Fast Rotation and Trailing Fragments of the Active Asteroid P/2012 F5 (Gibbs)

TL;DR

This study reports new observations of asteroid P/2012 F5, revealing fragments and a rapid rotation rate, supporting the idea that rotational instability causes activity in some main belt asteroids.

Contribution

First detection of fragments and measurement of rotation period in P/2012 F5, linking rapid spin to activity in main belt asteroids.

Findings

Detected 200-m fragments of P/2012 F5

Measured a rotation period of 3.24 hours

Confirmed rapid rotation as a common feature among active asteroids

Abstract

While having a comet-like appearance, P/2012 F5 (Gibbs) has an orbit native to the Main Asteroid Belt, and physically is a km-sized asteroid which recently (mid 2011) experienced an impulsive mass ejection event. Here we report new observations of this object obtained with the Keck II telescope on UT 2014 August 26. The data show previously undetected 200-m scale fragments of the main nucleus, and reveal a rapid nucleus spin with a rotation period of 3.24 $\pm$ 0.01 hr. The existence of large fragments and the fast nucleus spin are both consistent with rotational instability and partial disruption of the object. To date, many fast rotators have been identified among the minor bodies, which, however, do not eject detectable fragments at the present-day epoch, and also fragmentation events have been observed, but with no rotation period measured. P/2012 F5 is unique in that for the first time we detected fragments and quantified the rotation rate of one and the same object. The rapid spin rate of P/2012 F5 is very close to the spin rates of two other active asteroids in the Main Belt, 133P/Elst-Pizarro and (62412), confirming the existence of a population of fast rotators among these objects. But while P/2012 F5 shows impulsive ejection of dust and fragments, the mass loss from 133P is prolonged and recurrent. We believe that these two types of activity observed in the rapidly rotating active asteroids have a common origin in the rotational instability of the nucleus.