Gravity Driven Magnetic Field at ~1000 au Scales in High-mass Star
  Formation
  A full understanding of high-mass star formation requires the study of one of
the most elusive components of the energy balance in the interstellar medium:
magnetic fields. We report ALMA 1.2 mm, high-resolution (700 au) dust
polarization and molecular line observations of the rotating hot molecular core
embedded in the high-mass star-forming region IRAS 18089-1732. The dust
continuum emission and magnetic field morphology present spiral-like features
resembling a whirlpool. The velocity field traced by the H13CO+ (J=3-2)
transition line reveals a complex structure with spiral filaments that are
likely infalling and rotating, dragging the field with them. We have modeled
the magnetic field and find that the best model corresponds to a weakly
magnetized core with a mass-to-magnetic-flux ratio (lambda) of 8.38. The
modeled magnetic field is dominated by a poloidal component, but with an
important contribution from the toroidal component that has a magnitude of 30%
of the poloidal component. Using the Davis-Chandrasekhar-Fermi method, we
estimate a magnetic field strength of 3.5 mG. At the spatial scales accessible
to ALMA, an analysis of the energy balance of the system indicates that gravity
overwhelms turbulence, rotation, and the magnetic field. We show that high-mass
star formation can occur in weakly magnetized environments, with gravity taking
the dominant role.
