Binary Companions of Evolved Stars in APOGEE DR14: Orbital Circularization

Short-period binary star systems dissipate orbital energy through tidal interactions that lead to tighter, more circular orbits. Using a sample of binaries with subgiant, giant, and red clump star members that is nearly an order of magnitude larger than that of Verbunt & Phinney, we reexamine predictions for tidal circularization of binary stars with evolved members. We confirm that binary star systems in our sample predicted to have circular orbits (using equilibrium tide theory) generally have negligible measured eccentricities. At a fixed stellar mass, the transition period is correlated with the surface gravity (i.e., size) of the evolved member, indicating that the circularization timescale must be shorter than the evolutionary timescale along the giant branch. A few exceptions to the conclusions above are mentioned in the discussion. Some of these exceptions are likely systems in which the spectrum of the secondary biases the radial velocity measurements, but four appear to be genuine, short-period, moderate-eccentricity systems.