2.5 FK Com stars

FK Comae stars were first defined as a new group of active stars in the early 1980’s (Bopp and Rucinski, 1981

; Bopp, 1981

; Bopp and Stencel, 1981

). FK Comae stars are late-type giants with spectral types ranging from G to K and rotation periods of only a few days. This indicates very rapid rotation rates. Spectroscopic observations reveal $v sin i$ values of these stars of $-1 ~ 50 -150 km s$ . Still, these stars do not show any significant periodic radial velocity variations and are, therefore, most likely single. Three most active stars of this type are FK Com, V1794 Cyg (HD199178), and YY Men. A few other stars have been suggested as candidates for this group, but they exhibit milder activity and slower rotation.

The magnetic activity of FK Comae stars is revealed by rotationally modulated photometric variations with an amplitude in the $V$ -band of $0.1- 0.3 mag$ caused by cool asymmetrically distributed spots on the stellar surface. Other characteristics of these stars include a very strong and variable chromospheric emission in the $Ca II H & K$ lines and in the Balmer lines. The photometric and spectroscopic characteristics of FK Comae stars are very similar to those of the very active RS CVn stars (see Section 2.4), with the exception that RS CVn stars are close binary systems in which the tidal effects produce synchronous rotation and, therefore, also rapid rotation.

The origin of FK Comae stars is still debated. A few evolutionary scenarios have been proposed for explaining the odd rapid rotation of the late-type single giants. The first idea on the origin of FK Comae stars was that the stars may represent the further evolution of W UMa contact binaries (see Section 2.6) into a coalesced single star (Bopp and Rucinski, 1981 ; Bopp, 1981 ; Bopp and Stencel, 1981 ). Such a scenario was first considered by Webbink (1976 ) who calculated the evolution of a contact binary and showed that the mass ratio (secondary/primary) can decrease on an evolutionary time scale until the secondary is completely dissipated, during the primary’s initial ascent of the giant branch. The kinematics and age of FK Com support this hypothesis (Guinan and Robinson, 1986 ). However, as argued by Fekel and Balachandran (1993 ), the detection of lithium in these active giants is evidence against binary coalescence. They considered a scenario in which the surface convection zone reaches the rapidly rotating core just as a star begins its first ascent of the giant branch, and dredges both high angular momentum material and freshly synthesised Li to the surface. Still, the puzzle is not solved and requires further investigation of these objects.