Improving circumbinary planet detections by fitting their binary's apsidal precession

TL;DR

This paper enhances the detection of circumbinary planets by incorporating apsidal precession into radial velocity models, significantly improving sensitivity and enabling new measurements in real systems.

Contribution

We integrated apsidal precession modeling into the kima package, improving detection sensitivity for circumbinary planets and applying it to real systems for new insights.

Findings

Detection sensitivity improved by up to an order of magnitude.

Apsidal precession measured in KOI-126 and Kepler-16.

Derived expression for precession induced by outer planets.

Abstract

Apsidal precession in stellar binaries is the main non-Keplerian dynamical effect impacting the radial-velocities of a binary star system. Its presence can notably hide the presence of orbiting circumbinary planets because many fitting algorithms assume perfectly Keplerian motion. To first order, apsidal precession ($\dot{\omega}$) can be accounted for by adding a linear term to the usual Keplerian model. We include apsidal precession in the kima package, an orbital fitter designed to detect and characterise planets from radial velocity data. In this paper, we detail this and other additions to kima that improve fitting for stellar binaries and circumbinary planets including corrections from general relativity. We then demonstrate that fitting for $\dot{\omega}$ can improve the detection sensitivity to circumbinary exoplanets by up to an order of magnitude in some circumstances, particularly in the case of multi-planetary systems. In addition, we apply the algorithm to several real systems, producing a new measurement of aspidal precession in KOI-126 (a tight triple system), and a detection of $\dot{\omega}$ in the Kepler-16 circumbinary system. Although apsidal precession is detected for Kepler-16, it does not have a large effect on the detection limit or the planetary parameters. We also derive an expression for the precession an outer planet would induce on the inner binary and compare the value this predicts with the one we detect.