Ryutaro Iwakura | Searcharxiv

arXiv2026

Dynamical formation of Gaia neutron star binaries in star clusters as a phase-dependent process

Gaia has discovered dormant compact-object binaries in which the compact object shows no observable interaction with its luminous companion. These systems, hereafter Gaia BHs and Gaia NSs for black hole (BH) and neutron star (NS) companions, provide a new probe of compact-object binary formation. These detached systems are difficult to produce through standard isolated binary evolution alone, and their formation pathways remain unclear. Previous $N$-body studies have shown that Gaia BH-like objects are formed efficiently in star clusters, while Gaia NS-like objects are not. This has been attributed to the BH subsystem dominating the cluster core. We performed direct $N$-body simulations of $10^4 \mathrm{M}_\odot$ star clusters in a Galactic tidal field and investigated how NS-main-sequence (NS-MS) binaries form during the late BH-depleted phase. We found that NS-MS binaries satisfying Gaia-sensitive criteria form only after BH depletion. NSs are present in the core throughout the evolution, but the strong encounters there are monopolized by the BH subsystem until it is depleted. Our results show that the dynamical formation of Gaia-sensitive NS-MS systems is not prevented in star clusters, but delayed relative to Gaia BH-like systems.

astro-ph.SR↗

arXiv2025

A new approach for predicting the stability of hierarchical triple systems -- I. Coplanar Cases

Hierarchical triple systems play a crucial role in various astrophysical contexts, and therefore the understanding of their stability is important. Traditional empirical stability criteria rely on a threshold value of $Q$, the ratio between the outer orbit's pericenter distance and the inner orbit's semi-major axis. However, determining a single critical value of $Q$ is impossible because there is a range of the value of $Q$ for which both stable and unstable systems exist, referred to as the mixed-region. In this study, we introduce a novel method to assess the stability of triple systems within this mixed-region. We numerically integrate equal-mass, coplanar hierarchical triples within the mixed-region. By performing Fourier analysis of the time evolution of the semi-major axes ratio during the first 1000 inner orbital periods of the systems, we find notable features in stable systems: if the main peaks are periodically spaced in the frequency domain and the continuous components and irregularly spaced peaks are small, the system tends to be stable. This observation indicates that the evolution of stable triples is more periodic than that of unstable ones. We quantified the periodicity of the triples and investigated the correlation between the Fourier power distribution and the system's lifetime. Using this correlation, we show that it is possible to determine if a triple system in the mixed-region is stable or not with very high accuracy. These findings suggest that periodicity in orbital evolution can serve as a robust indicator of stability for hierarchical triples.

astro-ph.SR↗