The dust environment of comet 67P/Churyumov-Gerasimenko from Rosetta OSIRIS and VLT observations in the 4.5 to 2.9 AU heliocentric distance range inbound

¹ Instituto de Astrofísica de Andalucía, , CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: fernando@iaa.es
² Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, Buckinghamshire MK7 6AA, UK
³ European Southern Observatory, Karl-Schwarschild-Strasse 2, 85748 Garching bei München, Germany
⁴ Max-Planck Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany
⁵ Department of Physics and Astronomy G. Galilei, University of Padova, Vic. Osservatorio 3, 35122 Padova, Italy
⁶ Center of Studies and Activities for Space (CISAS) G. Colombo, University of Padova, via Venezia 15, 35131 Padova, Italy
⁷ Aix Marseille Université, CNRS, LAM (Laboratoire dAstro-physique de Marseille) UMR 7326, 13388 Marseille, France
⁸ Centro de Astrobiologia (INTA-CSIC), European Space Agency (ESA), European Space Astronomy Centre (ESAC), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁹ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁰ Research and Scientific Support Department, European Space Agency, 2201 Noordwijk, The Netherlands
¹¹ PAS Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
¹² Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
¹³ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁴ LESIA, Observatoire de Paris, CNRS, UPMC, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon Principal Cedex, France
¹⁵ LATMOS, CNRS/UVSQ/IPSL, 11 Bd d’Alembert, 78280 Guyancourt, France
¹⁶ INAF Osservatorio Astronomico di Padova, vic. dell’Osservatorio 5, 35122 Padova, Italy
¹⁷ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
¹⁸ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
¹⁹ Department of Industrial Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy
²⁰ University of Trento, via Sommarive 9, 38123 Trento, Italy
²¹ Physikalisches Institut, Sidlerstrasse 5, University of Bern, 3012 Bern, Switzerland
²² INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
²³ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstrasse 2, 12489 Berlin, Germany
²⁴ Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
²⁵ ESA/ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain
²⁶ Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy
²⁷ Astrophysics Research Centre, School of Physics and Astronomy, Queen’s University Belfast, Belfast BT7 1NN, UK
²⁸ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50 125 Firenze, Italy
²⁹ Institut d’Astrophysique et de Géophysique, Université de Liège, Sart-Tilman, 4000, Liège, Belgium
³⁰ Institute for Space Astrophysics and Planetology (IAPS), National Institute for AstroPhysics (INAF), via Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 15 October 2015
Accepted: 17 January 2016

Abstract

Context. The ESA Rosetta spacecraft, currently orbiting around comet 67P/Churyumov-Gerasimenko, has already provided in situ measurements of the dust grain properties from several instruments,particularly OSIRIS and GIADA. We propose adding value to those measurements by combining them with ground-based observations of the dust tail to monitor the overall, time-dependent dust-production rate and size distribution.

Aims. To constrain the dust grain properties, we take Rosetta OSIRIS and GIADA results into account, and combine OSIRIS data during the approach phase (from late April to early June 2014) with a large data set of ground-based images that were acquired with the ESO Very Large Telescope (VLT) from February to November 2014.

Methods. A Monte Carlo dust tail code, which has already been used to characterise the dust environments of several comets and active asteroids, has been applied to retrieve the dust parameters. Key properties of the grains (density, velocity, and size distribution) were obtained from Rosetta observations: these parameters were used as input of the code to considerably reduce the number of free parameters. In this way, the overall dust mass-loss rate and its dependence on the heliocentric distance could be obtained accurately.

Results. The dust parameters derived from the inner coma measurements by OSIRIS and GIADA and from distant imaging using VLT data are consistent, except for the power index of the size-distribution function, which is α = −3, instead of α = −2, for grains smaller than 1 mm. This is possibly linked to the presence of fluffy aggregates in the coma. The onset of cometary activity occurs at approximately 4.3 AU, with a dust production rate of 0.5 kg/s, increasing up to 15 kg/s at 2.9 AU. This implies a dust-to-gas mass ratio varying between 3.8 and 6.5 for the best-fit model when combined with water-production rates from the MIRO experiment.