The S stars’ zone of avoidance in the Galactic center

¹ Physics Department, Technion – Israel Institute of Technology, 3200003 Haifa, Israel
² Astrophysics Research Center of the Open University (ARCO), 4353701 Raanana, Israel
³ Max-Planck-Institut für Extraterrestrische Physik, 85748 Garching, Germany
⁴ Department of Physics, Technical University of Munich, 85748 Garching, Germany
⁵ Departments of Physics & Astronomy, University of California, Berkeley, CA 94720, USA
⁶ Instituto de Astrofísica de Andalucía, 18008 Granada, Spain

^★ Corresponding author; aleksey.generozov@gmail.com

Received: 3 December 2024
Accepted: 5 February 2025

Abstract

This paper investigates the origin and orbital evolution of S stars in the Galactic center using models of binary disruption and relaxation processes. We focus on explaining the recently discovered “zone of avoidance” in S-star orbital parameters, defined as a region where no S stars are observed with pericenters of log(r_p/AU) ≤ 1.57 + 2.6(1 − e) pc. We demonstrate that the observed S-star orbital distributions, including this zone of avoidance and their thermal eccentricity distribution, can be largely explained by the continuous disruption of binaries near the central supermassive black hole, followed by orbital relaxation. Our models consider binaries originating from large scales (5–100 pc) and incorporate empirical distributions of binary properties. We simulate close encounters between binaries and the black hole, tracking the remnant stars’ orbits. The initially highly eccentric orbits of disrupted binary remnants evolve due to nonresonant and resonant relaxation in the Galactic center potential. While our results provide insights into the formation mechanism of S stars, there are limitations, such as uncertainties in the initial binary population and mass function and simplifications in our relaxation models. Despite these caveats, our study demonstrates the power of using S-star distributions to probe the dynamical history and environment of the central parsec of our Galaxy.