Observations of Magnetic Fields Surrounding LkHα 101 Taken by the BISTRO Survey with JCMT-POL-2

Abstract

We report the first high spatial resolution measurement of magnetic fields surrounding LkH α 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 μ m 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 B POS  ∼ 115 μ G. 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 LkH α 101 region is subcritical, and the magnetic field is strong enough to prevent gravitational collapse. The inferred δ B / B 0  ∼ 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 α  = 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 (LkH α 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.