Origin of open clusters revealed by the evolution of the m_max$-$M_ecl relation

TL;DR

This study uses Gaia data and simulations to show that the evolution of the maximum star mass versus cluster mass relation supports subcluster coalescence as the main formation pathway for open clusters.

Contribution

It provides evidence that the m_max–M_ecl relation evolution aligns better with coalescence models than individual cluster evolution, highlighting coalescence as a key formation process.

Findings

Observed m_max–M_cluster relation deviates with age, indicating evolution.

Simulations show coalescence matches observed relations better than individual evolution.

Clusters older than 5 Myr exhibit significant deviations from initial relations.

Abstract

Using the Gaia DR3 open cluster catalog, we identified the most massive star in each observed cluster. Examining the m_max$-$M_cluster relations across different age ranges, we find that as clusters age, the relation gradually deviates from the initial m_max$-$M_ecl relation and eventually exhibits clear age stratification. We conducted N$-$body simulations for both individual cluster evolution and subcluster coalescence. Four gas expulsion modes were tested for individual clusters, and two scenarios were modeled for cluster coalescence. Under all four gas expulsion modes, the evolution of the m_max$-$M_cluster relation follows a similar trajectory, differing mainly in evolutionary speed. The coalescence simulations show comparable behavior but align better with the observations, as both exhibit systematically lower m_max$-$M_cluster relations than individual cluster simulations. This systematically lower observed m_max$-$M_cluster relation suggests slower cluster mass loss and smaller masses for the most massive stars$-$both conditions reproduced in the coalescence simulations. Observations also show that clusters older than 5 Myr have most massive stars significantly deviating from the initial m_max$-$M_ecl relation. From this perspective, the coalescence simulations also provide a better match to the observations. In conclusion, the evolution of the m_max$-$M_ecl relation supports subcluster coalescence as a dominant pathway for open cluster formation, consistent with our previous work.