Optical and Near-infrared Contemporaneous Polarimetry of C/2023 A3 (Tsuchinshan-ATLAS)

Abstract We conducted contemporaneous optical and near-infrared polarimetric and spectroscopic observations of C/2023 A3 (Tsuchinshan-ATLAS, hereafter T-A) from 2024 October 16 to December 17, covering a wide range of phase angles (20°–123°) and wavelengths (0.5–2.3 μ m). We paid special attention to gas contamination in the dust polarization using these data. As a result, we find the maximum polarization degree <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>P</mml:mi> <mml:mi>max</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>31.21</mml:mn> <mml:mo>%</mml:mo> <mml:mo>±</mml:mo> <mml:mn>0.05</mml:mn> </mml:math> %, 33.52% ± 0.06%, 35.12% ± 0.01%, 37.57% ± 0.01%, and 35.35% ± 0.01% in the Rc , Ic , J , H , and Ks bands, respectively. Although dust polarization shows a red slope at shorter wavelengths and can peak around 1.6 μ m, the phase angle at which maximum polarization occurs exhibits less dependence on wavelength ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>90</mml:mn> <mml:mo>°</mml:mo> </mml:math> –95°). Although only a few historically bright comets, such as West, Halley, and Hale–Bopp, have undergone such extensive dust-polarization observations, our measurements are generally consistent with those of two comets that possibly originated from the Oort Cloud (West and Halley). From these results, we conjecture that the optical properties and growth processes of dust in the presolar nebula, which formed these cometary nuclei, were likely uniform.