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Astron. Astrophys. 357, L53-L56 (2000)

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1. Introduction

The Centaurs are minor planets with orbits having semi-major axes between those of Jupiter and Neptune. Their orbits are unstable, with dynamical lifetimes measured in millions of years. Long-term orbital integrations (Stern 1995, Duncan et al. 1995, and Stern & Campins 1996) of the Trans-Neptunians Objects (TNOs) indicate that perturbations by giant planets or mutual collisions can change the orbits in such a way that they become Centaur-like. It is therefore believed that Centaurs originate in the Kuiper Belt. Furthermore, this population can be a source of short period comets (Levison & Duncan 1997).

To date, 17 objects of this particular population have been discovered, following the continuously updated list from the Minor Planet Center (Marsden 2000). Though a strict definition of this class does not exist, Jewitt & Kalas (1998) added to the list of Centaurs as defined in Marsden (2000) two comets: P/Schwassmann-Wachmann 1 and P/Oterma. Jedicke & Herron (1997) estimated that as many as two thousand Centaurs exist with sizes intermediate between comets and 2060 Chiron. This implies that the Centaur population can be as important, or even larger, than the Main Belt asteroid population in the same size range.

Although this new class of objects, the Centaurs, is interesting in its own right, it is also interesting because of its origin in the Kuiper Belt. As Centaurs are currently much closer to the Sun than TNOs, they are brighter than the brightest TNOs and can therefore be more easily studied. However, most of them are still quite faint in absolute terms, so considerable dedication is required to obtain information on their physical properties. Little work has been done so far to reveal their compositions, colors, shapes, and rotational properties. Reviews on the subject can be found in Davies et al. (1998), and Davies (1999). Concerning the surface composition, Luu & Jewitt (1996) were the first to show evidence for a wide dispersion in the colors of the Centaurs, very similar to what is found for TNOs. Barucci et al (1999) observed 5 Centaurs by visible spectroscopy and they also confirmed a diversity among the reflectances of the observed objects. They obtained spectra distributed between very flat for 2060 Chiron and very red for 7066 Nessus, which appears, in the visible region, to be nearly as red as 5145 Pholus, the reddest object in the Solar System so far. They did not find however any correlation between colors and perihelion distance, and there is currently no satisfactory explanation for such a color diversity.

Ices have been identified at the surface of Centaurs. Water ice has been detected on 1997 CU26 (now named 10199 Chariklo) (Brown & Koresko 1998, Brown et al 1998, and McBride et al 1999), on 5145 Pholus (Cruikshank et al. 1998), and on 2060 Chiron (Foster et al. 2000, Luu et al. 2000). Methanol ice, or another light hydrocarbon ice, has also been identified on 5145 Pholus (Cruikshank et al. 1998). Chiron is the only object of this group (official list of the Minor Planet Center) which has shown cometary activity (see, e.g., Meech et al. 1990). But other Centaurs, like 8405 Asbolus and 5145 Pholus, that come as close to the Sun as 2060 Chiron, or even closer, have not shown any sign of activity.

Since the TNOs represent the most primordial material of our solar system, and since dynamical links between TNOs and Centaurs have been established, we started an observational campaign at the ESO-Very Large Telescope (VLT) in Chile (Mount Paranal) to study the surface composition of these objects by near-infrared spectroscopy. The observations reported here were made during the first observing period available at the VLT with the Antu telescope (the first 8 meter telescope of the VLT built on Mount Paranal). Spectra of one of the brightest Centaurs, 1995 GO, now named 8405 Asbolus, were recorded in the J, K and H spectral ranges with the infrared spectrometer ISAAC. 8405 Asbolus has a highly elliptical orbit with a perihelion inside Saturn's orbit, and an aphelion slightly inside Neptune's orbit. Its main orbital characteristics, size and rotational period are listed in Table 1. The diameter of 8405 Absolus listed in Table 1 has been estimated by Jewitt & Kalas (1998) assuming that its albedo is 0.04, by analogy with 5145 Pholus, 1997 CU26, and cometary nuclei measured albedos (see Davies 2000).


Table 1. Orbital and physical characteristics of 8405 Asbolus

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000