Disc Tearing in a Be Star: Predicted 3D Observations

Abstract: We build on our previous work involving smoothed particle hydrodynamic simulations of Be stars, by using the model that exhibited disc tearing as input into the three-dimensional Monte Carlo radiative transfer code HDUST to predict observables from a variety of viewing angles throughout the disc tearing process. We run one simulation at the start of each orbital period from 20 to 72 orbital periods, which covers two complete disc tearing events. The resulting trends in observables are found to be dependent on the relative position of the observer and the tearing disc. The $\rm H\alpha$ equivalent width, $V$ magnitude, and polarization can all increase or decrease in any combination depending on the viewpoint of the observer. The $\rm H\alpha$ line profile also displays changes in strength and peak separation throughout the tearing process. We show how the outer disc of the torn system can have a large effect on the $\rm H\alpha$ line profile, and also contributes to a wavelength-dependent polarization position angle, resulting in a similar sawtooth shape to the polarization percentage. Finally, we compare our predictions to Pleione (28 Tau) where evidence has suggested that a disc tearing event has occurred in the past. We find that our tearing disc model can broadly match the trends seen in Pleione's observables, as well as produce the two-component $\rm H\alpha$ lines observed in Pleione. This is the strongest evidence, thus far, of Pleione's disc having indeed experienced a tearing event.