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W. H. Bauer (Wellesley Coll.), P. D. Bennett, A. Brown (CASA, U. Colorado)
VV Cephei is an eclipsing binary system with a 20.3-year period, consisting of an M2 Iab primary and a hot component (B0V?). The hot component has just emerged from total eclipse, and is passing behind successively higher layers of the M supergiant. This hot star is probably largely -- and variably -- obscured in the ultraviolet by a surrounding accretion region. However, its size is still much smaller than the M supergiant (R ~ 1600 R\sun), permitting us to use its orbital motion as a probe to study the structure of the outer atmosphere and wind of the M supergiant. The STIS spectrograph on the Hubble Space Telescope has been observing VV Cep since mid-totality in 1997 November, and has been following the spectrum through egress from eclipse.
During totality, a complex emission spectrum was observed, with a weak continuum that increased to shorter wavelengths. Strong emission lines such as Mg II (UV 1) and the strongest transitions in Fe II (UV 60, 62, 63, and 78) appear as double-peaked emission features with a stronger long-wavelength component, indicative of their formation in an expanding atmosphere. A narrow central absorption feature persists around the entire cycle and is likely due to the M supergiant wind.
Throughout the entire orbital cycle, the hot component remains shrouded in a rich absorption spectrum, mainly due to lines of singly-ionized elements. Absorption is seen from levels up to an excitation potential of ~ 5 eV. In general, these same lines are seen as single emission features during total eclipse.
For the first time, the spectrum as observed during totality can be subtracted from the IUE spectra obtained around nearly a full orbital period. We compare these IUE spectra with the STIS spectra observed during totality and through the beginning of egress from eclipse.
This work has been supported by STScI grant GO-07629.01-96A to the University of Colorado.
The author(s) of this abstract have provided an email address for comments about the abstract: wbauer@wellesley.edu