Rotation Periods of Binary Asteroids with Large Separations - Confronting the Escaping Ejecta Binaries Model with Observations

TL;DR

This study examines the rotation periods of binary asteroids with large separations to test the Escaping Ejecta Binaries model, finding that most are likely formed by rotational fission due to the YORP effect, with only one candidate fitting the original model.

Contribution

It provides observational evidence challenging the EEB formation model, supporting rotational fission via YORP as the primary mechanism for these binary asteroids.

Findings

Most binary asteroids with large separations have fast rotation rates.

The YORP effect timescale is shorter than the ages of relevant asteroid families.

Only one suspected EEB, (317) Roxane, fits the model based on size and formation evidence.

Abstract

Durda et al. (2004), using numerical models, suggested that binary asteroids with large separation, called Escaping Ejecta Binaries (EEBs), can be created by fragments ejected from a disruptive impact event. It is thought that six binary asteroids recently discovered might be EEBs because of the high separation between their components (~100 > a/Rp > ~20). However, the rotation periods of four out of the six objects measured by our group and others and presented here show that these suspected EEBs have fast rotation rates of 2.5 to 4 hours. Because of the small size of the components of these binary asteroids, linked with this fast spinning, we conclude that the rotational-fission mechanism, which is a result of the thermal YORP effect, is the most likely formation scenario. Moreover, scaling the YORP effect for these objects shows that its timescale is shorter than the estimated ages of the three relevant Hirayama families hosting these binary asteroids. Therefore, only the largest (D~19 km) suspected asteroid, (317) Roxane, could be, in fact, the only known EEB. In addition, our results confirm the triple nature of (3749) Balam by measuring mutual events on its lightcurve that match the orbital period of a nearby satellite in addition to its distant companion. Measurements of (1509) Esclangona at different apparitions show a unique shape of the lightcurve that might be explained by color variations.