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Based on kinematic data observed for very young, massive clusters that appearto be in dynamical equilibrium, it has recently been argued that such youngsystems set examples where the early residual gas-expulsion did not happen orhad no dynamical effect. The intriguing scenario of a star cluster formingthrough a single starburst has thereby been challenged. Choosing the case ofthe R136 cluster of the Large Magellanic Cloud, the most cited one in thiscontext, we perform direct N-body computations that mimic the early evolutionof this cluster including the gas-removal phase (on a thermal timescale). Ourcalculations show that under plausible initial conditions as consistent fromobservational data, a large fraction (> 60%) of a gas-expelled, expandingR136-like cluster is bound to regain dynamical equilibrium by its current age.Therefore, the recent measurements of velocity dispersion in the inner regionsof R136, that indicate that the cluster is in dynamical equilibrium, areconsistent with an earlier substantial gas expulsion of R136 followed by arapid re-virialization (in approx. 1 Myr). Additionally, we find that the lessmassive Galactic NGC 3603 Young Cluster (NYC), with a substantially longerre-virialization time, is likely to be found deviated from dynamicalequilibrium at its present age (approx. 1 Myr). The recently obtained stellarproper motions in the central part of the NYC indeed suggest this and areconsistent with the computed models. This work significantly extends previousmodels of the Orion Nebula Cluster which already demonstrated that there-virialization time of young post-gas-expulsion clusters decreases withincreasing pre-expulsion density.

Did the infant R136 and NGC 3603 clusters undergo residual gas expulsion?