(the following information provided by Dr. Pierre Chayer)
Drs. B.C. Monsignori Fossi, M. Landini, and G. Del Zanna from the Dipartimento di Astronomia e Scienza dello Spazio at the Universita di Firenze in Italy, and Dr. S. Bowyer from CEA, observed and analyzed the EUVE spectrum of AU Mic. AU Mic is a flaring and rapidly rotating M2 Ve star at a distance of 8.8 pc. EUVE detected AU Mic both in the scanning mode and with the Deep Survey/Spectrometer, which is equipped with a direct broadband imaging detector and three spectrometers to cover the short (70-190 A, SW), medium (140-380 A, MW), and long (280-760 A, LW) extreme-ultraviolet wavelengths. These instruments allow simultaneous imaging and spectroscopy with a spectral resolution of 0.5, 1, and 2 A in the SW, MW, and LW detectors, respectively.
Two flares were detected in the course of the observation of AU Mic. A first large flare was detected in all three spectrometer channels, followed by a decaying phase. A second, smaller flare was observed in the SW and MW channels. The large flare peaked very quickly, while the descending phase lasted about three hours. The star remained active between the two flares and returned to the quiescent level only after the second flare.
The EUVE spectrometers allow individual line identifications and detailed spectroscopic analysis. In the AU Mic observation, several highly ionized iron lines were well detected (Fe XV, XVI, XVIII, XIX, XX, XXI, XXII, XXIII, XXIV). These lines were used to characterize the temperature region logT = 5.8 - 7.3 and to provide the differential emission measure (DEM).
Dr. Monsignori Fossi and her colleagues also performed an analysis of the DEM as a function of time. They found that a high-temperature component is present in the flaring and decay phases of the flares that shifts toward higher temperatures at the flare peaks. The enhancement of this hot component may be easily followed during this time and is responsible for most of the flare brightening.
Finally, using the Fe XXI 142.2 A blend spectral line, Dr. Monsignori Fossi's group showed that both the line ratios and the synthetic spectrum computed during quiet and flaring conditions suggest an increase of electron density during flares, ranging between 3.0E+12 and 2.0E+13 cm^(-3). The densities obtained for the AU Mic flares are more than an order of magnitude larger than the values found in solar flares.
NOTE: The EUVE science team are deeply saddened by the loss of their colleague and friend, Brunella Monsignori Fossi, who died unexpectedly on 22 January 1996.