EXOSAT observations of variable coronal X-ray emission from the young K dwarf AB Doradus (= HD 36705)

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Abstract
We present the results of a search for rotationally-modulated X-ray emission from the young K0 dwarf AB Doradus, using the low- and medium-energy detectors on board the EXOSAT spacecraft. Two sets of observations were obtained, each covering all or nearly all of the 0.514 day rotation period of the star, in 1984 December and 1986 January. A single large stellar X-ray flare occurred during each observation. The long rise-times (∼6000 s) observed for both these flares suggest that they occurred in loop structures with lengths of several stellar radii. During quiescence, the amplitude of variability seen at low energies (0.05–2.0 keV) was significantly greater than that in the 2.0–5.0 keV band. The 1.0–10.0 keV pulse-height spectra obtained with the medium-energy experiment are well-fitted by thermal bremsstrahlung at ∼1.9×107 K, with possible excess emission at lower energies. The data are consistent with a model in which a range of coronal loop temperatures are present, the cooler loops tending to be confined closer to the stellar surface and thus being subject to rotational self-eclipse, while the hotter loops extend at least one stellar radius above the photosphere. The emission measure observed during quiescence was (1.5±0.3)×1053cm−3 for an assumed distance of 20pc. If the emitting region is to be confined within a pressure scale height or so above the photosphere, and if the pressure in the transition region at the base of each loop is to be no greater than 10–20 dyn cm−2, a high volume filling factor and hence some global ordering of the field configuration is likely.

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