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

TL;DR

V1371 Tau is characterized as a rare hierarchical triple comprising an inner eclipsing B-type binary and an outer classical Be star. By integrating TESS photometry, multi-epoch high-resolution spectroscopy, and interferometric distance information, the authors derive the inner orbit (P ≈ 33.62 days, e ≈ 0.27, i ≈ 88°, M_A ≈ 14 M☉, M_B ≈ 11 M☉) and atmospheric parameters (T_eff ≈ 25–28 kK; v sin i ≈ 160–250 km s⁻¹) for the EB components, while identifying the Be star as the fast-rotating outer companion (v sin i ≈ 250 km s⁻¹, v/v_crit ≈ 0.44–0.76). The Be component's disk shows month-scale Hα variability and a Keplerian signature, with the system located at ≈1.20 kpc and ≈10 Myr old. The work reveals apsidal motion and potential Kozai–Lidov inclination variations, underscoring the complexity of Be stars in multiple systems and motivating future spectral disentangling and interferometric campaigns to fully map the three-body dynamics and evolutionary history.

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$α$ 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.

Figures (13)

• Figure 1: The figure shows the aperture masks used to extract the flux of V1371 Tau (left panel) and the background flux (right panel), as observed by TESS in Sector 43. Black dots indicate field sources from the Gaia catalog with magnitude $G \leq 13$.
• Figure 2: Light curve of V1371 Tau observed by TESS in flux at BTJD (BJD-2,457,000).
• Figure 3: Comparison between the sum (orange solid line) of the best-fit TLUSTY spectral models for each component of V1371 Tau (non-solid lines), and the observed Goodman spectrum (black solid line) obtained on 2023-02-11. The laboratory wavelength of the HeI line, $\lambda$ 6678.152 Å, is indicated by the vertical black line.
• Figure 4: Distributions of effective temperature, surface gravity, projected rotational velocity, and $\chi^2$ values for the three stellar components, providing a view of both parameter discrete sampling strategy adopted in this analysis. The highlighted points mark the best-fit solutions (lowest $\chi^2$) for each parameter combination.
• Figure 5: Left and Right Panel: The dynamical spectra of the HeI $\lambda$4921.931 Å and HeI $\lambda$6678.152 Å in V1371 Tau, phased with the system’s orbital period of P = 33.6209 d. Colour-scales indicate the continuum-normalised flux F$_\lambda$. The orange and green dots are the observed points with their respective error bars, and the black solid line shows the radial-velocity curve model of the system.; Middle Panel: A corner plot generated using the emcee sampler illustrating one- and two-dimensional posterior distributions of free parameters derived from the Eq. \ref{['eq:rv']} model fitted to the V1371 Tau inner binary radial velocity points for HeI $\lambda$4921.931 Å and HeI $\lambda$6678.152 Å lines.