Binary companions of evolved stars in APOGEE DR14: Orbital circularization

TL;DR

This study analyzes a large sample of evolved binary stars from APOGEE DR14 to understand orbital circularization, confirming theoretical predictions and revealing correlations with stellar surface gravity, while identifying some exceptions.

Contribution

It significantly expands the sample size for studying tidal circularization in evolved binaries and confirms theoretical models with new observational evidence.

Findings

Circular orbits generally match equilibrium-tide theory predictions.

Circularization period correlates with surface gravity of evolved stars.

Some systems show unexpected eccentricities, possibly due to measurement biases or genuine orbital properties.

Abstract

Short-period binary star systems dissipate orbital energy through tidal interactions that lead to tighter, more circular orbits. When at least one star in a binary has evolved off of the main sequence, orbital circularization occurs for longer-period (~100 day) systems. Past work by (Verbunt & Phinney 1995) has shown that the orbital parameters and the circularization periods of a small sample of binary stars with evolved-star members can be understood within the context of standard tidal circularization theory. Using a sample of binaries with subgiant, giant, and red clump star members that is nearly an order of magnitude larger, we reexamine predictions for tidal circularization of binary stars with evolved members. We confirm that systems predicted by equilibrium-tide theory to have circular orbits generally have negligible measured eccentricities. The circularization 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.