ULTRAVIOLET LIGHT CURVES OF BETA-LYRAE - COMPARISON OF OAO A-2, IUE, AND VOYAGER OBSERVATIONS

The six-band ultraviolet light curves of beta Lyrae obtained with the Orbiting Astronomical Observatory A-2 in 1970 exhibited a very unusual behavior. The secondary minimum deepened at shorter wavelength, indic ating that one was not observing light variations caused primarily by the eclipses of two stars having a roughly Planckian energy distributi on. It was then suggested that the light variations were caused by a v iewing angle effect of an optically thick, ellipsoidal circumbinary ga s cloud. Since 1978 beta Lyrae has been observed with the Internationa l Ultraviolet Explorer (IUE) satellite. We have constructed ultraviole t light curves from the IUE archival data for comparison with the OAO A-2 results. We find that they are in substantial agreement with each other. The Voyager ultraviolet spectrometer was also used to observe t his binary during a period covered by IUE observations. The Voyager re sults agree with those of the two other satellite observatories at wav e-lengths longer than about 1350 Angstrom. However, in the wavelength region shorter than the Lyman-alpha line at 1216 Angstrom, the light c urves at 1085 and 965 Angstrom show virtually no light variation excep t an apparent flaring near phase 0.7, which is also in evidence at lon ger wavelengths. We suggest that the optically thick circumbinary gas cloud, which envelops the two stars completely, assumes a roughly sphe rical shape when observed at these shorter wavelengths.