The Zelima asteroid family: Resonant configuration and rotational fission clusters

Abstract

Asteroid families are groups of asteroids that are identifiable in domains of proper elements. They may be the outcome of a collision, or the product of rotational fission. Among collisional families, young asteroid families are of interest because they preserve information on the original ejection velocity field with which the asteroid fragments were expelled, and because they may host a significant population of clusters produced by rotational fission among their members. The recently identified Zelima asteroid family is unique because, apart from its very young age of less than 3 Myr, is also interacting with the z1 secular resonance, making it the third asteroid family in this peculiar configuration, after the Agnia and Padua families, and the only one for which all members are in z1 librating states. Because of its extremely young age, the orbits of its members are still very clustered in the domain (σ,dσ/dt), with σ=ϖ−ϖ6+Ω−Ω6 the resonant argument of the z1 resonance. This allow for precisely dating the family with a method not available for any other main belt group. The Zelima family, based on this argument, should be 2.5±1.5 Myr old, and conserved quantities of the z1 secular resonance show that its ejection velocity field is in a range of 1.1−0.2 +0.6 of the escape velocity from the parent body, in agreement with general results for cratering collisional families. Finally, 41.7% of the members of the Zelima family are also members of possible fission clusters, confirming the trend observed for other very young families, like the Jones and Lorre groups.