HAWC+/SOFIA Polarimetry in L1688: Relative Orientation of Magnetic Field and Elongated Cloud Structure

TL;DR

This study investigates the alignment between magnetic fields and elongated structures in the L1688 molecular cloud, revealing a transition from parallel to perpendicular orientation at a specific column density, supported by multi-instrument observations and simulations.

Contribution

It extends previous polarimetry studies to higher densities and combines HAWC+ and Planck data to identify the magnetic alignment transition in L1688.

Findings

Perpendicular magnetic field alignment at 0.02 pc scales.

Transition from parallel to perpendicular occurs at ~10^{21.7} cm^{-2} column density.

The transition corresponds to a volume density of ~10^{4} cm^{-3}.

Abstract

We present a study of the relative orientation between the magnetic field and elongated cloud structures for the $\rho$ Oph A and $\rho$ Oph E regions in L1688 in the Ophiuchus molecular cloud. Combining inferred magnetic field orientation from HAWC+ 154 $\mu$m observations of polarized thermal emission with column density maps created using Herschel submillimeter observations, we find consistent perpendicular relative alignment at scales of $0.02$ pc ($33.6"$ at $d \approx 137$ pc) using the histogram of relative orientations (HRO) technique. This supports the conclusions of previous work using Planck polarimetry and extends the results to higher column densities. Combining this HAWC+ HRO analysis with a new Planck HRO analysis of L1688, the transition from parallel to perpendicular alignment in L1688 is observed to occur at a molecular hydrogen column density of approximately $10^{21.7}$ cm$^{-2}$. This value for the alignment transition column density agrees well with values found for nearby clouds via previous studies using only Planck observations. Using existing turbulent, magnetohydrodynamic simulations of molecular clouds formed by colliding flows as a model for L1688, we conclude that the molecular hydrogen volume density associated with this transition is approximately $\sim10^{4}$ cm$^{-3}$. We discuss the limitations of our analysis, including incomplete sampling of the dense regions in L1688 by HAWC+.