NBODY meets stellar population — The HYDE - PARC Project

Aims. N-body simulations give us a rough idea of how the shape of a simulated object appears in three-dimensional space. From an observational point of view this may give us a misleading picture as a stellar system consists of both bright and faint stars. The faint stars may be the most common stars in the system but the morphological information obtained by observations of an object may be dominated by the color properties of the bright stars. Due to dynamical effects, such as energy equipartition, different masses of stars may populate different regions in the object. Since stars are evolving in mass the stellar evolution may also influence the dynamics of the system. Hence, if one is interested in simulating what the morphology will look like through a telescope, one needs to simulate in addition evolving stars and weight them by their luminosity. Methods. Therefore we need to combine simulations of the dynamical evolution and a stellar population synthesis at the same time. For the dynamical evolution part we use a parallel version of a direct N-body code, NBODY6++. This code also includes stellar evolution. We link the parameters from this stellar evolution routine to the BaSeL 2.0 stellar library. This allows us to obtain a spectrum and colors for each star in the simulated cluster. We call this the HYDE-PARC project, which means Hybrid Code for Dynamical Evolution and Population Analysis of StellaR Clusters. Results. We tested our method by simulating globular clusters with up to 50000 stars and investigating the integrated colors. For isolated clusters we found results assimilable to standard stellar population synthesis codes such as the PEGASE code. For clusters in a tidal field we found that the integrated colors become relatively bluer due to energy equipartition effects. In the time shortly before dissolution of the cluster the stellar M/L ratio becomes lower compared to isolated clusters. We compared the results of our simulations to integrated spectra of galactic globular clusters. For the cluster NGC 1851 we found good agreement between simulation and observation. For extragalactic clusters in M81 and M31 we looked at medium band spectral energy distributions and found for some clusters also a good agreement.