Rotation properties of four L5 Trojan asteroids from CCD photometry

Abstract

CCD observations made with the Cerro Tololo 0.9-m telescope of four L5 Trojan asteroids during June and July of 1986 are presented. Partial lightcurves for 1208 Troilus and 1867 Deiphobus suggest long periods ( P > 24 hr). Complete lightcurves were obtained for two asteroids, 1173 Anchises ( P = 11.6095 ± 0.0036 hr, Δm = 0.57 ± 0.01 mag) and 2674 Pandarus ( P = 8.4803 ± 0.0019, Δm = 0.58 ± 0.01). The large lightcurve amplitudes are unique: from a sample of 182 main-belt asteroids, A. W. Harris and J. A. Burns (1979, Icarus 40, 115–144) found none in a comparable size range (50–100 km) with amplitudes greater than 0.5 mag. We suggest that the Trojans may represent a primordial asteroid population, with shapes not significantly altered since the formation of the solar system. Sufficient phase angle coverage was attained for 1173 Anchises to determine a near-opposition phase coefficient. No opposition effect was observed. The two-parameter function of E. Bowell, A. W. Harris, and K. Lumme (1985, preprint) and a linear phase coefficient both fit the data well. However, the multiple scattering parameter derived from the Bowell et al. model implies a geometric albedo of near unity, in conflict with the low radiometric albedos observed for Anchises and other Trojans ( D. P. Cruikshank, 1977, Icarus 30, 224–230). The lack of an opposition effect implies that the surface of Anchises may be significantly less rough than other asteroid suface