Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS
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| Main Authors: | , , , , , , , , , , , , , , , |
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| Format: | Preprint |
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2025
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| author | Gray, Zuri Bagnulo, Stefano Borisov, Galin Kwon, Yuna G. Cellino, Alberto Kolokolova, Ludmilla Dorsey, Rosemary C. Fedorets, Grigori Granvik, Mikael MacLennan, Eric Muñoz, Olga Bendjoya, Philippe Devogèle, Maxime Ieva, Simone Penttilä, Antti Muinonen, Karri |
| author_facet | Gray, Zuri Bagnulo, Stefano Borisov, Galin Kwon, Yuna G. Cellino, Alberto Kolokolova, Ludmilla Dorsey, Rosemary C. Fedorets, Grigori Granvik, Mikael MacLennan, Eric Muñoz, Olga Bendjoya, Philippe Devogèle, Maxime Ieva, Simone Penttilä, Antti Muinonen, Karri |
| contents | We present the first polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1), obtained pre-perihelion with FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, over a phase angle range of 7.7-22.4°. This marks the second ever polarimetric study of an interstellar object, the first distinguishing 2I/Borisov from most Solar System comets by its higher positive polarisation. Our polarimetric measurements as a function of phase angle reveal that 3I is characterised by an deep and narrow negative polarisation branch, reaching a minimum value of -2.7% at phase angle 7°, and an inversion angle of 17° -- a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarisation phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatial resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2509_05181 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS Gray, Zuri Bagnulo, Stefano Borisov, Galin Kwon, Yuna G. Cellino, Alberto Kolokolova, Ludmilla Dorsey, Rosemary C. Fedorets, Grigori Granvik, Mikael MacLennan, Eric Muñoz, Olga Bendjoya, Philippe Devogèle, Maxime Ieva, Simone Penttilä, Antti Muinonen, Karri Earth and Planetary Astrophysics Astrophysics of Galaxies We present the first polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1), obtained pre-perihelion with FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, over a phase angle range of 7.7-22.4°. This marks the second ever polarimetric study of an interstellar object, the first distinguishing 2I/Borisov from most Solar System comets by its higher positive polarisation. Our polarimetric measurements as a function of phase angle reveal that 3I is characterised by an deep and narrow negative polarisation branch, reaching a minimum value of -2.7% at phase angle 7°, and an inversion angle of 17° -- a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarisation phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatial resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies. |
| title | Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS |
| topic | Earth and Planetary Astrophysics Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2509.05181 |