thermal and dynamic properties in high-mass star-forming regions reveal small
virial parameters, challenging the assumption of equilibrium star formation.
However, we estimate that a magnetic field strength of a fraction of a mG to
several mG in these objects could bring the dense gas close to a state of
equilibrium. Finally, (3) We find that the small number of sources with
hourglass-shaped magnetic field morphologies at 0.01 -- 0.1 pc scales cannot be
explained purely by projection effects, suggesting that while it does occur
occasionally, magnetically dominated core collapse is not the predominant mode
of low- or high-mass star formation. [Abridged]
