Exploring rotational properties and the YORP effect in asteroid families

The long-term dynamical evolution of asteroid families is governed by the interplay between orbital and rotational evolution driven by thermal forces and collision. We aim to observationally trace the rotational evolution of main-belt asteroid families over gigayear timescales. We analyzed rotational properties of 8739 asteroids with spin period measurements and 3794 asteroids with obliquity determinations across 28 asteroid families spanning ages from 14 Myrs to 3 Gyrs. We introduced a dimensionless timescale that normalizes each asteroid's family age by its classical YORP timescale, enabling a direct comparison of the rotational states across different evolutionary stages. We examined two key observables: the fraction of slow rotators (periods greater than or equal to 30 hours) and the polarization fraction (the degree to which asteroid spin poles align correctly with their position in the family's V-shape distribution according to the Yarkovsky theory). Evolution of both quantities were fit to identify characteristic transition timescales. We discovered that the slow-rotator fraction increases steeply with t and saturates at f_ ̊m slow ≃ 0. 25 around a break point of t_ ̊m bp ≃ 20. This implies a stochastic YORP timescale of τ_ ̊m YORP, stoc ̊m YORP in comparison with the rotational evolution models that include tumbling and weakened YORP torques. The polarization fraction reaches a maximum of ≃ 0. 8 at t ≃ 16 and then decays toward the random limit f_ ̊m pol ̊ightarrow 0. 5 for t ≳ 20, indicating an increasing dominance of collisional spin reorientation over time. The rotation properties within different asteroid families offer crucial clues to rotation evolution and can serve as a new dimension for the age estimation of asteroid families, particularly as more data will become available in the era of the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST).