Distortion of Magnetic Fields in BHR 71
  The magnetic field structure of a star-forming Bok globule BHR 71 was
determined based on near-infrared polarimetric observations of background
stars. The magnetic field in BHR 71 was mapped from 25 stars. By using a simple
2D parabolic function, the plane-of-sky magnetic axis of the core was found to
be $\theta_{\rm mag} = 125^{\circ} \pm 11^{\circ}$. The plane-of-sky mean
magnetic field strength of BHR 71 was found to be $B_{\rm pos} = 8.8 - 15.0$
$\mu$G, indicating that the BHR 71 core is magnetically supercritical with
$\lambda = 1.44 - 2.43$. Taking into account the effect of thermal/turbulent
pressure and the plane-of-sky magnetic field component, the critical mass of
BHR 71 was $M_{\rm cr} = 14.5-18.7$ M$_{\odot}$, which is consistent with the
observed core mass of $M_{\rm core} \approx 14.7$ M$_{\odot}$ (Yang et al.
2017). We conclude that BHR 71 is in a condition close to a kinematically
critical state, and the magnetic field direction lies close to the plane of
sky. Since BHR 71 is a star-forming core, a significantly subcritical condition
(i.e., the magnetic field direction deviating from the plane of sky) is
unlikely, and collapsed from a condition close to a kinematically critical
state. There are two possible scenarios to explain the curved magnetic fields
of BHR 71, one is an hourglass-like field structure due to mass accumulation
and the other is the Inoue \& Fukui (2013) mechanism, which proposes the
interaction of the core with a shock wave to create curved magnetic fields
wrapping around the core.
