CCD photometry of cometary nuclei, I: Observations from 1990-1995

CCD photometry of 18 Jupiter family comets, observed at medium and large he liocentric distances, was carried out between April 1990 and July 1995. Thi s is part of a long-term observational program designed to obtain their nuc lear magnitudes. The observations were made with the 1.54-m Danish Telescop e at ESO La Shia, the 2.5-m Nordic Optical, Telescope (NOT) at Observatorio del Rogue de los Muchachos (La Palma), the 2.0-m telescope at Pic du Midi, and the 2.15-m telescope at CASLEO, Argentina. Our estimates of the absolu te nuclear magnitudes are discussed in comparison with previous determinati ons. Estimates (sometimes upper limits) for the effective radius (R) of the nuclei are computed considering a typical geometric albedo p(v) = 0.04. Th e lowest radius found is the one of 37P/Forbes (R = 1.0 km) while the large st corresponds to 65P/Gunn (R = 11.0 km), but in this case the comet was ob served very active. Wherever possible, cometary image profiles are compared with stellar profil es, in order to determine the existence of a faint coma. Seven of the comet s were active, six of them at heliocentric distances larger than 4 AU, This unexpected activity is also discussed. We find a strong correlation betwee n activity at large heliocentric distances and recent downward jumps in per ihelion distance. (C) 2000 Academic Press.