The dragon simulations: globular cluster evolution with a million stars

Abstract

Introducing the DRAGON simulation project, we present direct N -body simulations of four massive globular clusters (GCs) with 10 6 stars and 5% primordial binaries at a high level of accuracy and realism. The GC evolution is computed with NBODY6++GPU and follows the dynamical and stellar evolution of individual stars and binaries, kicks of neutron stars and black holes, and the effect of a tidal field. We investigate the evolution of the luminous (stellar) and dark (faint stars and stellar remnants) GC components and create mock observations of the simulations (i.e. photometry, color-magnitude diagrams, surface brightness and velocity dispersion profiles). By connecting internal processes to observable features we highlight the formation of a long-lived 'dark' nuclear subsystem made of black holes (BHs), which results in a two-component structure. The inner core is dominated by the BH subsystem and experiences a core collapse phase within the first Gyr. It can be detected in the stellar (luminous) line-of-sight velocity dispersion profiles. The outer extended corecommonly observed in the (luminous) surface brightness profiles -shows no collapse features and is continuously expanding. We demonstrate how a King (1966) model fit to observed clusters might help identify the presence of post core-collapse BH subsystems. For global observables like core and half-mass radii the direct simulations agree well with Monte-Carlo models. Variations in the initial mass function can result in significantly different GC properties (e.g. density distributions) driven by varying amounts of early mass loss and the number of forming BHs.

Testing multimass dynamical models of star clusters with real data: mass segregation in three Galactic globular clusters

“…However, in spite of the impressive progress of the computing power in the last decades, the fit of star cluster observables with direct N-body simulations has been feasible only for open cluster-like objects (Hurley et al 2005;Harfst, Portegies Zwart & Stolte 2010) or small GCs (Zonoozi et al 2011(Zonoozi et al , 2014Heggie 2014;Wang et al 2016). This is because a single simulation with a number of particles consistent with that observed in GCs (10 5−6 ) requires months of computing time, and a large number of simulations are needed to tune the initial conditions in such a way to reproduce after a Hubble time the present-day structure and properties of a given GC.…”

The dynamical origin of multiple populations in intermediate-age clusters in the Magellanic Clouds

“…The N -body simulations of star clusters in this paper were carried out with NBODY6++GPU (Wang et al 2015(Wang et al , 2016 1 . This code is based on the well-known legacy code NBODY6 and its precursors, developed by S. Aarseth since the 1960s for direct N -body integration of stellar systems (see for an overview Aarseth 1999Aarseth , 2003.…”

“…Later, a GPU-based parallelized version (NBODY6GPU), designed for a desktop or a single computer node with GPU was developed by Nitadori & Aarseth (2012). Wang et al (2015Wang et al ( , 2016 provide a combination of MPI parallelization with the use of many GPUs (using GPU acceleration for every MPI process), with the added feature of OpenMP (or SSE/AVX) used for the neighbour forces. This latest variant, called NBODY6++GPU can be used across multiple GPUaccelerated nodes on supercomputers combining MPI, OpenMP and CUDA-based GPU parallel computing principles.…”

“…It is a code optimized for a truly hybrid architecture. This technical development enabled the first million-body simulations of globular clusters (Wang et al 2016). All codes include the single and binary stellar evolution recipes described by Hurley et al (2000Hurley et al ( , 2002, and galactic tidal fields.…”

“…All codes include the single and binary stellar evolution recipes described by Hurley et al (2000Hurley et al ( , 2002, and galactic tidal fields. In addition to these standard recipes the natal kicks of neutron stars have been updated (Hobbs et al 2005) in the treatment of velocity kicks for neutron stars (NSs) and black holes (BHs) when they form after supernova explosions, and employing a fallback scenario according to Belczynski et al (2002) for the formation of a certain mass range of stellar BHs (for more details see Wang et al 2016). Note that spectral synthesis routines (GALEV) are now coupled to our NBODY code, which will allow us also to present spectra and CMDs of our simulated star clusters in every desired waveband (Pang et al 2016).…”