Orion's Veil: Magnetic field strengths and other properties of a PDR in front of the Trapezium Cluster

TL;DR

This study analyzes the magnetic field strengths and physical conditions of the Orion Veil, a photodissociation region near the Trapezium Cluster, revealing strong magnetic fields and their potential role in the region's dynamics.

Contribution

First detailed Zeeman effect measurements of magnetic fields in the Orion Veil, linking magnetic properties to gas dynamics and star formation processes.

Findings

Magnetic fields in the atomic gas are -50 to -75 microgauss.

Magnetic field in a molecular region is -350 microgauss.

Veil's HI components are magnetically subcritical and denser than typical CNM.

Abstract

We present an analysis of physical conditions in the Orion Veil, a largely atomic PDR that lies just in front (about 2 pc) of the Trapezium stars. We have obtained 21 cm HI and 18 cm OH VLA Zeeman effect data. These data yield images of the line-of-sight magnetic field strength Blos in atomic and molecular regions of the Veil. We find Blos is typically -50 to -75 microgauss in the atomic gas across much of the Veil (25" resolution); Blos is -350 microgauss at one position in the molecular gas (40" resolution). The Veil has two principal HI velocity components. Magnetic and kinematical data suggest a close connection between these components. They may represent gas on either side of a shock wave preceding a weak-D ionization front. Magnetic fields in the Veil HI components are 3-5 times stronger than they are elsewhere in the ISM where N(H) and n(H) are comparable. The HI components are magnetically subcritical (magnetically dominated), like the CNM, although they are about 1 dex denser. Strong fields in the Veil HI components may have resulted from low turbulence conditions in the diffuse gas that gave rise to OMC-1. Strong fields may also be related to magnetostatic equilibrium that has developed in the Veil since star formation. We consider the location of the Orion-S molecular core, proposing a location behind the main Orion H+ region.