Submillimeter Array Observations of Magnetic Fields in G240.31+0.07: an
  Hourglass in a Massive Cluster-forming Core
  We report the first detection of an hourglass magnetic field aligned with a
well-defined outflow-rotation system in a high-mass star-forming region. The
observations were performed with Submillimeter Array toward G240.31+0.07, which
harbors a massive, flattened, and fragmenting molecular cloud core and a
wide-angle bipolar outflow. The polarized dust emission at 0.88 mm reveals a
clear hourglass-shaped magnetic field aligned within 20 degree of the outflow
axis. Maps of high-density tracing spectral lines, e.g., H13CO+ (4-3), show
that the core is rotating about its minor axis, which is also aligned with the
magnetic field axis. Therefore, both the magnetic field and kinematic
properties observed in this region are surprisingly consistent with the
theoretical predictions of the classic paradigm of isolated low-mass star
formation. The strength of the magnetic field in the plane of sky is estimated
to be about 1.1 mG, resulting in a mass-to-magnetic flux ratio of 1.4 times the
critical value and a turbulent to ordered magnetic energy ratio of 0.4. We also
find that the specific angular momentum almost linearly decreases from r~0.6 pc
to 0.03 pc scales, which is most likely attributed to magnetic braking.
