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 $\left(2.0\pm 0.1\right)\times 10^2$ m${}^2$ s${}^{-1}$ from the beginning to $\sim$70 d preperihelion in late March 2020, followed by a brightness fade and the comet gradually losing the central condensation. Meanwhile, the comet became progressively bluer, and was even bluer than the Sun ($g-r\approx 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, $A_1=\left(+2.25\pm 0.13\right)\times 10^{-7}$ 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 $\sim$5 kyr ago, during which its sibling C/1844 Y1 (Great Comet) was produced, with the in-plane component of the separation velocity $\gtrsim$1 m s${}^{-1}$. We constrained that the nucleus of C/2019 Y4 before disintegration was $\gtrsim$60 m in radius, and has been protractedly ejecting dust grains of $\sim$10-40 $\mu$m (assuming dust bulk density 0.5 g cm${}^{-3}$) with ejection speed $\sim$30 m s${}^{-1}$ in early March 2020 and increased to $\sim$80 m s${}^{-1}$ towards the end of the month for grains of $\sim$10 $\mu$m.

Accepted to Astronomical Journal; 9 figures, 4 tables

References in corpus (3)

• PHOTOMETRYPIPELINE: An Automated Pipeline for Calibrated Photometry
• Dynamic Sublimation Pressure and the Catastrophic Breakup of Comet ISON
• Split Active Asteroid P/2016 J1 (PANSTARRS)

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• Borderline hyperbolic comet C/2021 O3 (PANSTARRS) was fading as it approached the Sun
• Splitting of Long-Period Comet C/2018 F4 (PANSTARRS)
• Dust and Volatiles in the Disintegrating Comet C/2019 Y4 (ATLAS)