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Astron. Astrophys. 349, 649-659 (1999)

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1. Introduction

An understanding of the physical properties of cometary nuclei and their size distribution is important to constrain early solar system models. Unfortunately cometary nuclei are extremely difficult to observe. They can only be observed 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 whose scattering cross-section dominates the observed flux at small heliocentric distances. Nucleus size estimates have continued to decrease as more effective techniques emerge which are able to quantify the coma contamination. With the exception of the Centaur (2060) Chiron and C/1995 O1 (Hale-Bopp), modern measurements of nuclear radii range from 0.3 km [FORMULA] rN [FORMULA] 11.8 km (Meech 1998and references therein; Mueller 1992; Hainaut et al. 1998).

From recent work it is evident that comets exhibit coma activity at larger heliocentric distances than previously believed (Meech 1994). For example C/1995 O1 (Hale-Bopp) possessed a dust coma at Rh [FORMULA] 13.1 AU (McNaught & Cass 1995). Also Chiron was seen to display considerable activity near aphelion (at Rh [FORMULA] 17.8-18.8 AU) using pre-discovery photographic plate images taken between 1969-1977 (Bus et al. 1998). In fact this aphelion activity substantially exceeded, both in degree and duration, the observed activity as Chiron approached its most recent perihelion passage. Such observations have important implications. For example if comets are active throughout their entire orbit it would mean that their nuclei are being continuously resurfaced. The observed diversity of nucleus colours (Luu 1993) could be a result of this. Also if the total mass loss rate per orbit is underestimated then the total lifetimes of cometary nuclei would tend to be overestimated.

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. The largest study of cometary activity to date was by A'Hearn et al. (1995), but in this study only [FORMULA] 3% of the observations were at a heliocentric distance [FORMULA] 3 AU. We describe in this paper observations of distant comets that were obtained in order to increase our knowledge of activity in this region.

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© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999