We present the results of dust emission polarization measurements of Ophiuchus-B ( Oph-B ) carried out using the Submillimetre Common-User Bolometer Array 2 ( SCUBA-2 ) camera with its associated polarimeter ( POL-2 ) on the James Clerk Maxwell Telescope ( JCMT ) in Hawaii .
This work is part of the B-fields In Star-forming Region Observations ( BISTRO ) survey initiated to understand the role of magnetic fields in star formation for nearby star-forming molecular clouds .
We present a first look at the geometry and strength of magnetic fields in Oph-B .
The field geometry is traced over \sim 0.2 pc , with clear detection of both of the sub-clumps of Oph-B .
The field pattern appears significantly disordered in sub-clump Oph-B1 .
The field geometry in Oph-B2 is more ordered , with a tendency to be along the major axis of the clump , parallel to the filamentary structure within which it lies .
The degree of polarization decreases systematically towards the dense core material in the two sub-clumps .
The field lines in the lower density material along the periphery are smoothly joined to the large scale magnetic fields probed by NIR polarization observations .
We estimated a magnetic field strength of 630 \pm 410 \mu G in the Oph-B2 sub-clump using a Davis-Chandeasekhar-Fermi analysis .
With this magnetic field strength , we find a mass-to-flux ratio \lambda = 1.6 \pm 1.1 , which suggests that the Oph-B2 clump is slightly magnetically supercritical .