RS RS Canum Venaticorum stars

- This type is ascribed to close binary systems with spectra showing Ca II H and K in emission, their components having increased chromospheric activity that causes quasi-periodic light variability. The period of variation is close to the orbital one, and the variable amplitude is usually as great as 0".'2 in V. They are X-ray sources and rotating variables. RS CVn itself is also an eclipsing system. GCVS

"Canis Venatici, the Hunting Dogs, identified as Asterion and Chara, are usually depicted as greyhounds held on a leash by Bootes, the Herdsman.

As stated earlier, RS variables can be a confusing class of stars because this classification appears in the General Catalog of Variable Stars twice, in two of the main classes. As a group, these are relatively "new" stars since it wasn't until 1976 that Douglas Hall defined RS CVn binaries. RS CVn binaries have been, at least superficially, known as a subset of Algol-like eclipsing binaries, but with unusual properties that distinguish them from normal Algol-type variables.

Much speculation exists regarding the evolutionary status of these systems; in particular, regarding the number of stars with equal mass and because one star can be a red giant while the other is close to the main sequence. Such a configuration is difficult to understand. Several theories, some quite exotic, have been suggested to explain the formation of these interesting stars. They include: fission of a main sequence star, stars still in a state of pre-main sequence contraction, and evolved single stars with the more massive star losing some mass as it crosses the Hertzsprung gap.

RS variables, as a result of the underlying physics, can confound casual observation so extreme care must be taken when observing these stars. These chromospherically active stars vary in brightness on a variety of different time-scales: some are literally periodic, some are not strictly periodic and some can only be described in terms of a long time-scale. Some chromospherically active binaries, not showing eclipses, have been classified as ellipsoidal variables (see Chapter 6, "Rotating Variable Stars"). The variable star BH CVn is a non-eclipsing chromospherically active binary system that varies only as a result of the reflection effect. The reflection effect is described in Chapter 7, "Close Binary Eclipsing Systems."

Because these stars have large "spots," the brightness varies as the star rotates. This variation is commonly called the "wave" and can appear in the light curve superimposed on additional variability that might be resulting from eclipses, ellipticity, or reflection. Careful study of these stars can produce excellent data, including remarkably accurate rotation periods. Complex light curves should be expected but short periods in some systems allow detailed study of some of these stars.

Observation

^r Bright stars j^j Lorge amplitudes

A Mixed periods <g> Visual, CCD/PEP

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