Trojan, Hilda, and Cybele asteroids: New lightcurve observations and analysis

Abstract

We present lightcurve observations for 23 Trojan, Hilda, and Cybele asteroids, representing nearly one-half of the currently published data set for these objects. Previous researchers have suggested that Trojan and Hilda asteroids display significantly higher mean lightcurve amplitudes than their comparable diameter main-belt counterparts. We present a correction procedure for the use of multiple aspect asteroid lightcurve observations and perform a quantitative bias-corrected analysis of lightcurve amplitude distributions for all published data on Trojan, Hilda, and Cybele asteroids. Lightcurve samples are now in hand for all Trojan asteroids larger than 65 km. We report a new finding that the largest Trojans ( D > 90 km) have a higher mean lightcurve amplitude than their low albedo main-belt counterparts (98% confidence level). On the other hand, smaller Trojans, plus all Hildas and Cybeles, display lightcurve properties similar to comparable main-belt objects. A possible explanation for the Trojan and main-belt difference may be that the lower relative velocities in the Trojan regions allowed more irregular aggregates to form from planetesimals. Only the largest Trojans have been able to substantially retain their initial forms after subsequent collisional evolution. Thus among the Trojans, 90 km may represent a transition size between primordial objects and collision fragments. Within the main belt, previous observations have suggested an analogous transition occurs at a size near 125 km. Energy scaling based on relative collision velocities within these two populations predicts a ratio between the Trojan and main-belt transition diameters that is similar to the value derived purely from observations.