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Astron. Astrophys. 354, 1086-1090 (2000)

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1. Introduction

Observational discoveries and theoretical developments in the last few years have suggested that there may be an evolutionary link between comets and asteroids. These populations can be considered as a continuous suite of small objects with chemical, dynamical and physical interrelations.

Asteroids and comets have a close relationship with the planetesimals which formed in the solar nebula before 4.5 Gyrs ago. These objects are considered the remnants of the primordial processes which characterized the evolution of the Solar System. Small bodies should have preserved materials which witnessed the condensation and the early phases of the formation of the Solar System.

In the last years we have started the study of the interrelations between comets and asteroids because it can give new insights into the primordial evolution of the planetary bodies. Thermal evolution models of transition elements between comets and asteroids have been developed to see under which conditions a comet nucleus can become dormant or extinct, assuming an asteroidal appearance (Coradini et al. 1997a,b). The similarity between the spectra of comet nuclei and dark asteroids has been recognized on the basis of observational results. In particular the Trojans, while classified as asteroids on the basis of their lack of a detectable coma, seem to have physical properties close to those of the cometary nuclei. Jones et al. (1990) suggest that water ice can be incorporated in the surface of the supposed volatile-rich Trojan asteroids. Broadband visible photometry (Hartmann et al. 1987; Luu & Jewitt 1996) and spectroscopy (Jewitt & Luu 1990) showed that the distributions of the optical spectra of Trojans and cometary nuclei are similar.

A spectroscopic survey in the visible and near infrared of dark primitive asteroids belonging to the classes C, P and D has been performed (Barucci et al. 1994; Lazzarin et al. 1995). The spectra obtained have been used to better understand the relation between asteroids and comets, comparing these data to the available data on cometary nuclei founding no evidence of any distinction between the two populations, even if the data on comet nuclei are poor both in number and in quality (Luu 1993).

To improve the knowledge of cometary nuclei, we started to observe comets at high heliocentric distance where the coma might be still not present. Our observational campaign on cometary nuclei, performed at ESO and CFHT (Canadian-French-Hawaiian Telescope), started with the comet P/Gehrels 3.

The comet P/Gehrels 3 has a particular orbit with very low inclination, moderate eccentricity (Table 1) and a very high Tisserand invariant with respect to Jupiter all the time. For this reason the encounters with Jupiter are very important and the comet spends part of its life as a temporary satellite of Jupiter (Carusi et al. 1996). To better understand the evolution and the status of P/Gehrels 3, we have theoretically investigated the comet status during the observations, using a thermal evolution and chemical differentiation model of cometary nuclei (Coradini et al. 1997a,b; Capria et al. 1996; De Sanctis et al. 1999). The model was applied to a cometary nucleus with the estimated physical and dynamical characteristics of P/Gehrels 3, with various sets of initial physical conditions in order to infer the status and activity level of a body on such an orbit at the observation epoch.


Table 1. Dynamical parameters

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000