The Triple System V1371 Tau: An Eclipsing Binary with an Outer Be Star

TL;DR

This study confirms V1371 Tau as a rare triple system with an eclipsing binary and a Be star, revealing complex orbital dynamics, stellar properties, and variability, advancing understanding of triple star evolution.

Contribution

It provides detailed observational analysis of V1371 Tau as a triple system with a Be star, highlighting its orbital dynamics, stellar characteristics, and variability, which was previously poorly understood.

Findings

Confirmed V1371 Tau as a triple system with an eclipsing binary and Be star.

Identified orbital periods and variability patterns, including non-radial pulsations.

Suggested orbital inclination variations possibly caused by Kozai-Lidov cycles.

Abstract

Although triple systems are common, their orbital dynamics and stellar evolution remain poorly understood. We investigated the V1371 Tau system using TESS photometry, multi-epoch spectroscopy, and recent interferometric data, confirming it as a rare triple system consisting of an eclipsing binary orbited by a classical Be star, with a spectral classification of (B1V + B0V) + B0Ve. The eclipsing binary exhibits an orbital period of approximately 34 days, and the Be star orbits the inner pair on a timescale of a few years. Weak H$\alpha$ emission lines suggest the presence of a Keplerian disk with variability on a timescale of months around the Be star, and a nearly constant V/R ratio with no detectable asymmetry variations. Besides the eclipses, frequencies at 0.24 and 0.26 c/d dominate the photometric variability. Higher-frequency signals are present which appear associated with non-radial pulsation. The eclipsing pair ($i \approx 90^\circ$) shows projected rotational velocities of 160 and 200 km s$^{-1}$. The Be star's measured $v \sin i \approx 250$ km s$^{-1}$ implies a critical rotation fraction between 0.44 and 0.76 for plausible inclinations, significantly faster than the eclipsing components. The shallower eclipses in the KELT data compared to TESS suggest a variation in orbital inclination, possibly induced by Kozai-Lidov cycles from the outer Be star. The evolution analysis suggests that all components are massive main-sequence stars, with the secondary star in the eclipsing binary being overluminous. This study emphasizes the complexity of triple systems with Be stars and provides a basis for future research on their formation, evolution, and dynamics.