Preparing the next gravitational million-body simulations: evolution of single and binary stars in nbody6++gpu , mocca , and mcluster

Abstract

We present the implementation of updated stellar evolution recipes in the codes Nbody6++GPU, MOCCA and McLuster. We test them through numerical simulations of star clusters containing 1.1 × 105 stars (with 2.0 × 104 in primordial hard binaries) performing high-resolution direct N-body (Nbody6++GPU) and Monte Carlo (MOCCA) simulations to an age of 10 Gyr. We compare models implementing either delayed or core-collapse supernovae mechanisms, a different mass ratio distribution for binaries, and white dwarf natal kicks enabled/disabled. Compared to Nbody6++GPU, the MOCCA models appear to be denser, with a larger scatter in the remnant masses, and a lower binary fraction on average. The MOCCA models produce more black holes (BHs) and helium white dwarfs (WDs), whilst Nbody6++GPU models are characterised by a much larger amount of WD-WD binaries. The remnant kick velocity and escape speed distributions are similar for the BHs and neutron stars (NSs), and some NSs formed via electron-capture supernovae, accretion-induced collapse or merger-induced collapse escape the cluster in all simulations. The escape speed distributions for the WDs, on the other hand, are very dissimilar. We categorise the stellar evolution recipes available in Nbody6++GPU, MOCCA and Mcluster into four levels: the one implemented in previous Nbody6++GPU and MOCCA versions (level A), state-of-the-art prescriptions (level B), some in a testing phase (level C), and those that will be added in future versions of our codes.

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

“…For the first time we have studied the impact of initial bulk rotation, realistic stellar evolution mass loss models (Kamlah et al 2022) in combination with primordial binaries and stars drawn from a continuous IMF (Kroupa 2001) as well as a tidal field mass loss on the global dynamics of the star clusters and the development, evolution and coupling of the gravothermal and the gravogyro catastrophes using direct 𝑁-body methods. We have therefore expanded upon but also greatly surpassed any previous study on this phenomenon in astrophysical realism (Einsel & Spurzem 1999;Kim et al 2002Kim et al , 2004Ernst et al 2007;Kim et al 2008;Fiestas et al 2006;Fiestas & Spurzem 2010;Hong et al 2013;Szölgyen & Kocsis 2018;Szölgyen et al 2019Szölgyen et al , 2021Tiongco et al 2022;Livernois et al 2022).…”

“…The star clusters are initialised with M L (Kuepper et al 2011;Kamlah et al 2022;Leveque et al 2022). This code is used to either set up initial conditions for 𝑁-body computations or to generate artificial star clusters for direct investigation (Kuepper et al 2011).…”

“…The initial models from M L (Kuepper et al 2011;Kamlah et al 2022;Leveque et al 2022) are constructed as smaller mock models of the Milky Way GC NGC3201 and are shown in Tab.1. The initial number of objects is set to 10 5 with a binary fraction of 0.1.…”

“…It might be suspected that 𝑛 BHesc should be similar for all models just like the evolution of 𝑛 NSesc . However, the double-core collapse hump in combination with the fallback-dependent scaling of the natal kicks produces a larger diversity (see also Fryer et al (2012); Kamlah et al (2022)).…”

“…Whilst the observational evidence of rotating flattened clusters accumulates, the majority of numerical and theoretical models of star clusters still rely on the simplistic assumption of non-rotating spherical symmetry (e.g. Wang et al (2016); Askar et al (2017); Rizzuto et al (2021b,a); Kamlah et al (2022)), which are supported by a wide range of models with fully self-consistent energy and angular momentum distribution functions (e.g. Plummer (1911); King (1962); Wilson (1975)).…”