Dust and Volatiles in the Disintegrating Comet C/2019 Y4 (ATLAS)

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, C 2 , C 3 , and NH 2 , which are superimposed on a dust-scattering continuum, typical of cometary spectra in the optical. The production rates of CN, C 2 , and C 3 derived using the Haser model and the corresponding C 2 /CN and C 3 /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 Af ρ quantities. Its dust-scattering reflectivity is slightly red, with a gradient of ∼5% per 10 3 Å. We model the reflectivity gradient in terms of porous dust and find that the red color is accounted for by porous dust.