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abstract
We have measured some 500 equivalent widths of red and violet H$\alpha$ emission lobes in nine long-period Algol binaries with periods from 7 to 262 days. In most systems, both lobes underwent large, nearly chaotic, variations in times less than or comparable to an orbital period. On one occasion in AD Herculis, emission in both lobes nearly disappeared in just over half the period. Fractional variations generally decreased toward longer period. No clear correlation exists between variations and orbital phase. There was a general increase in emission equivalent width with increasing binary period, as the volume between the mass-gaining star and its Roche lobe increases. The Huang-Struve relation between disk orbital velocity and period holds to P=30 days.
There is no evidence of an actual physical collapse of disk matter; rather, variations in kinetic temperature probably account for emission variations. Contemporary observations of the disk absorption line O I $\lambda$ 7774 show no significant temporal variations. H$\alpha$ emission originates largely in the inner disk, while oxygen absorption arises from the outer parts of the disk, where scale heights are large. Therefore, accretion-disk instability occurs mainly in the inner disk. This behavior mimics that seen in some cataclysmic binaries.
