Astron. Astrophys. 329, 1145-1151 (1998)

1. Introduction

In the last years theoretical studies and observational results have pointed out the presence of a class of objects that could represent a transitional state between asteroids and comets. Most of these bodies are classified as asteroids, but they can show some cometary characteristics: possible outgassing activity, cometary-type orbits, meteor-shower association, and taxonomic type consistent with a carbon-rich/primitive composition (i.e. low albedo).

From the dynamical point of view, a class of transition objects between comets and asteroids has been identified: such bodies, called "cometary asteroids", have asteroidal appearance and exhibit dynamical characteristics typical of Jupiter-family short-period comets (Harris and Bailey, 1996). Moreover, Marzari et al. (1995) have suggested that a few tens of the currently observed short-period comets may have originated in the Trojan clouds. They showed that typical family-forming Trojan collisions eject a significant percentage of the resulting fragments into unstable orbits: such unstable Trojan fragments experience close encounters with Jupiter, becoming indistinguishable from i) Jupiter-family comets, ii) comets undergoing temporary captures by Jupiter, iii) objects with Jupiter-crossing or -approaching orbits. The activity timescale of a periodic comet, computed on the basis of the mean-rate decrease of the luminosity in a century, can be about yr (150-500 revolutions; Kresák and Kresáková, 1990). The final stage of the evolution of such objects is not yet well known: the discoveries of a possible faint cometary activity in the Apollo asteroids (1566) Icarus and (2201) Oljato (McFadden, 1994), and the coincidence between asteroid (4015) 1979 VA and comet Wilson-Harrington (1949 III) (Bowell, 1992) could be the first verifications of the hypothesized phenomenon of dormant comets, showing the existence of residual sporadic cometary activity.

Among the possible asteroid-comet transition objects, the dark asteroids, classified as C, P and D type (Barucci et al., 1987; Tholen 1984; Tedesco et al., 1989) seem to play a fundamental role. In the last few years, spectroscopic and photometric observations together with theoretical analyses have led to the conclusion that outer-belt asteroids, and in particular the reddest ones (D-class objects), which dominate the Trojan region and are thought to have preserved part of their original composition, show spectral and mineralogical characteristics very similar to those of cometary nuclei (Hartmann et al., 1987; Jewitt and Luu, 1990). As proposed by Jones et al. (1990), accretion of ice and anhydrous silicates from the solar nebula, combined with relic interstellar organic kerogen, formed very dark, volatile-rich asteroids, of which low-albedo asteroids, especially D-type, may be representative. If dark and distant asteroids could be exhausted comet nuclei, some of the low-albedo asteroids may still contain water ice and organic compounds.

Information about these objects is very poor: the aim of our work is to improve the data set of these bodies and to investigate their physical and dynamical properties, in order to sort out a possible link between asteroids and comets, and explore the possible mechanisms responsible for comet-asteroid transitions. Here we present:

• analysis of the dynamical evolution of three objects [(6042) 1990 WW₂ , (6144) 1994 EQ₃ , 1995 QY₂ ], classified as asteroids and having "unusual" orbits;
• spectroscopic observations of these objects to achieve information about their mineralogy.

© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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