Disintegration of Long-period Comet C/2019 Y4 (ATLAS). I. Hubble Space Telescope Observations

Abstract

The near-Sun comet C/2019 Y4 (ATLAS) is the first member of a long-period comet group observed to disintegrate well before perihelion. Here we present our investigation into this disintegration event using images obtained in a three-day Hubble Space Telescope campaign. We identify two fragment clusters produced by the initial disintegration event, corresponding to fragments C/2019 Y4-A and C/2019 Y4-B identified in groundbased data. These two clusters started with similar integrated brightness but exhibit different evolutionary behavior. C/2019 Y4-A was much shorter-lived compared to C/2019 Y4-B and showed signs of significant mass loss and changes in size distribution throughout the three-day campaign. The cause of the initial fragmentation is undetermined by the limited evidence but crudely compatible with either the spin-up disruption of the nucleus or runaway sublimation of subsurface supervolatile ices, either of which would lead to the release of a large amount of gas as inferred from the significant bluing of the comet observed shortly before its disintegration. Gas can only be produced by the sublimation of volatile ices, which must have survived at least one perihelion passage at a perihelion distance of q = 0.25 au. We speculate that Comet ATLAS is derived from the ice-rich interior of a nonuniform, kilometer-wide progenitor that split during its previous perihelion. This suggests that comets down to a few kilometers in diameter can still possess complex, nonuniform interiors that can protect ices against intense solar heating.

Splitting of Long-period Comet C/2018 F4 (PANSTARRS)

“…Fragmentation and disintegration are common fates for comets. Over the past 150 yr, more than 40 comets have been observed to split (Boehnhardt 2004), and the number is still growing (e.g., Fernández 2009;Li & Jewitt 2015;Jewitt et al 2016;Ye et al 2021). However, the mechanisms by which they fragment are usually far from clear, except in the case of D/ 1993 F2 (Shoemaker-Levy 9), which was torn apart by the tidal force of Jupiter (Scotti & Melosh 1993;Sekanina et al 1994).…”

Physical properties and mini-outburst of 64P/Swift-Gehrels

“…Though the typical color of comets turns out to be redder than the Sun, several snapshot color measurements of different comets were reported showing blue color (Korsun et al 2010;Zubko et al 2014). As for long timescale variation, a similar bluing phenomenon was recorded recently on the comet C/2019 Y4 (AT-LAS) before its splitting event, indicating a large amount of gas released (Hui & Ye 2020;Ye et al 2021). Compared to the previous examples, the minimum V − R color of our observation for 64P lasted over 100 days, exceeding other bluing events recorded, and the tendency of gradual change is not recognized.…”

The Lingering Death of Periodic Near-Sun Comet 323P/SOHO

“…The rapid changing observing geometry of 323P in 2021 March prevented us from directly using the sky-plane component of the separation distances between the fragments and the primary nucleus as the proxy to study the fragment kinematics (e.g., Jewitt et al 2016;Ye et al 2021), otherwise one would find that both of the fragments were spatially approaching the primary, resulting in a precarious argument that they were trapped in the gravitational field of the primary. To evaluate the likelihood, we estimate the apparent angle subtended by the Hill radius to the HST…”