We report 850 \mu m dust polarization observations of a low-mass ( \sim 12 M _ { \odot } ) starless core in the \rho Ophiuchus cloud , Ophiuchus C , made with the POL-2 instrument on the James Clerk Maxwell Telescope ( JCMT ) as part of the JCMT B-fields In STar-forming Region Observations ( BISTRO ) survey .
We detect an ordered magnetic field projected on the plane of sky in the starless core .
The magnetic field across the \sim 0.1 pc core shows a predominant northeast-southwest orientation centering between \sim 40 \degr to \sim 100 \degr , indicating that the field in the core is well aligned with the magnetic field in lower-density regions of the cloud probed by near-infrared observations and also the cloud-scale magnetic field traced by Planck observations .
The polarization percentage ( P ) decreases with an increasing total intensity ( I ) with a power-law index of - 1.03 \pm 0.05 .
We estimate the plane-of-sky field strength ( B _ { \mathrm { pos } } ) using modified Davis-Chandrasekhar-Fermi ( DCF ) methods based on structure function ( SF ) , auto-correlation ( ACF ) , and unsharp masking ( UM ) analyses .
We find that the estimates from the SF , ACF , and UM methods yield strengths of 103 \pm 46 \mu G , 136 \pm 69 \mu G , and 213 \pm 115 \mu G , respectively .
Our calculations suggest that the Ophiuchus C core is near magnetically critical or slightly magnetically supercritical ( i.e .
unstable to collapse ) .
The total magnetic energy calculated from the SF method is comparable to the turbulent energy in Ophiuchus C , while the ACF method and the UM method only set upper limits for the total magnetic energy because of large uncertainties .