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 applies histograms of relative orientation (HRO) to dense cores in the Ophiuchus cloud to link magnetic-field orientation with density structure using plane-of-sky geometry. By combining JCMT POL-2 850 μm polarisation maps and Herschel N(H2) maps with simple Plummer-core models under linear and hourglass magnetic-field morphologies, the authors interpret observed HROs and assess the presence of hourglass fields. They find that high-aspect-ratio, magnetically dominated cores (e.g., ρ Oph A, IRAS 16293) are consistent with linear fields parallel to the density gradient, while others show weaker or more complex signals; no hourglass signatures are detected. Gaussian fits to column-density maps enable tailored modelling, revealing that HROs can diagnose magnetic topology in well-behaved cores, though complexity and noise constrain interpretations in more intricate regions. The work highlights the utility and limits of HRO-based diagnostics for magnetic fields in star-forming cores and sets the stage for broader surveys and more sophisticated models.

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 $μ$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 $ξ$. Cores with linear fields oriented $< 45^{\circ}$ from their minor axis produce constant HROs with $-1 < ξ< 0$, indicating 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, $ξ\approx 0$ 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 $ξ\approx 0$. $ρ$ 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.

Figures (11)

• Figure 1: HRO shape parameter $\xi$ for a model cores of eccentricity 0 (i.e. circular, top), 0.5 (middle) and 0.8 (bottom) for different magnetic field structures with no noise. Profiles for an hourglass field and multiple linear magnetic fields at varying rotation angles from the minor axis -- parallel to axis, $30^{\circ}$, $60^{\circ}$, and perpendicular -- are all shown.
• Figure 2: HRO shape parameter $\xi$ for a model cores of eccentricity 0 (i.e. circular, top), 0.5 (middle) and 0.8 (bottom) for different magnetic field structures with added noise. Profiles for an hourglass field and multiple linear magnetic fields at varying rotation angles from the minor axis -- parallel to axis, $30^{\circ}$, $60^{\circ}$, and perpendicular -- are all shown.
• Figure 3: HRO shape parameter $\xi$ (blue) and associated uncertainty (equations (\ref{['eq:shape_parameter']}) and (\ref{['eq:shape_parameter_uncertainty']})) determined for hydrogen column density bins in each region of Ophiuchus. We also show HROs produced from re-sampling the observational data (light blue) and the least-squares regression best-fit line (dashed red) for the observational HROs, along with the corresponding values of $C_{\text{HRO}}$ and $X_{\text{HRO}}$. From $\rho$ Oph A to L1689B, we also show the HRO for a linear magnetic field calculated from a Gaussian fit on the regions (green) and the uncertainty (light green) based on re-sampling. We further show the HRO for an hourglass magnetic field (yellow). The region IRAS 16293-2422* shows a second HRO calculated for IRAS 16293 from a second data cut (see Section \ref{['sec:gaussian_fitting']}). The black dashed line represents the $\xi = 0$ line. The HRO for L1688 used the combined data from $\rho$ Oph A, B and C as shown in Fig. \ref{['fig:L1688_gaussians']}. Similarly, the HRO for L1689 used the combined data from IRAS 16293-2422, L1689A and L1689B in Fig. \ref{['fig:L1689_gaussians']}. The HRO for Ophiuchus then used the combined data from L1688 and L1689.
• Figure 4: The regions $\rho$ Oph A (top), $\rho$ Oph B (middle) and $\rho$ Oph C (bottom) within the Ophiuchus cloud L1688. Background data show the Herschel column density map for each region. The POL-2 magnetic field pseudo-vectors are overlaid in black, with the Gaussian fit for each region shown in solid red and its minor axis in dashed red. The solid yellow line represents the mean magnetic field direction.
• Figure 5: The regions IRAS 16293 (top), L1689A (middle) and L1689B (bottom) within the Ophiuchus cloud L1689. Background data show the Herschel column density map for each region. The POL-2 magnetic field line pseudo-vectors are overlaid in black, with the Gaussian fit for each region shown in solid red and its minor axis in dashed red. The solid yellow line represents the mean magnetic field direction. For IRAS 16293, an additional data cut limited magnetic field vectors to those in orange. Their mean direction is shown in dashed yellow.