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 Å) 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.

Beginning of activity in 67P/Churyumov-Gerasimenko and predictions for 2014–2015

“…The dashed line shows a prediction for the total magnitude based on a water production rate model (see section 4.3), while the dotted lines show a power law fit to the data. age (V − R) = 0.53 ± 0.02 and (R − I) = 0.48 ± 0.02 (Lamy et al 2006;Tubiana et al 2008Tubiana et al , 2011Lowry et al 2012). This implies that the grains in the coma are smaller than the dominant surface grains, preferentially scattering bluer wavelengths.…”

“…Different symbols show data taken during different apparitions, and highlight 'our data' presented in section 3 (which were taken at the start of the 2009 apparition). The solid black line shows the expected magnitude of the bare nucleus, based on the absolute magnitude reported by Tubiana et al (2011). The dashed line shows a prediction for the total magnitude based on a water production rate model (see section 4.3), while the dotted lines show a power law fit to the data.…”

“…3, which also shows the predicted nuclear magnitude based on the observations of the inactive nucleus in July 2007 and measurement of the nucleus phase function by Tubiana et al (2008Tubiana et al ( , 2011. Our VLT observations were short sequences (covering only a small fraction on the comet's ∼ 12.7 hour rotation period), and we are looking for small changes in brightness (less than the ∆m ≈ 0.4 mag.…”

“…In this paper we present data taken in 2007 and 2008 around the time of the expected onset, between the inactive nucleus observations taken in 2007 (Tubiana et al 2011;Lowry et al 2012) and the active comet observations in 2008 (Tozzi et al 2011). 20 × 30 (0) R 27/3/08 3.39 3.80 14.6 3×15 + 4×60 (6) VRI 29/3/08 3.38 3.76 14.9 3×15 + 4×60 (6) VRI 30/3/08 3.38 3.74 15.0 3×15 + 4×60 (6) VRI (a) Heliocentric distance (AU).…”

“…Schulz et al (2004), Lamy et al (2006) and Lara et al (2005) obtained the first dedicated characterisation observations after it was selected, when the comet was on the outbound leg of its orbit, and still highly active, following its perihelion passage in 2002. As the comet passed through aphelion its inactive nucleus was studied by Tubiana et al (2008Tubiana et al ( , 2011 and Lowry et al (2012), before it was observed in an active state again as it returned to perihelion in 2008 (Tozzi et al 2011;Lara et al 2011). Further authors have studied the longer lived dust trail associated with the comet (Ishiguro 2008;Kelley et al 2008;Agarwal et al 2010), the dust's polarisation (Hadamcik et al 2010), the nucleus thermal properties (Kelley et al 2006;Lamy et al 2008;Kelley et al 2009), and the morphology (Vincent et al 2013) and composition (Schleicher 2006) of its coma over multiple apparitions.…”