2.7 Algols

Algols are eclipsing binaries consisting of a hot, main-sequence primary star of spectral class B to F and a cool, evolved, less massive secondary. Cool stars in Algols are usually G or K subgiants with outer convection envelopes which are in contact with their Roche lobes. Tidal interactions force most of these stars to rotate synchronously with their orbital motions, producing typical rotational velocities between about 30 and 100 km s–1. The rapid rotation combined with deep convection envelopes produces a variety of magnetic activity phenomena including starspots in these stars, similar to RS CVn (see Section 2.4), BY Dra (see Section 2.1), and W UMa binaries (see Section 2.6). Hall (1989Jump To The Next Citation Point) summarised observations indicating magnetic activity in Algols. In addition to strong Ca ii H & K emission, X-ray, and radio emission was detected from a number of Algol binaries which indicated the presence of hot coronae associated with the cool stars (White and Marshall, 1983Stewart et al., 1989Umana et al., 1989).

Brightness variations due to starspots can be observed in Algols only during primary total eclipses, when the luminous hot components are hidden. Therefore, because of synchronised rotation, only one hemisphere of the cool Algol stars can be observed and the photometric data collected are less detailed than for other spotted binaries. Photospheric continuum variations were nevertheless detected in cool components and attributed to the effect of cool starspots (Richards, 1990Olson and Etzel, 1993Varricatt, 2000).

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