Asteroid families in the first-order resonances with Jupiter

Asteroids residing in the first-order mean motion resonances with Jupiter hold important information about the processes that set the final architecture of giant planets. Here, we revise current populations of objects in the J2/1 (Hecuba-gap group), J3/2 (Hilda group) and J4/3 (Thule group) resonances. The number of multi-opposition asteroids found is 274 for J2/1, 1197 for J3/2 and three for J4/3. By discovering a second and third object in the J4/3 resonance (186024) 2001 QG₂₀₇ and (185290) 2006 UB₂₁₉, this population becomes a real group rather than a single object. Using both hierarchical clustering technique and colour identification, we characterize a collisionally born asteroid family around the largest object (1911) Schubart in the J3/2 resonance. There is also a looser cluster around the largest asteroid (153) Hilda. Using N-body numerical simulations we prove that the Yarkovsky effect (infrared thermal emission from the surface of asteroids) causes a systematic drift in eccentricity for resonant asteroids, while their semimajor axis is almost fixed due to the strong coupling with Jupiter. This is a different mechanism from main belt families, where the Yarkovsky drift affects basically the semimajor axis. We use the eccentricity evolution to determine the following ages: (1.7 +/- 0.7)Gyr for the Schubart family and >~4Gyr for the Hilda family. We also find that collisionally born clusters in the J2/1 resonance would efficiently dynamically disperse. The steep size distribution of the stable population inside this resonance could thus make sense if most of these bodies are fragments from an event older than ~=1Gyr. Finally, we test stability of resonant populations during Jupiter's and Saturn's crossing of their mutual mean motion resonances. In particular, we find primordial objects in the J3/2 resonance were efficiently removed from their orbits when Jupiter and Saturn crossed their 1:2 mean motion resonance.