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The majority of massive stars ($>8$ $\rm{M_{\odot}}$) in OB associations arefound in close binary systems. Nonetheless, the formation mechanism of theseclose massive binaries is not understood yet. Using literature data, wemeasured the radial-velocity dispersion ($\sigma_\mathrm{RV}$) as a proxy forthe close binary fraction in ten OB associations in the Galaxy and the LargeMagellanic Cloud, spanning an age range from 1 to 6 Myrs. We find a positivetrend of this dispersion with the cluster's age, which is consistent withbinary hardening. Assuming a universal binary fraction of $f_\mathrm{bin}$ =0.7, we converted the $\sigma_\mathrm{RV}$ behavior to an evolution of theminimum 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. Ourresults suggest that binaries are formed at larger separations, and they hardenin around 1 to 2 Myrs to produce the period distribution observed in fewmillion year-old OB binaries. Such an inward migration may either be driven byan interaction with a remnant accretion disk or with other young stellarobjects present in the system. Our findings constitute the first empiricalevidence in favor of migration as a scenario for the formation of massive closebinaries.

astronomy and astrophysics

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