67P/Churyumov-Gerasimenko at large heliocentric distance

Abstract

Aims. The Jupiter family comet (JFC) 67P/Churyumov-Gerasimenko (67P/C-G) is the target of ESA's ROSETTA mission. Observational campaigns and theoretical investigations were performed to characterise 67P/C-G in terms of nucleus properties (e.g. size, surface colours, rotational period), gas and dust production, and plasma environment in preparation for the rendezvous with the spacecraft; however, there are still open questions that need to be addressed. Our observations of 67P/C-G are important not only for a good planning of the rendezvous of the ROSETTA spacecraft with 67P/C-G, but also for providing valuable information on the basic physical properties of the nuclei of JFCs at large heliocentric distances. Moreover, this information will help to identify to what extent 67P/C-G is a typical JFC. Methods. We performed broad-band imaging and low-resolution spectroscopy of 67P/C-G in the visible wavelength range during five periods between April 2004 and July 2007 at the ESO Very Large Telescope (VLT) with the FORS2 instrument. At the time of the observations 67P/C-G was at heliocentric distance r > 4.6 AU. The imaging data were used to search for a faint coma, to improve the phase function of the nucleus, to constrain its rotational period, and to analyse the neck-line of dust close to the nucleus. Results. The comet appears point-like and no coma signature was found around the nucleus. The most realistic representative of the phase function of 67P/C-G is the linear approximation: This could be interpreted to mean that the opposition effect is not very pronounced for 67/C-G in the phase angle range between 0.5° and 10°. We determined that the magnitude dependence on the phase angle is very steep, with the linear phase coefficient in the range β = 0.061-0.076 mag/°. The colour indices and reflectance spectrum show that the nucleus of 67P/C-G is slightly redder than the Sun (spectral slope ∼11%/1000 A) and uniform with the rotational phase. A tail-like structure of heavy grains is detected in April 2004, June 2004, and May 2006. Based on its orientation and the variations of its surface brightness we interpreted it as a neck-line phenomenon.