Down But Not Out: The Case of Long-Period Comet C/2021 O3 (Panstarrs)

TL;DR

This study combines ground and space observations to analyze long-period comet C/2021 O3, revealing it survived perihelion without nucleus destruction, with insights into its activity, debris, and nucleus properties.

Contribution

It provides new observational evidence that the comet's nucleus survived perihelion, challenging expectations of destruction, and offers detailed analysis of its activity and debris characteristics.

Findings

Nucleus survived perihelion without destruction.

Pre-perihelion activity showed a smaller heliocentric dependence than typical.

Nucleus radius estimated between 1.0 and 1.7 km.

Abstract

We combine ground- and space-based observations of long-period comet C/2021 O3 (Panstarrs) (perihelion distance 0.287 au) in order to investigate its reported near-perihelion destruction. Pre-perihelion photometric observations show a remarkably small heliocentric dependence of the scattered light, $\propto r_H^{-s}$ with $s = 2.59\pm0.21$, distinct from values reported in other long-period comets, for which $s$ = 4 is the canonical standard. The index is smaller than expected of coma production by equilibrium sublimation of either supervolatiles (for which $s \sim$ 4 is expected), or water ice ($s \sim$ 6 to 8) across the $\sim$4 au to 2 au range. The absolute magnitude deduced from the pre-perihelion data is $H$ = 13.0$\pm$0.3 (coma scattering cross-section $\sim$225 km$^2$ for an assumed geometric albedo 0.04) while, after perihelion, the cross-section fades by a factor of 25 to $H$ = 16.5 ($\sim$9 km$^2$). STEREO spacecraft observations near perihelion show a long debris trail whose properties are consistent with forward scattering from radius $\sim$7 $\mu$m particles. The data show that the nucleus of C/2021 O3 was not destroyed at perihelion. Although the lightcurve from 3.9 au inbound to 0.8 au outbound cannot be uniquely interpreted, a simple and plausible explanation is provided by seasonal dimming on a nucleus having high obliquity and an asymmetric distribution of near-surface volatiles. The survival of the nucleus against rotational disruption suggests a pre-perihelion nucleus radius $r_n \gtrsim$ 1.0 km while the photometric limit to the radius of the nucleus after perihelion is $r_n < 1.7$ km (geometric albedo 0.04 assumed).