EPIC 203868608: A low-mass quadruple star system in the Upper Scorpius OB association

TL;DR

This study reveals that EPIC 203868608, previously thought to be a triple star system, is actually a quadruple system with a spectroscopic binary and an eclipsing binary, providing insights into star formation and orbital dynamics.

Contribution

The paper uncovers the quadruple nature of EPIC 203868608 and develops a new analytic framework to measure obliquity in eclipsing binaries.

Findings

EPIC 203868608 is a quadruple star system with a spectroscopic binary and an eclipsing binary.

The obliquity of the eclipsing binary is approximately -57 degrees.

The system's properties constrain tidal interactions in low-mass stars.

Abstract

Young multiple star systems provide excellent testing grounds for theories of star formation and evolution. EPIC 203868608 was previously studied~\citep{David2016} as a triple star system in the Upper Scorpius OB association, but the follow-up Keck NIRC2/HIRES/NIRSPAO observations reported here reveal its quadruple nature. We find that the system consists of a double-lined spectroscopic binary (SB2) Aab (M5+M5) and an eclipsing binary (EB) Bab with a total mass that is lower than that of the SB2. Furthermore, we measure the obliquity of the EB using the Doppler tomography technique during the primary eclipse. EPIC 203868608 Bab is likely on an inclined orbit with a projected obliquity of $-57^{+40}_{-36}$ degrees. The inclined orbit is used to constrain the tidal quality factor for low-mass stars and the evolution of the quadruple system. The analytic framework to infer obliquity that has been developed in this paper can be applied to other EB systems as well as transiting planets.