New evidence supporting past dust ejections from active asteroid (4015) Wilson-Harrington

TL;DR

This study provides new polarimetric and dust tail evidence supporting that active asteroid (4015) Wilson-Harrington experienced dust ejections likely caused by rotational mass shedding, confirming its asteroid nature with comet-like activity.

Contribution

The paper presents new polarimetric observations, reanalysis of historical images, and dust tail modeling to reveal dust ejection activity in (4015) Wilson-Harrington, supporting its classification as an active asteroid.

Findings

Polarization characteristics similar to low-albedo asteroids.

Brightness increase around the nucleus in 1949 images.

Dust ejection of ~9 x 10^5 kg at low velocity.

Abstract

Context. (4015) Wilson-Harrington (hereafter, WH) was discovered as a comet in 1949 but has a dynamical property consistent with that of a near-Earth asteroid. Although there is a report that the 1949 activity is associated with an ion tail, the cause of the activity has not yet been identified. Aims. This work aims to reveal the mysterious comet-like activity of the near-Earth asteroid. Methods. We conducted new polarimetric observations of WH from May 2022 to January 2023, reanalyses of the photographic plate images taken at the time of its discovery in 1949, and dust tail simulation modelings, where the dust terminal velocity and ejection epoch are taken into account. Results. We found that this object shows polarization characteristics similar to those of low-albedo asteroids. We derived the geometric albedo ranging from pV = 0.076 +- 0.010 to pV = 0.094 +- 0.018 from our polarimetry (the values vary depending on the data used for fitting and the slope-albedo relationship coefficients). In addition, the 1949 image showed an increase in brightness around the nucleus. Furthermore, we found that the color of the tail is consistent with sunlight, suggesting that the 1949 activity is associated with dust ejection. From the dust tail analysis, ~9 x 10^5 kg of material was ejected episodically at a low velocity equivalent to or even slower than the escape velocity. Conclusions. We conclude that WH is most likely an active asteroid of main belt origin and that the activity in 1949 was likely triggered by mass shedding due to fast rotation.