Fragmentation Kinematics in Comet 332P/Ikeya-Murakami

TL;DR

This study uses Hubble observations to analyze the fragmentation process of comet 332P/Ikeya-Murakami, revealing details about fragment sizes, ejection times, and the nucleus's smaller-than-expected size and mass, suggesting rotational instability influences fragmentation.

Contribution

First detailed time-resolved analysis of comet 332P/Ikeya-Murakami's fragmentation, estimating smaller nucleus size and mass, and highlighting rotational instability as a key factor.

Findings

Fragments recede at 0.06 to 3.5 m/s with ejection times in late 2015.

Small fragments are less abundant than expected from a simple power-law distribution.

Nucleus radius estimated at ≤275 m, much smaller than previous estimates.

Abstract

We present initial time-resolved observations of the split comet 332P/Ikeya-Murakami taken using the Hubble Space Telescope. Our images reveal a dust-bathed cluster of fragments receding from their parent nucleus at projected speeds in the range 0.06 to 3.5 m s$^{-1}$ from which we estimate ejection times from October to December 2015. The number of fragments with effective radii $\gtrsim$20 m follows a differential power law with index $\gamma$ = -3.6$\pm$0.6, while smaller fragments are less abundant than expected from an extrapolation of this power-law. We argue that, in addition to losses due to observational selection, torques from anisotropic outgassing are capable of destroying the small fragments by driving them quickly to rotational instability. Specifically, the spin-up times of fragments $\lesssim$20 m in radius are shorter than the time elapsed since ejection from the parent nucleus. The effective radius of the parent nucleus is $r_e \le$ 275 m (geometric albedo 0.04 assumed). This is about seven times smaller than previous estimates and results in a nucleus mass at least 300 times smaller than previously thought. The mass in solid pieces, $2\times10^9$ kg, is about 4% of the mass of the parent nucleus. As a result of its small size, the parent nucleus also has a short spin-up time. Brightness variations in time-resolved nucleus photometry are consistent with rotational instability playing a role in the release of fragments.