The Active Centaurs

TL;DR

This study observes 23 Centaurs, finding nine active ones with activity likely driven by thermal processes such as amorphous to crystalline ice conversion, suggesting they contain amorphous ice and are influenced by their proximity to the Sun.

Contribution

It provides the first observational evidence linking Centaur activity to thermal processes and amorphous ice, supporting the hypothesis of their icy composition and activity driven by sublimation or crystallization.

Findings

Nine out of 23 Centaurs are active with measurable mass-loss rates.

Active Centaurs have smaller perihelia than inactive ones, indicating thermal influence.

Crystallization of amorphous ice may trigger activity, consistent with observed perihelion distribution.

Abstract

The Centaurs are recent escapees from the Kuiper belt that are destined either to meet fiery oblivion in the hot inner regions of the Solar system or to be ejected to the interstellar medium by gravitational scattering from the giant planets. Dynamically evolved Centaurs, when captured by Jupiter and close enough to the Sun for near-surface water ice to sublimate, are conventionally labeled as "short-period" (specifically, Jupiter-family) comets. Remarkably, some Centaurs show comet-like activity even when far beyond the orbit of Jupiter, suggesting mass-loss driven by a process other than the sublimation of water ice. We observed a sample of 23 Centaurs and found nine to be active, with mass-loss rates measured from several kg/s to several tonnes/s. Considered as a group, we find that the "active Centaurs" in our sample have perihelia smaller than the inactive Centaurs (median 5.9 AU vs. 8.7 AU), and smaller than the median perihelion distance computed for all known Centaurs (12.4 AU). This suggests that their activity is thermally driven. There are several possibilities for the origin of the mass-loss from the active Centaurs. We consider the possibility that activity in the Centaurs is triggered by the conversion of amorphous ice into the crystalline form accompanied by the release of trapped gases, including carbon monoxide. By imposing the condition that crystallization should occur when the crystallization time is shorter than the orbital period we find a qualitative match to the perihelion distribution of the active Centaurs and conclude that the data are consistent with the hypothesis that the Centaurs contain amorphous ice.