Clusters are not evolving in isolation. They dynamically evolve internally plus interact with the surrounding gravitational field. Both effects together lead to the preferential loss of low-mass stars from the clusters, constantly changing the stellar mass function in the cluster. This impacts on prime observables such as magnitudes, colours, masses and mass-to-light ratios. In addition, physical properties like ages (and star cluster formation histories of galaxies) derived from the analysis of star cluster observations are strongly affected.
I will present new spectrophotometric evolutionary synthesis models which take into account these changing mass functions, based on detailed N-body simulations, hence removing one source of significant systematic uncertainities for studies of evolved stellar clusters.
For example, comparing the new models with models without the evolving mass function treatment reveils systematic effects of a factor ~3-10 in mass-to-light ratios and ~20-100% overestimated ages using incorrectly the standard models for dissolving star clusters.
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