X-ray spectral diagnostics for coronal loop sizes and heating rates in the active K dwarf AB Doradus

Abstract

We present a set of theoretical X-ray spectra derived from models of coronal loops in thermal and hydrostatic equilibrium on the rapidly-rotating K0 dwarf AB Doradus. This star is a typical representative of active X-ray bright stars. The coronal loop models are based on EXOSAT and IUE observations of AB Dor. The resulting synthetic spectra are folded through the expected responses of the XMM and XSPECT missions. Useful constraints on the DEM(T) distribution could be obtained for a star as bright as AB Dor with the grating spectrometer of XMM in 10 3 to 10 4 s exposure times. The ratio of the He-type (O VII) forbidden and intercombination lines at 0.56 keV will provide density diagnostics sufficient to determine whether, for example, the X-rays originate mainly in compact, dense flare loops or in more extended loops of lower density. The strong O VIII line at 0.65 keV can be observed with the Bragg crystal spectrometers of both missions, in 10 4 s, and may be used for studies of rotational modulation and of flow velocities in flaring loops. At the Iron 6.7 keV lines, the crystal spectrometers and the lower-resolution CCD and GSPC instruments can be used to set constraints on the hottest gas at the loop summits.