The Rise and Fall of μ Velorum: A Remarkable Flare on a Yellow Giant Star Observed with the Extreme Ultraviolet Explorer
Abstract
The close visual double μ Velorum (HD 93497; G6 III+dF) consists of a yellow giant and a fainter companion currently 2" apart. Recently μ 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, μ Vel A is a 3 Msolar star, in the ``rapid braking zone'' redward of G0 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 Å 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 μ Vel event suggests that it occurred in a significantly elongated structure of moderate density, ne<~109 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 μ 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1999
- DOI:
- 10.1086/308001
- Bibcode:
- 1999ApJ...526..445A
- Keywords:
-
- STARS: ACTIVITY;
- STARS: CORONAE;
- STARS: FLARE;
- STARS: INDIVIDUAL (HD 93497;
- HD 4128);
- STARS: LATE-TYPE;
- ULTRAVIOLET: STARS;
- Stars: Activity;
- Stars: Coronae;
- Stars: Flare;
- stars: individual (HD 93497;
- Stars: Late-Type;
- Ultraviolet: Stars