CCD photometry of distant comets. III. Ensemble properties of Jupiter-family comets

We describe the results of a ground-based observational "snapshot" study of Jupiter-family comets in the heliocentric range 2.29 AU < Rh < 5.72 AU. Results are presented based on observations from the 1m JKT on the island of La Palma. A total of 25 comets were targeted with 15 being positively detected. Broad-band VRI photometry was performed to determine dimensions, colour indices, and dust production rates in terms of the "Af-rho" formalism. The results for selected comets are compared with previous investigations. Ensemble properties of the Jupiter-family population have been investigated by combining the results presented here with those of Lowry et al. (1999), and Lowry &amp; Fitzsimmons (2001). We find that the cumulative size distribution of the Jupiter-family comets can be described by a power law of the form: ZIGMA(> r) 'propto' r^(-1.6 ± 0.1). This size distribution is considerably shallower than that found for the observed Edgeworth-Kuiper belt objects, which may reflect either an intrinsic difference at small km-sizes in the belt, or the various processes affecting the nuclei of comets as their orbits evolve from the Edgeworth-Kuiper belt to the inner Solar system. Also, there would appear to be no correlation between nuclear absolute magnitude and perihelion distance. Finally, for the sample of active comets, there is a distinct correlation between absolute R band magnitude and perihelion distance, which can be explained by either a discovery bias towards brighter comets or in terms of "rubble" mantle formation.