Interferometric mapping of the 3.3-mm continuum emission of comet 17P/Holmes after its 2007 outburst^⋆,⋆⋆

¹ Istituto di Radioastronomia, INAF, via Gobetti, 101 Bologna, Italy
e-amil: boissier@ira.inaf.it
² ESO, Karl Schwarzschild St. 2, 85748 Garching, Muenchen, Germany
³ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France

Received: 9 February 2012
Accepted: 12 April 2012

Abstract

Context. Comet 17P/Holmes underwent a dramatic outburst in October 2007, caused by the sudden fragmentation of its nucleus and the production of a large quantity of grains scattering sunlight.

Aims. We report on 90 GHz continuum observations carried out with the IRAM Plateau de Bure interferometer on 27.1 and 28.2 October 2007 UT, i.e., 4−5 days after the outburst. These observations probed the thermal radiation of large dust particles, and therefore provide the best constraints on the mass in the ejecta debris.

Methods. The thermal emission of the debris was modelled and coupled to a time-dependent description of their expansion after the outburst. The analysis was performed in the Fourier plane. Visibilities were computed for the two observing dates and compared to the data to measure their velocity and mass. Optical data and 250-GHz continuum measurements published in the literature were used to further constrain the dust kinematics and size distribution.

Results. Two distinct dust components in terms of kinematic properties are identified in the data. The large-velocity component, with typical velocities V₀ of 50−100 m s^-1 for 1 mm particles, displays a steep size distribution with a size index estimated to q = −3.7 (±0.1), assuming a minimum grain size of 0.1 μm. It corresponds to the fast expanding shell observed in optical images. The slowly-moving “core” component (V₀ = 7−9 m s^-1) detected near the nucleus has a size index |q|  < 3.4 and contains a higher proportion of large particles than the shell. The dust mass in the core is in the range 0.1−1 that of the shell. Using optical constants pertaining to porous grains (50% porosity) made of astronomical silicates mixed with water ice (48% in mass), the total dust mass M_dust injected by the outburst is estimated to 4−14  ×  10¹¹ kg, corresponding to 3−9% the nucleus mass.