Observations of Disintegrating Long-Period Comet C/2019 Y4 (ATLAS) -- A Sibling of C/1844 Y1 (Great Comet)
Abstract
We present a study of C/2019 Y4 (ATLAS) using Sloan gri observations from mid-January to early April 2020. During this timespan, the comet brightened with a growth in the effective cross-section of (2.0 0.1 ) × 102 m2 s-1 from the beginning to 70 d preperihelion in late March 2020, 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 ≈ 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, A1 = (+2.25 0.13) × 10-7 au d-2 in the heliocentric motion of the comet. Taking all of these finds into consideration, we conclude that the comet has disintegrated since mid-March 2020. By no means was the split new to the comet, as we quantified that the comet had undergone another split event around last perihelion 5 kyr ago, during which its sibling C/1844 Y1 (Great Comet) was produced, with the in-plane component of the separation velocity 1 m s-1. We constrained that the nucleus of C/2019 Y4 before disintegration was 60 m in radius, and has been protractedly ejecting dust grains of 10-40 μm (assuming dust bulk density 0.5 g cm-3) with ejection speed 30 m s-1 in early March 2020 and increased to 80 m s-1 towards the end of the month for grains of 10 μm.