Dust Properties of the Interstellar Object 3I/ATLAS Revealed by Optical and Near-Infrared Polarimetry

TL;DR

This study uses optical and near-infrared polarimetry to analyze the dust properties of interstellar object 3I/ATLAS, revealing its primitive, refractory dust composition and intrinsic optical characteristics distinct from typical Solar System comets.

Contribution

First near-infrared polarimetric measurements of 3I/ATLAS, showing its unique polarization behavior and dust properties indicative of an interstellar origin.

Findings

PPC of 3I/ATLAS differs from Solar System comets with large polarization amplitude.

No significant change in polarization across perihelion, suggesting intrinsic dust properties.

PCC peaks at 1.5-2.0 μm, indicating dust aggregates of submicron monomers.

Abstract

We present independent polarimetric observations of the interstellar object 3I/ATLAS, including the first near-infrared polarimetric measurements. Using imaging polarimeters, we measured the degree of linear polarization from the visible RC band (0.64 {\mu}m) to the near-infrared KS band (2.25 {\mu}m), and investigated its dependence on solar phase angle (polarization phase curve; PPC) and wavelength (polarization color curve; PCC). We confirm that the PPC of 3I/ATLAS differs significantly from those of typical Solar System comets, showing an unusually large polarization amplitude. This PPC shows no significant change in the RC band across perihelion passage, despite the perihelion lying within the water snow line. This indicates that the unusual polarimetric behavior of 3I/ATLAS is unlikely to be driven by transient volatile activity, but instead reflects intrinsic optical properties of refractory dust particles. The PCC increases with wavelength over 0.6-1.2 {\mu}m and peaks at 1.5-2.0 {\mu}m, suggesting that the dominant scattering units are dust aggregates composed of submicron-sized monomers, broadly consistent with interstellar dust and solar-system cometary aggregates. Taken together, our results indicate that 3I/ATLAS preserves polarimetric properties characteristic of a primitive cometary planetesimal formed in another planetary system, with a refractory dust composition that differs from that typically observed among Solar System comets, despite sharing a similar size scale of the aggregate building blocks.