A study of Cybele asteroids ^*

II. Spectral properties of Cybele asteroids

¹ Astronomical Observatory, Box 515, 751 20 Uppsala, Sweden e-mail: Claes-Ingvar.Lagerkvist@astro.uu.se
² DLR, Optical Information Systems, Rutherfordstrasse 2, 12489 Berlin, Germany
³ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany
⁴ DLR, Institute of Planetary Research, Rutherfordstrasse 2, 12489 Berlin, Germany
⁵ Observatorio Astronomico, Alfonso XII 3, 28014 Madrid, Spain

Received: 23 April 2004
Accepted: 25 October 2004

Abstract

We have used the 3.5 m New Technology Telescope at ESO, La Silla, to obtain spectra of 18 asteroids belonging to the Cybele group. One additional Cybele asteroid was observed with the ESO 3.6 m telescope. From the spectra we have derived spectral slopes and taxonomy classifications. Our observations show that spectrally red D-type Cybeles tend to be smaller than more spectrally neutral P- and C-type objects from this group. Similar colour-diameter trends have previously been reported for other outer belt low albedo asteroids (Hildas and Trojans). We discuss possible reasons for this trend. In particular, the observed dominance of red objects for small diameters is consistent with a space weathering scenario, where irradiation of asteroid regoliths with solar wind plasma neutralizes their surface colours due to carbonization of originally reddish organic components. Collisional disruption of such large greyish “aged” P-type objects would produce a number of redder D-type fragments lacking mature regoliths. In addition, the observed colour-diameter trend may be due to compositional differences between D-, P- and C-type asteroids. P- and C-types may be lacking at small diameters, since their materials are less susceptible to collisional break-up than spectrally red D-type material. A simultaneous contribution of both factors (compositional differences and space weathering) to the observed trend is possible as well.