Photodynamical Modeling of the Fascinating Eclipses in the Triple-Star System KOI-126

Abstract

We explore the fascinating eclipses and dynamics of the compact hierarchical triple star system KOI-126 (KIC 5897826). This system is comprised of a pair of M-dwarf stars (KOI-126 B and C) in a 1.74 day orbit which revolve around an F-star (KOI-126 A) every 34 days. Complex eclipse shapes are created as the M stars transit the F star, due to two effects: (i) the duration of the eclipse is a significant fraction of the M-star orbital period, so the prograde or retrograde motion of the M stars in their orbit lead to unusually short or long duration eclipses; (ii) due to 3-body dynamics, the M-star orbit precesses with an astonishingly quick timescale of 1.74 years for the periastron (apsidal) precession, and 2.73 years for the inclination and nodal angle precession. Using the full Kepler data set, supplemented with ground-based photometry, plus 29 radial velocity measurements that span 6 years, our photodynamical modeling yields masses of $M_A=1.2713\pm 0.0047M_{\odot }$ (0.37%), $M_B=0.23529\pm 0.00062M_{\odot }$ (0.26%), and $M_C=0.20739\pm 0.00055M_{\odot }$ (0.27%) and radii of $R_A=1.9984\pm 0.0027R_{\odot }$ (0.14%), $R_B=0.25504\pm 0.00076R_{\odot }$ (0.3%), and $R_C=0.23196\pm 0.00069R_{\odot }$ (0.3%). We also estimate the apsidal motion constant of the M-dwarfs, a parameter that characterizes the internal mass distribution. While not particularly precise, we measure a mean apsidal motion constant, $\overline{k_2}$, of $0.046_{-0.028}^{+0.046}$, which is approximately 2-$\sigma$ lower than the theoretical model prediction of 0.150. We explore possible causes for this discrepancy.

30 pages, 17 figures, submitted to ApJ

References in corpus (9)

• Modules for Experiments in Stellar Astrophysics (MESA)
• The Transiting Exoplanet Survey Satellite
• KOI-126: A Triply-Eclipsing Hierarchical Triple with Two Low-Mass Stars
• Absolute properties of the low-mass eclipsing binary CM Draconis
• Discovery of a Third Transiting Planet in the Kepler-47 Circumbinary System
• 2MASS J05162881+2607387: A New Low-Mass Double-Lined Eclipsing Binary
• The Compact Triply Eclipsing Triple Star TIC 209409435 Discovered with TESS
• Apsidal motion in the massive binary HD 152248 -- Constraining the internal structure of the stars
• The Interior Structure Constants as an Age Diagnostic for Low-Mass, Pre-Main Sequence Detached Eclipsing Binary Stars

Cited by in corpus (7)

• Triply eclipsing triple stars in the northern TESS fields: TICs 193993801, 388459317 and 52041148
• THE BANANA PROJECT. VI. Close double stars are well aligned with noticeable exceptions; results from an ensemble study using apsidal motion and Rossiter-McLaughlin measurements
• Improving circumbinary planet detections by fitting their binary's apsidal precession
• Spectroscopy of Eclipsing Compact Hierarchical Triples I
• A differentiable N-body code for transit timing and dynamical modelling -- II. Photodynamics
• NGTS-EB-8: A double-lined eclipsing M+M binary discovered by citizen scientists
• Discovery of a compact hierarchical triple main-sequence star system while searching for binary stars with compact objects