Collisional family structure within the Nysa–Polana complex

Abstract The Nysa–Polana complex is a group of low-inclination asteroid families in the inner main belt, bounded in semimajor axis by the Mars-crossing region and the Jupiter 3:1 mean-motion resonance. This group is important as the most likely source region for the target of the OSIRIS-REx mission, (101955) Bennu; however, family membership in the region is complicated by the presence of several dynamically overlapping families with a range of surface reflectance properties. The large S-type structure in the region appears to be associated with the parent body (135) Hertha, and displays an ( e P , a P ) correlation consistent with a collision event near true anomaly of ∼180° with ejecta velocity v ej ∼ 285 m / s . The ejecta distribution from a collision with these orbital properties is predicted to have a maximum semimajor axis dispersion of δ a ej = 0.005 ± 0.008 AU , which constitutes only a small fraction (7%) of the observed semimajor axis dispersion, the rest of which is attributed to the Yarkovsky effect. The age of the family is inferred from the Yarkovsky dispersion to be 300 - 50 + 60  My. Objects in a smaller cluster that overlaps the large Hertha family in proper orbital element space have reflectance properties more consistent with the X-type (135) Hertha than the surrounding S-type family. These objects form a distinct Yarkovsky “V” signature in ( a P , H ) space, consistent with a more recent collision, which appears to also be dynamically connected to (135) Hertha. Production of two families with different reflectance properties from a single parent could result from the partial differentiation of the parent, shock darkening effects, or other causes. The Nysa–Polana complex also contains a low-albedo family associated with (142) Polana (called “New Polana” by Walsh et al. (Walsh, K.J. et al. [2013]. Icarus 225, 283–297)), and two other low-albedo families associated with (495) Eulalia. The second Eulalia family may be a high- a P , low- e P , low- i P component of the first Eulalia family-forming collision, possibly explained by an anisotropic ejection field.