LAUSR

Previous studies pointed out that many observed samples of short-period binaries display a cutoff period, Pcut, such that almost all binaries with periods shorter than Pcut have circular orbits. This feature is probably due to long-term circularization processes induced by tidal interaction between the two stars of each binary. It seemed as if coeval main-sequence (MS) samples of open clusters display Pcut that depends on the sample age. Using the unprecedentedly large sample of MS spectroscopic orbits recently released by Gaia we have found that the Pcut does not depend on the stellar age but, instead, varies with stellar temperature, decreasing linearly from 6.5 day at Teff ∼ 5700 K to ∼2.5 day at 6800 K. Pcut was derived by a new algorithm that relied on clear upper envelopes displayed in the period-eccentricity diagrams. Our Pcut determines both the border between the circular and eccentric binaries and the location of the upper envelope. The results are inconsistent with the theory which assumes circularization occurs during the stellar MS phase, a theory that was adopted by many studies. The circularization has probably taken place at the pre-main-sequence phase, as suggested already in 1989 by Zahn and Bouchet, and later by Khaluillin and Khaluillina in 2011. Our results suggest that the weak dependence of Pcut on the cluster age is not significant, and/or might be due to the different temperatures of the samples. If indeed true, this has far-reaching implications for the theory of binary and exoplanet circularization, synchronization, and alignment.