The impact of stellar evolution on rotating star clusters: the gravothermal-gravogyro catastrophe and the formation of a bar of black holes

Abstract

We present results from a suite of eight direct N-body simulations, performed with N 6++GPU, representing realistic models of rotating star clusters with up to 1.1 × 10 5 stars. Our models feature primordial (hard) binaries, a continuous mass spectrum, differential rotation, and tidal mass loss induced by the overall gravitational field of the host galaxy. We explore the impact of rotation and stellar evolution on the star cluster dynamics. In all runs for rotating star clusters we detect a previously predicted mechanism: an initial phase of violent relaxation followed by the so-called gravogyro catastrophe. We find that the gravogyro catastrophe reaches a finite amplitude, which depends in strength on the level of the bulk rotation, and then levels off. After this phase the angular momentum is transferred from high-mass to low-mass particles in the cluster (both stars and compact objects). Simultaneously, the system becomes gravothermally unstable and collapses, thus undergoing the so-called gravothermal-gravogyro catastrophe. Comparing models with and without stellar evolution, we find an interesting difference. When stellar evolution is not taken into account, the whole process proceeds at a faster pace. The population of heavy objects tend to form a triaxial structure that rotates in the cluster centre. When stellar evolution is taken into account, we find that such a rotating bar is populated by stellar black holes and their progenitors. The triaxial structure becomes axisymmetric over time, but we also find that the models without stellar evolution suffer repeated gravogyro catastrophes as sufficient angular momentum and mass are removed by the tidal field.

Study on the detectability of gravitational radiation from single-binary encounters between black holes in nuclear star cluster: the case of hyperbolic flybys

“…encounters between two or three unbound BH [Arca- Sedda et al 2018, Arca Sedda et al 2020, Askar et al 2017, Banerjee 2021, Banerjee et al 2010, Fragione et al 2019, Fragione & Kocsis 2018, Fragione et al 2022, Kamlah et al 2022, Kremer et al 2019, Mapelli 2016, Mapelli et al 2013, O'Leary et al 2009, Rastello et al 2021, Rizzuto et al 2022, Rodriguez et al 2016, Samsing 2018a,b, Tanikawa 2013, Ziosi et al 2014, Zwart & McMillan 1999. Moreover, we selected the hyperbolic encounters, since the resonant encounters require more careful numerical analyses to calculate the associated GW signals and their impact on the rate.…”

“…This kind of interactions on unbound orbits between isolated BHs have already been the subject of studies with the scope of providing both the analytical [Capozziello et al 2008, Cho et al 2018, De Vittori et al 2014, 2012, García-Bellido & Nesseris 2018, Majár et al 2010, Morrás et al 2022, Mukherjee et al 2021 and numerical [Damour et al 2014, Nagar et al 2021) tools to determine the GW emission of a fly-by or of a dynamical capture. Dynamical friction in star clusters (SCs) causes the segregation of BHs, and the very high densities reached in the SC core triggers both the formation of binary black holes [BBHs; see, e.g., Mapelli 2020, for a review) and close encounters with other stars and BHs [Arca- Sedda et al 2018, Arca Sedda et al 2020, Askar et al 2017, Banerjee 2021, Banerjee et al 2010, Fragione et al 2019, Fragione & Kocsis 2018, Fragione et al 2022, Kamlah et al 2022, Kremer et al 2019, Mapelli 2016, Mapelli et al 2013, O'Leary et al 2009, Rastello et al 2021, Rizzuto et al 2022, Rodriguez et al 2016, Samsing 2018a,b, Tanikawa 2013, Ziosi et al 2014, Zwart & McMillan 1999. Therefore, since the rate of binary-single encounters scales with the local density of stars , we also expect a significant contribution from BBH-BH triple encounters in dense stellar environments.…”

BIFROST: simulating compact subsystems in star clusters using a hierarchical fourth-order forward symplectic integrator code

“…Tree and FMM solvers are typically employed in galaxy-or cosmologicalscale simulations (Springel et al 2020) allowing for very large particle numbers, but also recently in collisional star cluster simulations as well (e.g. PeTar, Wang et al 2020band Taichi, Mukherjee et al 2021, 2022. We plan to include either tree or FMM solver for BIFROST as well in further studies.…”

“…Due to the negative specific heat capacity of gravitating systems (Lynden-Bell & Wood 1968;Lynden-Bell & Eggleton 1980) hard binaries release energy to other stars in close few-body encounters by further shrinking to smaller separations. This has far-reaching consequences: a ★ E-mail: anttiran@mpa-garching.mpg.de relatively small number of binary systems can halt the runaway contraction of the core of a star cluster (Sugimoto & Bettwieser 1983;Bettwieser & Sugimoto 1984;Heggie 1993;Kamlah et al 2022).…”