The evolution of two stellar populations in globular clusters I. The dynamical mixing timescale

We investigate the long-term dynamical evolution of two distinct stellar populations of low-mass stars in globular clusters in order to study whether the energy equipartition process can explain the high number of stars harbouring abundance anomalies seen in globular clusters. We analyse N-body models by artificially dividing the low-mass stars (m<0.9 Msun) into two populations: a small number of stars (second generation) consistent with an invariant IMF and with low specific energies initially concentrated towards the cluster-centre mimic stars with abundance anomalies. These stars form from the slow winds of fast-rotating massive stars. The main part of low-mass (first generation) stars has the pristine composition of the cluster. We study in detail how the two populations evolve under the influence of two-body elaxation and the tidal forces due to the host galaxy.Stars with low specific energy initially concentrated toward the cluster centre need about two relaxation times to achieve a complete homogenisation throughout the cluster. For realistic globular clusters, the number ratio between the two populations increases only by a factor 2.5 due to the preferential evaporation of the population of outlying first generation stars. We also find that the loss of information on the stellar orbital angular momentum occurs on the same timescale as spatial homogenisation.Comment: 9 pages, 9 figures, accepted for publication in A&A, references adde

The nature of some doubtful open clusters as revealed by Hipparcos

We discuss the nature of some galactic open clusters by using proper motions and parallaxes from the Hipparcos and ACT catalogues. We show that the clusters Collinder 399, Upgren 1, NGC 1252 and Melotte 227 do not exist. Collinder 132 is found to be mainly composed out of members of an OB association, but there may be a star cluster present in this area too. Roser & Bastian (1994) proposed that NGC 2451 consists of two star clusters. We show that NGC 2451 A definitively does exist, NGC 2451 B may exist. A star cluster may also be present in the area of Roslund 5. The Hipparcos data finally confirm the reality of Collinder 135

The initial distribution and evolution of globular cluster systems

This work considers the evolution of globular cluster systems in galaxies, Here globular cluster systems start with power-law mass functions phi similar to M-alpha with slopes around 2.0, similar to what has been observed for the young luminous clusters seen in merging and interacting galaxies, We then follow the orbits of the clusters through their parent galaxy, allowing various destruction mechanisms to dissolve them, In comparing the surviving distribution to the observed one, we show that our model can reproduce several aspects of present day globular cluster systems

Star Cluster Survival in Star Cluster Complexes under Extreme Residual Gas Expulsion

After the stars of a new, embedded star cluster have formed they blow the remaining gas out of the cluster. Especially winds of massive stars and definitely the on-set of the first supernovae can remove the residual gas from a cluster. This leads to a very violent mass-loss and leaves the cluster out of dynamical equilibrium. Standard models predict that within the cluster volume the star formation efficiency (SFE) has to be about 33 per cent for sudden (within one crossing-time of the cluster) gas expulsion to retain some of the stars in a bound cluster. If the efficiency is lower the stars of the cluster disperse mostly. Recent observations reveal that in strong star bursts star clusters do not form in isolation but in complexes containing dozens and up to several hundred star clusters, i.e. in super-clusters. By carrying out numerical experiments for such objects placed at distances >= 10 kpc from the centre of the galaxy we demonstrate that under these conditions (i.e. the deeper potential of the star cluster complex and the merging process of the star clusters within these super-clusters) the SFEs can be as low as 20 per cent and still leave a gravitationally bound stellar population. Such an object resembles the outer Milky Way globular clusters and the faint fuzzy star clusters recently discovered in NGC 1023.Comment: 21 pages, 8 figures, accepted by Ap

Theoretical and Observational Agreement on Mass Dependence of Cluster Life Times

Observations and N-body simulations both support a simple relation for the disruption time of a cluster as a function of its mass of the form: t_dis = t_4 * (M/10^4 Msun)^gamma. The scaling factor t_4 seems to depend strongly on the environment. Predictions and observations show that gamma ~ 0.64 +/- 0.06. Assuming that t_dis ~ M^0.64 is caused by evaporation and shocking implies a relation between the radius and the mass of a cluster of the form: r_h ~ M^0.07, which has been observed in a few galaxies. The suggested relation for the disruption time implies that the lower mass end of the cluster initial mass function will be disrupted faster than the higher mass end, which is needed to evolve a young power law shaped mass function into the log-normal mass function of old (globular) clusters.Comment: 2 pages, to appear in "The Formation and Evolution of Massive Young Star Clusters", 17-21 November 2003, Cancun (Mexico