AAS 205th Meeting, 9-13 January 2005
Session 12 Stellar Atmospheres
Poster, Monday, January 10, 2005, 9:20am-6:30pm, Exhibit Hall

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[12.02] FK Comae: King of Spin

T.R. Ayres (CASA), H. Korhonen (AIP), G.M. Harper, A. Brown (CASA), S. Redfield (UT)

Fast-rotating early-G giants often display ultraviolet profiles of ``hot lines,'' like O~{\small VI} (3\times105~K), up to {\em twice}\/ as broad as anticipated from the photospheric \upsilon\sin{i}. This peculiar behavior has been attributed to highly extended coronal outer atmospheres.

{\em FUSE}\/ recently has contributed fundamentally to the exploration of these ``super-rotational'' effects by observing the fascinating object FK Comae Berenices (G5~III). It is prototype of a class of rapidly rotating single giants which display spectacular emission activity from X-rays to radio. FK~Com has a spin period of only 2.4~d, and a remarkable \upsilon\sin{i} of 160 km s-1. The origins of such ultra-fast--rotating stars are controversial. They might result from a coalesced contact binary, cannibalism of a ``hot Jupiter,'' or simply represent the top of the natural spin distribution of moderate-mass Hertzsprung gap giants.

In February 2004, {\em FUSE}\/ obtained a 13 ks exposure of FK~Com in the 920--1180~Å\ range, the first UV observation of this unusual object since the {\em IUE}\/ era a decade ago, and by far the best quality spectrum to date. The FUV emissions of O~{\small VI} \lambda1031 and C~{\small III} \lambda977 are enormously broad, asymmetric, but nearly identical in shape, aside from a blue-shifted absorption component in the latter. The FHWM's are a remarkable 600 km s-1, about twice the broadest FUV profile of any late-type star observed up to that point.

The blueshifted C~{\small III} feature might represent a wind at ~3\times104~K, or alternatively a scattering structure in the highly extended coronal envelope, something like the ``prominences'' seen already in H\alpha. The asymmetric O~{\small VI} profile might indicate an outflow at higher ~3\times 105~K temperatures; or simply reflects a skewed distribution of high-altitude activity in the equatorial zones of FK~Com. The relationship between the hot lines and photospheric active regions---deduced from contemporaneous optical Doppler mapping---also will be discussed.

This work was supported by {\em FUSE}\/ Guest Investigator grant NNG04GH25G.

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