The rise and fall of μ Velorum:: A remarkable flare on a yellow giant star observed with the Extreme Ultraviolet Explorer

The close visual double mu Velorum (HD 93497; G6 III+dF) consists of a yellow giant and a fainter companion currently 2" apart. Recently mu Vel was the source of a large flare recorded by the Extreme Ultraviolet Explorer. The long 1.5 day decay phase was like the extremes seen on hyperactive RS CVn-type binaries. The primary, mu Vel A is a 3 M. star, in the "rapid braking zone" redward of GO III. Yellow giants are not commonly reported as flare stars, perhaps because the first-crossers are relatively rare and not well represented in the observational samples. The secondary star is classified G2 V, but the 1700 Angstrom energy distribution places it earlier on the main sequence, probably F4 or F5 V, in a class also not usually known for coronal variability. The long duration of the mu Vel event suggests that it occurred in a significantly elongated structure of moderate density, n(e) less than or similar to 10(9) cm(-3). If it was a magnetic plasmoid, like a coronal mass ejection on the Sun, then such events: might play a role in shedding angular momentum from active evolved stars. The associated spin-down could control the activity survival time of red giants (in later stages of evolution than the first-crosser mu Vel) whose dynamos were rejunvenated by dredge-up of angular momentum from the interior, or more exotic sources, such as cannibalism of close-in substellar companions during the first or second ascent.