Young star clusters in the Antennae galaxies and NGC 1487

International audienceWe estimate the dynamical masses of several young ({≈ 10} Myr) massive star clusters in two interacting galaxies, NGC 4038/4039 (``The Antennae") and NGC 1487, under the assumption of virial equilibrium. These are compared with photometric mass estimates. A dynamical mass substantially higher than the photometric estimate could indicate non-virial motion of the stars in the cluster, and potentially lead to cluster disruption. All but one of the Antennae clusters have dynamical and photometric mass estimates which are within a factor ≈ 2 of one another, implying both that standard IMFs provide a good approximation to the IMF of these clusters, and that there is no significant extra-virial motion, as would be expected if they were rapidly dispersing. These results suggest that almost all of the clusters in our sample have survived the gas removal phase as bound or marginally bound objects. But a few targets (two in NGC 1487 and one in the Antennae) have M_dyn estimates which are significantly larger than the photometric mass estimates. At least two of those clusters may be actively in the process of dissolving. The dissolution candidates in both galaxies are amongst the clusters with the lowest pressures/densities measured in our sample