Spectroscopic observations of Edgeworth-Kuiper Belt (EKB) objects

To increase our knowledge of the physical properties of the Edgeworth-Kuiper Belt (EKB) objects, we started an observational program at the European Southern Observatory with the 3.5 m NTT. The aim of our observations is to obtain UBVRI colors for a large sample to examine the spectral diversity of the population and identify candidate objects for higher resolution studies of the surface composition, to determine precise magnitudes from which diameter estimates can be made, and to measure their lightcurves from which rotation rates and shape constraints can be derived. To minimize systematic errors in the colors caused by rotational lightcurve variations, we interpolated observations through other filters with multiple observations through the same filter so that interpolation can be performed (e.g. V-B-V-R-V-I-V). The first results concern six objects (1994 TB, 1994 JR1, 1995 QY9, 1996 TL66,1996 TP66, and 1996 TO66). Two of them (1996 TL66 and 1996 TO66) have a spectrum similar to those of dark D-type asteroids, while the other have redder spectra. We analysed all the BVRI color data available in the literature (about 20 EKB objects). The data tend to be noisy, due to their faint apparent magnitudes, but there are discrepancies in the results obtained by different observers of the same object that exceed the stated errors by significant amounts, which could be due to contamination by faint background sources. When multiple observations of the same objects were available, we adopted the maximum semi-dispersion for the error bar. Looking for groupings, we did not find any cluster of objects in either the three-dimensional (B-V, V-R, R-I) space or in the projected two-color planes, though considerable spectral diversity appears to exist.