A Determination of the Coronal Emission Measure Distribution in the Young Solar Analog EK Draconis fromASCA/EUVESpectra

Abstract

We present the results of a coronal differential emission measure (DEM) analysis of the nearby analog of the young Sun, EK Draconis, using data obtained with the ASCA and EUVE satellites. Various methods (including a CLEAN algorithm, a polynomial fit, a direct inversion method, a genetic algorithm, and a multitemperature fit) have been applied to reconstruct the DEM between 0.1 and 100 MK. The spectra from the four ASCA detectors and the two spectra from the short-wavelength and the medium-wavelength detectors of EUVE were subject jointly to the algorithms, taking into account both emission lines and continua. All methods converge to a DEM distribution that is essentially bimodal: we find two significant peaks near 7 MK and near 18 MK with a deep minimum around 10 MK. Little plasma is found at temperatures below 3-4 MK, despite EUVE's sensitivity to this temperature regime. We argue that the DEM distribution seen in EK Dra is induced by the properties of the radiative cooling function of a thermal, optically thin plasma. From an elemental abundance analysis, an Fe abundance of 0.83 times the corresponding solar photospheric abundance is found, and an abundance of Mg of 1.69 times the solar photospheric abundance, which may indicate for this specific element a first ionization potential effect similar to that found in the solar corona.