BU Tauri (Pleione) Transtioning from Be-shell Phase to Becoming a Pure Be Star

Volume 53 number 2 (2025)

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Ernst Pollmann

Eml-Nolde-Str, 12, 51375 Leverkusen, Germany; ernst-pollmann@t-online.de

Abstract

This report presents observations of the onset of a rare spectroscopic phase in the classical Be star 28 Tau (BU Tauri, HD 23862, Pleione), characterized by the temporary disappearance of the central absorption dip in the Hα emission profile. This phenomenon is interpreted as a consequence of the precessional motion of the circumstellar disk, driven by tidal torques from an unseen binary companion. The precession leads to a gradual change in the disk inclination relative to the observer’s line of sight, from a near edge-on to a near pole-on orientation, corresponding to an inclination variation of Δi ≈ 30°. This geometrical evolution results in distinct spectroscopic signatures, including systematic variations in the equivalent width (EW) of the Hα line and in the depth of the central depression (CD). The full precessional cycle, together with its associated spectroscopic variability, unfolds over a timescale of approximately 34 years.