Modeling Dense Stellar Systems: Background

I provide some background about recent efforts made in modeling dense stellar systems, within the context of the MODEST initiative. During the last four years, we have seen more than fifteen MODEST workshops, with an attendance between twenty and a hundred participants, and topics ranging from very specialized discussions to rather general overviews. 1. Dense Stellar Systems The study of star clusters, and of dense stellar systems in general, has recently seen great progress, through observations as well as simulations, as is evident from the papers in the proceedings of this meeting, JD14. The label ‘dense’ is given when stars are close enough that significant interactions between them occur on a time scale short compared to the age of the stellar system. A star forming region is dense in this sense, because the contracting protostellar clouds have a high probability to interact with each other during star formation. Old open clusters are called dense when their age exceeds their half-mass relaxation time. Most globular clusters in our galaxy are dense for that reason, and in addition, many globulars have a central density high enough for physical collisions between stars to occur frequently. The most spectacular type of dense stellar system is that found in the nucleus of most galaxies. Our own Milky Way galaxy is no exception: in the central parsec around the central supermassive black hole, there are frequent collisions between the stars, which have a total mass of a few million solar masses. 2. Multi-Scale Simulations Twenty-five years ago, stellar dynamics was split up in a number of different subfields that could be studied independently. Planetary dynamics, simulations of star forming regions, star cluster dynamics, modeling of galactic nuclei, the study of interacting galaxies, and cosmological simulations formed six different areas of research that had rather little in common. In contrast, all six areas are now firmly integrated. In many cases, it makes little sense to study only of these in isolation. Starting at the smallest scales, a detailed simulation of planet formation may have to take into account the influence of neighboring stars within the same star forming region. Or looking from the largest scales, that of cosmological simulations, the most detailed modeling efforts resolve the encounters between individual galaxies; and in turn, a detailed simulation of such an encounter shows how new dense star clusters are formed in the process. As a result, detailed simulations now routinely span multiple scales, on which the same physical laws show rather different emerging properties. In stellar dynamics, relaxation effects between stars can be ignored on galactic scales, yet are essential in the more dense