DYNAMIC PROCESSES IN Be STAR ATMOSPHERES. Ⅲ. RAPID MULTIWAVELENGTH VARIABILITY IN γ CASSIOPEIAE

摘要

A review of the characteristics of X-ray emission from γ Cas suggests various weaknesses with conventional scenarios that invoke Be wind interactions with a putative binary companion. To explore whether this emission might arise instead from sporadic high-energy events on the Be star itself, we have examined the temporal behavior of Voyager 1 FUV spectra and He i λ6678 line profiles. The FUV continuum light curves show a continuous rapid variability with a range of ≥ 0.2 mag over a timescale of ～1 hr. All indications are that this variability is real. The difference spectrum of the inferred fluctuating component is identical to the photo-spheric spectrum, ruling out an origin much hotter than the star's effective temperature. A histogram of flux differences formed from consecutive observations shows a flat-topped distribution out to 0.2 mag. This result bears a striking resemblance to histograms constructed from EXOSAT and Ginga data, suggesting that the X-ray and FUV variabilities could have a common origin. Evidence is also presented of rapid variability in optical spectral lines associated with the star's photosphere. Time series of difference profiles of the λ6678 line confirm the existence of migrating subfeatures reported by earlier studies. The comoving average of these profiles shows that these features have a shape resembling a symmetrical "wave packet." Wave packets appear to be preceded and rejuvenated by a sharp central absorption feature. We also report a transient emission feature in λ6678 that is similar to brief emissions once reported by Slettebak & Snow in UV resonance line profiles of Si Ⅳ and Mg Ⅱ. We argue that the FUV and optical variability may find an explanation in terms of magnetic flares. Solar white-light flares offer a possible paradigm. However, problems remain with such explanations, in particular their high mean energy, frequency, and temperature.