EDP Sciences
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Volume 387, Number 2, May IV 2002
Page(s) 580 - 594
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20020453

A&A 387, 580-594 (2002)
DOI: 10.1051/0004-6361:20020453

Properties and nature of Be stars

XXI. The long-term and the orbital variations of = 59 Cyg
P. Harmanec1, 2, H. Bozic2, 3, J. R. Percy4, S. Yang5, D. Ruzdjak3, D. Sudar3, M. Wolf1, L. Iliev6, L. Huang7, C. Buil8 and P. Eenens9

1  Astronomical Institute of the Charles University, V Holesovickách 2, 180 00 Praha 8, Czech Republic
2  Astronomical Institute, Academy of Sciences, 251 65 Ondrejov, Czech Republic
    e-mail: hec(bozic)@sunstel.asu.cas.cz
3  Hvar Observatory, Faculty of Geodesy, Kaciceva 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

(Received 25 January 2002 / Accepted 25 March 2002)

 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 28 $^{\rm d}\!\!.$1971 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 V/R variations, the duplicity of which has been proven, the four other cases being $\zeta$ Tau, V923 Aql, $\gamma$ Cas and X Per. Therefore, the hypothesis that the long-term V/R 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 V/R variations of the H $\alpha$ 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, $\varphi$ Per. 4. We derived the orbital elements and found that in spite of the remaining uncertainties, the basic physical properties of the 28 $^{\rm d}\!\!.$2 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 (B-V) and reddest in (U-B) at the same time. This may indicate that a part of the optically thick regions of the envelope is eclipsed at these orbital phases.

Key words: stars: emission-line, Be -- stars: binaries: close -- stars: binaries: spectroscopic -- stars: fundamental parameters -- stars: individual:

Offprint request: P. Harmanec, hec@sunstel.asu.cas.cz

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