Phase-dependent velocity shift of the O VI broad emission component

The chromospheric and transition region emission lines of the most active stars have routinely been shown to consist of very broad wings superimposed on the narrower, rotationallybroadened emission core. If these wings are attributed to a single emission mechanism in the stellar atmosphere, the implied thermal broadening often strains credibility. If they are due to active regions or loops rotating into and out of view, they should be present only for a small range of rotational phase. They have also been attributed to continuous flaring and to circumstellar material (loops, disks, etc.). We have been analyzing spectra of very active stars in the FUSE continuous viewing zone, allowing them to be observed continuously over one or more complete rotations. This helps to distinguish between spatial (repeatable, phase-dependent) and temporal variations and provides further insight into the mechanism(s) responsible for the broad wing emission.