Analysis of Karin and Koronis2 asteroid families: new findings and challenges

TL;DR

This study refines the age and membership of the Karin asteroid family using backward orbit integration and introduces a novel convergence metric, revealing insights into asteroid family evolution and Yarkovsky/YORP effects.

Contribution

It presents a new method for estimating asteroid family ages based on orbital convergence and analyzes the Yarkovsky and YORP influences on family members.

Findings

Karin family is approximately 5.72 million years old.

The size distribution of Karin family follows a power-law with slope -3.20.

Koronis2 family members' convergence suggests it may be older than previously estimated.

Abstract

We use our home catalog of the asteroid proper elements to study the Karin family. The hierarchical clustering method provides formal identification with 3,863 members, but this set also includes objects from the neighboring Koronis2 and Kuitaisi families, as well as interlopers originating from the much older Koronis family. By tracking the trajectories of cluster objects backward in time, we identified 2,161 asteroids whose orbits converged with that of their parent body (832) Karin at $5.72\pm 0.09$ My ago ($95$\% C.L.). This method of calculating the family's age is based on a novel convergence metric that is directly related to the velocities at which fragments were ejected from (832) Karin. We analyze the extent to which members $\leq 1.5$ km in diameter had drifted in semimajor axis due to Yarkovsky thermal forces and find it reflects the tilt of their rotation poles away from the ecliptic, recording the influence of the YORP torque. Karin's size frequency distribution in the $\simeq(0.8-3)$ km range follows a power-law with a cumulative slope index $-3.20\pm 0.01$. Removing members of the Karin family from the original group, we examine the Koronis2 family, whose members are associated with (158) Koronis. We find it difficult for large members of the Koronis2 family to converge with the orbit of (158) Koronis within its previously estimated age of $7.6$ My. Achieving such convergence would require the Koronis2 family to be older than $10$ My, but our result must be verified with a direct numerical approach in the future.