Properties and nature of Be stars ^*,**,***

XXI. The long-term and the orbital variations of = 59 Cyg

¹ Astronomical Institute of the Charles University, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
² Astronomical Institute, Academy of Sciences, 251 65 Ondřejov, Czech Republic e-mail: hec(bozic)@sunstel.asu.cas.cz
³ Hvar Observatory, Faculty of Geodesy, Kačićeva 26, 10000 Zagreb, Croatia e-mail: hbozic(dsudar,domagoj)@hvar.geof.hr
⁴ Erindale College and Department of Astronomy, University of Toronto, Mississauga, ON L5L IC6, Canada e-mail: jpercy@erin.utoronto.ca
⁵ Department of Physics and Astronomy, University of Victoria, PO Box 3055 STN CSC, Victoria, B.C., Canada V8W 3P6 e-mail: yang@uvastro.phys.uvic.ca
⁶ National Astronomical Observatory, Rozhen, Bulgaria and Isaak Newton Institute of Chile, Bulgarian Branch e-mail: liliev@libra.astro.bas.bg
⁷ Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, PR China e-mail: hlin@class1.bao.ac.cn
⁸ Association des Utilisateurs de Détecteurs Électroniques (AUDE), 28 rue du Pic du Midi, 31130 Quint-Fonsegrives, France e-mail: christian.buil@cnes.fr
⁹ Dept. of Astronomy, University of Guanajuato, 36000 Guanajuato, GTO, Mexico e-mail: eenens@astro.ugto.mx

Corresponding author: P. Harmanec, hec@sunstel.asu.cas.cz

Received: 25 January 2002
Accepted: 25 March 2002

Abstract

 An analysis of numerous homogenized UBV photoelectric observations and red spectra of the Be star from several observatories led to the following principal findings: 1. Pronounced long-term light and colour variations of result from a combination of two effects: from the gradual formation of a new Be envelope, and from an asymmetry and a slow revolution of the envelope (or its one-armed oscillation). The colour variations associated with the envelope formation are characterized by a positive correlation between brightness and emission strength, typical for stars which are not seen roughly equator-on. 2. The V magnitude observations prewhitened for the long-term changes follow a sinusoidal orbital light curve with a small amplitude and a period of 281971 which is derived from observations spanning 43 years. This independently confirms a 12-year old suggestion that the star is a spectroscopic binary with a 29-d period. thus becomes the fifth known Be star with cyclic long-term variations, the duplicity of which has been proven, the four other cases being ζ Tau, V923 Aql, γ Cas and X Per. Therefore, the hypothesis that the long-term variations may arise due to the attractive force of the binary companion at certain phases of the envelope formation is still worth considering as a viable alternative to the model of one-armed oscillation. 3. We have shown that the RV and variations of the Hα and He I 6678 emission lines are all roughly in phase. In particular, the He I 6678 emission also moves with the Be primary which differs from what was found for another Be binary, φ Per. 4. We derived the orbital elements and found that in spite of the remaining uncertainties, the basic physical properties of the 282 binary are well constrained. 5. The light minimum of the orbital light curve occurs at elongation when the Be star is approaching us and the object becomes bluest in and reddest in at the same time. This may indicate that a part of the optically thick regions of the envelope is eclipsed at these orbital phases.