Exploring rotational properties and the YORP effect in asteroid families

TL;DR

This study analyzes the long-term rotational evolution of asteroid families, revealing how YORP and collisional effects influence spin states over Gyr timescales, and introduces a new method for asteroid family age estimation.

Contribution

It provides observational evidence of rotational evolution in asteroid families and introduces a normalized timescale to compare different families' evolution stages.

Findings

Slow rotator fraction increases with family age and saturates at 25%.

Polarization fraction peaks around 16-20 Myrs before declining.

Discovered a stochastic YORP timescale approximately twice the classical YORP timescale.

Abstract

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 Gyr 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 direct comparison of 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 fitted to identify characteristic transition timescales. We discovered that the slow-rotator fraction increases steeply with $t$ and saturates at $f_{\rm slow} \simeq 0.25$ around a breakpoint $t_{\rm bp} \simeq 20$. This implies a stochastic YORP timescale $\tau_{\rm YORP,stoc} \simeq 10\,\tau_{\rm YORP}$ by comparison with rotational evolution models that include tumbling and weakened YORP torques. The polarization fraction reaches a maximum of $\simeq 0.8$ at $t \simeq 16$ and then decays toward the random limit $f_{\rm pol} \rightarrow 0.5$ for $t \gtrsim 20$, 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 age estimation of asteroid families with more data in the LSST era.