X‐Ray Spectroscopy of the Unsteady Quiescent Corona of AD Leonis withChandra

We present the analysis and interpretation of an observation of the flarestar AD Leo (dM3e) with Chandra/LETG. The high resolution X-ray spectrumallowed us to infer the plasma emission measure distribution (EMD) vs.temperature, and the abundances of individual elements in the corona of thismagnetically-active star, during a typical state characterized by significantvariability but no large flaring event. We have also measured plasma densitiesat various temperatures using spectroscopic diagnostics provided by He-liketriplets and Fe XXI lines. We show that the present EMD is similar to thosepreviously obtained from EUVE spectra during quiescent and flaring states,confirming the long-term stability of the corona of AD Leo. At variance withthe case of other active stars, the EMD of AD Leo is characterized by asignificantly shallower slope, compatible with that predicted by static modelsof isobaric loops with constant cross-section and uniform heating. We discusssuch coronal modeling to infer the average properties of the corona in terms ofloop populations, including estimates of the surface filling factor. We alsoshow that the EMD is compatible with the model of a corona continuously heatedby 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 solarcoronal metallicity.Comment: 37 pages, 13 figures; to appear in ApJ, main journa