Observations of magnetic fields surrounding LkHa 101 taken by the BISTRO survey with JCMT-POL-2

Abstract

We report the first high spatial resolution measurement of magnetic fields surrounding LkHa 101, part of the Auriga- California molecular cloud. The observations were taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope within the framework of the B-fields In Star-forming Region Observations (BISTRO) survey. Observed polarization of thermal dust emission at 850 μm is found to be mostly associated with the redshifted gas component of the cloud. The magnetic field displays a relatively complex morphology. Two variants of the Davis-Chandrasekhar- Fermi method, unsharp masking and structure function, are used to calculate the strength of magnetic fields in the plane of the sky, yielding a similar result of BPOS~ 115 μG. The mass-to-magnetic-flux ratio in critical value units, λ~0.3, is the smallest among the values obtained for other regions surveyed by POL-2. This implies that the LkHa 101 region is subcritical, and the magnetic field is strong enough to prevent gravitational collapse. The inferred dB/B0~0.3 implies that the large-scale component of the magnetic field dominates the turbulent one. The variation of the polarization fraction with total emission intensity can be fitted by a power law with an index of a =0.82±0.03, which lies in the range previously reported for molecular clouds. We find that the polarization fraction decreases rapidly with proximity to the only early B star (LkHa 101) in the region. Magnetic field tangling and the joint effect of grain alignment and rotational disruption by radiative torques can potentially explain such a decreasing trend.