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

¹ Istituto di Radioastronomia – INAF, via Gobetti, 101 Bologna, Italy
e-mail: sboissier@ira.inaf.it
² ESO, Karl Schwarzschild Str. 2, 85748 Garching bei Muenchen, Germany
³ LESIA – Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁴ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁵ Institut de radioastronomie millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’ Hères, France

Received: 27 February 2013
Accepted: 23 July 2013

Abstract

Context. 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.

Aims. We attempted to measure the nucleus size of the Oort Cloud comet C/2009 P1 (Garradd).

Methods. 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).

Results. Whereas the observations at 266 GHz were not usable due to bad atmospheric conditions, we derived a 3σ 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%.