Introducing the Eulalia and new Polana asteroid families: Re-assessing primitive asteroid families in the inner Main Belt

Abstract

The so-called Nysa–Polana complex of asteroids is a diverse and widespread group studied by Cellino et al. (Cellino, A., Zappalà, V., Doressoundiram, A., di Martino, M., Bendjoya, P., Dotto, E., Migliorini, F., 2001. Icarus 152, 225–237) as a dynamically linked asteroid family. It carries the name of two asteroids because it appears to be two overlapping families of different asteroid taxonomies: (44) Nysa is an E-type asteroid with the lowest number in the midst of a predominantly S-type cluster and (142) Polana is a B-type asteroid near the low-albedo B-and C-type cluster. The latter has been shown to be a very important source of primitive near Earth asteroids.Using the data from the Wide-field Infrared Survey Explorer (WISE) mission we have re-analyzed the region around the Nysa–Polana complex in the inner Main Belt, focusing on the low-albedo population. (142) Polana does not appear to be a member of the family of low-albedo asteroids in the Nysa–Polana complex. Rather, the largest is Asteroid (495) Eulalia. This asteroid has never before been linked to this complex for an important dynamical reason: it currently has a proper eccentricity slightly below the range of most of the family members. However, its orbit is very close to the 3:1 mean motion resonance with Jupiter and is in a weak secular resonance. We show that its osculating eccentricity varies widely (e=0.06–0.19) on short timescales (∼1Myr) and the averaged value diffuses (between e=0.11–0.15) over long timescales (∼100Myr). The diffusive orbit, low-albedo, taxonomic similarity and semimajor axis strongly suggests that despite its current proper eccentricity, (495) Eulalia could have recently been at an orbit very central to the family. Hierarchical Clustering Method tests confirm that at an eccentricity of e=0.15, (495) Eulalia could be the parent of the family. The “Eulalia family” was formed between 900 and 1500Myr ago, and likely resulted from the breakup of a 100 to 160km parent body.There is also compelling evidence for an older and more widespread primitive family in the same region of the asteroid belt parented by Asteroid (142) Polana. This family, the “new Polana family”, is more extended in orbital elements, and is older than 2000Myr.