Coalescence and star formation in collapsing flattened systems

We report on the results of N-body simulations of star formation resulting from mergers during the collapse of dynamically cold, flattened systems of cloudlets. Such evolution is expected to occur in several models of cluster star formation. As was found previously in the case of spherical systems, the resulting star clusters have half-mass radii that are significantly sma ller than the initial values for the systems. Stars that form early in the collapse have large final orbital radii and retain a strong memory of the i nitial ellipticity of the system. Stars that form later have smaller final radii and retain less memory. If we examine only those stars within the fin al half-mass radii of the models, we find that they follow a much less elli psoidal distribution than do stars at larger radii. Their ellipticities, ho wever, generally exceed those of young, dynamically unevolved massive star clusters in the LMC. The best comparisons with the observed ellipticities a re found in models for which the initial system minor-to-major axis ratios b/a greater than or similar to 0.2, system kinetic-to-potential energy rati os Q greater than or similar to 0.1, covering factors f(tau) less than or s imilar to 0.2, or initial cloud masses M-i much less than M-G. In the above , f(tau) is the fraction of the projected area of the system covered by the cloudlets, and M-G is the critical mass for gravitational instability of t he cloudlets, which are assumed to evolve isothermally. The ratio M-i/M-G i s taken to be one of the model parameters. All of these ranges of parameter s tend to delay the onset of star formation until late in the collapse, whe n the system is less elliptical. Systems with M-i much less than M-G tend t o produce very flat stellar initial mass functions, much flatter than obser ved. Unless, therefore, f(tau) less than or similar to 0.2 results from the fragmentation of the parent cloud, systems such as the young LMC clusters are unlikely to have formed from extremely flattened or dynamically cold in itial conditions, even in the presence of dissipation during star formation .