The Discovery and Evolution of a Possible New Epoch of Cometary Activity by the Centaur (2060) Chiron

TL;DR

This paper reports on a significant brightening event of Centaur (2060) Chiron in 2021, suggesting a possible new epoch of cometary activity, analyzed through multiple observational datasets and considering various potential causes.

Contribution

It introduces a comprehensive multi-instrument observational analysis of Chiron's 2021 brightening, proposing a new or increased cometary activity epoch as the most likely cause.

Findings

Chiron's brightening is unlikely due to rotation or surface features.

No coma detected in follow-up observations, but faint coma cannot be ruled out.

The event may indicate a new phase of cometary activity in Chiron.

Abstract

Centaurs are small Solar System objects on chaotic orbits in the giant planet region, forming an evolutionary continuum with the Kuiper belt objects and Jupiter-family comets. Some Centaurs are known to exhibit cometary activity, though unlike comets this activity tends not to correlate with heliocentric distance and the mechanism behind it is currently poorly understood. We utilize serendipitous observations from the Asteroid Terrestrial-impact Last Alert System (ATLAS), Zwicky Transient Facility (ZTF), Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), Dark Energy Survey (DES), and Gaia in addition to targeted follow-up observations from the Las Cumbres Observatory, TRAnsiting Planets and PlanetesImals Small Telescope South (TRAPPIST-South), and Gemini North telescope to analyze an unexpected brightening exhibited by the known active Centaur (2060) Chiron in 2021. This is highly indicative of a cometary outburst. As of 2023 February, Chiron has still not returned to its pre-brightening magnitude. We find Chiron's rotational lightcurve, phase curve effects, and possible high-albedo surface features to be unlikely causes of this observed brightening. We consider the most likely cause to be an epoch of either new or increased cometary activity, though we cannot rule out a possible contribution from Chiron's reported ring system, such as a collision of as-yet unseen satellites shepherding the rings. We find no evidence for coma in our Gemini or TRAPPIST-South observations, though this does not preclude the possibility that Chiron is exhibiting a coma that is too faint for observation or constrained to the immediate vicinity of the nucleus.