A Relation Between The Radial Velocity Dispersion Of Young Clusters And Their Age: Evidence For Hardening As The Formation Scenario Of Massive Close Binaries

The majority of massive stars (> 8 M-circle dot) 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(1D)) 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 Myr. 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(bin) = 0.7, we converted the sigma(1D) behavior to an evolution of the minimum orbital period P-cutoff from similar to 9.5 years at 1 Myr to similar to 1.4 days for the oldest clusters in our sample at similar to 6 Myr. Our results suggest that binaries are formed at larger separations, and they harden in around 1 to 2 Myr 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.