Observations of Disintegrating Long-period Comet C/2019 Y4 (ATLAS): A Sibling of C/1844 Y1 (Great Comet)

We present a study of C/2019 Y4 (ATLAS) using Sloangriobservations from 2020 mid-January to early April. During this time span, the comet brightened with a growth in the effective cross-section of (2.0 +/- 0.1) x 10(2) m(2) s(-1)from the beginning to similar to 70 days preperihelion in late 2020 March, followed by a brightness fade and the comet gradually losing the central condensation. Meanwhile, the comet became progressively bluer, and was even bluer than the Sun (g - r approximate to 0.2) when the brightness peaked, likely due to activation of subterranean fresh volatiles exposed to sunlight. With the tailward bias-corrected astrometry we found an enormous radial nongravitational parameter, A(1) = (+2.25 +/- 0.13) x 10(-7) au day(-2) in the heliocentric motion of the comet. Taking all of these findings into consideration, we conclude that the comet has disintegrated since 2020 mid-March. By no means was the split new to the comet, as we quantified that the comet had undergone another split event around last perihelion similar to 5 kyr ago, during which its sibling C/1844 Y1 (Great Comet) was produced, with the in-plane component of the separation velocity greater than or similar to 1 m s(-1). We constrained that the nucleus of C/2019 Y4 before disintegration was greater than or similar to 60 m in radius, and has been protractedly ejecting dust grains of similar to 10-40 mu m (assuming a dust bulk density of 0.5 g cm(-3)) with an ejection speed of similar to 30 m s(-1) in early 2020 March and increased to similar to 80 m s(-1) toward the end of the month for grains of similar to 10 mu m.