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Astron. Astrophys. 356, 913-928 (2000)


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Properties and nature of Be stars

XIX. Spectral and light variability of 60 Cygni

P. Koubský 1, P. Harmanec 1,2, A.M. Hubert 3, M. Floquet 3, J. Kubát 1, D. Ballereau 3, J. Chauville 3, H. Boi 4, D. Holmgren 1,9, S. Yang 8,10, H. Cao 5, P. Eenens 6, L. Huang 5 and J.R. Percy 7

1 Astronomický ústav, Akademie vd eské republiky, 251 65 Ondejov, Czech Republic (koubsky, hec, kubat; david@sunstel.asu.cas.cz)
2 Astronomický ústav, Matematicko-fyzikální fakulta, Karlova Univerzita, V Holeovikách 2, 180 00 Praha 8, Czech Republic (hec@sirrah.troja.mff.cuni.cz)
3 DASGAL/UMR 8633 du CNRS, Observatoire de Paris-Meudon, 92195 Meudon Principal Cedex, France (Dominique.Ballereau, Jacques.Chauville, Michele.Floquet, Anne-Marie.Hubert@obspm.fr)
4 Opservatorij Hvar Geodetskog fakulteta Sveuilita u Zagrebu, Kaieva 26, 10000 Zagreb, Croatia (hbozic@geodet.geof.hr)
5 Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, P.R. China (hling@bao01.bao.ac.cn)
6 Departamento de Astronomia, Universidad de Guanajuato, Apartado 144, 36000 Guanajuato, GTO, Mexico (eenens@carina.astro.ugto.mx)
7 Erindale College and Department of Astronomy, University of Toronto, Mississauga, ON L5L IC6, Canada (jpercy@erin.utoronto.ca)
8 Department of Physics and Astronomy, University of Victoria, P.O. Box 3055 STN CSC, Victoria B.C. V8W 3P6, Canada (yang@uvastro.phys.uvic.ca)
9 Department of Physics and Astronomy, Brandon University, Brandon, MA  R7A 6A9, Canada (holmgren@brandonu.ca)
10 Visiting Astronomer at the Dominion Astrophysical Observatory, operated by the National Research Council of Canada

Received 24 September 1999 / Accepted 7 January 2000

Abstract

An analysis of electronic spectra secured between 1992 and 1999 at the Haute Provence, Ondejov and Dominion Astrophysical Observatories and of differential UBV measurements of 60 Cyg obtained between 1984 and 1999 at Hvar, San Pedro Mártir, Toronto and Xinglong Observatories, the all-sky Hipparcos satellite [FORMULA] photometry transformed to Johnson V and B magnitudes, and all-sky UBV observations published by several authors and dating back to fifties, led to the following findings:

1. 60 Cyg exhibits pronounced long-term spectral variations characterized by the B [FORMULA] Be [FORMULA] B phase transitions. These long-term spectral changes of 60 Cyg are also accompanied by corresponding, though rather mild, secular light and colour variations. The character of these variations is indicative of a positive correlation between the brightness and emission-line strength.

2. NLTE model atmosphere analysis of spectra secured during the quiescence state (B phase) of 60 Cyg shows that the star has overabundance of helium. Best results were obtained for [FORMULA].

3. The presence of periodic medium-term changes, with a period of 146[FORMULA]6 [FORMULA] 0[FORMULA]6 was found in the radial-velocity of the H[FORMULA] and He I 6678 Å lines. If confirmed by future observations, these variations could indicate that 60 Cyg is a spectroscopic binary.

4. There are clear rapid periodic line-profile changes of (a) overall line asymmetry, and (b) weak sub-features passing across the line profiles every about 0[FORMULA]1. The radial velocity and asymmetry of He I lines vary with a period of 1[FORMULA]0647 and a double-wave curve. There is no evidence of this period in photometry, however.

5. The rapid light variations of 60 Cyg are dominated by rapid changes with a full amplitude of almost 0[FORMULA]1. A period analysis of V magnitude data prewhitened for the long-term changes indicates a period of 0[FORMULA]2997029, reported earlier. The most interesting finding is that also all recorded series of moving sub-features in the line profiles can be reconciled with this period: the sub-features reappear at the same phase intervals of the 0[FORMULA]2997 period in the line profiles over an interval of several years. Considering the acceleration of these sub-features, 1900 km s-1 d-1, it is conceivable that the true physical (super) period of these changes is either 0[FORMULA]8991 or 1[FORMULA]1988.

6. The findings mentioned in points 4 and 5 represent a challenge for the NRP scenario since the light changes would be dominated by a high-order mode instead of a low-order one.

Key words: stars: early-type – stars: emission-line, Be – stars: individual: 60 Cyg – stars: oscillations – stars: variables: general

Send offprint requests to: P. Koubský

© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000

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