Initial virial-state distribution of Milky Way open clusters

Determine the distribution of initial virial states for Milky Way open clusters, including how those states vary across the modeled cluster population, to explain the observed early disruption of young clusters.

Background

The simulations assume that all stellar clusters are born in virial equilibrium, with virial ratio Q = 0.5. However, the observed age function indicates substantial cluster disruption within approximately 1 Myr, which the equilibrium models do not reproduce. Rerunning simulations with a supervirial initial virial ratio of Q = 1.25 produces a young-cluster age-function slope that agrees more closely with observations, although the fitted breakpoint remains discrepant.

The results suggest that lower-mass clusters may be born more supervirial than higher-mass clusters, potentially because differences in star-formation efficiency and feedback-driven gas expulsion affect their initial dynamical states. The appropriate distribution of initial virial ratios, and any dependence on initial cluster mass, therefore remains unresolved.

References

However, it is not clear from this experiment what the distribution of the initial virial states should be, and further investigation is needed.

— $N$-body simulations of the open cluster population in the Milky Way and the impact of GMC encounters  (2608.27359 - Jørgensen et al., 27 Aug 2026) in Section 3, subsection “Age functions”

The present census cannot determine whether the stars were briefly bound before being released or were never bound and inherited the motions of an already dispersing clump.

— The dispersal law of young star clusters: free expansion -- A Gaia/eROSITA census of Orion and Sco--Cen  (2609.05179 - Alves et al., 4 Sep 2026) in Section 4, Discussion; Section 5, Conclusions