View here: Details for Spectroscopy requested for the Be star BU Tau = Pleione
Abstract: Spectroscopist Ernst Pollmann (Leverkusen, Germany) writes: For Be star observers: A rare event is imminent: The Hα spectrum of the Be star BU Tau (= 28 Tau = Pleione) will soon appear almost completely devoid of central shell absorption lines.
This phenomenon is linked to the precession of the circumstellar disk, caused by the tidal torque of the companion star, which alters the disk inclination from a nearly edge-on to a nearly pole-on orientation by approximately i ≈ 30°.
This change is reflected in variations of the Hα equivalent width (EW) and the central absorption features.
The process recurs on a timescale of approximately 30 years.
Justification: To study this rare event, spectra are urgently desired from now through July 2026.