Nucleus of the active Centaur C/2011 P2 (PANSTARRS)

Aims. In this paper we present observations of the active Centaur C/2011 P2 (PANSTARRS), showing a compact comet-like coma at the heliocentric distance of rh= 9 au. The observations were obtained in the framework of a wider program on Centaurs aimed at searching for comet-like activity in several targets outside Jupiter's aphelion. Methods. We analysed visible images of the Centaur taken at the TNG telescope in the R filter to investigate the level of coma contributing to the target brightness and to derive information on its nucleus size. Results. Centaur C/2011 P2 (PANSTARRS) shows a faint but still detectable comet-like activity, which accounts for more than 50% to the observed brightness. The coma contribution has been subtracted in order to derive an estimate for the Centaur's diameter of D ~ 16 km, assuming an albedo of A = 0.07 (average of albedo measured within the Centaur group). The results for Centaur C/2011 P2 (PANSTARRS) fit in the general picture of the group: Centaurs with smaller perihelion distance q and semi-major axis a are smaller than those remaining farther from the Sun during their orbital path, thus reinforcing the idea that active Centaurs are "comets in fieri".