Rotational properties of outer belt asteroids

We present new photometric observations of 13 asteroids (153, 190, 225, 279, 334, 420, 483, 528, 588, 692, 721, 940, 1583) orbiting in the outer part of the belt, beyond the Hecuba gap at 3.3 AU from the Sun. The resulting periods and amplitudes are then used to compare the properties of a main-belt asteroid sample and an outer-belt sample at similar sizes. The outer-belt sample includes objects which are likely to have been affected less by collisional evolution and to have retained a better record of their "primordial" features. The distributions of rotational periods are not different at a statistically significant level; however, the outer-belt sample does not include the "population" of show rotators which make the main-belt distribution markedly non-Maxwellian. As already pointed out by L. M. French (1987 Icarus 72,325) and W. K. Hartmann and coworkers (1988, Icarus 73, 487) for Trojans and Hildas, amplitudes in the outer belt are clearly higher than in the main belt. We suggest that elongated shapes might have been frequent among planetesimals in the outer belt, as a consequence of smaller impact velocities and lower densities than for the present main-belt asteroids.