Formation of asteroid pairs by rotational fission

TL;DR

This paper provides observational evidence supporting the rotational fission model for asteroid pair formation, showing that primaries with low mass ratios spin rapidly, and as the ratio nears 0.2, their spin periods lengthen, consistent with theoretical predictions.

Contribution

The study offers new photometric data confirming the rotational fission mechanism as the formation process for asteroid pairs, especially near the critical mass ratio of 0.2.

Findings

Primaries with low mass ratios spin near critical frequency.

Spin period increases as mass ratio approaches 0.2.

No asteroid pairs found with mass ratios above 0.2.

Abstract

Asteroid pairs sharing similar heliocentric orbits were found recently. Backward integrations of their orbits indicated that they separated gently with low relative velocities, but did not provide additional insight into their formation mechanism. A previously hypothesized rotational fission process4 may explain their formation - critical predictions are that the mass ratios are less than about 0.2 and, as the mass ratio approaches this upper limit, the spin period of the larger body becomes long. Here we report photometric observations of a sample of asteroid pairs revealing that primaries of pairs with mass ratios much less than 0.2 rotate rapidly, near their critical fission frequency. As the mass ratio approaches 0.2, the primary period grows long. This occurs as the total energy of the system approaches zero requiring the asteroid pair to extract an increasing fraction of energy from the primary's spin in order to escape. We do not find asteroid pairs with mass ratios larger than 0.2. Rotationally fissioned systems beyond this limit have insufficient energy to disrupt. We conclude that asteroid pairs are formed by the rotational fission of a parent asteroid into a proto-binary system which subsequently disrupts under its own internal system dynamics soon after formation.