Monte Carlo simulations of star clusters - II. Tidally limited, multi-mass systems with stellar evolution

A revision of Stod\{'o}{\l}kiewicz's Monte Carlo code is used to simulate evolution of large star clusters. A survey of the evolution of N-body systems influenced by the tidal field of a parent galaxy and by stellar evolution is presented. The results presented are in good agreement with theoretical expectations and the results of other methods (Fokker-Planck, Monte Carlo and N-body). The initial rapid mass loss, due to stellar evolution of the most massive stars, causes expansion of the whole cluster and eventually leads to the disruption of less bound systems ($W_0=3$). Models with larger $W_0$ survive this phase of evolution and then undergo core collapse and subsequent post-collapse expansion, like isolated models. The expansion phase is eventually reversed when tidal limitation becomes important. The results presented are the first major step in the direction of simulating evolution of real globular clusters by means of the Monte Carlo method.Comment: 13 pages, 18 figures, 3 tables, submitted to MNRA

Monte-Carlo Simulations of Star Clusters I. First Results

A revision of Stodolkiewicz's Monte-Carlo code is used to simulate evolution of star clusters. The new method treats each superstar as a single star and follows the evolution and motion of all individual stellar objects. The first calculations for isolated, equal-mass N-body systems with three-body energy generation according to Spitzer's formulae show good agreement with direct N-body calculations for N=2000, 4096 and 10000 particles. The density, velocity, mass distributions, energy generation, number of binaries etc. follow the N-body results. Only the number of escapers is slightly too high compared to N-body results and there is no level off anisotropy for advanced post-collapse evolution of Monte-Carlo models as is seen in N-body simulations for N 10000 gravothermal oscillations are clearly visible. The calculations of N=2000, 4096, 10000, 32000 and 100000 models take about 2, 6, 20, 130 and 2500 hours, respectively. The Monte-Carlo code is at least 10^5 times faster than the N-body one for N=32768 with special-purpose hardware (Makino 1996ab). Thus it becomes possible to run several different models to improve statistical quality of the data and run individual models with N as large as 100000. The Monte-Carlo scheme can be regarded as a method which lies in the middle between direct N-body and Fokker-Planck models and combines most advantages of both methods.Comment: 11 pages, 8 PS-figures, To appear in MNRA

COCOA Code for Creating Mock Observations of Star Cluster Models

We introduce and present results from the COCOA (Cluster simulatiOn Comparison with ObservAtions) code that has been developed to create idealized mock photometric observations using results from numerical simulations of star cluster evolution. COCOA is able to present the output of realistic numerical simulations of star clusters carried out using Monte Carlo or \textit{N}-body codes in a way that is useful for direct comparison with photometric observations. In this paper, we describe the COCOA code and demonstrate its different applications by utilizing globular cluster (GC) models simulated with the MOCCA (MOnte Carlo Cluster simulAtor) code. COCOA is used to synthetically observe these different GC models with optical telescopes, perform PSF photometry and subsequently produce observed colour magnitude diagrams. We also use COCOA to compare the results from synthetic observations of a cluster model that has the same age and metallicity as the Galactic GC NGC 2808 with observations of the same cluster carried out with a 2.2 meter optical telescope. We find that COCOA can effectively simulate realistic observations and recover photometric data. COCOA has numerous scientific applications that maybe be helpful for both theoreticians and observers that work on star clusters. Plans for further improving and developing the code are also discussed in this paper.Comment: 18 pages, 12 figures, accepted for publication in MNRAS. Revised manuscript has a new title, better quality figures and many other improvements. COCOA can be downloaded from: https://github.com/abs2k12/COCOA (comments are welcome

Comparing Direct N-Body Integration with Anisotropic Gaseous Models of Star Clusters

We compare the results for the dynamical evolution of star clusters derived from anisotropic gaseous models with the data from N-body simulations of isolated and one-component systems, each having modest number of stars. The statistical quality of N-body data was improved by averaging results from many N-body runs, each with the same initial parameters but with different sequences of random numbers used to initialize positions and velocities of the particles. We study the development of anisotropy, the spatial evolution and energy generation by three-body binaries and its N-dependence. We estimate the following free parameters of anisotropic gaseous models: the time scale for collisional anisotropy decay and the coefficient in the formulae for energy generation by three-body binaries. To achieve a fair agreement between N-body and gaseous models for the core in pre- as well as in post-collapse only the energy generation by binaries had to be varied by N. We find that anisotropy has considerable influence on the spatial structure of the cluster particularly for the intermediate and outer regions.Comment: 24 pages (25 figures appended as postsript files