Revisiting the universality of (multiple) star formation in present-day star formation regions

Populations of multiple stars inside clustered regions are known to change through dynamical interactions. The efficiency of binary disruption is thought to be determined by stellar density. King and collaborators recently investigated the multiplicity properties in young star-forming regions and in the Galactic field. They concluded that stellar-density-dependent modification of a universal initial binary population (the standard or null hypothesis model) cannot explain the observations. We revisit their results, analysing the data within the framework of different model assumptions, namely non-universality without dynamical modification and universality with dynamics. We illustrate that the standard model does account for all known populations if regions were significantly denser in the past. Some of the effects of using present-day cluster properties as proxies for their past values are emphasized and that the degeneracy between age and density of a star-forming region cannot be omitted when interpreting multiplicity data. A new analysis of the Corona Australis region is performed within the standard model. It is found that this region is likely as unevolved as Taurus and an initial density of ≈190 M⊙ pc−3 is required to produce the presently observed binary population, which is close to its present-day density.