Close Encounters of Tight Binary Stars with Stellar-mass Black Holes

Taeho Ryu, Rosalba Perna, Ruediger Pakmor, Jing-Ze Ma, Rob Farmer, Selma E. de Mink

Submitted on 4 November 2022


Strong dynamical interactions among stars and compact objects are expected in a variety of astrophysical settings, such as star clusters and the disks of active galactic nuclei. Here, via a suite of 3D hydrodynamics simulations using the moving-mesh code {\small AREPO}, we investigate the effect of close encounters between an equal-mass circular binary star with mass of 2M or 20M and single 20M black hole (BH), focusing on the formation of transient phenomena and their properties. Stars can be disrupted by the BH during three-body dynamical interactions, naturally producing electromagnetic transient phenomena. Encounters with impact parameters smaller than the semimajor axis of the initial binary frequently lead to a variety of transients whose electromagnetic signatures are qualitatively different from those of ordinary tidal disruption events involving just two bodies. These include the simultaneous or successive full disruptions of both stars and one full disruption of one star accompanied by successive partial disruptions of the other star. On the other hand, when the impact parameter is larger than the semimajor axis of the initial binary, the binary is either simply tidally perturbed or dissociated into bound and unbound single stars (``micro-Hills'' mechanism). We found that the dissociation of binaries consisting of 10M stars can produce the formation of a runaway star and an active isolated BH moving away from one another. Also, one of the unbound stars produced in the binary dissociation can either form an interacting binary with the BH, or a non-interacting, hard binary (which may later shrink via weak encounters); both of these could be candidates of BH high- and low-mass X-ray binaries with periodic luminosity modulation.


Comment: 13 pages, 8 figures, 2 tables. submitted to MNRAS. Comments welcome!

Subjects: Astrophysics - High Energy Astrophysical Phenomena; Astrophysics - Solar and Stellar Astrophysics


The radial density profile of the main-sequence stars with $M_{\star}=1\Msol$ (blue) and $10\Msol$ (red) relaxed for five stellar dynamical times, as a function of mass. The dashed lines indicate the profiles for the {\sc MESA} models, which are just sitting below the solid lines.