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Colloquium
A Hundred Sleeping Beasts: Dynamical Triggers of Cometary Activity in Centaurs

Abstract

The Centaur population is comprised of small, likely icy bodies on dynamically unstable orbits transitioning from the trans-Neptunian region to the inner Solar system. Centaurs represent an interesting stage in the dynamical evolution of primordial small bodies scattered by planetary perturbations from the deep freeze of the TNO reservoir where they have been stored for billions of years. They enter the giant planet region as largely primordial objects and carry information about the composition of their parent populations to our doorstep.

Here we present a thermodynamical study of Centaurs, complemented with a visual survey conducted with the Gemini telescopes aimed to search for cometary activity and to identify possible activity triggers in the members of the Centaur population. Through the duration of our survey only one Centaur in our sample - C/2014 OG392 (PANSTARRS) appeared active. Both results from our survey and the thermodynamical modeling indicate that the orbital history plays a critical role in the thermal processing of Centaurs and their potential for onset of cometary activity. Our in-depth analysis of orbital evolution revealed that within the past several hundred years every known active Centaur underwent a rapid orbital change leading to significant decrease in semi-major axis we call an `a-jump’, which led to a major increase in the average-per orbit insolation. Such a-jumps typically occur after close encounters with Jupiter or Saturn and are not present in recent orbital histories of inactive Centaurs. A single Centaur can typically undergo several a-jumps throughout its lifetime leading to the depletion of volatiles and thermal processing of the nucleus. Our results also show that most of the targets of our survey dynamically evolved interior to ~14 AU over the past 1000 years, which is a region where several processes could trigger phase transitions of volatiles leading to outgassing. However, the apparent inactivity of inspected Centaurs, including objects with perihelia near Jupiter, indicates they are either dormant or volatile-depleted, supported by the fact that apart from C/2014 OG392 their orbital history doesn’t show significant a-jumps typical for active Centaurs, adding further evidence that the orbital evolution of Centaurs is the key to understanding the cometary activity in members of the population.