Distortion of Magnetic Fields in the Dense Core CB81 (L1774, Pipe 42) in
  the Pipe Nebula
  The detailed magnetic field structure of the starless dense core CB81 (L1774,
Pipe 42) in the Pipe Nebula was determined based on near-infrared polarimetric
observations of background stars to measure dichroically polarized light
produced by magnetically aligned dust grains in the core. The magnetic fields
pervading CB81 were mapped using 147 stars and axisymmetrically distorted
hourglass-like fields were identified. On the basis of simple 2D and 3D
magnetic field modeling, the magnetic inclination angles in the plane-of-sky
and line-of-sight directions were determined to be $4^{\circ} \pm 8^{\circ}$
and $20^{\circ} \pm 20^{\circ}$, respectively. The total magnetic field
strength of CB81 was found to be $7.2 \pm 2.3$ $\mu{\rm G}$. Taking into
account the effects of thermal/turbulent pressure and magnetic fields, the
critical mass of CB81 was calculated to be $M_{\rm cr}=4.03 \pm 0.40$
M$_{\odot}$, which is close to the observed core mass of $M_{\rm core}=3.37 \pm
0.51$ M$_{\odot}$. We thus conclude that CB81 is in a condition close to the
critical state. In addition, a spatial offset of $92''$ was found between the
center of magnetic field geometry and the dust extinction distribution; this
offset structure could not have been produced by self-gravity. The data also
indicate a linear relationship between polarization and extinction up to $A_V
\sim 30$ mag going toward the core center. This result confirms that
near-infrared polarization can accurately trace the overall magnetic field
structure of the core.
