Triaxial equilibrium ellipsoids among the asteroids

A physical model is presented to explain the existence of a class of large-lightcurve-amplitude, rapidly rotating asteroids found most commonly among objects in the size range 100-300 km diameter. A significant correlation between rotation period and lightcurve amplitude exists for asteroids in this size range in the sense that those with larger amplitudes spin more rapidly and hence these objects have high rotational angular momenta. Since this is a property of Jacobi ellipsoids, the question of whether these asteroids might be examples of triaxial equilibrium ellipsoids has been investigated. It is found that objects rotating with periods of 6 hr must have densities between 1.1 and 1.4 g/cu cm, while those rotating in 4 hr would have densities between 2.4 and 3.2 g/cu cm. If this model is valid then at least some of these asteroids have rather low mean densities. The reality of this result and its interpretation in terms of collisional evolution of the asteroids is discussed.