Properties of the nuclei and comae of 13 ecliptic comets from Hubble Space Telescope snapshot observations

Abstract

Context. Knowledge of the size distribution of cometary nuclei and, more generally, of their physical properties is important for constraining models of the formation and evolution of the Solar System. Aims. We report on our on-going effort to determine the ensemble properties of comets based on our success in detecting the nuclei of active comets with the Hubble Space Telescope (HST). Methods. During cycle 8 (July 1999 to June 2000), we observed 13 ecliptic comets with the planetary camera 2 through at least two filters (V and R) and up to four (B, V, R, I) for the brightest ones. The ∼30 min of HST observational time devoted to each comet did not permit a proper determination of light curves, so our "snapshot" observations yield effective radii, not shapes. In our present sample, eight out of thirteen nuclei have sub-kilometer radii. The average color of the observed nuclei is (V − R) = 0.52 ± 0.04, which is significantly redder than the Sun. We determined the dust activity parameter A f ρ of their coma in the R band, and estimated the dust production rates. The average reflectivity gradient of the dust comae of six comets is S [670, 792 nm] = 15.2 ± 2.3% per kÅ.

67P/Churyumov-Gerasimenko at large heliocentric distance

“…Moreover, the nuclear colours we determined are slightly redder than the mean colours determined for ecliptic comets: <V − R > = 0.42 mag, <R − I > = 0.38 mag (Lamy et al 2009). The absence of absorption or emission features in the spectra and a fairly constant spectral slope agrees with the results of Luu (1993), who obtained visible spectra of a small number of inactive comets.…”

“…The colour indices show that the nucleus of 67P/C-G is slightly redder than the Sun as expected for cometary nuclei (Lamy et al 2009;Snodgrass et al 2011). Moreover, the nuclear colours we determined are slightly redder than the mean colours determined for ecliptic comets: <V − R > = 0.42 mag, <R − I > = 0.38 mag (Lamy et al 2009).…”

“…In July 2007, thanks to observations also in the B Bessell filter, we determined the B − R colour index as well (see Table 3). The colour indices show that the nucleus of 67P/C-G is slightly redder than the Sun ((V −R) = 0.36 mag, (R−I) = 0.33 mag, (B−R) = 1.03 mag), as expected for cometary nuclei (Lamy et al 2009;Snodgrass et al 2008). V, I, and B filter images of the comet were obtained at different rotational phases, thus the resulting colour indices sample different regions of the nucleus surface.…”

“…Moreover, this information will help to identify to what extent 67P/C-G is a typical Jupiter family comet (JFC), comparing our results (e.g. rotational period, phase function, nucleus colors) with the average values determined by Lamy et al (2009) and Snodgrass et al (2011) for JFCs.…”

The dust coma environment of the short period comets 32P/Comas Solá, 56P/Slaughter-Burnham, and 78P/Gehrels 2 from ground-based observations

“…Colour indexes for target comets, integrated in the innermost 1.4 × 10 4 km to minimize effects of the sky background. (Fink et al 1997, Cartwright et al 1997, Epifani et al 1999, 50P/Arend Weissman 2003, Lamy et al 2009), 59P/Kearns-Kwee (Lamy et al 2009), 63P/Wild 1 (Lamy et al 2009), 67P/ChuryumovGerasimenko (Schulz et al 2004, Lara et al 2005 the colours obtained at the optical aperture of radius ρ = 7.2 × 10 3 km (i.e., in the innermost 1.4 × 10 4 km), chosen to minimize the effects of the sky background. This aperture is also suitable for taking most of the coma flux for the faintest comet (56P/Slaughter-Burnham) into account in the faintest filter (B filter), to allow a straightforward analysis of coma colours 1 .…”