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Scott L. Cully (1,2), George H. Fisher (1), Mark J. Abbott (2), and Oswald H.W. Siegmund (1,2)
1) Space Sciences Laboratory, University of California,
Berkeley, CA 94720
2) Center for EUV Astrophysics, 2150 Kittredge Street, University
of California, Berkeley, CA 94720
The dM1e flare star AU Microscopii (AU Mic) was observed by the EUVE Deep Survey Instrument on 1992 July 14 - 18. A large flare was detected in the Deep Survey Lexan/Boron (DS Lex/B) (65 - 190 Å) band and the SW (70 - 190 Å) and MW (140 - 380 Å) spectrometers. The flare consisted of a sharp impulsive peak lasting approximately 2 hours followed by a decaying tail lasting about a day. We present a simple, single temperature, dynamic model for the flare decay which is consistent with the the DS Lex/B light curve and reproduces the strongest, high temperature spectral lines in the released EUVE spectra. In this model, we assume the long decay time is due to an ejected, magnetically confined, low beta plasmoid expanding self similarly in the ambient medium in a manner reminiscent of solar coronal mass ejections. We demonstrate that the long tail of the DS Lex/B light curve can be explained by rapid expansion, causing the plasma to become tenuous sufficiently quickly that it avoids catastrophic radiative cooling. From this model, we estimate the mass of the plasmoid to be 1E+20 g and the total energy of the event to be 1E+36 ergs. These values are approximately 1E+04 times as large as those seen during the largest solar CME events. We argue that the results of our model are consistent with other measurements of stellar flare parameters. We also estimate a mass loss rate of a few times 10-13 MSolar yr-1 and discuss the role of mass loss from dMe stars in the mass balance of the interstellar medium. We estimate the rotational braking time scale from these events to be less than 500 million years and suggest that CME's may be an important source of angular momentum loss from late type stars.
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