The Active Centaurs

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 −1 to several tonnes s −1 . 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. We consider several possibilities for the origin of the mass-loss from the active Centaurs. Most are too cold for activity at the observed levels to originate via the sublimation of crystalline water ice. Solid carbon monoxide and carbon dioxide have the opposite problem: they are so volatile that they should drive activity in Centaurs at much larger distances than observed. 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.• The perihelion distance, q, and the semimajor axis, a, satisfy a J < q < a N and a J < a < a N , respectively, where a J = 5.2 AU is the semimajor axis of Jupiter and a N = 30.0 AU is the semimajor axis of Neptune.• The orbit is not in 1:1 mean-motion resonance with any planet.This definition has the practical advantage of simplicity; it efficiently isolates a sub-set of the Solar system small-body population whose members are short-lived by virtue of their gravitational interactions with the giant planets. At the time of writing there are, by the above definition, 92 known Centaurs.In this paper, we focus attention on Centaurs which display comet-like mass-loss. Our purpose is to identify common properties of the activity and to seek evidence concerning its origin. The active Centaurs have, so far, escaped systematic study as a group. One likely reason is that the cometary classification afforded to such objects makes no distinction between them and other, long-studied comets in the Jupiter family, Halley family and Long-Period populations. We seek answers to such questions as 1) How do the pr...

Observational results for eight long-period comets observed far from the Sun

“…This is similar to the dust environment assumed for several SPC tails (see a summary in Fulle 1999) and for active Centaurs (Jewitt 2009).…”

“…The method is derived from the one used by Jewitt (2009) to compute the dust production rate of active Centaurs in the region between 5 and 12 AU; it was adapted and already successfully applied by our group to cometary environment. We refer to Geometric albedo p R : we adopted the typical value for comet nuclei of p R = 0.04; this is an important factor in the uncertainty in our calculations since the actual albedo of dust grains is unknown.…”

Dust environment and dynamical history of a sample of short-period comets

“…This comet was study by Mazzotta Epifani & Palumbo (2011) in its previous perihelion passage in October 2004. The authors estimated that the dust production rate at perihelion with values within Q d = 14−345 kg s −1 using a method derived from the one used by Jewitt (2009) to compute the dust production rate of active Centarus. They also obtained A f ρ = 846 ± 55 cm in an aperture of radius ρ = 7.3 × 10 3 km, and they concluded that this comet is more active than the average at a given heliocentric distance.…”

The dust coma environment of the short period comets 32P/Comas Solá, 56P/Slaughter-Burnham, and 78P/Gehrels 2 from ground-based observations

“…The method is derived from the one used by Jewitt (2009) to compute the dust production rate of active Centaurs in the region between 5 and 12 AU, and was already successfully applied to estimating the dust production rate of the Oort comet C/2007 D1 (LINEAR) (Mazzotta Epifani et al 2010). The method consists of computing the apparent magnitude of the pure coma m d in an annulus between an inner φ and a conveniently chosen outer φ as…”