Short-term variability and mass loss in Be stars

Abstract

Context. Classical Be stars have been established as pulsating stars. Space-based photometric monitoring missions contributed significantly to that result. However, whether Be stars are just rapidly rotating SPB or β Cep stars, or whether they have to be understood differently, remains debated in the view of their highly complex power spectra. Aims. Kepler data of three known Be stars are re-visited to establish their pulsational nature and assess the properties of additional, non-pulsational variations. The three program stars turned out to be one inactive Be star, one active, continuously outbursting Be star, and one Be star transiting from a non-outbursting into an outbursting phase, thus forming an excellent sample to distill properties of Be stars in the various phases of their life-cycle. Methods. The Kepler data was first cleaned from any long-term variability with Lomb-Scargle based pre-whitening. Then a LombScargle analysis of the remaining short-term variations was compared to a wavelet analysis of the cleaned data. This offers a new view on the variability, as it enables us to see the temporal evolution of the variability and phase relations between supposed beating phenomena, which are typically not visualized in a Lomb-Scargle analysis. Results. The short-term photometric variability of Be stars must be disentangled into a stellar and a circumstellar part. The stellar part is on the whole not different from what is seen in non-Be stars. However, some of the observed phenomena might be to be due to resonant mode coupling, a mechanism not typically considered for B-type stars. Short-term circumstellar variability comes in the form of either a group of relatively well-defined, short-lived frequencies during outbursts, which are called Štefl frequencies, and broad bumps in the power spectra, indicating aperiodic variability on a time scale similar to typical low-order g-mode pulsation frequencies, rather than true periodicity. Conclusions. From a stellar pulsation perspective, Be stars are rapidly rotating SPB stars, that is they pulsate in low order g-modes, even if the rapid rotation can project the observed frequencies into the traditional high-order p-mode regime above about 4 c/d. However, when a circumstellar disk is present, Be star power spectra are complicated by both cyclic, or periodic, and aperiodic circumstellar phenomena, possibly even dominating the power spectrum.

γCas stars: Normal Be stars with discs impacted by the wind of a helium-star companion?

“…There is no such report for γ Cas (apart from the optical broad-band flux). Radial pulsations are not known in Be stars but both short periods are well within the range of NRPs found in other Be stars (Rivinius et al 2016;Baade et al 2017;Semaan et al 2018). Since the 1.216 d variability faded while the 0.403 d variability rose, it is plausible to believe that both are of the same nature, which can, then, only be NRPs.…”

“…Since the 1.216 d variability faded while the 0.403 d variability rose, it is plausible to believe that both are of the same nature, which can, then, only be NRPs. In fact, space photometry (Rivinius et al 2016;Baade et al 2017;Semaan et al 2018) has detected multiple low-order NRP modes in many Be stars over the full range of B-type stars.…”

Signatures of internal rotation discovered in theKeplerdata of five slowly pulsating B stars

“…Because of the complex beating patterns originating in many closely spaced frequencies and the limited duration of the outbursts, it is impossible to use a window width for the STFT that is long enough to avoid visual artefacts, but short enough so that it recovers some of the temporal changes that occur during an outburst. When the temporal resolution is chosen to be too small, such as in the wavelet analysis performed in Rivinius et al (2016), dense frequency groups remain unresolved. This implies that the observed changes in amplitude are simply a result of the beating of the unresolved modes.…”

The accelerating rotation of the magnetic He-weak star HD 142990

“…This allows for much better background and trend corrections, since these are individual to each camera. The computational methods for this are described by Rivinius et al (2016). In detail the dataset was split into three per-camera subsets.…”

“…Our analysis of the new reduction of the SMEI light curve, obtained between 2003 and 2011, finds Prot = 0.97890(5) d from the full dataset. The analysis follows the same methods described by Rivinius et al (2016), but due to the relatively low data quality the wavelet analysis is hardly distinguishable from noise. The data were analysed in several subsets, by camera, by year, and over several years.…”