Disintegration of Long-period Comet C/2019 Y4 (ATLAS). II. Post-Perihelion Remnant Recovery

TL;DR

This study investigates the fate of the disrupting near-Sun comet C/2019 Y4 (ATLAS) after its 2020 fragmentation by combining LDT imaging (3–5 months post-disruption) with ZTF survey searches to locate or constrain remnants. It constrains the primary remnant C/2019 Y4-B to $D<0.5$ km (via $H>20.5$) and finds no detectable activity, while fragment A remains undetected with $D\lesssim 2$ km, suggesting continued disintegration since perihelion. The work highlights substantial uncertainty in the post-disruption state of such comets and contrasts this case with six other reportedly disrupted long-period comets, about half of which lack negative follow-up confirmations. The results underscore the importance of deep, timely follow-up observations to clarify whether remnants persist and how disintegration proceeds in these extreme bodies.

Abstract

We present an investigation into the fate of disrupting near-Sun comet C/2019 Y4 (ATLAS). Imaging observations with the Lowell Discovery Telescope (LDT), obtained 3--5 months after the reported disruption and the last sighting, constrained the primary component C/2019 Y4-B to an absolute magnitude of $H>20.5$ or a diameter of $D<0.5$~km (assuming a geometric albedo of 0.04) at an outbound heliocentric distance of 2.71~au. A search of shallower data obtained from the Zwicky Transient Facility (ZTF) survey, conducted when components A and B receded from 2.5 to 2.9~au from the Sun, yielded no detections, suggesting that the fragments had either ceased to exist or were not substantially active during this period. The uncertain fate of C/2019 Y4 highlights the challenge in understanding the state of presumably disrupted comets. Our review of six other presumably disrupted long-period comets reveals that negative confirmations were not reported for half of them, leaving their disruption presumptions inconclusive.

Figures (3)

• Figure 1: ZTF nightly stacks for C/2019 Y4-A on 2020 October 14 (panel a) and 15 (panel b). Also shown are 3$\sigma$ uncertainty ellipses for fragment A calculated using JPL orbit solution #7. Each image is $5'\times5'$ in size with up points to celestial north. The white rectangles are masked stars and subtraction residuals.
• Figure 2: Debris mass of fragment A as a function of differential size index $\gamma$. Two lower bound cases are plotted for $a_1=1 \, \mu\mathrm{m}$ and $a_1=500 \, \mu\mathrm{m}$. The dashed horizontal line, $M_d=(3.2\pm0.6)\times10^{11}$ kg, is derived in Paper I, with the highlighted region representing the constraint's uncertainty.
• Figure 3: The LDT composite, star-subtracted image on 2020 October 16 centered at the nominal position of C/2019 Y4-B. The 3$\sigma$ uncertainty ellipse based on JPL orbit solution #9 is shown.