From Poloidal to Toroidal: Detection of Well-ordered Magnetic Field in
  High-mass Proto-cluster G35.2-0.74N
  We report on detection of an ordered magnetic field (B field) threading a
massive star-forming clump in the molecular cloud G35.2-0.74, using
Submillimeter Array observations of polarized dust emission. Thanks to the
sensitive and high-angular-resolution observations, we are able to resolve the
morphology of the B field in the plane of sky and detect a great turn of 90
degree in the B field direction: Over the northern part of the clump, where a
velocity gradient is evident, the B field is largely aligned with the long axis
of the clump, whereas in the southern part, where the velocity field appears
relatively uniform, the B field is slightly pinched with its mean direction
perpendicular to the clump elongation. We suggest that the clump forms as its
parent cloud collapses more along the large scale B field. In this process, the
northern part carries over most of the angular momentum, forming a fast
rotating system, and pulls the B field into a toroidal configuration. In
contrast, the southern part is not significantly rotating and the B field
remains in a poloidal configuration. A statistical analysis of the observed
polarization dispersion yields a B field strength of ~ 1 mG, a
turbulent-to-magnetic energy ratio of order unity, and a mass-to-magnetic flux
ratio of ~ 2--3 in units of the critical value. Detailed calculations support
our hypothesis that the B field in the northern part is being rotationally
distorted. Our observations, in conjunction with early single-dish data,
suggest that the B field may play a critical role in the formation of the dense
clump, whereas rotation and turbulence could also be important in further
dynamical evolution of the clump. The observations also provide evidence for a
wide-angle outflow driven from a strongly rotating region whose B field is
largely toroidal.
