Coalescence and Star Formation in Collapsing Flattened Systems

Abstract

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 smaller 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 initial ellipticity of the system. Stars that form later have smaller final radii and retain less memory. If we examine only those stars within the final half-mass radii of the models, we find that they follow a much less ellipsoidal distribution than do stars at larger radii. Their ellipticities, however, generally exceed those of young, dynamically unevolved massive star clusters in the LMC. The best comparisons with the observed ellipticities are found in models for which the initial system minor-to-major axis ratios b/a≳0.2, system kinetic-to-potential energy ratios Q≳0.1, covering factors fτ≲0.2, or initial cloud masses Mi≪MG. In the above, fτ is the fraction of the projected area of the system covered by the cloudlets, and MG is the critical mass for gravitational instability of the cloudlets, which are assumed to evolve isothermally. The ratio Mi/MG is taken to be one of the model parameters. All of these ranges of parameters tend to delay the onset of star formation until late in the collapse, when the system is less elliptical. Systems with Mi≪MG tend to produce very flat stellar initial mass functions, much flatter than observed. Unless, therefore, fτ≲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 initial conditions, even in the presence of dissipation during star formation.