N-body Simulations Of Clusters Research Articles

Evidence for the strong effect of gas removal on the internal dynamics of young stellar clusters

Abstract We present detailed luminosity profiles of the young massive clusters M82-F, NGC 1569-A and NGC 1705-1 which show significant departures from equilibrium (King and Elson, Fall & Freeman) profiles. We compare these profiles with those from N-body simulations of clusters that have undergone the rapid removal of a significant fraction of their mass as a result of gas expulsion. We show that the observations and simulations agree very well with each other, suggesting that these young clusters are undergoing violent relaxation and are also losing a significant fraction of their stellar mass. That these clusters are not in equilibrium can explain the discrepant mass-to-light ratios observed in many young clusters with respect to simple stellar population models without resorting to non-standard initial stellar mass functions as claimed for M82-F and NGC 1705-1. We also discuss the effect of rapid gas removal on the complete disruption of a large fraction of young massive clusters (‘infant mortality’). Finally, we note that even bound clusters may lose >50 per cent of their initial stellar mass as a result of rapid gas loss (‘infant weight-loss’).

An analytical description of the disruption of star clusters in tidal fields with an application to Galactic open clusters

We present a simple analytical description of the disruption of star clusters in a tidal field. The cluster disruption time, defined as tdis, depends on the mass M of the cluster as tdis with for clusters in a tidal field, as shown by empirical studies of cluster samples in different galaxies and by N-body simulations. Using this simple description we derive an analytic expression for the way in which the mass of a cluster decreases with time due to stellar evolution and disruption. The result agrees very well with those of detailed N-body simulations for clusters in the tidal field of our galaxy. The analytic expression can be used to predict the mass and age histograms of surviving clusters for any cluster initial mass function and any cluster formation history. The method is applied to explain the age distribution of the open clusters in the solar neighbourhood within 600 pc, based on a new cluster sample that appears to be unbiased within a distance of about 1 kpc. From a comparison between the observed and predicted age distributions in the age range between 10 Myr to 3 Gyr we find the following results: (1) The disruption time of a 104 cluster in the solar neighbourhood is about Gyr. This is a factor of 5 shorter than that derived from N-body simulations of clusters in the tidal field of the galaxy. Possible reasons for this discrepancy are discussed. (2) The present star formation rate in bound clusters within 600 pc of the Sun is Myr-1, which corresponds to a surface star formation rate of bound clusters of yr-1 pc-2. (3) The age distribution of open clusters shows a bump between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher than before and after. (4) The present star formation rate in bound clusters is about half that derived from the study of embedded clusters. The difference suggests that about half of the clusters in the solar neighbourhood become unbound within about 10 Myr. (5) The most massive clusters within 600 pc had an initial mass of about . This is in agreement with the statistically expected value based on a cluster initial mass function with a slope of -2, even if the physical upper mass limit for cluster formation is as high as 106 .

Core Evolution of Rich Stellar Clusters in the LMC

Observations of rich LMC clusters reveal an increasing spread in core radius with increasing age. We use N-body simulations of clusters on elliptical orbits about the LMC to show that a time-varying tidal field can result in significant core expansion. The observed spread in core radii may therefore reflect a spread in orbital parameters between clusters.