Rotational Mass Shedding from Asteroid (6478) Gault

TL;DR

This study investigates asteroid (6478) Gault's episodic dust ejections, providing evidence that its mass loss is driven by rapid rotation near the point of rotational breakup, supported by new photometric observations.

Contribution

The paper offers new observational data confirming Gault's rapid rotation and models its mass shedding as a consequence of near-breakup spin rates, using the Froude number to analyze mass loss regimes.

Findings

Gault's lightcurve shows a 2.55-hour period with minimal amplitude.

Dust ejection velocities are low, around 0.15 m/s.

Gault's rotation likely causes episodic mass loss near the point of rotational breakup.

Abstract

The $\sim$4 km diameter main belt asteroid 6478 Gault has ejected dust intermittently since at least 2013. The character of the emission, including its episodic nature and the low speed of the ejected particles ($V \sim $ 0.15 m s$^{-1}$), is most consistent with mass loss from a body rotating near rotational breakup. Owing to dust contamination of the nucleus signal, this conclusion has not yet been confirmed. To test this idea, we have obtained new images of Gault in August 2020, in the absence of dust. Our photometry shows a lightcurve having a very small amplitude (maximum $\sim 0.05$ mag) and a periodicity of $ 2.55 \pm 0.10$ hours. The new observations are consistent with a model in which Gault is rotating near breakup, with centrifugal forces responsible for its episodic mass loss. Approximated as a strengthless (fluid) spherical body, the implied density is $\rho$ = 1700 kg m$^{-3}$. We use the Froude number $Fr$, defined here as the ratio between centrifugal force and gravitational force, as a way to investigate mass loss regimes in fast spinning asteroids and find that mass shedding starts at $Fr \sim 0.5$.