Binary stars in the Milky Way nuclear stellar cluster

TL;DR

This study uses simulations to explore the evolution, migration, and outcomes of binary stars in the Milky Way's nuclear stellar cluster, revealing their potential roles in stellar phenomena and cluster dynamics.

Contribution

It provides the first detailed simulation-based analysis of binary star evolution, migration, and merger outcomes in the Milky Way's nuclear stellar cluster.

Findings

Approximately 0.3% of evaporated binaries fall into the SMBH's loss cone.

At least 1% of mergers produce blue straggler stars or red giants.

About 1% of binaries at 0.1 pc merge within the inner arcsec.

Abstract

Intermediate-mass galaxies, including the Milky Way, typically host both a supermassive black hole (SMBH) and a nuclear stellar cluster (NSC). Binaries in an NSC evolve via close encounters with surrounding stars and secular processes related to the SMBH. We study moderately soft and hard binaries ($0.03$-$2.5\,\mathrm{au}$, $M \lesssim 2\,M_\odot$) initially at galactocentric radii 0.1 and 0.3 pc using three-body simulations including von Zeipel-Lidov-Kozai oscillations and tidal dissipation over $\sim 10$ Gyr. Binaries migrate both inward and outward as a consequence of kicks received in the three-body encounters. Inward migration leads to destruction via mergers and evaporation, while outward migration is a pathway to retaining intact binaries for $\gtrsim 10$ Gyr. All surviving binaries are hard and circular, but outcomes for binaries initially at the hard-soft boundary are stochastic. We find that: (i) about $0.3$ percent of evaporated binaries fall into the SMBH's loss cone, (ii) at least $1$ percent of mergers occur late enough to appear as blue straggler stars (BSSs) on the main sequence or as recently evolved red giants, (iii) about $1$ percent of binaries initially at 0.1 pc merge within the inner arcsec of the NSC, and (iv) less than about $80$ percent of field-star collisions with a binary star lead to a subsequent merger; a three-body pile up, which are relatively common in the first 1-2 Gyr and could serve as a way to form more massive BSSs in the NSC. We predict that a small fraction of binaries originate closer to the SMBH than their present-day orbits, and vice versa for evaporated binaries and BSSs. The mergers confined to the inner arcsec occur after $\gtrsim 300$ Myr, too long to be directly related to the formation of the S-stars or G-objects, but suggest that the inner arcsec is contaminated with BSSs from earlier star formation events.