RS Canum Venaticorum star

RS Canum Venaticorum star

(kay -nŭm vĕ- nat-ă-kor -ŭm, -koh -rŭm) (RS CVn star) A short-period binary star system that contains a subgiant of spectral type G or K exhibiting intense activity at radio, ultraviolet, and X-ray wavelengths; the other component is a normal F or G main-sequence star. The orbital period can range from 1 day to 2 weeks. The two stars are not undergoing mass transfer (they are a detached system), but tidal forces have locked the rotation of the subgiant so that it is forced to rotate once in each orbital period, much faster than a normal subgiant. This rapid rotation apparently causes strong magnetic fields, which then produce the observed activity.

The light curve of these stars shows a continuously changing brightness during the orbital period, believed to be due to dark starspots extensively blanketing one half of the subgiant's surface. Measurements of the Zeeman effect in light from Lambda Andromedae show that the spotted hemisphere has a field strength of over 0.1 tesla, similar to that of sunspots. Some of these stars, including the prototype RS CVn, are also eclipsing binaries; the light curve shows regular dips of about a magnitude as the subgiant eclipses the main-sequence star. The subgiant's magnetic field also causes intense radio flares (compare flare stars), strong ultraviolet emission lines from the chromosphere, and powerful X-ray emission from a very hot corona.

As the subgiant swells to giant size, it begins to transfer mass and become a W Serpentis star and then an Algol variable.