Eccentricities of Close Stellar Binaries

Orbits of stellar binaries are in general eccentric. These eccentricities encode information about their early lives. Here, we use thousands of main-sequence binaries from the Gaia DR3 catalog to reveal that binaries inward of a few astronomical units exhibit a simple Rayleigh distribution with a mode of σ_e ≃ 0.3. We find the same distribution for binaries from M to A spectral types, and from tens of days to thousands of days (possibly extending to tens of astronomical units). This observed distribution is most likely primordial and its invariance suggests a single universal process. One possibility is eccentricity excitation by circumbinary disks. Another, as is suggested by the Rayleigh form, is weak scattering and ejection of brown-dwarf objects. We explore this latter scenario and find that the binary eccentricities reach an equipartition value of σe≃√MBD/M∗. So to explain the observed mode, the brown dwarfs will have to be of order one-tenth the stellar masses, and be at least as abundant in the Galaxy as in the close binaries. The veracity of both proposals remains to be tested.