Eccentricities of Close Stellar Binaries

Abstract

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 inwards of a few AU exhibit a simple Rayleigh distribution with a mode sigma_e ~ 0.3. We find the same distribution for binaries from M to A spectral types, and from tens of days to a thousand days (possibly extending to tens of AU). 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 equi-partition value of sigma_e ~ sqrt{M_bd 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 the close binaries. The veracity of both proposals remains to be tested.