Photometry and taxonomy of trans-Neptunian objects and Centaurs in support of aHerschelkey program

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Context. The investigation of Centaurs and trans-Neptunian objects (TNOs) provides essential information about the early conditions and evolution of the outer solar system. The radiometric technique combines measurements in the visible and thermal infrared; with these one can estimate the size and albedo of Centaurs and TNOs. Aims. Our aim is to obtain visible photometry of a sample of Centaurs and TNOs, a subset of the targets of the “TNOs are cool” key program at the Herschel Space Observatory. Methods. We carried out visible photometry of Centaurs and TNOs, making use of the DOLORES instrument at the Telescopio Nazionale Galileo (TNG, La Palma, Spain). Results. We report photometric observations of 20 objects and present the computed absolute magnitudes. We derive the taxonomy of our targets (nine are classified for the first time, the results for five objects agree with the literature, the other targets are tentatively classified based on incomplete datasets) and combine the results with the literature, searching for correlations between taxonomy and dynamics. We look for comet-like activity in our Centaur sample, including (248835) 2006 SX368, which was previously described as active. Conclusions. We provide an accurate determination of the absolute magnitude and of the relative error for each of our targets. These values can be readily used in combination with thermal infrared data. The surface of TNO (65489) Ceto seems to be heterogeneous. Our results seem to support an evolutionary origin for the color dichotomy of Centaurs, and the occurrence of a strong mixing after the TNO formation. No evident cometary activity is detected around the five Centaurs in our sample; assuming that an unresolved coma is present around (248835) 2006 SX368, we use the “photometric model” to derive the possible dust production rate, finding that Qdust is in the range 1–31 kg/s.