The Remarkable Far‐Ultraviolet Spectrum of FK Comae Berenices: King of Spin

A Far Ultraviolet Spectroscopic Explorer (FUSE) pointing on the ultrafast rotating yellow giant FK Comae Berenices (HD 117555; v sin i ~ 163 km s-1) recorded emission profiles of C III λ977 (T ~ 8 × 104 K) and O VI λ1031 (T ~ 3 × 105 K) that are exceptionally broad and asymmetric, but nearly identical in shape, aside from a blueward absorption component in the latter (identified as interstellar O I, rather than, say, a C III outflow feature). The FWHMs exceed 500 km s-1, twice the broadest far-UV line shape of any normal late-type star observed to date, but similar to the Hα profiles of FK Com, and following the trend of other fast spinning early G giants that often display "superrotational" broadening of their UV "hot" lines. Although the red-asymmetric O VI λ1031 profile is suggestive of an outflow at ~3 × 105 K, the weaker member of the doublet, λ1037, does not display the differential absorption pattern expected from a warm wind. Furthermore, at times the chromospheric Mg II λ2796 + λ2803 composite profile, from a collection of International Ultraviolet Explorer (IUE) echellegrams obtained two decades earlier, is nearly identical in shape to red-asymmetric O VI λ1031. A contemporaneous optical Doppler map places the photospheric dark spots mainly in the polar regions of the approaching hemisphere. The dominantly redward biased profiles of C III and O VI could be explained if the associated emission zones were leading the starspots in phase and partially rooted in lower latitudes.