Families

1. Families What can we learn from them?

2. 20th Century Families

3. 21th Century Families

4. 20th Century Families: General Ideas Families exist and can be reliably identified Families have a collisional origin Family members dominate the asteroid inventory at small sizes The original ejection velocity fields can be reconstructed Families ejected collisional fragments at high velocities Families can be (or have been) important sources of NEOs Families trigger the collisional evolution of the whole population The original parent bodies were not differentiated Families have anisotropic structures

5. Major Contributors Zappalà et al., Bendjoya et al., Williams, Lindblad (identification) Zappalà et al. 1996 (reconstruction velocity fields) Tanga et al. 1999, Campo Bagatin and Petit (size distribution) Cellino et al. 1999 (size-velocity relation) Dell’Oro et al. 2000 (Role of families in triggering collision probability) Bus, Lazzaro and many authors (spectroscopic studies) Morbidelli et al. 1995, Zappalà et al. 1998; 2000; Gladman et al. 1997 (effects of nearby resonances)

6. Families’ size distributions are steeper than Dohnanyi equilibrium slope. Background objects’ size distributions are shallower than Dohanyi’s value

7. Size-velocity relation interpreted in terms of a maximum possible amount of kinetic energy delivered to fragments

8. The Maria Family

9. 21th Century Families: General Ideas Families exist and can be reliably identified Families have a collisional origin Family members do not dominate the asteroid inventory Families did noteject collisional fragments at high velocities Families have been strongly modified by evolutionary mechanisms The original ejection velocity fields can hardly be reconstructed Family ages can be evaluated Family members are mostly reaccumulated Most original parent bodies were not differentiated Family members may show a preferential spin axis alignement

10. Major Contributors “The Yarkovsky team” (Bottke, Vokhroulickí, Morbidelli, Nesvorný, Broz, et al.) “The hydrocode team” (Benz, Michel, et al.) “The reaccumulation team” (Tanga, Michel, Richardson, et al.) “The Cornell team” (Carruba, Burns et al.) The SDSS and Subaru (Yoshida et al.) teams

11. Other Contributions Zappalà et al. 2002 (No large NEAs from purely collisional mechanisms) Sliwan and Binzel 2000, 2002 (Spins of Koronis family members) Nesvorný et al. 2002 (Discovery of Karin Family) Cellino et al. 2001 (Separation of former Nysa-Polana clan) Milani and Knežević (Steady supply of new proper elements) Dell’Oro et al. 2002 (Primordial inter-family collisions) Carruba et al. 2003 (Role of close encounters with large asteroids) Spitale and Greenberg 2000, 2002 (Analysis of Yarkovsky effect) Other recent analyses (this meeting)

12. The Yarkovsky concept Newly-born families are compact Very soon small family members start to “fly away” Escaping family members are easy targets of resonant traps

13. Advantages of 21th Century Family Concepts Previously unrecognized phenomena are now fully taken into account Much better agreement with hydrocode results Possibility of deriving family ages Agreement with observational bias estimates and observations (SDSS, Subaru) Natural explanation of the fact that Families do not cross major resonances

14. Problems !? Up to which size Yarkovsky is really effective? Explanations of H vs. Proper elements relations Need of putting together the effects of events having different, and size-dependent time scales (resonance crossing, resonance-driven dynamical evolution, spin axis collisional realignment. Real families have e and i distributions which look often more dispersed than preliminary simulations. Initial family structures not known a priori. Can they be estimated from the distributions of the largest members?

15. Family Ages Short time for Yarkovsky evolution Short time for collisional evolution Current properties diagnostic of original events Young (Koronis?) Plenty of time to evolve Big families and surrounding background objects should exhibit the same kind of ageing features (size distribution, etc.) Old

16. Thesis (20th Century Families) Synthesis ?? Antithesis (21th Century Families) W.F. Hegel

17. Urgent tasks New Families Needed (some attempts already done) Difficulty in identification due to mutual overlapping Role of Spectroscopy Need of better estimates of Yarkovsky effectiveness Interpretation of Proper elements vs. H plots Better assessment of Primordial vs. Yarkovsky evolution New photometric data