Near-Infrared Imaging Polarimetry of M42: Aperture Polarimetry of Point-like Sources

TL;DR

This study presents wide-field near-infrared polarimetry of the Orion Nebula Cluster, revealing magnetic field structures and identifying sources with intrinsic polarization, including young brown dwarfs, to understand magnetic influences and circumstellar environments.

Contribution

First comprehensive NIR aperture polarimetry of ~500 stars in M42, linking NIR and FIR/submillimeter polarization to magnetic field structures and identifying intrinsic polarization sources.

Findings

Magnetic fields show an hourglass shape consistent with models of gas contraction and rotation.

Most NIR polarizations agree with FIR/submillimeter data, tracing magnetic fields.

Detected ~50 sources with likely intrinsic polarization, including young brown dwarfs.

Abstract

We have conducted aperture polarimetry of ~500 stars of the Orion Nebula Cluster (ONC) in M42 based on our wide-field (~8'\times 8') $JHKs$ band polarimetry. Most of the near-infrared (NIR) polarizations are dichroic, with position angles of polarization agreeing, both globally and locally, with previous far-infrared (FIR) and submillimeter observations, having taken into account the 90$^\circ $ difference in angles between dichroic absorption and emission. This is consistent with the idea that both NIR dichroic polarizations and FIR/submillimeter thermal polarizations trace the magnetic fields in the OMC-1 region. The magnetic fields inferred from these observations show a pinch at scales less than 0.5 pc with a centroid near IRc2. The hourglass-shaped magnetic field pattern is explained by the models in which the magnetic field lines are dragged along with the contracting gas and then wound up by rotation in a disk. The highly polarized region to the northwest of IRc2 and the low-polarized region near the bright bar are also common among NIR and FIR/submillimeter data, although a few regions of discrepancy exist. We have also discerned ~50 possible highly polarized sources whose polarizations are more likely to be intrinsic rather than dichroic. Their polarization efficiencies ($P(H)/A(H)$) are too large to be explained by the interstellar polarization. These include 10 young brown dwarfs that suggest a higher polarization efficiency, which may present geometrical evidence for (unresolved) circumstellar structures around young brown dwarfs.