Kozai-driven mass loss of the circumbinary disk in D9 in orbit around the supermassive black hole Sgr A*

Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands

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Received: 18 December 2025
Accepted: 17 April 2026

Abstract

Context. The supermassive black hole (Sgr A*) in the Galactic center is surrounded by the S-star cluster consisting of young stars on eccentric orbits. Recently, the an S-star binary system, called D9, was found. Its detection was based on periodic emission in Brackettγ (Brγ). Since Brγ is considered a signature of accretion, this emission could originate from the interaction of a binary and its circumbinary disk. However, due to the gravitational interaction between Sgr A* and the binary, the disk could be short-lived.

Aims. We investigated the evolution of the disk around a stellar binary while orbiting Sgr A*.

Methods. We used the AMUSE framework for coupling a gravity solver (for the binary and Sgr A*) with a hydrodynamics solver (for the disk).

Results. We find that, irrespective of the initial disk inner and outer radii, it eventually settles between 5.2a_in and 0.28 Hill radii of the binary. Here, a_in is the semimajor axis of the binary D9. The inclination of the circumbinary disk follows that of the binary, which evolves due to the von Zeipel-Lidov-Kozai (vZLK) mechanism induced by Sgr A*. The mean eccentricity of the disk is approximately in antiphase with the eccentricity evolution of the binary. We find a vZLK timescale of T_vZLK ≈ 62.5 kyr, which is two orders of magnitude shorter than the value reported earlier. As a consequence, D9 has undergone multiple vZLK oscillations in its lifetime of 2.7 Myr. We find that the disk shows periodic bursts of mass loss on the vZLK timescale, suggesting that the mass loss itself is in part driven by the vZLK mechanism.

Conclusions. The secular evolution observed in both the binary and the disk are consistent with theoretical predictions. We find the disk loses ~7% ± 2% of its mass every vZLK cycle. If we extrapolate this mass loss, the disk will have 1% of its current mass left after another ~4 Myr. D9 will then be ~6.7 Myr old, which is on the same order as the current average age of S cluster members. The vZLK-driven mass loss could therefore explain the absence of Brγ emission from other S cluster members.