Revisiting the Tertiary-induced Binary Black Hole Mergers: the Role of Superthermal Wide Tertiary Eccentricity Distributions

TL;DR

This paper investigates how superthermal eccentricity distributions in outer binaries of triple systems influence black hole merger rates, revealing that such distributions can significantly affect merger pathways even without classical ZLK effects.

Contribution

It demonstrates that superthermal eccentricity distributions persist after supernova mass loss and modestly impact black hole merger rates via the tertiary channel, expanding understanding of merger conditions.

Findings

Superthermal eccentricities can remain in BH triples post-supernova.

Outer eccentricity distributions influence merger likelihood.

Merger rates are modestly affected by extreme outer eccentricities.

Abstract

Recent studies show that the eccentricity distribution of wide binaries (semimajor axis $\gtrsim10^3\;\mathrm{AU}$) observed by \emph{Gaia} tends to favor large eccentricities more strongly than the canonical thermal distribution ($P(e) \propto e$) -- such distributions are termed "superthermal". Motivated by this observation, we revisit the formation channel of black hole (BH) binary mergers in triple stellar systems and study the impact of superthermal eccentricity distributions in the outer binaries. We explore the persistence of the highly eccentric outer orbits after each component in a stellar triple has undergone mass loss due to supernova explosions. We find that the outer eccentricity distribution can remain significantly superthermal for modestly hierarchical BH triples satisfying $a_{\rm in}/a_{\rm out}\gtrsim 0.005$ (where $a_{\rm in}$ and $a_{\rm out}$ are the semimajor axes of the inner and outer orbits), and are otherwise shaped by mass-loss induced kicks and dynamical instability. We then study the impact of these different outer eccentricity distributions of the remaining BH triples on mergers via the tertiary-induced channel. Of interest, we find that mergers can sometimes be produced even when the initial stellar orbits are near alignment (not subject to the von-Zeipel-Lidov-Kozai effect; ZLK effect) as long as the system is sufficiently hierarchical. On the other hand, although the impact of the octupole-order ZLK effect is much greater when the outer binary is more eccentric, we find that the merger fraction only changes modestly for extreme outer eccentricity distributions, as the largest eccentricities tend to lead to dynamical instability.