Submillimeter Array Observations of Magnetic Fields in G240.31+0.07: an Hourglass in a Massive Cluster-forming Core

TL;DR

This study presents the first observation of an hourglass-shaped magnetic field in a high-mass star-forming core, aligning with outflow and rotation axes, supporting theories of magnetic influence in massive star formation.

Contribution

It provides the first direct detection of an hourglass magnetic field in a high-mass star-forming region, aligning with theoretical models and demonstrating magnetic regulation of core dynamics.

Findings

Magnetic field strength estimated at 1.1 mG.

Magnetic field aligned within 20° of outflow axis.

Angular momentum decreases with scale, indicating magnetic braking.

Abstract

We report the first detection of an hourglass magnetic field aligned with a well-defined outflow-rotation system in a high-mass star-forming region. The observations were performed with Submillimeter Array toward G240.31+0.07, which harbors a massive, flattened, and fragmenting molecular cloud core and a wide-angle bipolar outflow. The polarized dust emission at 0.88 mm reveals a clear hourglass-shaped magnetic field aligned within 20 degree of the outflow axis. Maps of high-density tracing spectral lines, e.g., H13CO+ (4-3), show that the core is rotating about its minor axis, which is also aligned with the magnetic field axis. Therefore, both the magnetic field and kinematic properties observed in this region are surprisingly consistent with the theoretical predictions of the classic paradigm of isolated low-mass star formation. The strength of the magnetic field in the plane of sky is estimated to be about 1.1 mG, resulting in a mass-to-magnetic flux ratio of 1.4 times the critical value and a turbulent to ordered magnetic energy ratio of 0.4. We also find that the specific angular momentum almost linearly decreases from r~0.6 pc to 0.03 pc scales, which is most likely attributed to magnetic braking.