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Session 43 - Binary Stars.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
We present an observation of the eclipsing binary V471 Tauri by the Extreme Ultraviolet Explorer\/ (EUVE\/). The extreme ultraviolet (EUV) spectrum of V471 Tauri is dominated by that of the hot white dwarf but, in addition, a small fraction of the flux is radiated in emission lines with He II at \lambda304 Å\ being the strongest line. We find the former line to be in correlation with the orbital phase of the binary system with the maximal intensity observed when the K star is at superior conjunction. We discuss the possibility that a fraction of the emission is fluorescence-induced and results from the illumination of the K star by the continuum of the hot white dwarf. In addition to a clear observation of the eclipse of the white dwarf at phase 0.0, we saw a dip (\approx50% depth) appearing at orbital phase -0.12 and fading away 3 orbits of the system later. The dip in the light curve is detected only in the short wavelength (70--190 Åand medium wavelength (140--380 Åspectrometers; the non-detection in the long wavelength (300--750 Åspectrometer suggests that the white dwarf EUV flux is scattered by ionized plasma at a temperature of about 10^6 K\@. We speculate that the occulter is a large coronal loop from the K star located between the two stars. We have also derived an interstellar column density by modeling the white dwarf continuum and obtain a surprisingly low value of about 10^18 cm^-2 for N_H. This work is supported by NASA contract NAS5-29298.
