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Abstract: Many young extra-galactic clusters have a measured velocity dispersion thatis too high for the mass derived from their age and total luminosity, which hasled to the suggestion that they are not in virial equilibrium. Most of theseclusters are confined to a narrow age range centred around 10 Myr because ofobservational constraints. At this age the cluster light is dominated byluminous evolved stars, such as red supergiants, with initial masses of ~13-22Msun for which primordial binarity is high. In this study we investigate towhat extent the observed excess velocity dispersion is the result of theorbital motions of binaries. We demonstrate that estimates for the dynamicalmass of young star clusters, derived from the observed velocity dispersion,exceed the photometric mass by up-to a factor of 10 and are consistent with aconstant offset in the square of the velocity dispersion. This can bereproduced by models of virialised star clusters hosting a massive starpopulation of which ~25 is in binaries, with typical mass ratios of ~0.6 andperiods of ~1000 days. We conclude that binaries play a pivotal role inderiving the dynamical masses of young ~10 Myr moderately massive and compact<1e5 Msun; > 1 pc star clusters.



Author: M. Gieles 1, H. Sana 1,2, S.F. Portegies Zwart 3 1 ESO, 2 Amsterdam, 3 Leiden

Source: https://arxiv.org/







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