Polarimetric and Photometric Characteristics of the Interstellar Comet 3I/ATLAS: Main Features

The aim of this study is to determine the polarimetric and photometric characteristics of the interstellar comet 3I/2025 N1 (ATLAS), as well as to model the physical properties of particles in its coma. Observations of the comet were carried out using the POLSHAKH polarimeters on the 2.6-m telescope of the Crimean Astrophysical Observatory, the telescope Zeiss-2000 of shared research facility “Terskol observatory” of Institute of Astronomy of the RAS, and the SCORPIO-2 instrument on the 6-m telescope of the Special Astrophysical Observatory during the period from November 18, 2025 to January 26, 2026. The following main results were obtained. • Among all comets observed to date, comet 3I/ATLAS showed the highest absolute values of the degree of linear polarization on the negative and positive branches of the phase curve. The following parameters of the polarization phase curve were determined: Pmin = –3.01% ± 0.05%, αmin = 5.46° ± 0.05°, polarimetric slope h = 0.53 ± 0.01 (% per angular degree) and inversion angle αinv = 17.6° ± 0.8°. • The spectral polarization gradient ΔP(λ)/Δλ = 0.004 ± 0.001 (% per nm) at a phase angle of about 25°. This is less than the average spectral polarization gradient of 0.008 (% per nm) for comets in the Solar System. • In November 2025, the B–V, V–R, and R–I colors were ≈0.4m, and g–r = 0.71m. • Dust productivity Afρ amounted to 5461 ± 70 cm for the r‑sdss filter and a coma diameter of 22696 km at a heliocentric distance of 1.710 AU. • The best fit to the observed polarization phase curve is achieved by modeling conjugate random Gaussian particles with roughness G_50⁷( 0.5 ) for a particle mixture consisting of 95% amorphous carbon, 4.5% magnesium-rich silicates, and 0.5% water ice.