“Be Stars in Binary Systems”

Abstract

Be stars are fast-rotating B-type stars that display sometimes Balmer emission lines, especially Hα, produced in circumstellar decretion disks. In this thesis, we studied the role of binarity in these objects, aiming to determine their stellar and orbital parameters and to assess how companions affect disk dynamics, angular momentum, and variability. While rotation and non-radial pulsations are relevant, observational evidence shows that binary interaction is crucial for the Be phenomenon. To investigate this, we combined TESS photometry, BeSS/SOAR spectroscopy, and Gaia DR3 astrometry, applying frequency analysis, spectral modeling, and orbital solutions with Wilson–Devinney and MCMC methods. From our main sample list of 158 targets, we refined to 50 confirmed Be binaries, only two of which are eclipsing, while excluding non-Be binaries, non-binaries, and poorly observed systems. We found that the Be stars rotate at a large fraction of their critical velocity, with a range of values of v sin i/vcrit ≈ 0.30 − 0.85. The frequency analysis of TESS light curves revealed 3 dominant periodicities per target on average, and we identified harmonics up to the 5th order in one system. The Be stars exhibit relatively large effective temperatures and radii when compared to typical values observed for main-sequence stars. A highlight of this work is the system V1371 Tau, a hierarchical triple. For this system, we determined an eccentric inner orbit with Porb ∼ 34 days and e ∼ 0.27, and we detected a third body that significantly affects the dynamics. The system consists of a detached eclipsing B-type binary (B1V + B0V) orbited by a classical Be star (B0Ve). Photometric analysis reveals dominant low-frequency signals near 0.24–0.26 c/d, alongside higher-frequency signals attributed to non-radial pulsations of the Be star. Its projected rotation (v sin i ≈ 250 km s−1) corresponds to a critical rotation fraction between 0.44 and 0.76, substantially faster than the inner components (v sin i ∼ 160–200 km s−1). This study emphasizes the complexity of systems with Be stars and provides a basis for future research on their formation, evolution, and dynamics.