Ultraviolet spectrophotometry of variable early-type Be and B stars derived from high-resolution IUE data.
Abstract (from CDS):
High-dispersion IUE data encode significant information about aggregate line absorptions that cannot be conveniently extracted from individual stellar spectra. Here 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 division of binned spectra obtained during a ``bright-star'' phase by spectra from a ``faint-star'' phase leads to a ratioed spectrum that contains information about the mechanism responsible for a star's variability. The most likely candidate mechanisms are either the periodic or episodic occultations of the star by ejected matter or a change in photospheric structure, e.g., from pulsation. We model the variations caused by these mechanism by means of model atmosphere and absorbing-slab codes. Line absorptions strength changes are rather sensitive to physical conditions in circumstellar shells and ``clouds'' at temperatures of 8000-13,000 K, which is the regime expected for circumstellar structures of early B stars. To demonstrate proofs of this concept, we construct spectral ratios for circumstellar structures associated with flux variability in various Be stars: (1) Vela X-1 has a bow-shock wind trailing its neutron star companion; at successive phases and hence in different sectors, the wind exhibits spectrophotometric signatures of a 13,000 or 26,000 K medium; (2) 88 Her undergoes episodic ``outbursts'' during which its UV flux fades, followed a year later by a dimming at visible wavelengths as well; the ratioed spectrum indicates the ``phase lag'' is a result of a nearly gray opacity that dominates all wavelengths as the shell expands from the star and cools, permitting the absorptions in the visible to ``catch up'' to those in the UV; and (3) ζ Tau and 60 Cyg exhibit periodic spectrum and flux changes, which match model absorptions for occulting clouds but are actually most easily seen from selective variations of various resonance lines. In addition, ratioed UV spectra of radial and large-amplitude nonradial pulsating stars show unique spectrophotometric signatures, which can be simulated with model atmospheres. An analysis of ratioed spectra obtained for a representative sample of 18 classical Be stars known to have rapid periodic flux variations indicates that 13 of them have ratioed spectra that are relatively featureless or have signatures of pulsation. Ratioed spectra of three others in the sample exhibit signatures that are consistent with the presence of corotating clouds.