FUSE observations of atmospheric eclipsing binaries

We have obtained FUSE observations of the zeta Aur systems HR 6902 and 22 Vul near and at primary eclipse. These systems axe comprised of a G-type luminous primary star and a late B-type main sequence companion, which can be used as a probe of the outer atmosphere of the G star during partial eclipse phases. They axe objects of interest since the G stars are on different sides of the dividing line in the HR diagram between coronal and non-coronal stars. The spectra of these stars are quite different in the FUSE wavelength region. Outside of eclipse, with the secondary at a height of 2.5 R-G*, 22 Vul already shows numerous absorption features from the G star wind superimposed on the B star spectrum. Surprisingly, O VI emission is seen above the local photospheric level with a pronounced self-reversal, which does not change between the pre-eclipse and totality spectra. During primary eclipse, the spectrum of 22 Vul is dominated by emission lines such as Fe II and Fe III arising from the ground state, caused by resonance scattering of the B star photons in the cool star wind. In contrast, HR 6902 shows very little O VI emission and the wind-scattered lines at totality axe weak. However, interstellar reddening and strong H-2 absorption mask some of the diagnostic lines.