Shape model and rotation acceleration of (1685) Toro and (85989) 1999 JD6 from optical observations

TL;DR

This study measures the YORP effect on two near-Earth asteroids, updating their rotation acceleration data and analyzing the broader implications for asteroid spin evolution and detection biases.

Contribution

It provides new YORP acceleration measurements for (1685) Toro and (85989) 1999 JD6, and discusses the overall distribution of YORP effects among near-Earth asteroids.

Findings

(1685) Toro's YORP acceleration confirmed and updated.

(85989) 1999 JD6's rotation period and pole position determined.

Most detected YORP effects are positive, indicating possible detection bias or a physical trend.

Abstract

The Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is a net torque caused by solar radiation directly reflected and thermally re-emitted from the surface of small asteroids and is considered to be crucial in their dynamical evolution. By long-term photometric observations of selected near-Earth asteroids, it's hoped to enlarge asteroid samples with a detected YORP effect to facilitate the development of a theoretical framework. Archived light-curve data are collected and photometric observations are made for (1685) Toro and (85989) 1999 JD6, which enables measurement of their YORP effect by inverting the light curve to fit observations from a convex shape model. For (1685) Toro, a YORP acceleration $\upsilon=(3.2\pm0.3)\times10^{-9}\ \rm{rad\cdot d^{-2}}(1\sigma\ error)$ is updated, which is consistent with previous YORP detection based on different light-curve data; for (85989) 1999 JD6, it is determined that the sidereal period is $7.667749\pm 0.000009$ h, the rotation pole direction locates is at $\lambda=232\pm 2^{\circ},\ \beta = -59\pm 1^{\circ}$, the acceleration is detected to be $\upsilon = (2.4\pm0.3)\times10^{-8}\ \rm{rad\cdot d^{-2}}(1\sigma\ error)$ and in addition to obtaining an excellent agreement between the observations and model. YORP should produce both spin-up and spin-down cases. However, including (85989) 1999 JD6, the $\rm{d}\omega/\rm{d}t$ values of eleven near-Earth asteroids are positive totally, which suggests that there is either a bias in the sample of YORP detections or a real feature needs to be explained.