A New Secular Instability of Eccentric Stellar Disks Around Supermassive Black Holes, with Application to the Galactic Center

TL;DR

This paper discovers a new secular instability in eccentric stellar disks around supermassive black holes, explaining the observed eccentricity distribution in the Galactic center and its implications for star disruption and high-velocity stars.

Contribution

It identifies a novel secular instability mechanism driven by stellar cusp-induced precession, with simulations showing its impact on orbital eccentricities.

Findings

The instability can significantly alter stellar eccentricities within a precession timescale.

The observed bimodal eccentricity distribution in the Galactic center aligns with the instability's predictions.

High-eccentricity stars resulting from the instability could be tidally disrupted, producing S-stars and high-velocity stars.

Abstract

We identify a new secular instability of eccentric stellar disks around supermassive black holes. We show that retrograde precession of the stellar orbits, due to the presence of a stellar cusp, induces coherent torques that amplify deviations of individual orbital eccentricities from the average, and thus drive all eccentricities away from their initial value. We investigate the instability using N-body simulations, and show that it can drive individual orbital eccentricities to significantly higher or lower values on the order of a precession time-scale. This physics is relevant for the Galactic center, where massive stars are likely to form in eccentric disks around the SgrA* black hole. We show that the observed bimodal eccentricity distribution of disk stars in the Galactic center is in good agreement with the distribution resulting from the eccentricity instability and demonstrate how the dynamical evolution of such a disk results in several of its stars acquiring high (1-e < 0.1) orbital eccentricity. Binary stars on such highly eccentric orbits would get tidally disrupted by the SgrA* black hole, possibly producing both S-stars near the black hole and high-velocity stars in the Galactic halo.