Title: Asteroid family ages Author: Federica Spoto, Andrea Milani, Zoran Knezevic

A new family classification, based on a catalog of proper elements with ~384,000 numbered asteroids and on new methods is available. For the 45 dynamical families with >250 members identified in this classification, we present an attempt to obtain statistically significant ages: we succeeded in computing ages for 37 collisional families. We used a rigorous method, including a least squares fit of the two sides of a V-shape plot in the proper semimajor axis, inverse diameter plane to determine the corresponding slopes, an advanced error model for the uncertainties of asteroid diameters, an iterative outlier rejection scheme and quality control. The best available Yarkovsky measurement was used to estimate a calibration of the Yarkovsky effect for each family. The results are presented separately for the families originated in fragmentation or cratering events, for the young, compact families and for the truncated, one-sided families. For all the computed ages the corresponding uncertainties are provided. We found 2 cases where two separate dynamical families form together a single V-shape with compatible slopes, thus indicating a single collisional event. We have also found 3 examples of dynamical families containing multiple collisional families, plus a dubious case. We have found 2 cases of families containing a conspicuous subfamily, such that it is possible to measure the slope of a distinct V-shape, thus the age of the secondary collision. We also provide data on the central gaps appearing in some families. The ages computed in this paper are obtained with a single and uniform methodology, thus the ages of different families can be compared, providing a first example of collisional chronology of the asteroid main belt.

Title: Asteroid Family Physical Properties Author: Joseph Masiero, Francesca DeMeo, Toshihiro Kasuga, Alex H. Parker

An asteroid family is typically formed when a larger parent body undergoes a catastrophic collisional disruption, and as such family members are expected to show physical properties that closely trace the composition and mineralogical evolution of the parent. Recently a number of new datasets have been released that probe the physical properties of a large number of asteroids, many of which are members of identified families. We review these data sets and the composite properties of asteroid families derived from this plethora of new data. We also discuss the limitations of the current data, and the open questions in the field.

Title: Asteroid families classification: exploiting very large data sets Author: Andrea Milani, Alberto Cellino, Zoran Knezevic, Bojan Novakovic, Federica Spoto, Paolo Paolicchi

The number of asteroids with accurately determined orbits increases fast. The catalogues of asteroid physical observations have also increased, although the number of objects is still smaller than in the orbital catalogues. 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 catalogues, with respect to classifications using also physical observations for a smaller number of asteroids. Our work is based on the large catalogue of the high accuracy synthetic proper elements (available from AstDyS). 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 catalogue, 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. By using information from absolute magnitudes, we take advantage of the larger size range in some families to analyse their shape in the proper semimajor axis vs. inverse diameter plane. This leads to a new method to estimate the family age (or ages). The results from the previous steps are then analysed, using also auxiliary information on physical properties including WISE albedos and SDSS colour indexes. This allows to solve some difficult cases of families overlapping in the proper elements space but generated by different collisional events. We analyse some examples of cratering families (Massalia, Vesta, Eunomia) which show internal structures, interpreted as multiple collisions. We also discuss why Ceres has no family.

NASA's WISE Mission Finds Lost Asteroid Family Members

Data from NASA's Wide-field Infrared Survey Explorer (WISE) have led to a new and improved family tree for asteroids in the main belt between Mars and Jupiter. Astronomers used millions of infrared snapshots from the asteroid-hunting portion of the WISE all-sky survey, called NEOWISE, to identify 28 new asteroid families. The snapshots also helped place thousands of previously hidden and uncategorised asteroids into families for the first time. The findings are a critical step in understanding the origins of asteroid families, and the collisions thought to have created these rocky clans. Read more

Title: A multi-domain approach to asteroid families identification Authors: V. Carruba, R. C. Domingos, D. Nesvorný, F. Roig, M. E. Huaman, D. Souami

Previous works have identified families halos by an analysis in proper elements domains, or by using Sloan Digital Sky Survey-Moving Object Catalogue data, fourth release (SDSS-MOC4) multi-band photometry to infer the asteroid taxonomy, or by a combination of the two methods. The limited number of asteroids for which geometric albedo was known until recently discouraged in the past the extensive use of this additional parameter, which is however of great importance in identifying an asteroid taxonomy. The new availability of geometric albedo data from the Wide-field Infrared Survey Explorer (WISE) mission for about 100,000 asteroids significantly increased the sample of objects for which such information, with some errors, is now known. In this work we proposed a new method to identify families halos in a multi-domain space composed by proper elements, SDSS-MOC4 (a*,i-z) colours, and WISE geometric albedo for the whole main belt (and the Hungaria and Cybele orbital regions). Assuming that most families were created by the breakup of an undifferentiated parent body, they are expected to be homogeneous in colours and albedo. The new method is quite effective in determining objects belonging to a family halo, with low percentages of likely interlopers, and results that are quite consistent in term of taxonomy and geometric albedo of the halo members.

Title: The Size Distributions of Asteroid Families in the SDSS Moving Object Catalog 4 Authors: Alex H. Parker, Zeljko Ivezic, Mario Juric, Robert H. Lupton, Michael D. Sekora, Adam F. Kowalski

Asteroid families, traditionally defined as clusters of objects in orbital parameter space, often have distinctive optical colours. We show that the separation of family members from background interlopers can be improved with the aid of SDSS colours as a qualifier for family membership. Based on an ~88,000 object subset of the Sloan Digital Sky Survey Moving Object Catalogue 4 with available proper orbital elements, we define 37 statistically robust asteroid families with at least 100 members using a simple Gaussian distribution model in both orbital and colour space. The interloper rejection rate based on colours is typically ~10% for a given orbital family definition, with four families that can be reliably isolated only with the aid of colours. About 50% of all objects in this data set belong to families, and this fraction varies from about 35% for objects brighter than an H magnitude of 13 and rises to 60% for objects fainter than this. The fraction of C-type objects in families decreases with increasing H magnitude for H > 13, while the fraction of S-type objects above this limit remains effectively constant. This suggests that S-type objects require a shorter timescale for equilibrating the background and family size distributions via collisional processing. The size distributions for 15 families display a well-defined change of slope and can be modelled as a "broken" double power-law. Such "broken" size distributions are twice as likely for S-type families than for C-type families, and are dominated by dynamically old families. The remaining families with size distributions that can be modelled as a single power law are dominated by young families. When size distribution requires a double power-law model, the two slopes are correlated and are steeper for S-type families.