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X-ray spectroscopy of the unsteady quiescent corona of AD Leo with Chandra

We present the analysis and interpretation of an observation of the flare star AD Leo (dM3e) with Chandra/LETG. The high resolution X-ray spectrum allowed us to infer the plasma emission measure distribution (EMD) vs. temperature, and the abundances of individual elements in the corona of this magnetically-active star, during a typical state characterized by significant variability but no large flaring event. We have also measured plasma densities at various temperatures using spectroscopic diagnostics provided by He-like triplets and Fe XXI lines. We show that the present EMD is similar to those previously obtained from EUVE spectra during quiescent and flaring states, confirming the long-term stability of the corona of AD Leo. At variance with the case of other active stars, the EMD of AD Leo is characterized by a significantly shallower slope, compatible with that predicted by static models of isobaric loops with constant cross-section and uniform heating. We discuss such coronal modeling to infer the average properties of the corona in terms of loop populations, including estimates of the surface filling factor. We also show that the EMD is compatible with the model of a corona continuously heated by flares. The coronal composition is such that the element abundances, relative to solar values, tend to increase with the First Ionization Potential, with few exceptions. The line to continuum ratios suggest a nearly solar coronal metallicity.