An accretion model for the X-ray emission of classical T Tauri stars

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Both classical as well as weak-line T Tauri stars are known to be copious X-ray emitters. Because of their deep convection zones and rapid rotation magnetic activity was thought to underly the observed X-ray activity phenomena. Recently obtained high-resolution X-ray spectroscopy of classical T Tauri stars (cTTS) shows, however, clear differences in some key spectroscopic features between the X-ray spectra of cTTS as compared to normal stars, suggesting other or at least additional X-ray production mechanism(s). Since cTTS are still accreting material from a circumstellar disk, an accretion shock at the bottom of a magnetically funnelled accretion column is an obvious candidate for such an additional X-ray production mechanism. We have developed a one-dimensional model of such an accretion shock column to compute its emergent spectrum and confronted this model with actual X-ray observations. In this fashion we can to derive infall velocities, shock densities, chemical abundances of the infalling material, mass accretion rates and filling factors from X-ray observations alone. With the proposed project we plan to improve this model by incorporating additional physics and extend its scope to line fluxes and line profiles in the UV. This will greatly enhance our ability to divide the observed emission between coronal and accretion dominated sources and to characterise further objects.