Knowledge of the size distribution of the various cometary groups is vital to constrain early solar system accretion models. Determining the size distribution of the Jupiter-family comets may be the only way to probe the size distribution of the smaller Edgeworth-Kuiper belt objects too faint to be seen with current instrumentation. Although this family of comets have generally spent some time in the inner solar system, some of this population are recent arrivals and thus represent the least altered samples from the Edgeworth-Kuiper belt.
Unfortunately cometary nuclei are extremely difficult to observe. One technique
is to observe them at large heliocentric distances where the sublimation of
surface volatiles is so low that any photometric measurements made are
dominated by light reflected from the nucleus rather than from the dust coma.
Indeed, nucleus size estimates have continued to decrease as more effective
techniques emerge which are able to more accurately quantify the coma
contamination. With the exception of the Centaurs and C/1995 O1 (Hale-Bopp),
current measurements of nuclear radii range from 0.35 km
km
(Meech 2002 and references therein; Mueller 1992;
Hainaut et al. 1998; Boehnhardt et al. 1999;
Lamy et al. 1998; Lamy et al. 1999).
Few studies of distant comets have been carried out in the past. Meech &
Hainaut (1997) have an on-going long-term programme
to obtain CCD images of short and long period comets over a wide range of
heliocentric distances to compare activity levels and obtain information about
primordial and evolutionary differences between comets with different dynamical
histories. Other long-term ground based observational programs of distant comets
currently underway include those of Licandro et al. (2000)
and Fernández et al. (1999). Also, the Hubble Space Telescope
has been used to obtain high spatial resolution CCD images of comets
(Lamy & Toth 1995), and has so far proved extremely effective.
The largest study of cometary activity to date was the photoelectric photometry
performed by A'Hearn et al. (1995), but in
this study only 
3% of the observations were at
a heliocentric distance >3 AU.
We present here broad-band CCD photometry of distant Jupiter-family comets obtained using the 1m Jacobus Kapteyn Telescope (JKT) in June 1999. These observations were obtained to supplement those discussed in Lowry et al. (1999), and Lowry and Fitzsimmons (2001) (hereafter referred to as Papers I and II respectively). The observations presented here and in Papers I and II are "snapshot'' in nature. In other words, we are observing the comets at only one point in the rotational lightcurve. "Snapshot'' observations have the disadvantage of sacrificing rotational information but have the advantage that a larger number of comets can be sampled at any given time.
Section 2 briefly describes the observations, the instrumental set-up, and the photometric calibration procedure. Section 3 outlines the analysis of each cometary observation and the results obtained. In order to be consistent with Papers I and II we have divided this section, with each subsection describing separately the results and analysis of comets that were undetected, unresolved, and active. In Sect. 4 we discuss in more detail the results obtained, as well as comparing our results with those of previous investigations. In Sect. 5 we have combined the results obtained here with those presented in Papers I and II in an attempt to describe the ensemble properties of the Jupiter-family comet population. A brief summary and some concluding remarks are provided in Sect. 6.
Copyright ESO 2003