A Far-Infrared Observational Test of the Directional Dependence in Radiative Grain Alignment

Abstract

The alignment of interstellar dust grains with magnetic fields provides a key method for measuring the strength and morphology of the fields. In turn, this provides a means to study the role of magnetic fields from diffuse gas to dense star-forming regions. The physical mechanism for aligning the grains has been a long-term subject of study and debate. The theory of radiative torques, in which an anisotropic radiation field imparts sufficient torques to align the grains while simultaneously spinning them to high rotational velocities, has passed a number of observational tests. Here we use archival polarization data in dense regions of the Orion molecular cloud (OMC-1) at 100, 350, and $850\mu$m to test the prediction that the alignment efficiency is dependent upon the relative orientations of the magnetic field and radiation anisotropy. We find that the expected polarization signal, with a 180-degree period, exists at all wavelengths out to radii of 1.5 arcminutes centered on the BNKL object in OMC-1. The probabilities that these signals would occur due to random noise are low ($\lesssim$1\%), and are lowest towards BNKL compared to the rest of the cloud. Additionally, the relative magnetic field to radiation anisotropy directions accord with theoretical predictions in that they agree to better than 15 degrees at $100\mu$m and 4 degrees at $350\mu$m.

16 pages, 4 figures, ApJL, in press

References in corpus (4)

• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• Radiative torques: Analytical Model and Basic Properties
• Placing Confidence Limits on Polarization Measurements
• Radiative torque alignment: Essential Physical Processes

Cited by in corpus (8)

• Planck 2018 results. XII. Galactic astrophysics using polarized dust emission
• A unified model of grain alignment: radiative alignment of interstellar grains with magnetic inclusions
• Submillimeter Polarization Spectrum in the Vela C Molecular Cloud
• A systematic study of radiative torque grain alignment in the diffuse interstellar medium
• Probing the cold magnetized Universe with SPICA-POL (B-BOP)
• Comparing submillimeter polarized emission with near-infrared polarization of background stars for the Vela C molecular cloud
• The Mechanical Alignment of Dust (MAD) I: On the spin-up process of fractal grains by a gas-dust drift
• Effect of alignment on polarized infrared emission from polycyclic aromatic hydrocarbons