2024ApJ...963...90Z

C/2019 Y4 (ATLAS) is an Oort cloud comet with an orbital period of ∼5895 yr. Starting in 2020 March, its nucleus underwent disintegration. In order to investigate the gas and dust properties of C/2019 Y4 (ATLAS) during its disintegration, we obtained long-slit spectra at 3600-8700 Å and BVRI multiband images with the Xinglong 2.16 m Telescope in 2020 April. Our observations revealed that C/2019 Y4 (ATLAS) exhibited strong emission bands of CN, C₂, C₃, and NH₂, which are superimposed on a dust-scattering continuum, typical of cometary spectra in the optical. The production rates of CN, C₂, and C₃ derived using the Haser model and the corresponding C₂/CN and C₃/CN ratios suggest that C/2019 Y4 (ATLAS) is a “typical” Oort cloud comet under the A'Hearn classification, although it appears less dusty, as revealed by the Afρ quantities. Its dust-scattering reflectivity is slightly red, with a gradient of ∼5% per 10³Å. We model the reflectivity gradient in terms of porous dust and find that the red color is accounted for by porous dust.