Visible and near-IR observations of transneptunian objects - Results from ESO and Calar Alto Telescopes

We present visible (BVRI) and near-IR (JHK(s)) broadband photometry and visible low-dispersion spectroscopy of Transneptunian Objects (TNOs) and Centaurs. In total, 16 TNOs and 1 Centaur were observed over the past two years at ESO telescopes in La Silla and Paranal in Chile as well as at the Calar Alto Observatory in Spain. The sample consists of objects measured for the first time and those for which comparison data is available from literature. The targets were: 1992QB1, 1993RO, 1994EV3, 1995HM5, 1995SM55, 1996RQ20, 1996TL66, 1996TO66, 1996TP66, 1997CQ29, 1997CS29, 1998HK151, 1998TF35, 1998VG44, 1998WH24, 1998XY95, 1999TC36. The spectra of 5 TNOs (1995SM55, 1996TO66, 1997CQ29, 1997CS29, 1998HK151) show almost constant gradients over the visible wavelength range with only marginal indication for a flatter slope beyond 750-800 nm. The photometric colour gradients obtained quasi-simultaneously are in good agreement with the spectral data. This suggests that in general photometric colour gradients are a valuable diagnostic tool for spectral type classification of TNOs. The photometric study revealed a number of new objects with neutral and red colours. For re-measured objects the published broadband colours were - in general -confirmed, although a few remarkable exceptions exist. Two TNOs appear to be outlyers according to the available broadband colours: 1993EV3 and 1995HM5. 1995SM55 is the bluest TNO measured so far. No clear global correlation between V - I colour and absolute R filter brightness of our TNO targets is found. However, the data for the 5 brightest TNOs (brighter than 5 mag absolute magnitude) could also be interpreted with a linear increase of V - I colour by about 0.75 mag per brightness magnitude. The colour-colour diagrams show continuous reddening of the TNOs in V - R vs. B - V, R - I vs. B - V and R - I vs. V - R. The bimodality suggested from earlier measurements of Tegler & Romanishin (1998) is not confirmed. According to our colour gradient statistics (number of objects per gradient interval) most of the TNOs have surface reddening between 0 and 40%/100 nm. For the Cubewanos the major population falls between 20-40%/100 nm. The Plutinos and Centaurs show a bifold grouping, i.e. a neutral/slightly reddish group (reddening <20%/100 nm) and a red group (reddening 30-40%/100 nm). The statistical significance of the various populations found is suffering - for the Centaurs and scattered disk objects very severely - from the small number of objects measured. However, the diversity of the reddening distribution of Centaurs/Plutinos and Cubewanos, if confirmed by new observations, may indicate a different balancing of resurfacing processes for these object types: for instance, for Centaurs a possibility is that re-condensed frost from coma activity may be dominant over impact re-surfacing and high energy surface processing.