Short-term variability and mass loss in Be stars

Abstract

Context. Be stars are physically complex systems that continue to challenge theory to understand their rapid rotation, complex variability, and decretion disks. γ Cassiopeiae (γ Cas) is one such star but is even more curious because of its unexplained hard thermal X-ray emission. Aims. We aim to examine the optical variability of γ Cas and thereby to shed more light on its puzzling behaviour. Methods. We analysed 321 archival Hα spectra from 2006 to 2017 to search for frequencies corresponding to the 203.5 day orbit of the companion. Space photometry from the SMEI satellite from 2003 to 2011 and the BRITE-Constellation of nano-satellites from 2015 to 2019 were investigated in the period range from a couple of hours to a few days. Results. The orbital period of the companion of 203.5 days is confirmed with independent measurements from the structure of the Hα line emission. A strong blue versus red asymmetry in the amplitude distribution across the Hα emission line could hint at a spiral structure in the decretion disk. With the space photometry, the known frequency of 0.82 d−1 is confirmed in data from the early 2000s. A higher frequency of 2.48 d−1 is present in the data from 2015 to 2019 and possibly in the early 2000s as well. A third frequency at 1.25 d−1 is proposed to exist in both SMEI and BRITE data. Seemingly, only a non-radial pulsation interpretation can explain all three variations. The two higher frequencies are incompatible with rotation.

TESS light curves of γ Cas stars

“…Using fifteen seasons of ground-based APT data, Smith et al (2006) and Henry & Smith (2012) detected a frequency of 0.82 d −1 with variable amplitude. The signal appeared lower in 2004-2011compared to 1997-2003. Borre et al (2020 confirmed these results using SMEI space-based data taken over the same years.…”

“…strength of this signal seems to have further fainted, becoming undetectable. The dominant frequency now seems to be a signal at 2.48 d −1 frequency (Borre et al 2020). The same authors also reported an additional signal at 1.25 d −1 as well as potential detections near 0.97 and 5.06 d −1 .…”

“…The targets display isolated frequencies or a few frequency groups, sometimes in harmonic relation and always appearing at low frequencies ( f < 5 d −1 -only a few faint high-frequency signals are de-tected). The high-cadence, space-borne photometry of three γ Cas stars had already been studied (γ Cas, Borre et al 2020;HD 110432 and π Aqr, Nazé et al 2020b), as well as the high-quality photometric datasets of several "normal" (i.e. non-γ Cas) Be stars (e.g.…”

“…Indeed, the variability directly relates to the disk or the star itself hence it could pinpoint their specificities, if the γ Cas phenomenon arises from the Be star and/or its disk. In Nazé et al (2020b), we already reported the detection of coherent variations from π Aqr and HD 110432 (BZ Cru) while Borre et al (2020) analyzed the case of γ Cas itself. We now extend the study to a larger set of γ Cas analogs.…”

Optical spectroscopy of Galactic field classical Be stars

“…HD 45910 is a known interacting binary system (Koubský et al 2012(Koubský et al , 2011, whereas Cas is also suggested to be a binary system having a possible compact object as a companion (Smith et al 2017). On the contrary, Borre et al (2020) suggested that the X-ray emission in case of Cas might be coming from the CBe star itself, thus ruling out the binarity scenario.…”