The Remarkable Far-Ultraviolet Spectrum of FK Comae Berenices: King of Spin
Abstract
A Far Ultraviolet Spectroscopic Explorer (FUSE) pointing on the ultrafast rotating yellow giant FK Comae Berenices (HD 117555; vsini~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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2006
- DOI:
- 10.1086/503522
- Bibcode:
- 2006ApJ...644..464A
- Keywords:
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- Stars: Coronae;
- stars: individual (HD 117555);
- stars: individual (HD 111812);
- stars: individual (HD 113001B);
- stars: individual (vZ 1128);
- Ultraviolet: Stars