Three years in the coronal life of AB Dor - I. Plasma emission measure distributions and abundances at different activity levels

The young active star AB Dor (K1 IV-V) has been observed 16 times in the last three years with the XMM-Newton and Chandra observatories, totalling 650 ks of high-resolution X-ray spectra. The XMM/RGS observations with the highest and lowest average emission levels have been selected to study the coronal properties of AB Dor in two different activity levels. We compare the results based on the XMM data with those obtained from a higher resolution Chandra/HETG spectrum, using the same line-based analysis technique. We have reconstructed the plasma Emission Measure Distribution vs. temperature (EMD) in the range log T(K)similar to6.1-7.6, and we have determined the coronal abundances of AB Dor, obtaining consistent results between the two instruments. The overall shape of the EMD is also consistent with the one previously inferred from EUVE data. The EMD shows a steep increase up to the peak at log T(K)similar to6.9 and a substantial amount of plasma in the range log T(K)similar to6.9-7.3. The coronal abundances show a clear trend of increasing depletion with respect to solar photospheric values, for elements with increasing First Ionization Potential (FIP), down to the Fe value ([Fe/H]=-0.57), followed by a more gradual recovery of the photospheric values for elements with higher FIP. He-like triplets and Fe XXI and Fe XXII lines ratios indicate electron densities log n(e)similar to10.8 cm(-3) at log T(K)similar to6.3 and log n(e)similar to12.5 cm(-3) at log T(K)similar to7, implying plasma pressures steeply increasing with temperature. These results are interpreted in the framework of a corona composed of different families of magnetic loop structures, shorter than the stellar radius and in isobaric conditions, having pressures increasing with the maximum plasma temperature, and which occupy a small fraction (fsimilar to10(-4)-10(-6)) of the stellar surface.