Short-term variability and mass loss in Be stars

III. BRITE and SMEI satellite photometry of 28 Cygni^★,★★

¹ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
e-mail: dbaade@eso.org
² Astronomical Institute, Wrocław University, Kopernika 11, 51-622 Wrocław, Poland
³ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Casilla 19001, Santiago 19, Chile
⁴ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil
⁵ Observatório Nacional, Rua General José Cristino 77, São Cristóvão RJ-20921-400, Rio de Janeiro, Brazil
⁶ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
⁷ Institut für Kommunnikationsnetze und Satellitenkommunikation, Technical University Graz, Inffeldgasse 12, 8010 Graz, Austria
⁸ Institute of Astrophysics, University of Vienna, Universitätsring 1, 1010 Vienna, Austria
⁹ Département de physique and Centre de Recherche en Astrophysique du Québec (CRAQ), Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, H3C 3J7, Canada
¹⁰ Institute of Automatic Control, Silesian University of Technology, Akademicka 16, Gliwice, Poland
¹¹ Department of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, Ontario K7K 7B4, Canada
¹² Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstrasse 25, 6020 Innsbruck, Austria

Received: 17 May 2017
Accepted: 6 October 2017

Abstract

Context. Be stars are important reference laboratories for the investigation of viscous Keplerian discs. In some cases, the disc feeder mechanism involves a combination of non-radial pulsation (NRP) modes.

Aims. We seek to understand whether high-cadence photometry can shed further light on the role of NRP modes in facilitating rotation-supported mass loss.

Methods. The BRITE-Constellation of nanosatellites obtained mmag photometry of 28 Cygni for 11 months in 2014–2016. We added observations with the Solar Mass Ejection Imager (SMEI) in 2003–2010 and 118 Hα line profiles, half of which were from 2016.

Results. For decades, 28 Cyg has exhibited four large-amplitude frequencies: two closely spaced frequencies of spectroscopically confirmed g modes near 1.5 c/d, one slightly lower exophotospheric (Štefl) frequency, and at 0.05 c/d the difference (Δ) frequency between the two g modes. This top-level framework is indistinguishable from η Cen (Paper I), which is also very similar in spectral type, rotation rate, and viewing angle. The circumstellar (Štefl) frequency alone does not seem to be affected by the Δ frequency. The amplitude of the Δ frequency undergoes large variations; around maximum the amount of near-circumstellar matter is increased and the amplitude of the Štefl frequency grows by a factor of a few. During such brightenings dozens of transient spikes appear in the frequency spectrum; these spikes are concentrated into three groups. Only 11 frequencies were common to all years of BRITE observations.

Conclusions. Be stars seem to be controlled by several coupled clocks, most of which are not very regular on timescales of weeks to months but function for decades. The combination of g modes to the slow Δ variability and/or the atmospheric response to it appears significantly non-linear. As in η Cen, the Δ variability seems to be mainly responsible for the modulation of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of Δ frequencies may reach the longest known timescales of the Be phenomenon.