AAS 204th Meeting, June 2004
Session 5 Eclipsing Binaries and Friends
Poster, Monday, May 31, 2004, 9:20am-6:30pm, Ballroom

## [5.12] Six Years of HST/STIS Observations of the Eclipsing Binary VV Cephei

P.D. Bennett, A. Brown (University of Colorado), W.H. Bauer (Wellesley College)

VV Cephei (M2~Iab + B0.5~V) is the brightest M~supergiant in an eclipsing binary system in the sky (V=4.90). Its orbital period of 20.3 years is one of the longest known for an eclipsing binary. This system is of great interest because of the possible use of the eclipse-mapping technique developed for the \zeta~Aurigae binaries to construct empirical models of the M~supergiant's extended atmosphere and wind. The method uses circumstellar absorption and scattering features seen superimposed on the early-type companion's continuum near eclipse (when the hot companion passes behind the cool supergiant) to map absorber column densities along the line of sight. During ingress, the line of sight to the hot companion sweeps through increasingly thick layers of the M~supergiant's chromosphere prior to second contact; this sequence proceeds again in reverse during egress. Observing in the ultraviolet is particularly advantageous since there the cool supergiant contributes negligibly to the flux, thereby avoiding the problem of disentangling composite spectra. To this end, we have obtained HST/STIS echelle observations of the ultraviolet spectrum of VV~Cep at 21 epochs over a 6-year period from mid-eclipse in late 1997 through quadrature in 2003. The application of the eclipse-mapping technique to VV~Cep is not straightforward. The high mass loss rate and corresponding massive wind column densities produce a complex, heavily blended forest of circumstellar absorption lines, mainly from singly-ionized iron group elements. Accretion near the hot companion produces additional absorption and a highly-variable accretion continuum. Nevertheless, the science objectives of the project have been realized: there are sufficient, unblended lines of appropriate strengths (near optical depth unity) to permit the reconstruction of column-density maps along the various sightlines, and thus the construction of detailed models of the supergiant's chromosphere and wind. We show some sample spectra and present preliminary modeling results in this poster.