Volume 581, September 2015
|Number of page(s)||22|
|Section||Stellar structure and evolution|
|Published online||07 September 2015|
1 Astronomický ústav, Akademie věd České republiky, Fričova 298, 25165 Ondřejov, Czech Republic
2 Departamento de Espectroscopía Estelar, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (UNLP), Paseo del Bosque s/n, B1900FWA, La Plata, Argentina
3 Instituto de Astrofísica de La Plata, CCT La Plata, CONICET-UNLP, Paseo del Bosque s/n, B1900FWA, La Plata, Argentina
4 Dr. Remeis Sternwarte, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
5 Astronomical Institute, Wrocław University, Kopernika 11, 51-622 Wrocław, Poland
6 Matematicko fyzikální fakulta, Univerzita Karlova, 12116 Praha, Czech Republic
7 Tartu Observatory, Tõravere, 61602 Tartumaa, Estonia
8 Matematički Institut SANU, Kneza Mihaila 36, 11001 Beograd, Serbia
9 LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
10 Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, 5030 Casilla Valparaíso, Chile
11 Astronomical Observatory Institute, Faculty of Physics, A. Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland
Received: 21 November 2014
Accepted: 28 June 2015
Context. Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the star’s evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind.
Aims. To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind.
Methods. We modeled the H, He i, Si ii, and Si iii lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations.
Results. The Hα line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7–2 on different timescales. We also observe changes in the ionization rate of Si ii and determine a multiperiodic oscillation in the He i absorption lines, with periods ranging from a few hours to 22.5 days.
Conclusions. We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of α Cyg variables.
Key words: stars: early-type / supergiants / stars: winds, outflows / stars: mass-loss / stars: individual: 55 Cygnus / stars: activity
Based on observations taken with the Perek 2m telescope at Ondřejov Observatory, Czech Republic, and the Poznan Spectroscopic Telescope 2 at the Winer Observatory in Arizona, USA.
Tables 1 and 2, Figs. 3 to 7 are available in electronic form at http://www.aanda.org
© ESO, 2015
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