Properties of the nuclei and comae of 10 ecliptic comets from Hubble Space Telescope multi‐orbit observations ★

We report on our on‐going effort to detect and characterize cometary nuclei with the Hubble Space Telescope ( HST ). During cycle 9 (2000 July to 2001 June), we performed multi‐orbit observations of 10 ecliptic comets with the Wide Field Planetary Camera 2. Nominally, eight contiguous orbits covering a time interval of ∼11 h were devoted to each comet but a few orbits were occasionally lost. In addition to the standard R band, we could additionally observe four of them in the V band and the two brightest ones in the B band. Time series photometry was used to constrain the size, shape and rotational period of the 10 nuclei. Assuming a geometric albedo of 0.04 for the R band, a linear phase law with a coefficient of 0.04 mag deg −1 and an opposition effect similar to that of comet 19P/Borrelly, we determined the following mean values of the effective radii 47P/Ashbrook–Jackson: 2.86±0.08 km, 61P/Shajn–Schaldach: 0.62±0.02 km, 70P/Kojima: 1.83±0.05 km, 74P/Smirnova–Chernykh: 2.23±0.04 km, 76P/West–Kohoutek–Ikemura: 0.30±0.02 km, 82P/Gehrels 3: 0.69±0.02 km, 86P/Wild 3: 0.41±0.03 km, 87P/Bus: 0.270.01 km, 110P/Hartley 3: 2.15±0.04 km and 147P/Kushida–Muramatsu: 0.21±0.01 km. Because of the limited time coverage (∼11 h), the rotational periods could not be accurately determined, multiple solutions were sometime found and three periods were not constrained at all. Our estimates range from ∼5 to ∼32 h. The lower limits for the ratio a / b of the semi‐axis of the equivalent spheroids range from 1.10 (70P) to 2.20 (87P). The four nuclei for which we could measure ( V − R ) are all significantly redder than the Sun, with 86P/Wild 3 ( V − R ) = 0.86 ± 0.10 appearing as an ultrared object. We finally determined the dust activity parameter Af ρ of their coma in the R band, the colour indices and the reflectivity spectra of four of them.