The JCMT BISTRO Survey: The magnetised evolution of star-forming cores in the Ophiuchus Molecular Cloud interpreted using Histograms of Relative Orientation

TL;DR

This study investigates the orientation of magnetic fields in dense star-forming cores within the Ophiuchus Molecular Cloud using polarisation data, revealing correlations between core shape, magnetic field geometry, and density structure.

Contribution

The paper introduces a detailed geometrical analysis of magnetic field orientations in dense cores, employing HROs and modeling different field configurations, which advances understanding of magnetic influence in star formation.

Findings

High-aspect-ratio cores show strong HRO signals with fields parallel to density gradients.

Linear fields are prevalent, with no evidence of hourglass configurations at core scales.

Core shape influences magnetic field orientation and the HRO shape parameter $\xi$.

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, prestellar and protostellar cores. We use James Clerk Maxwell Telescope (JCMT) POL-2 850 $\mu$m polarisation 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 high-aspect-ratio ellipsoidal cores produce strong HRO signals, as measured using the shape parameter $\xi$. Cores with linear fields oriented $< 45^{\circ}$ from their minor axis produce constant HROs with $-1 < \xi < 0$, indicating fields are preferentially parallel to column density gradients. Fields parallel to the core minor axis produce the most negative value of $\xi$. For low-aspect-ratio cores, $\xi \approx 0$ for linear fields. Hourglass fields produce a minimum in $\xi$ 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. $\rho$ Oph A and IRAS 16293 have high aspect ratios and preferentially negative HROs, consistent with moderately strong-field behaviour. $\rho$ Oph C, L1689A and L1689B have low aspect ratios, and $\xi \approx 0$. $\rho$ 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.