The recent Chandra Orion Ultradeep project (COUP) has shown that the X-ray luminosity of T Tauri stars increases with stellar mass. We have modelled this by taking a surface magnetic field with a realistic degree of complexity and extrapolating it into the corona. We then determine the structure of the X-ray emitting corona and the location of the accretion funnels (Jardine et al 2006) .

We find that the higher-mass stars have coronae that do not extend out to the inner edge of the disk (at the co-rotation radius) and so they accrete along open field lines (upper picture). The opposite is true of lower-mass stars which accrete along closed field lines (lower picture).

The outer edges of the coronae of these lower mass stars are stripped by the interaction with a disk (if present) or simply by centrifugal forces (Jardine, 2004). This X-ray suppression leads to the observed mass dependence.