Properties and nature of Be stars^*

27. Orbital and recent long-term variations of the Pleiades Be star Pleione = BU Tauri

¹ Astronomical Institute of the Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Praha 8, Czech Republic e-mail: janicka.ari@seznam.cz
² Astronomical Institute of the Academy of Sciences, 251 65 Ondřejov, Czech Republic
³ Institute of Astronomy, Bulgarian Academy of Sciences, 1784, 72 Tsarigradsko Chaussee Blvd., Sofia, Bulgaria
⁴ Physics & Astronomy Department, University of Victoria, PO Box 3055 STN CSC, Victoria, BC, V8W 3P6, Canada
⁵ Observatório do Instituto Geográfico do Exército, R. Venezuela 29, 3 Esq. 1500-618, Lisboa, Portugal

Received: 16 December 2009
Accepted: 19 February 2010

Abstract

Radial-velocity variations of the Hα emission measured on the steep wings of the Hα line, prewhitened for the long-time changes, vary periodically with a period of 218025 ± 0022, confirming the suspected binary nature of the bright Be star BU Tau, a member of the Pleiades cluster. The orbit seems to have a high eccentricity over 0.7, but we also briefly discuss the possibility that the true orbit is circular and that the eccentricity is spurious owing to the phase-dependent effects of the circumstellar matter. The projected angular separation of the spectroscopic orbit is large enough to allow the detection of the binary with large optical interferometers, provided the magnitude difference primary – secondary is not too large. Since our data cover the onset of a new shell phase up to development of a metallic shell spectrum, we also briefly discuss the recent long-term changes. We confirm the formation of a new envelope, coexisting with the previous one, at the onset of the new shell phase. We find that the full width at half maximum of the Hα profile has been decreasing with time for both envelopes. In this connection, we briefly discuss Hirata's hypothesis of precessing gaseous disk and possible alternative scenarios of the observed long-term changes.