Close Encounters of Stars with Stellar-mass Black Hole Binaries

Abstract

Many astrophysical environments, from star clusters and globular clusters to the disks of Active Galactic Nuclei, are characterized by frequent interactions between stars and the compact objects that they leave behind. Here, using a suite of 3 − 𝐷 hydrodynamics simulations, we explore the outcome of close interactions between 1𝑀 stars and binary black holes (BBHs) in the gravitational wave regime, resulting in a tidal disruption event (TDE) or a pure scattering, focusing on the accretion rates, the back reaction on the BH binary orbital parameters and the increase in the binary BH effective spin. We find that TDEs can make a significant impact on the binary orbit, which is often different from that of pure scattering. Binaries experiencing a prograde (retrograde) TDE tend to be widened (hardened) by up to 20%. Initially circular binaries become more eccentric by 10% by a prograde or retrograde TDE, whereas the eccentricity of initially eccentric binaries increases (decreases) by a retrograde (prograde) TDE by 5%. Overall a single TDE can generally result in changes of the gravitational wave-driven merger time scale by order unity. The accretion rates of both black holes are very highly super-Eddington, showing modulations (preferentially for retrograde TDEs) on a time scale of the orbital period, which can be a characteristic feature of BBH-driven TDEs. Prograde TDEs result in the effective spin parameter 𝜒 to vary by 0.02 while 𝜒 −0.005 for retrograde TDEs.

Gravitational waves or x‐ray counterpart? No need to choose

“…Hints of tidal disruption event (TDE) disks have been reported in observations (e.g., Lin et al 2017) and the subsequent formation of accretion structures studied numerically (e.g., Bonnerot et al 2016). Even though TDEs are facilitated by the large tidal radius of massive BHs, the large number of stellar-mass BBHs in the Galaxy (Lamberts et al 2018) could be an asset to expect TDEs from stellar-mass BBHs in young stellar clusters (Kremer et al 2020, see also Ryu et al 2022). Up to now, stellar-mass BBHs harboring a post-TDE disk may not be distinguishable from a low-mass x-ray binary system, that is, a system hosting only one compact object.…”

“…Bonnerot, Rossi, Lodato, & Price 2016). Even though TDEs are facilitated by the large tidal radius of massive BHs, the large number of stellar-mass BBHs in the Galaxy (Lamberts et al, 2018) could be an asset to expect TDEs from stellar-mass BBHs in young stellar clusters (Kremer et al 2020, see also Ryu, Perna, & Wang 2022). Up to now, stellar-mass BBHs harboring a post-TDE disk may not be distinguishable from a low-mass X-ray binary system, i.e.…”