The Role of Disk Tearing and Precession in the Observed Variability of Pleione

We acquired Hα spectroscopic observations from 2005 to 2019 showing Pleione has transitioned from a Be phase to a Be-shell phase during this period. Using the radiative transfer code HDUST, we created a grid of ~100,000 disk models for Pleione. We successfully reproduced the observed transition with a disk model that varies in inclination while maintaining an equatorial density of ${\rho }_{0}{(r)=3\,\times \,{10}^{-11}(r/{R}_{\mathrm{eq}})}^{-2.7}\,{\rm{g}}\,{\mathrm{cm}}^{-3}$ , and an Hα-emitting region extending to 15 R _eq. We use a precessing disk model to extrapolate the changing disk inclination over 120 yr and follow the variability in archival observations. The best-fit disk model precesses over a line-of-sight inclination between ~25° and ~144° with a precessional period of ~80.5 yr. Our precessing models match some of the observed variability but fail to reproduce all of the historical data available. Therefore, we propose an ad hoc model based on our precessing disk model inspired by recent smoothed particle hydrodynamics simulations of similar systems, where the disk tears due to the tidal influence of a companion star. In this model, a single disk is slowly tilted to an angle of 30° from the stellar equator over 34 yr. Then, the disk is torn by the companion's tidal torque, with the outer region separating from the innermost disk. The small inner disk returns to the stellar equator as mass injection remains constant. The outer disk precesses for ~15 yr before gradually dissipating. The process repeats every 34 yr and reproduces all trends in Pleione's variability.