ESO Large Programme on Physical Studies of Trans-Neptunian Objects and Centaurs: Visible Spectroscopy

Abstract

We present the first results from a visible spectroscopic investigation of trans-Neptunian objects (TNOs) and Centaurs, performed within an ESO Large Programme started in 2001 April to spectrophotometrically study these pristine objects in the visible and near-infrared. So far, spectra of 12 TNOs and Centaurs have been obtained using the FORS1 instrument at the Very Large Telescope. The principal preliminary results are differences in the spectral gradient—the gradients obtained indicate the existence of a range of values from moderately red to very red—and the presence of absorption features on two of the observed objects of as yet unexplained origin. The spectral gradients of the objects are also compared with photometric slopes obtained from quasi-simultaneous BVRI magnitudes of the objects (where available). An analysis of the spectral gradients with respect to the perihelion distance of the objects suggests that Centaurs (with the possible exception of 1999 OX3) occupy a zone of lower reflectance slope compared with the TNOs, probably indicating stronger resurfacing effects from cometary activity and, though less likely, collisions.