Peter Quigley

The origin of the near-critical rotation of classical Be stars remains uncertain. Binary mass transfer provides a formation channel, but it cannot explain Be stars that are observed without a close companion. We investigate V1371 Tau, a hierarchical triple containing an inner B+B binary and an outer Be star. With n -body simulations, we first show that the observed inclination variations of the B+B binary do not require von Zeipel–Lidov–Kozai (ZLK) oscillations, as nodal precession alone can produce substantial changes in inclination relative to the line of sight. We then consider whether V1371 Tau formed from a primordial 2+2 quadruple, in which a progenitor binary of the Be star underwent ZLK-driven eccentricity excitation and merged. We find that mergers occur over a broad range of progenitor binary parameters while leaving the observed B+B binary intact. The merger remnant is expected to retain the orbital angular-momentum direction at the time of merger, producing a potentially observable misalignment between the Be-star spin axis and its orbit around the surviving B+B binary. These results demonstrate that ZLK-driven mergers in higher-order multiple systems provide a viable formation pathway for rapidly rotating Be stars without present-day close companions.

Angular momentum transport is a fundamental process shaping the structure, evolution, and lifespans of stars and disks across a wide range of astrophysical systems. Be stars offer a valuable environment for studying viscous transport of angular momentum in massive stars, thanks to their rapid rotation, observable decretion disks, and likely absence of strong magnetic fields. This study analyzes angular momentum loss in 40 Be binary simulations spanning a range of orbital separations and companion masses, using a smoothed-particle hydrodynamics (SPH) code. A novel framework is introduced to define the outer disk edge based on the behavior of the azimuthal velocity, streamlining the analysis of angular momentum transport within the system. Applying this framework reveals that systems with smaller truncation radii tend to reaccrete a larger fraction of their angular momentum during dissipation, thereby inhibiting the stars ability to regulate its surface rotation. Modification of this rate may alter the star's mass-injection duty cycle or long-term evolutionary track. Finally, a subset of the simulations were post-processed using the Monte Carlo radiative transfer code \texttt{HDUST}, generating synthetic observables including H$α$ line profiles, V-band polarization, and UV polarization. Suggestions for observational verification of the dynamical results are demonstrated using the simulated data.