Rotational properties of Centaurs and Trans-Neptunian objects. Lightcurves and densities

Context. The knowledge of the rotational periods of the small bodies in the outer Solar System is a useful tool for retrieving information on the internal structure of the observed objects and for having hints on the collisional evolution state of the whole population. Aims. In order to investigate the physical nature of Centaurs and TNOs, we analysed the rotational properties of a selected sample. Methods. Photometric observations of 2 Centaurs (12929 1999 TZ1 and 95626 2002 GZ32 ), and 5 Trans-Neptunian Objects (42355 Typhon, 47932 2000 GN171 , 65489 Ceto, 90568 2004 GV9 , and 120132 2003 FY128 ) were carried out with the New Technology Telescope (NTT) at the European Southern Observatory La Silla (Chile) in the framework of the ESO Large Programme 178.C-0036 (PI. M.A. Barucci). Results. These observations allow us to estimate the rotational rate of three objects, to confirm previously published periods of two bodies, and to have the first single night lightcurves of 42355 Typhon and 120132 2003 FY128 . Conclusions. These data allow us to improve the available sample of determined rotational periods of TNOs and Centaurs. For 5 out of the 7 observed objects we have estimated the axis ratio a/b, hence the density. Our new results seem to confirm the density/dimension trend, found by Sheppard et al. (2008, in The Solar System Beyond Neptune (Univ. Arizona press), 129), with larger (brighter) objects having higher densities.