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

Seungwon Choi, Masateru Ishiguro, Jun Takahashi, Tomoki Saito, Yoonsoo P. Bach, Bumhoo Lim, Hiroyuki Naito, Jooyeon Geem, Sunho Jin, Jinguk Seo, Hyeonwoo Ju, Hiroshi Akitaya, Koji S. Kawabata, Mahito Sasada, Kazuya Doi, Hisayuki Kubota, Seiko Takagi, Makoto Watanabe, Tomohiko Sekiguchi, Myungshin Im

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 R <sub>C</sub> band (0.64  μ m) to the near-infrared K <sub>s</sub> band (2.25  μ 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 R <sub>C</sub> 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  μ m and peaks at 1.5–2.0  μ 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.

The Astrophysical Journal Letters

1000

1

L29

10.3847/2041-8213/ae497c