Abstract
The majority of massive stars (> 8M⊙) 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 (σ1D) 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 offbin= 0.7, we converted theσ1Dbehavior to an evolution of the minimum orbital periodPcutofffrom ∼9.5 years at 1 Myr to ∼1.4 days for the oldest clusters in our sample at ∼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.
Highlights
It is well established that the vast majority of massive stars (M > 8 M ) come in pairs or as higher-order multiples (e.g., Mason et al 2009; Chini et al 2012; Peter et al 2012; Kiminki & Kobulnicky 2012; Kobulnicky et al 2014; Sana et al 2014; Dunstall et al 2015)
In order to compare the multi-epoch radial velocity (RV) data provided by Sana et al (2012) with the single epoch data of M 17, M 8, NGC 6357, and Westerlund 2 (Wd2), we drew the RV measured for each star in a given cluster in a random epoch and we computed the RV dispersion
Binary fraction and minimum period We focus on the effect that the binary fraction, fbin, and minimum orbital period, Pcutoff, have on the observed σ1D
Summary
It is well established that the vast majority of massive stars (M > 8 M ) come in pairs or as higher-order multiples (e.g., Mason et al 2009; Chini et al 2012; Peter et al 2012; Kiminki & Kobulnicky 2012; Kobulnicky et al 2014; Sana et al 2014; Dunstall et al 2015). For 2−4 Myr clusters, with binary fractions >0.5 and minimum periods of ∼1.4 days, a dispersion of 30 to 50 km s−1 is typical (e.g., Kouwenhoven et al 2007; Sana et al 2008, 2012; Sota et al 2014; Kobulnicky et al 2014) The latter is in stark contrast with our observation of. ∼5 and ∼70 M and the age of this cluster is between 1 and 3 Myr. In G333.6−0.2, the main sequence population ranges in mass between ∼5 and ∼35 M and the estimated age of this region is
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