The Rapidly Rotating Giants of the FK Comae-type

Abstract

We present new spectroscopic and photometric observations and discuss a small but important group of rapidly rotating G - K giants (FK Com=HD 117555, UZ Lib=BD−8°3999, HD 199178 possibly others) which are photometrically variable, show slightly redshifted and variable Ca II H and K emission and variable Hα emission, show rotationally broadened spectral lines (50–100 km/s), and yet show no large radial velocity variations. Possible explanations of the properties of these stars are discussed and coalescence of a W UMa binary system seems to be the most probable evolutionary state.The most extreme of the group, FK Com is discussed in detail. Its type, G2IIIa (Keenan, priv.comm.) implying MV=−1 (the IR indices are consistent with a giant) suggests a reduction of log g (relative to Sun) −1.5 to −2.5; this can be only marginally reconciled with the dimensions of about 5 Ro resulting from the photometric period 2.4 d. and Vrot sin i about 100 km/s. The photometric variability (0.05 in I, 0.07 in R) is most probably due to spots asymmetrically distributed in longitude. The brightness minima observed by Chugainov in 1966 and 1974 and the new minimum observed in 1979 (JD 2443949.025) can be phased with one period 2.3995±0.0002 d. The erratic behavior of rotationally broadened spectral lines precludes obtaining a radial velocity orbit but an upper limit of 25 km/s for any periodic variations implies that the secondary must be a very low mass object. The strong Hα emission is variable in relative intensity of its double peaks but has a constant full width in excess of 20 A. The full half width at base implies rotational velocities of the order of 570 km/s; the half separation of peaks implies 300 km/s. Existence of an excretion disc of the type suggested by Webbink is possible.