Magnetic fields in the massive dense cores of DR21 filament: weakly
  magnetized cores in a strongly magnetized filament
  We present Submillimeter Array 880 $\mu$m dust polarization observations of
six massive dense cores in the DR21 filament. The dust polarization shows
complex magnetic field structures in the massive dense cores with sizes of 0.1
pc, in contrast to the ordered magnetic fields of the parsec-scale filament.
The major axes of the massive dense cores appear to be aligned either parallel
or perpendicular to the magnetic fields of the filament, indicating that the
parsec-scale magnetic fields play an important role in the formation of the
massive dense cores. However, the correlation between the major axes of the
cores and the magnetic fields of the cores is less significant, suggesting that
during the core formation, the magnetic fields below 0.1 pc scales become less
important than the magnetic fields above 0.1 pc scales in supporting a core
against gravity. Our analysis of the angular dispersion functions of the
observed polarization segments yields the plane-of-sky magnetic field strengths
of 0.4--1.7 mG of the massive dense cores. We estimate the kinematic, magnetic,
and gravitational virial parameters of the filament and the cores. The virial
parameters show that in the filament, the gravitational energy is dominant over
magnetic and kinematic energies, while in the cores, the kinematic energy is
dominant. Our work suggests that although magnetic fields may play an important
role in a collapsing filament, the kinematics arising from gravitational
collapse must become more important than magnetic fields during the evolution
from filaments to massive dense cores.
