The Explosion in Orion-KL as Seen by Mosaicking the Magnetic Field with ALMA

TL;DR

This study uses ALMA to map the magnetic field in Orion-KL, revealing a transition from a radial to hourglass pattern and showing the outflow's dominance over magnetic energy.

Contribution

First to perform linear-polarization mosaicked observations of Orion-KL with ALMA, revealing detailed magnetic field morphology and its interaction with explosive outflows.

Findings

Magnetic field strength estimated at 9.4 mG.

Field pattern transitions from radial to hourglass shape.

Magnetic energy is much less than outflow energy.

Abstract

We present the first linear-polarization mosaicked observations performed by the Atacama Large Millimeter/submillimeter Array (ALMA). We mapped the Orion-KLeinmann-Low (Orion-KL) nebula using super-sampled mosaics at 3.1 and 1.3 mm as part of the ALMA Extension and Optimization of Capabilities (EOC) program. We derive the magnetic field morphology in the plane of the sky by assuming that dust grains are aligned with respect to the ambient magnetic field. At the center of the nebula, we find a quasi-radial magnetic field pattern that is aligned with the explosive CO outflow up to a radius of approximately 12 arc-seconds (~ 5000 au), beyond which the pattern smoothly transitions into a quasi-hourglass shape resembling the morphology seen in larger-scale observations by the James-Clerk-Maxwell Telescope (JCMT). We estimate an average magnetic field strength $\langle B\rangle = 9.4$ mG and a total magnetic energy of 2 x 10^45 ergs, which is three orders of magnitude less than the energy in the explosive CO outflow. We conclude that the field has been overwhelmed by the outflow and that a shock is propagating from the center of the nebula, where the shock front is seen in the magnetic field lines at a distance of ~ 5000 au from the explosion center.