The impact of mass loss on star cluster formation – II. Numerical N-body integration and further applications

We subject to an N-body numerical investigation our analysis of Paper I on the survival of stellar clusters undergoing rapid mass loss. We compare analytical tracks of bound mass fraction versus star-formation efficiency∈ to those obtained with N-body integration. We use these to argue that stellar clusters must develop massive cores of high-binding energy if they are to remain bound despite a star-formation efficiency as low as 30 per cent or lower suggested by observations. The average local virial ratio (σ 2 /‖Φ‖) is introduced to classify bound clusters as a function of their critical ∈ for dissolution. Clusters dissolving at lower ∈ achieve the lowest ratio. We have applied this classification parameter successfully to Michie-King and Hernquist-type distribution functions. The Plummer sphere is exceptional in that it defies this and other classification parameters we have tried. The reasons for the discrepancy include less effective energy redistribution during the expansion phase for this case.