Relativity and the evolution of the Galactic center S-star orbits

TL;DR

This paper investigates the orbital evolution of S-stars near the Galactic center's supermassive black hole, incorporating relativistic effects and resonant relaxation, to understand their origins and orbital distributions.

Contribution

It introduces the first combined analysis of Newtonian and relativistic perturbations, including frame dragging and stellar torques, affecting S-star orbital evolution.

Findings

Most S-stars can penetrate the Schwarzschild barrier within 50 Myr.

The probability of tidal disruption of S-stars by the black hole is less than 1%.

Relativistic effects and resonant relaxation significantly influence S-star orbital dynamics.

Abstract

We consider the orbital evolution of the S-stars, the young main-sequence stars near the supermassive black hole (SBH) at the Galactic center (GC), and put constraints on competing models for their origin. Our analysis includes for the first time the joint effects of Newtonian and relativistic perturbations to the motion, including the dragging of inertial frames by a spinning SBH as well as torques due to finite-N asymmetries in the field-star distribution (resonant relaxation, RR). The evolution of the S-star orbits is strongly influenced by the Schwarzschild barrier (SB), the locus in the (E,L) plane where RR is ineffective at driving orbits to higher eccentricities. Formation models that invoke tidal disruption of binary stars by the SBH tend to place stars below (i.e., at higher eccentricities than) the SB; some stars remain below the barrier, but most stars are able to penetrate it, after which they are subject to RR and achieve a nearly thermal distribution of eccentricities. This process requires roughly 50 Myr in nuclear models with relaxed stellar cusps, or >~10 Myr, regardless of the initial distribution of eccentricities, in nuclear models that include a dense cluster of 10 M_Sun black holes. We find a probability of <~1% for any S-star to be tidally disrupted by the SBH over its lifetime.