Dynamical formation of Gaia BH1 in a young star cluster

Abstract

Gaia BH1, the first quiescent black hole (BH) detected from Gaia data, poses a challenge to most binary evolution models: its current mass ratio is ≈0.1, and its orbital period seems to be too long for a post-common envelope system and too short for a non-interacting binary system. Here, we explore the hypothesis that Gaia BH1 formed through dynamical interactions in a young star cluster (YSC). We study the properties of BH-main sequence (MS) binaries formed in YSCs with initial mass 3 × 102–3 × 104 M⊙ at solar metallicity, by means of 3.5 × 104 direct N-body simulations coupled with binary population synthesis. For comparison, we also run a sample of isolated binary stars with the same binary population synthesis code and initial conditions used in the dynamical models. We find that BH-MS systems that form via dynamical exchanges populate the region corresponding to the main orbital properties of Gaia BH1 (period, eccentricity, and masses). In contrast, none of our isolated binary systems match the orbital period and MS mass of Gaia BH1. Our best-matching Gaia BH1-like system forms via repeated dynamical exchanges and collisions involving the BH progenitor star, before it undergoes core collapse. YSCs are at least two orders of magnitude more efficient in forming Gaia BH1-like systems than isolated binary evolution.

A Triple Scenario for the Formation of Wide Black Hole Binaries Such as Gaia BH1

“…Indeed, it has already been suggested that stellar mergers in triples can produce blue straggler binaries (Perets & Fabrycky 2009). Rastello et al (2023), propose an alternative definition of Gaia-BH1-like binaries with more stringent requirements on the component masses. In particular, they require the BH to be 8-12 M e and the companion to be 0.5-1.5 M e .…”

“…Previous work has estimated the formation rate of BH-MS binaries in open star clusters (Di Carlo et al 2023;Rastello et al 2023;Tanikawa et al 2024). For example, Rastello et al (2023) find a formation efficiency of…”

ESPRESSO Observations of Gaia BH1: High-precision Orbital Constraints and no Evidence for an Inner Binary

“…This scenario is difficult to test because it makes no predictions for present-day observables that could distinguish a dynamically assembled BH binary from a primordial one. Several recent works have predicted that dynamical formation in intermediate-or high-mass clusters could dominate the formation rate of Gaia BH1-like binaries (Di Carlo et al 2023;Rastello et al 2023;Tanikawa et al 2024). However, the models considered so far (a) appear rather fine-tuned, involving multiple protagonists, stellar mergers, and a calibrated BH natal kick, (b) do not actually avoid a common envelope event, which the binary may not survive, and (c) predict binaries with periods P orb ∼ 100 days to form inefficiently compared to both shorter and longer periods (Rastello et al 2023).…”

“…Several recent works have predicted that dynamical formation in intermediate-or high-mass clusters could dominate the formation rate of Gaia BH1-like binaries (Di Carlo et al 2023;Rastello et al 2023;Tanikawa et al 2024). However, the models considered so far (a) appear rather fine-tuned, involving multiple protagonists, stellar mergers, and a calibrated BH natal kick, (b) do not actually avoid a common envelope event, which the binary may not survive, and (c) predict binaries with periods P orb ∼ 100 days to form inefficiently compared to both shorter and longer periods (Rastello et al 2023). Further work is required to explore the sensitivity of these predictions to initial cluster mass and primordial binary population.…”

Constraining the Binarity of Black Hole Candidates: A Proof-of-concept Study of Gaia BH1 and Gaia BH2

“…Indeed, star-BH binary candidates, Gaia BH1 and Gaia BH2, recently discovered from Gaia Data Data Release 3 astrometric data (Gaia Collaboration et al 2023;El-Badry et al 2023bChakrabarti et al 2023;Tanikawa et al 2023a) would provide a good opportunity to directly check the methodology. Gaia BH1 is a binary of a ∼1 M e main-sequence star and a ∼10 M e dark companion, with an orbital period P obs ∼ 190 days (El-Badry et al 2023a;Chakrabarti et al 2023; see also Rastello et al 2023 andTanikawa et al 2023b). Gaia BH2 was first discovered by Tanikawa et al (2023a) using Gaia astrometry and later more robustly identified combining the follow-up RV observations by El-Badry et al (2023b).…”