Fragmenting Filaments and Evolving Cores -- Insights from Dust Polarisation Study of a filament in Northern Orion B

Abstract

We present an analysis of polarised dust emission at 850 micron for a parsec long filament in the northern part of the Orion B molecular cloud. The region was observed by the JCMT SCUBA-2/POL-2 polarimeter. The filament has a line mass (~80 Msun/pc) larger than the critical (magnetic) line mass (~37 Msun/pc); and hosts one starless, three prestellar, and four protostellar cores, with masses in the range 0.13 to 9.13 Msun. The mean (debiased) polarisation fraction of the filament and core pixels was calculated to be 5.3+/-0.3% and 3.2+/-0.3%, respectively, likely reflecting their distinct physical conditions. The polarisation fraction for the cores does not depend on the type of core, and was found to decrease with increasing column density, varying from 6-11% at the filament edges to 1$^{+0.7}_{-0.1}$% in the denser parts ($N_{H2}\gtrsim$2x10$^{22}$cm$^{-2}$). Magnetic field orientation of the protostellar cores, in contrast to prestellar cores, appears to be relatively aligned with the magnetic field orientation of the local filament in this region. Using the Davis-Chandrasekhar-Fermi formalism the plane-of-sky magnetic field strength for the protostellar cores (~39-110 microG) was found to be higher than that of the prestellar cores (~22-61 microG); and weakest for the starless core (~6 microG). The average value for the filament was found to be ~31 microG. The magnetic field-volume density relation for the prestellar/starless cores and protostellar cores suggests a transition from weak field case to strong field case as the cores evolve from prestellar to protostellar phase.