Observed Orbital Eccentricities. II. Late-Type Stars

The distribution of individual eccentricities for binaries shows a large scatter between zero (circular orbits) and nearly 1 (highly elliptical), but mean eccentricities show systematic effects. For 296 F0-M5 IV or V primaries with known spectroscopic orbital elements and 482 similar primaries with known visual orbital elements, I collected mean eccentricities as a function of orbital periods. The results show complete circularization for periods up to 1.4 days for B dwarfs, 1.6-2.2 days for A dwarfs, 2.0 days for F dwarfs, and 4.3 days for G-M dwarfs. However, the evidence of maximum circularization periods of 4.3 to 18 days for late-type stars with increasing age suggests that these limiting periods depend primarily on age, rather than on primary mass. The mean eccentricities as a function of period approach e = 0.52 ± 0.02 asymptotically for the longest periods and for all types of stars. For long periods the distribution of eccentricities is statistically flat, indicating that for wide binaries, all eccentricities are equally probable, as was found previously for BA dwarfs in binaries. For intermediate periods the higher eccentricities disappear, and for short periods all eccentricities are zero. Double-lined spectroscopic binaries have statistically larger eccentricities than single-lined ones of the same periods, consistent with Kepler's third law. Late-type giants follow similar distributions, with complete circularization occurring below 70 days.