Physical Characterization of the December 2017 Outburst of the Centaur 174P/Echeclus

TL;DR

This study characterizes the December 2017 outburst of Centaur 174P/Echeclus through imaging and spectroscopy, revealing dust grain properties and debris ejection, with implications for understanding Centaur activity.

Contribution

It provides the first detailed optical and infrared analysis of Echeclus's 2017 outburst and models the dust and debris ejection processes involved.

Findings

Echeclus's coma is dominated by large blue dust grains.

A secondary peak indicates a large debris cloud ejected during the outburst.

No significant orbital changes were observed post-outburst.

Abstract

The Centaurs are the small solar system bodies intermediate between the active inner solar system Jupiter Family Comets and their inactive progenitors in the trans-Neptunian region. Among the fraction of Centaurs which show comet-like activity, 174P/Echeclus is best known for its massive 2005 outburst in which a large apparently active fragment was ejected above the escape velocity from the primary nucleus. We present visible imaging and near-infrared spectroscopy of Echeclus during the first week after its December 2017 outburst taken at the Faulkes North & South Telescopes and the NASA IRTF, the largest outburst since 2005. The coma was seen to be highly asymmetric. A secondary peak was seen in the near-infrared 2D spectra, which is strongly hinted at in the visible images, moving hyperbolically with respect to the nucleus. The retrieved reflectance spectrum of Echelcus is consistent with the unobscured nucleus but becomes bluer when a wider extraction aperture is used. We find that Echeclus's coma is best explained as dominated by large blue dust grains, which agrees with previous work. We also conducted a high-resolution orbital integration of Echeclus's recent evolution and found no large orbital changes that could drive its modern evolution. We interpret the second peak in the visible and near-infrared datasets as a large cloud of larger-than-dust debris ejected at the time of outburst. If Echeclus is typical of the Centaurs, there may be several debris ejection or fragmentation events per year on other Centaurs that are going unnoticed.