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P.D. Bennett (CASA, U.Colorado), T.B. Ake (JHU/CSC), G.M. Harper (CASA/ARL, U.Colorado)
Epsilon Aurigae, a bright third-magnitude star in the northern sky, has puzzled astronomers for most of the last century. The optical and ultraviolet spectrum longward of Lyman-\alpha is that of an F~supergiant. It is distinguished by having the longest known period, 27.1 years, of any eclipsing binary. The spectrum implies a massive primary (15--30 M\odot) and the orbital solution implies a comparably massive companion. Although the two-year long primary eclipse is flat-bottomed, which should mean the primary star is totally occulted, no secondary stellar spectrum is seen. Shortward of 1600~Å\ and longward of 5~\mum, the eclipse is shallower. The most favored model for this system, which explains the optical light curve and excess infrared flux, is that the secondary object is a large, dark, cold disk (presumably with an optically-obscured star at its center), seen nearly edge-on, which partially occults the F~star during eclipse.
Recent FUSE observations of the far ultraviolet spectrum of \epsilon~Aur show an emission line spectrum and scaled surface flux quite unlike that of the comparable supergiant \alpha~Car, suggesting the presence of a hot component. The rich emission line spectrum is reminiscent of \zeta~Aurigae stars in eclipse. The \zeta~Aur binaries are supergiant stars that eclipse their main-sequence companions. In these binaries, the emission line spectrum seen during totality is produced by scattering of hot star continuum photons in the wind of the cool supergiant. For \epsilon~Aur, the anomalous FUV spectrum and flux suggest a similar formation: continuum photons from a hot companion embedded in the occulting disk are scattered by the wind of the F~supergiant. The presence of scattered photons down to 1050 Å\ implies the companion has a spectral type earlier than B5. We use a variety of multi-wavelength observations to constrain the parameters of the newly unveiled B-star in \epsilon~Aur.
The author(s) of this abstract have provided an email address for comments about the abstract: pbennett@casa.colorado.edu
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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.