Short-term variability and mass loss in Be stars

Abstract

Context. Empirical evidence for the involvement of nonradial pulsations (NRPs) in the mass loss from Be stars ranges from (i) a singular case (μ 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 η and μ Cen. Results. In the low-inclination star μ 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 η 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 highamplitude circumstellar activity running with the incoherent and slightly lower so-called Štefl frequency. The mass-loss-modulation cycles are tightly coupled to variations in the value of the Štefl 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 Štefl frequencies. Bursts in power spectra during mass-loss events can be understood as the noise inherent to these gas flows.

Beyond the Kepler/K2 bright limit: variability in the seven brightest members of the Pleiades

“…We note, however that the oscillation spectra of these presumed SPB pulsations are qualitatively quite different from those seen in Alcyone and Electra, as well as Taygeta, which we discuss next, both in terms of amplitude and the spacing between modes. The oscillations in Merope show more similarity with pulsations detected in other Be stars from the ground, including the beating between several frequencies, which has been advanced as a possible explanation for the production of the decretion disc (Rivinius et al 2003;Kee et al 2014;Baade et al 2016). Of course, the SPB pulsations seen in Alcyone, Electra, and Taygeta are at such a low amplitude that they would not be observed from the ground.…”

Studying the photometric and spectroscopic variability of the magnetic hot supergiant ζ Orionis Aa

“…Various processes could lead to increased mass loss, yet only a limited amount would do this in a periodic manner. Beating effects between NRPs would be such a process, similar to the mass-loss mechanism responsible for the Be phenomenon (e.g., Rivinius et al 1998Rivinius et al , 2003Neiner et al 2013;Baade et al 2016). However, we did not detect any stellar pulsations for ζ Ori Aa.…”

“…In the case of differences between the onboard stacking, the data should be treated with care, and detailed techniques or weighting are advised. A common method is to merge each satellite passage to one individual datapoint (e.g., Baade et al 2016). This is only advisable, however, when one is not interested in the high-frequency region (i.e., above f sat /2), the low-frequency regime (i.e., below ∼0.5 d −1 ) or when only considering model fitting.…”

Be discs in coplanar circular binaries: Phase-locked variations of emission lines

“…The most probable scenario is that mass is ejected due to instabilities caused by non-radial pulsations (NRP; Rivinius et al 2003), in a probably non-continuous and non-isotropical manner. The NRP frequencies are connected to the short-term variability of Be stars (Baade et al 2016), probably because of the non-axisymmetric ejections of mass that they cause. The rapid rotation of Be stars assists the mass particles with escaping the photosphere, which makes the disc start being built up.…”