Post-Eruptive Flare Energy Release as Detected on AU Mic by EUVE

Abstract

The long-duration emission arising after the impulsive rise and decay in a flaring event observed by the Extreme Ultraviolet Explorer on the red dwarf star AU Mic is discussed. The decay of the intensity in the Deep Survey 65–190 Å band and in the Fe XVIII line during this prolonged event is 10 times slower than the time of radiative cooling of coronal loops with the typical for the flare plasma density. The temporal behavior of the emission measure is determined for both the 65–190 Å band and the Fe XVIII line fluxes. The total energy emitted in the 1–2000 Å region over nearly 12 hrs is 3 · 1035 ergs. We first point out some difficulties with earlier explanations proposed for this event; we then propose the following physical model: the source of the prolonged emission is a system of high coronal loops, the size of which is more than the active region scale, but less than the stellar radius. Such systems are observed in soft X-rays during large solar flares after coronal mass ejections. Some additional post-flare energy input into this high coronal loop system can be caused by reconnection in a vertical current sheet, and this post-eruptive energy release provides prolonged and intensive EUV emission.Apparently, we are faced here with new kind of the surface activity on late-type stars which is intermediate between impulsive flares on red dwarfs and long-duration, powerful events the subgiants components of the RS CVn binaries.