Optical and infrared observations of the Centaur 1997 CU26

Minor planet 1997 CU26 is a Centaur, and is probably undergoing dynamical evolution inwards from the Kuiper Belt. We present optical and infrared (VRIJHK) photometry which gives mean colours of V - R = 0.46 +/- 0.02, V - I = 1.02 +/- 0.02, V - J = 1.74 +/- 0.02, V - H = 2.15 +/- 0.02 and V - K = 2.25 +/- 0.02. The resulting relative reflectance spectrum lies between those of Chiron and Pholus (although closer to that of Chiron). A 1.6-2.6 mu m spectrum confirms the broad absorption feature at 2.05 mu m associated with water ice reported by Brown et al. 1997 CU26 displays no significant light curve variation and (unlike Chiron) has no observable coma. We place an upper limit to the dust production rate of 1.5 kg s(-1). J-band data taken at phase angles of 1.degrees 7 to 4.degrees 0 give a phase parameter of G(J) = 0.36 +/- 0.1, and are consistent with a phase parameter of G = 0.15 in the V band (a value often assigned to low-albedo objects when no other information is available) if we assume a phase reddening of 0.017 mag deg(-1) in the J band. We find V(1; alpha = 4.degrees 1) = 7.022 +/- 0.02, from which we deduce, by assuming G = 0.15 +/- 0.1, an absolute visual magnitude of H-V = 6.64 +/- 0.04.