Ultraviolet Spectrophotometry of Variable Early-Type Be and B stars Derived from High-Resolution IUE Data

Abstract

High-dispersion IUE data encode significant information about aggregate line absorptions that cannot be conveniently extracted from individual spectra. We apply a new technique in which fluxes from each echelle order of a short wavelength IUE spectrum are binned together to construct low-resolution spectra of a rapidly varying B or Be star. The ratio of binned spectra obtained bright- star and faint-star phases contains information about the mechanism responsible for a star's variability, such as from pulsations or occultations of the star by ejected matter. We model the variations caused by these mechanism by means of model atmosphere and absorbing-slab codes. Line absorptions strength changes are sensitive to conditions in circumstellar clouds with T = 8,000--13,000K. To demonstrate proofs of concept, we construct spectral ratios for circumstellar structures associated with flux variability in various Be stars: (1) Vela X1 has bow-shock wind trailing its neutron star companion and shows signatures of gas at 13,000K or 26,000K medium in different sectors, (2) 88 Her undergoes episodic outbursts as its UV flux fades, followed a year later by a dimming in visible wavelengths, a result of a gray opacity that dominates as the shell expands and cools, and (3) zeta Tau and 60 Cyg exhibit periodic spectrum and flux changes, which match model absorptions for occulting clouds. Also, ratioed UV spectra of strongly pulsating stars show unique spectrophotometric signatures which can be simulated with models. An analysis of ratioed spectra obtained for a typical sample of 18 classical Be stars known to have rapid periodic flux variations indicates that 13 of them have ratioed spectra which are relatively featureless or have signatures of pulsation. Ratioed spectra of 3 others in the sample are consistent with the presence of co-rotating clouds.