The JCMT BISTRO Survey: the magnetized evolution of star-forming cores in the Ophiuchus molecular cloud interpreted using histograms of relative orientation

Abstract

The relationship between B-field orientation and density structure in molecular clouds is often assessed using the histogram of relative orientations (HRO). We perform a plane-of-the-sky geometrical analysis of projected B-fields, by interpreting HROs in dense, spheroidal, pre-stellar, and protostellar cores. We use James Clerk Maxwell Telescope POL-2 850 μm polarization maps and Herschel column density maps to study dense cores in the Ophiuchus molecular cloud complex. We construct two-dimensional core models, assuming Plummer column density profiles and modelling both linear and hourglass B-fields. We find that high-aspect ratio ellipsoidal cores produce strong HRO signals, as measured using the shape parameter ξ. Cores with linear fields oriented <45◦ from their minor axis produce constant HROs with −1 < ξ < 0, indicating that fields are preferentially parallel to column density gradients. Fields parallel to the core minor axis produce the most negative value of ξ. For low-aspect ratio cores, ξ ≈ 0 for linear fields. Hourglass fields produce a minimum in ξ at intermediate densities in all cases, converging to the minor-axis-parallel linear field value at high and low column densities. We create HROs for six dense cores in Ophiuchus. ρ Oph A and IRAS 16293 have high aspect ratios and preferentially negative HROs, consistent with moderately strong field behaviour. ρ Oph C, L1689A, and L1689B have low aspect ratios, and ξ ≈ 0. ρ Oph B is too complex to be modelled using a simple spheroidal field geometry. We see no signature of hourglass fields, agreeing with previous findings that dense cores generally exhibit linear fields on these size scales.