Asteroid families classification: Exploiting very large datasets

Abstract

The number of asteroids with accurately determined orbits increases fast, and this increase is also accelerating. The catalogs of asteroid physical observations have also increased, although the number of objects is still smaller than in the orbital catalogs. Thus it becomes more and more challenging to perform, maintain and update a classification of asteroids into families. To cope with these challenges we developed a new approach to the asteroid family classification by combining the Hierarchical Clustering Method (HCM) with a method to add new members to existing families. This procedure makes use of the much larger amount of information contained in the proper elements catalogs, with respect to classifications using also physical observations for a smaller number of asteroids.Our work is based on the large catalog of the high accuracy synthetic proper elements (available from AstDyS), containing data for > 330 000 numbered asteroids. By selecting from the catalog a much smaller number of large asteroids, we first identify a number of core families; to these we attribute the next layer of smaller objects. Then, we remove all the family members from the catalog, and reapply the HCM to the rest. This gives both halo families which extend the core families and new independent families, consisting mainly of small asteroids. These two cases are discriminated by another step of attribution of new members and by merging intersecting families. This leads to a classification with 128 families and currently 87095 members; the number of members can be increased automatically with each update of the proper elements catalog. By using information from absolute magnitudes, we take advantage of the larger size range in some families to analyze their shape in the proper semimajor axis vs. inverse diameter plane. This leads to a new method to estimate the family age, or ages in cases where we identify internal structures. The analysis of the plot above evidences some open problems but also the possibility of obtaining further information of the geometrical properties of the impact process. The results from the previous steps are then analyzed, using also auxiliary information on physical properties including WISE albedos and SDSS color indexes. This allows to solve some difficult cases of families overlapping in the proper elements space but generated by different collisional events.The families formed by one or more cratering events are found to be more numerous than previously believed because the fragments are smaller. We analyze some examples of cratering families (Massalia, Vesta, Eunomia) which show internal structures, interpreted as multiple collisions. We also discuss why Ceres has no family. 6 According to [Ivezić et al. 2001] the first principal component in the r − i vs g − r plane is defined as a * = 0.89(g − r) + 0.45(r − i) − 0.57. 7 Every asteroid is affected by chaotic effects over timescales comparable to the age of the solar system, but this does not matter for family classification.

On the current distribution of main belt objects: Constraints for evolutionary models

“…Another possibility for the predominance of volatilepoor asteroids is the breakup of a volatile-poor parent body inside the boomerang region. In fact, an S-type family, 5 Astraea, is identified in this region by Milani et al (2014). However, the family is oddly shaped, and the fact that the dynamics of the region by itself seems able to produce an accumulation of objects in this regions casts some doubts on whether Astrea is a real collisional family.…”

A Dark Asteroid Family in the Phocaea Region

“…The method of family age estimation based on V-shapes requires a Yarkovsky calibration, that is, an estimate for the maximum value of the Yarkovsky driven secular drift (da/dt) max for a hypothetical family member of diameter D = 1 km (Milani et al 2014). …”

Asteroid Family Associations of Active Asteroids

“…Of course, due to its much larger size (r e ∼ 470 km; Carry et al 2008;Park et al 2016) relative to the other objects we are considering here, the physical regime occupied by the object is certainly very different from those occupied by other active asteroids. No family has been identified for Ceres to date (e.g., Milani et al 2014;Rivkin et al 2014), although Carruba et al (2016b) proposed that Ceres family members might simply be highly dispersed and therefore undetectable by standard family identification techniques.…”

“…As ongoing surveys continue to discover more asteroids, and particularly as future surveys discover smaller and fainter asteroids than are detectable now, continued searches for tightly clustered young families should be performed (e.g., Milani et al 2014), perhaps using osculating or mean elements rather than proper elements for detecting very young clusters (e.g., Nesvorny et al 2006;Pravec & Vokrouhlický 2009;Pravec et al 2017;Rosaev & Plávalová 2017). Furthermore, as new young families are identified -especially ones associated with known MBCs, believed to contain primitive asteroids, or found in the outer main belt (i.e., a between the 5J:2A MMR at 2.8252 AU and the 2J:1A MMR at 3.2783 AU) -targeted observations or at least targeted close examination of survey data of members of these young families should be conducted to search for new MBCs.…”