V994 Her: A Unique Triply Eclipsing Sextuple Star System

TL;DR

This paper reports the discovery and analysis of V994 Her as a unique sextuple star system with three eclipsing binaries, using TESS and ground-based data to characterize their orbits and mutual interactions.

Contribution

It is the first detailed characterization of a fully eclipsing (2+2)+2 sextuple system, revealing complex orbital dynamics and mutual perturbations.

Findings

V994 Her is a triply eclipsing sextuple system.

Orbital period of the inner quadruple is approximately 1062 days.

Detected eclipse timing variations indicate mutual star perturbations.

Abstract

We report the discovery with $TESS$ of a third set of eclipses from V994 Herculis (TIC 424508303), previously only known as a doubly-eclipsing system. The key implication of this discovery and our analyses is that V994 Her is the second fully-characterized (2+2) + 2 sextuple system, in which all three binaries eclipse. In this work, we use a combination of ground-based observations and $TESS$ data to analyze the eclipses of binaries A and B in order to update the parameters of the inner quadruple's orbit (with a derived period of 1062 $\pm$ 2d). The eclipses of binary C that were detected in the $TESS$ data were also found in older ground-based observations, as well as in more recently obtained observations. The eclipse timing variations of all three pairs were studied in order to detect the mutual perturbations of their constituent stars, as well as those of the inner pairs in the (2+2) core. At the longest periods they arise from apsidal motion, which may help constraining parameters of the component stars' internal structure. We also discuss the relative proximity of the periods of binaries A and B to a 3:2 mean motion resonance. This work represents a step forward in the development of techniques to better understand and characterize multiple star systems, especially those with multiple eclipsing components.