Extreme Ultraviolet Explorer Spectra of the 1993 March Flares on AD Leonis: The Differential Emission Measure and Implications for Coronal Structure

The flare star AD Leonis was observed by the Extreme Ultraviolet Explorer (EUVE) from 1993 March 1 to 3 UT. Two flares were detected by the EUVE Deep Survey detector and spectrometer and also seen in optical photometry on 1993 March 2 UT. The DS Lexan/boron-band and optical results have been discussed in the previous paper by Hawley et al. In this paper, we describe the spectra observed by EUVE during quiescence, the peaks of the flares, and the decay phase following the first flare and analyze the spectra to investigate the stellar atmospheric structure during these time periods. The spectra show that the observed EUV emission from AD Leo is dominated by iron lines from a hot coronal plasma. Two methods were used to estimate the differential emission measure distribution (DEM) of the stellar corona. In the "Pottasch" method, we fitted Gaussian line profiles to the strongest lines in the spectra and estimated the DEM at the formation temperature of those lines. Upper limits to the DEM were obtained in the case of no detection. We also used a regularized inversion technique, together with a weighting scheme based on information contained in the plasma-emission model and on the signal-to-noise ratio of the data, to find the DEM. The weighting was designed to prevent the noisy pixels in our low-signal-to-noise ratio data from dominating the solution. The results produced by the two methods are consistent in the temperature regimes where strong lines are present. The inversion method provides additional information where no strong single lines dominate the spectra. The ability to use lines from the entire wavelength region covered by the spectra allowed us to investigate the hydrogen column NH and iron abundance [Fe/H]. We found that [Fe/H] in the corona of AD Leo was essentially unconstrained by our data, but NH was well determined, yielding NH ~ (3 ± 1) × 1018 cm-2. We assumed both a solar-coronal value of [Fe/H] and a value one tenth of this and computed the DEM distribution of the stellar corona for both cases. The DEM of the quiescent corona is dominated by a broad plateau of emission ranging from 106.8 to 107.2 K, with the DEM of plasma near 106.2 K about an order of magnitude less. We interpret the plateau of the DEM in terms of a broad distribution of loops with differing peak temperatures. We discuss and compare these results with those of Giampapa et al., who analyzed ROSAT soft X-ray data from AD Leo taken during a different time period. The DEM of the flare plasma is strongly peaked at temperatures greater than 107 K, indicative of hot flare loops, while that of the decay phase consists of a smaller peak at temperatures less than 107 K, as might be expected from the cooling and condensation of previously heated flare loops. These results are consistent with a flare model that includes strong evaporation and condensation as in our previous paper. The EUVE spectral analysis leads to lower peak flare temperatures than those used in our previous paper, but the basic conclusion reached—that the dominant flaring emission originates from long loops with L ~ R* and with peak flare densities ranging from 109 to 1011 cm-3—remains unchanged. This conclusion is not qualitatively affected by the value of [Fe/H] used in our DEM analysis.