The dragon simulations: globular cluster evolution with a million stars

Wang, Long and Spurzem, Rainer and Aarseth, Sverre and Giersz, Mirek and Askar, Abbas and Berczik, Péter and Naab, Thorsten and Schadow, Riko and Kouwenhoven, M. B. N. (2016) The dragon simulations: globular cluster evolution with a million stars. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 458 (2). pp. 1450-1465. ISSN 0035-8711

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Abstract

Introducing the DRAGON simulation project, we present direct N -body simulations of four massive globular clusters (GCs) with 106 stars and 5% primordial bina- ries 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 stel- lar remnants) GC components and create mock observations of the simulations (i.e. photometry, color-magnitude diagrams, surface brightness and velocity dispersion pro- files). By connecting internal processes to observable features we highlight the forma- tion 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 core - commonly 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 sub- systems. 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.

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