Distortion of Magnetic Fields in BHR 71

Abstract

The magnetic field structure of a star-forming Bok globule BHR 71 was determined based on near-infrared polarimetric observations of background stars. The magnetic field in BHR 71 was mapped from 25 stars. By using a simple 2D parabolic function, the plane-of-sky magnetic axis of the core was found to be $\hat{I}{¸}_{\mathrm{mag}}=125^{\circ }\pm 11^{\circ }$. The plane-of-sky mean magnetic field strength of BHR 71 was found to be $B_{\mathrm{pos}}=8.8-15.0$ $\hat{I}¼$G, indicating that the BHR 71 core is magnetically supercritical with $\hat{I}»=1.44-2.43$. Taking into account the effect of thermal/turbulent pressure and the plane-of-sky magnetic field component, the critical mass of BHR 71 was $M_{\mathrm{cr}}=14.5-18.7$ M${}_{\odot }$, which is consistent with the observed core mass of $M_{\mathrm{core}}\approx 14.7$ M${}_{\odot }$ (Yang et al. 2017). We conclude that BHR 71 is in a condition close to a kinematically critical state, and the magnetic field direction lies close to the plane of sky. Since BHR 71 is a star-forming core, a significantly subcritical condition (i.e., the magnetic field direction deviating from the plane of sky) is unlikely, and collapsed from a condition close to a kinematically critical state. There are two possible scenarios to explain the curved magnetic fields of BHR 71, one is an hourglass-like field structure due to mass accumulation and the other is the Inoue \& Fukui (2013) mechanism, which proposes the interaction of the core with a shock wave to create curved magnetic fields wrapping around the core.

Accepted for publication in ApJ