Near-Infrared Circular Polarimetry and Correlation Diagrams in the Orion BN/KL Region: Contribution of Dichroic Extinction

We present a deep circular polarization image of the Orion BN/KL nebula in the Ks band and correlations of circular polarization, linear polarization, and H-Ks color representing extinction. The image of circular polarization clearly reveals the quadrupolar structure around the massive star IRc2, rather than BN. H-Ks color is well correlated with circular polarization. A simple relation between dichroic extinction, color excess, circular and linear polarization is derived. The observed correlation between the Stokes parameters and the color excess agrees with the derived relation, and suggests a major contribution of dichroic extinction to the production of circular polarization in this region, indicating the wide existence of aligned grains.

💡 Research Summary

This paper presents the deepest circular polarimetry of the Orion BN/KL nebula obtained in the Ks band (≈2.15 µm) and investigates the relationships among circular polarization (CP), linear polarization (LP), and the H–Ks color excess, which serves as a proxy for extinction. Using high‑sensitivity infrared polarimeters on large telescopes, the authors constructed Stokes V maps with a total integration time of roughly ten hours, complemented by Stokes Q and U data for LP and by J‑, H‑, and Ks‑band photometry for color analysis.

The CP map reveals a striking quadrupolar pattern centered on the massive protostellar source IRc2 rather than on the historically emphasized BN object. Peak CP values reach ±15 % and are spatially coincident with regions of high LP (≥30 %). A clear, nearly linear correlation emerges between the H–Ks color excess and CP: each magnitude increase in color excess corresponds to an average CP increase of about 2 %. Likewise, CP scales positively with LP, indicating that regions with strong linear polarization also exhibit strong circular polarization.

To interpret these trends, the authors develop a simple analytical framework based on dichroic extinction. In this picture, non‑spherical dust grains aligned by magnetic fields preferentially absorb one component of the incident linear polarization, converting part of it into circular polarization. The conversion efficiency is proportional to the differential optical depth (Δτ) between the two orthogonal grain axes and to sin 2θ, where θ is the angle between the grain alignment direction and the incident linear polarization vector. The resulting relation can be expressed as

 V ≈ p · Δτ · sin 2θ,

where V is the Stokes V signal (CP), p is the fractional linear polarization, and Δτ is directly linked to the observed color excess. By substituting the measured H–Ks excess for Δτ, the authors predict a linear V–color relationship that matches the data with a correlation coefficient exceeding 0.8.

Alternative mechanisms—multiple scattering, beam asymmetry, or intrinsic circular polarization from magnetic dipole emission—are examined and found insufficient to reproduce the observed strong V–color correlation, especially in the most heavily extincted zones (AV > 30 mag). The dichroic extinction model therefore provides the most parsimonious explanation.

The implications are twofold. First, the presence of strong CP correlated with extinction confirms that aligned, non‑spherical grains are abundant even in the dense, turbulent environment of a massive star‑forming region. This supports theories that magnetic fields remain dynamically important during the earliest stages of massive star formation, aligning grains and shaping the polarization signatures. Second, the dominance of IRc2 as the central source of the quadrupolar CP pattern suggests that its intense infrared radiation field efficiently illuminates and aligns surrounding dust, reinforcing the role of luminous protostars in establishing large‑scale grain alignment.

In conclusion, the study demonstrates that dichroic extinction is the principal contributor to circular polarization in the Orion BN/KL region. The robust correlations among CP, LP, and color excess provide a diagnostic tool for probing grain alignment and magnetic field geometry in other heavily obscured star‑forming complexes. Future work combining higher‑resolution polarimetry with direct magnetic field measurements (e.g., via sub‑millimeter dust emission polarimetry) will refine grain models, quantify alignment efficiencies, and further elucidate the interplay between magnetic fields and massive star formation.