Visible spectroscopy of the new ESO large programme on?trans-Neptunian objects and Centaurs: final results

摘要

Context. A second large programme (LP) for the physicalstudies of TNOs and Centaurs, started at ESO Cerro Paranal on October2006 to obtain high-quality data, has recently been concluded. In thispaper we present the spectra of these pristine bodies obtained in thevisible range during the last two semesters(November2007-November2008) of the LP. Aims. We investigate the spectral behaviour of the TNOs andCentaurs observed, and we analyse the spectral slopes distribution ofthe full data set coming from this LP and from the literature. Methods. Spectroscopic observations in the visible range werecarried out at the UT1 (Antu) telescope using the instrument FORS2. Wecomputed the spectral slope for each observed object, and searched forpossible weak absorption features. A statistical analysis was performedon a total sample of 73 TNOs and Centaurs to look for possiblecorrelations between dynamical classes, orbital parameters, andspectral gradient. Results. We obtained new spectra for 28 bodies (10 Centaurs, 6classical, 5 resonant, 5 scattered disk, and 2 detached objects), 15 ofwhich were observed for the first time. All the new presented spectraare featureless, including 2003 AZ84, for which a faint and broadabsorption band possibly attributed to hydrated silicates on itssurface has been reported. The data confirm a wide variety of spectralbehaviours, with neutral-grey to very red gradients. An analysis of thespectral slopes available from this LP and in the literature for atotal sample of 73 Centaurs and TNOs shows that there is a lack of veryred objects in the classical population. We present the results of thestatistical analysis of the spectral slope distribution versus orbitalparameters. In particular, we confirm a strong anticorrelation betweenspectral slope and orbital inclination for the classical population.Nevertheless, we do not observe a change in the slope distribution at ,the boundary between the dynamically hot and cold populations, but we find that objects with show no correlation between spectral slope and inclination, as hasalready been noticed on the colour-inclination relation for classicalTNOs. A strong correlation is also found between the spectral slope andorbital eccentricity for resonant TNOs, with objects having higherspectral slope values with increasing eccentricity. Key words: methods: observational - methods: statistical - techniques: spectroscopic - Kuiper Belt