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. Results: Assuming spherical nuclei with a geometric albedo of 0.04 for the R band (except 0.024 for 10P/Tempel 2, as independently measured) and a phase law of 0.04 mag/deg, we obtained the following effective radii: 4P/Faye: 1.77 km, 10P/Tempel 2: 5.98 km, 17P/Holmes: 1.71 km, 37P/Forbes: 0.81 km 44P/Reinmuth 2: 1.61 km, 50P/Arend: 0.95 km, 59P/Kearns-Kwee: 0.79 km, 63P/Wild 1: 1.46 km, 71P/Clark: 0.68 km, 84P/Giclas: 0.90 km, 106P/Schuster: 0.94 km, 112P/Urata-Niijima: 0.90 km, 114P/Wiseman-Skiff: 0.78 km. 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. © 2009 ESO.