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 = 0.52± 0.04, which is significantly redder than the Sun. We determined the dust activity parameter 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 kA.