A Curved Magnetic field in the ring-like shell of bubble N4
  We report the detection of a curved magnetic field in the ring-like shell of
the bubble N4, derived from near-infrared polarization of reddened diskless
stars located behind this bubble. The magnetic field in the shell is curved and
parallel to the ring-like shell, and its strength is estimated to be
$\sim120\,\mu$G in the plane of the sky. The magnetic field strength in the
shell is significantly enhanced compared to the local field strength. We
calculate the mass-to-flux ratio for the submillimeter clumps in the shell and
find that they are all magnetically subcritical. Our results demonstrate that
the magnetic field strengthens as the interstellar medium is compressed into a
shell, and suggest that the magnetic field has the potential to hinder star
formation triggered by \ion{H}{2} region expansion.
