Von Zeipel - Lidov - Kozai cycles in action: $Kepler$ triples with eclipse depth variations: KICs 6964043, 5653126, 5731312 and 8023317

Abstract

We report the results of the photodynamical analyses of four compact, tight triple stellar systems, KICs 6964043, 5653126, 5731312, 8023317, based largely on $Kepler$ and $TESS$ data. All systems display remarkable eclipse timing and eclipse depth variations, the latter implying a non-aligned outer orbit. Moreover, KIC 6964043 is also a triply eclipsing system. We combined photometry, ETV curves, and archival spectral energy distribution data to obtain the astrophysical parameters of the constituent stars and the orbital elements with substantial precision. KICs 6964043 and 5653126 were found to be nearly flat with mutual inclinations $i_{mut}=4.1$ deg and $12.3$ deg, respectively, while KICs 5731312, 8023317 ($i_{mut}=39.4$ deg and $55.7$ deg, respectively) are found to lie in the high $i_{mut}$ regime of the von Zeipel-Kozai-Lidov (ZKL) theorem. We show that currently both high inclination triples exhibit observable unusual retrograde apsidal motion. Moreover, the eclipses will disappear in all but one of the four systems within a few decades. Short-term numerical integrations of the dynamical evolution reveal that both high inclination triples are currently subject to ongoing, large amplitude ($\Delta e\sim0.3$) inner eccentricity variations on centuries-long timescales, in accord with the ZKL theorem. Longer-term integrations predict that two of the four systems may become dynamically unstable on $\sim$ Gyr timescales, while in the other two triples common envelope phases and stellar mergers may occur. Finally we investigate the dynamical properties of a sample of 71 KIC/TIC triples statistically, and find that the mutual inclinations and outer mass ratios are anti-correlated at the 4$\sigma$ level. We discuss the implications for the formation mechanisms of compact triples.