Disk Evolution of Classical Be Stars with Misaligned Binary Companions

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Classical Be stars are rapidly rotating B type stars that are surrounded by a gaseous circumstellar decretion disk. Like many massive stars, Be stars are frequently found to exist in binary systems, with a wide array of orbital configurations. Recently, we have computed three- dimensional (3D) smoothed particle hydrodynamics (SPH) models of Be stars with equal-mass binary companions whose orbit is misaligned to the initial plane of the disk and found that the misaligned companion can cause the disk to undergo phenomena of Kozai–Lidov oscillations, and disk-tearing, in addition to the expected tilting and warping of the disk. We now show that these phenomena are not unique to equal-mass systems, but that there is a range of mass ratios for which disk-tearing and Kozai–Lidov oscillations can occur. We also find that an increase in viscosity can suppress these phenomena. Next, we combine our SPH models with the 3D nonlocal thermodynamic equilibrium Monte Carlo radiative transfer code HDUST to produce synthetic spectra of these systems. We show how both phenomena can change the Hα emission line profile, and how disk-tearing creates a detectable polarization signature. We also present a comparison of the observable trends in our disk-tearing model, and how these mimic the changes observed in Pleione (28 Tau). Overall our results demonstrate how phenomena from a misaligned binary companion can cause variations in observables across the whole disk, and that these phenomena need to be considered when analyzing the time evolution of Be star spectra.