Short-term variability and mass loss in Be stars II. Physical taxonomy of photometric variability observed by the Kepler spacecraft

TL;DR

This study analyzes Kepler data of three Be stars to distinguish stellar pulsations from circumstellar variability, revealing that Be stars are rapidly rotating SPB stars with complex photometric behaviors influenced by their circumstellar disks.

Contribution

It provides a detailed physical taxonomy of photometric variability in Be stars, highlighting the interplay between stellar pulsations and circumstellar phenomena observed by Kepler.

Findings

Be stars are rapidly rotating SPB stars pulsating in low-order g-modes.

Circumstellar variability manifests as short-lived frequencies and broad power spectrum bumps.

Power spectra are dominated by circumstellar phenomena when disks are present.

Abstract

Context: [abbreviated] 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: [abbreviated] 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 \v{S}tefl 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.