Short-term variability and mass loss in Be stars III. BRITE and SMEI satellite photometry of 28 Cygni

TL;DR

This study analyzes long-term photometric data of 28 Cygni, revealing complex variability patterns, coupled oscillations, and their influence on mass transfer processes in Be stars, with implications for understanding their long-term behavior.

Contribution

It provides a detailed analysis of 28 Cygni's variability, identifying multiple coupled oscillation modes and their role in star-disc interactions, extending knowledge of Be star dynamics.

Findings

Identification of four large-amplitude frequencies in 28 Cygni

Large amplitude variations linked to increased circumstellar matter

Complex, nonlinear interactions among oscillation modes

Abstract

The BRITE Constellation of nanosatellites obtained mmag photometry of 28 Cygni for 11 months in 2014-2016. Observations with the Solar Mass Ejection Imager in 2003-2010 and 118 H$\alpha$ line profiles were added. 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 (Stefl) frequency, and at 0.05 c/d the difference frequency between the two g modes. This top-level framework is indistinguishable from eta Cen (Paper I), which is also very similar in spectral type, rotation rate, and viewing angle. The Stefl frequency is the only one that does not seem to be affected by the difference frequency. The amplitude of the latter undergoes large variations; around maximum the amount of near-circumstellar matter is increased, and the amplitude of the Stefl frequency grows by some factor. During such brightenings dozens of transient spikes appear in the frequency spectrum, concentrated in three groups. Only eleven frequencies were common to all years of BRITE observations. 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 low difference frequency and/or the atmospheric response to it appears significantly nonlinear. Like in eta Cen, the difference-frequency variability seems the main responsible for the modulation of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of difference frequencies may reach the longest timescales known of the Be phenomenon.