Fkcom Fk Comae Bernices6 stars

- These are rapidly rotating giants with nonuniform surface brightnesses, which have spectral types G-K with broad H and K Ca 11 emission and sometimes Ha. They may also be spectral binary systems. Periods of light variation (up to several days) are equal to rotational periods, and amplitudes are several tenths of a magnitude. It is not excluded that these objects are the product of further evolution of EW (W UMa) close binary systems. GCVS

FK Comae variables are rapidly rotating giant stars varying as a result of irregular surface brightness. A region of cool spots localized on one hemisphere of the star causes the irregular brightness. As originally defined, the class included late-type giants with very high rotation speed (short rotation period), evidence of extreme chromospheric activity, but displaying no evidence of large velocity variations (Figure 6.3). The GCVS allows binaries to be included in the class.

FK Comae itself rotates so rapidly that the most reasonable evolutionary scenario involves the coalescence of a W UMa-type binary and a surrounding optically thick spun-up envelope. Other stars assigned to this class do not rotate so rapidly and may be simply evolved single A-type stars, which have not lost much of

Observation

Key f Mixed stars

Small amplitudes ^ Mixed periods CCD or PEP

6Coma Berenices, the Hai r of Berenice. Berenice is the wife of Ptolemy Euergetes, king of Egypt who vowed to sacrificc her hair if her husband was successful in waging war on the Assyrians.

Figure 6.3. Artist's conception of a FK Corn-type variable showing the fast orbiting nature and irregular surface brightness of these binary systems Copyright: Gerry A. Good their original rapid main sequence rotation. If binaries are allowed in the class, then their rapid rotation will be a result simply of synchronization with a rather short orbital period.

Again, because we're observing light variation caused by spots rotating across the face of distant stars, the light amplitude for these stars is in the hundredths to tenths of a magnitude. Definitely work for instruments.

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