Millimetre continuum observations of comet C/2009 P1 (Garradd)

TL;DR

This study used millimetre observations to set an upper limit on comet C/2009 P1 (Garradd)'s nucleus size and activity, providing insights into its thermal properties and activity level.

Contribution

First millimetre thermal emission measurements of comet Garradd, establishing upper limits on nucleus size and activity using the Plateau de Bure Interferometer.

Findings

Upper limit of 5.6 km for nucleus radius

Garradd's nucleus was over 50% active

Dust contribution to millimetric emission is negligible

Abstract

Little is known about the physical properties of the nuclei of Oort cloud comets. Measuring the thermal emission of a nucleus is one of the few means for deriving its size and constraining some of its thermal properties. We attempted to measure the nucleus size of the Oort cloud comet C/2009 P1 (Garradd). We used the Plateau de Bure Interferometer to measure the millimetric thermal emission of this comet at 157 GHz (1.9 mm) and 266 GHz (1.1 mm). Whereas the observations at 266 GHz were not usable due to bad atmospheric conditions, we derived a 3-sigma upper limit on the comet continuum emission of 0.41 mJy at 157 GHz. Using a thermal model for a spherical nucleus with standard thermal parameters, we found an upper limit of 5.6 km for the radius. The dust contribution to our signal is estimated to be negligible. Given the water production rates measured for this comet and our upper limit, we estimated that Garradd was very active, with an active fraction of its nucleus larger than 50%.