Main-Belt Comet P/2008 R1 (Garradd)

TL;DR

This study characterizes main-belt comet P/2008 R1 (Garradd), revealing its physical and dynamical properties, suggesting it is an icy asteroid from the outer belt with recent sublimation activity triggered by orbital changes.

Contribution

First detailed physical and dynamical analysis of P/2008 R1, demonstrating its recent origin from the asteroid belt and its sublimation-driven activity.

Findings

Effective nucleus radius < 0.7 km

Dust coma fading observed over time

Mass loss rate < 1.5 kg/s

Abstract

We present a study of the newly-discovered main-belt comet P/2008 R1 (Garradd), an object with the dynamical characteristics of an asteroid and the physical characteristics of a comet. Photometry sets a limit to the effective radius of the nucleus at r_e < 0.7 km (red geometric albedo 0.05 assumed). The coma shows a secular fading in our data caused by the escape of dust particles from the near-nucleus environment. The optical reflection spectrum is a nearly neutral continuum devoid of gaseous emission lines, from which we derive a limit to the cyanide (CN) radical production rate of Q_CN <1.4e23/s and infer a mass loss rate <1.5 kg/s at the time of our observations. Unlike the first-reported main-belt comets, P/2008 R1 is not dynamically stable. The nearby 8:3 mean-motion resonance with Jupiter induces dynamical instability on timescales 20 to 30 Myr. Hence, we conclude that P/2008 R1 has recently arrived from a more stable source elsewhere. The high Tisserand parameter of the orbit (in fact, with T_J = 3.216 it is the highest of any comet) points to a source in the asteroid belt itself, instead of in the Kuiper belt (putative source of the Jupiter family comets). We infer that P/2008 R1 is an icy body from the outer asteroid belt in which sublimation has been triggered by rising temperatures resulting from a decreasing perihelion distance.