S-stars in the Galactic center and hypervelocity stars in the Galactic halo: two faces of the tidal breakup of stellar binaries by the central massive black hole?

TL;DR

This study explores the connection between hypervelocity stars in the Galactic halo and S-stars near the Galactic center, proposing they originate from the tidal breakup of stellar binaries by the central black hole, supported by numerical simulations.

Contribution

It demonstrates that the properties of HVSs and S-stars can be explained by binary tidal breakup models, with specific predictions on their numbers, distributions, and velocities.

Findings

Reproduces observed properties of HVSs and S-stars with binary injection models.

Suggests a top-heavy initial mass function (~-1.6 slope) fits the data.

Predicts a population of captured companions with specific orbital parameters.

Abstract

In this paper, we investigate the link between the hypervelocity stars (HVSs) discovered in the Galactic halo and the S-stars moving in the Galactic center (GC), under the hypothesis that they are both the products of the tidal breakup of the same population of stellar binaries by the central massive black hole (MBH). By adopting several hypothetical models for binaries to be injected into the vicinity of the MBH and doing numerical simulations, we realize the tidal breakup processes of the binaries and their follow-up evolution. We find that many statistical properties of the detected HVSs and S-stars can be reproduced under some binary injecting models, and their number ratio can be reproduced if the stellar initial mass function is top-heavy (e.g., with slope ~-1.6). The total number of the captured companions is ~50 that have masses in the range ~3-7Msun and semimajor axes <~4000 AU and survive to the present within their main-sequence lifetime. The innermost one is expected to have a semimajor axis ~300-1500 AU and a pericenter distance ~10-200 AU, with a significant probability of being closer to the MBH than S2. Future detection of such a closer star would offer an important test to general relativity. The majority of the surviving ejected companions of the S-stars are expected to be located at Galactocentric distances <~20 kpc, and have heliocentric radial velocities ~-500-1500 km/s and proper motions up to ~5-20 mas/yr. Future detection of these HVSs may provide evidence for the tidal-breakup formation mechanism of the S-stars.