A relation between the radial velocity dispersion of young clusters and their age: Evidence for hardening as the formation scenario of massive close binaries

TL;DR

This study finds that the radial velocity dispersion of young star clusters increases with age, supporting the idea that massive close binaries form at wider separations and harden over time through migration.

Contribution

It provides the first empirical evidence linking binary hardening to cluster age, supporting migration as a key process in forming massive close binaries.

Findings

Radial velocity dispersion increases with cluster age.

Minimum orbital period decreases from 9.5 years to 1.4 days within 6 Myr.

Binary hardening occurs within 1-2 Myrs, likely via migration.

Abstract

The majority of massive stars ($>8$ $\rm{M_{\odot}}$) in OB associations are found in close binary systems. Nonetheless, the formation mechanism of these close massive binaries is not understood yet. Using literature data, we measured the radial-velocity dispersion ($\sigma_\mathrm{RV}$) as a proxy for the close binary fraction in ten OB associations in the Galaxy and the Large Magellanic Cloud, spanning an age range from 1 to 6 Myrs. We find a positive trend of this dispersion with the cluster's age, which is consistent with binary hardening. Assuming a universal binary fraction of $f_\mathrm{bin}$ = 0.7, we converted the $\sigma_\mathrm{RV}$ behavior to an evolution of the minimum orbital period $P_\mathrm{cutoff}$ from $\sim$9.5 years at 1 Myr to $\sim$1.4 days for the oldest clusters in our sample at $\sim$6 Myr. Our results suggest that binaries are formed at larger separations, and they harden in around 1 to 2 Myrs to produce the period distribution observed in few million year-old OB binaries. Such an inward migration may either be driven by an interaction with a remnant accretion disk or with other young stellar objects present in the system. Our findings constitute the first empirical evidence in favor of migration as a scenario for the formation of massive close binaries.