Short-term variability and mass loss in Be stars I. BRITE satellite photometry of η and μ Centauri

Context. Empirical evidence for the involvement of nonradial pulsations (NRPs) in the mass loss from Be stars ranges from (i) a singular case (mu Cen) of repetitive mass ejections triggered by multi-mode beating to (ii) several photometric reports about enormous numbers of pulsation modes that suddenly appear during outbursts and on to (iii) effective single-mode pulsators. Aims. The purpose of this study is to develop a more detailed empirical description of the star-to-disk mass transfer and to check the hypothesis that spates of transient nonradial pulsation modes accompany and even drive mass-loss episodes. Methods. The BRITE Constellation of nanosatellites was used to obtain mmag photometry of the Be stars eta and mu Cen. Results. In the low-inclination star mu Cen, light pollution by variable amounts of near-stellar matter prevented any new insights into the variability and other properties of the central star. In the equator-on star eta Cen, BRITE photometry and HEROS echelle spectroscopy from the 1990s reveal an intricate clockwork of star-disk interactions. The mass transfer is modulated with the frequency difference of two NRP modes and an amplitude three times as large as the amplitude sum of the two NRP modes. This process feeds a high-amplitude circumstellar activity running with the incoherent and slightly lower so-called Stefl frequency. The mass-loss-modulation cycles are tightly coupled to variations in the value of the Stefl frequency and in its amplitude, albeit with strongly drifting phase differences. Conclusions. The observations are well described by the decomposition of the mass loss into a pulsation-related engine in the star and a viscosity-dominated engine in the circumstellar disk. Arguments are developed that large-scale gas-circulation flows occur at the interface. The propagation rates of these eddies manifest themselves as Stefl frequencies. Bursts in power spectra during mass-loss events can be understood as the noise inherent to these gas flows.