KIC 2856960: the impossible triple star

TL;DR

This study analyzes the complex eclipsing behavior of KIC 2856960, revealing that a simple triple star model cannot explain the observations, suggesting the need for more complex or unseen stellar configurations.

Contribution

The paper demonstrates the difficulty of modeling KIC 2856960 with a triple star system and explores alternative quadruple star models requiring an additional intermediate period.

Findings

Triple star model cannot reproduce observed dips while satisfying Kepler's laws.

Quadruple star models require an undetected intermediate period of 5-20 days.

Spectroscopy indicates the system is dominated by a K3/K4-type star.

Abstract

KIC 2856960 is a star in the Kepler field which was observed by Kepler for 4 years. It shows the primary and secondary eclipses of a close binary of 0.258d as well as complex dipping events that last for about 1.5d at a time and recur on a 204d period. The dips are thought to result when the close binary passes across the face of a third star. In this paper we present an attempt to model the dips. Despite the apparent simplicity of the system and strenuous efforts to find a solution, we find that we cannot match the dips with a triple star while satisfying Kepler's laws. The problem is that to match the dips the separation of the close binary has to be larger than possible relative to the outer orbit given the orbital periods. Quadruple star models can get round this problem but require the addition of a so-far undetected intermediate period of order 5 -- 20d that has be a near-perfect integer divisor of the outer 204d period. Although we have no good explanation for KIC 2856960, using the full set of Kepler data we are able to update several of its parameters. We also present a spectrum showing that KIC 2856960 is dominated by light from a K3- or K4-type star.