  We have modelled the X-ray emission of T Tauri stars assuming that they have
isothermal, magnetically-confined coronae. These coronae extend outwards until
either the pressure of the hot coronal gas overcomes the magnetic field, or, if
the corona interacts with a disk before this happens, by the action of the disk
itself. This work is motivated by the results of the Chandra Orion Ultradeep
Project (COUP) that show an increase in the X-ray emission measure with
increasing stellar mass. We find that this variation (and its large scatter)
result naturally from the variation in the sizes of the stellar coronae. The
reduction in the magnitude of the X-ray emission due to the presence of a disk
stripping the outer parts of the stellar corona is most pronounced for the
lower mass stars. The higher mass stars with their greater surface gravities
have coronae than typically do not extend out as far as the inner edge of the
disk and so are less affected by it. For these stars, accretion takes place
along open field lines that connect to the disk. By extrapolating surface
magnetograms of young main sequence stars we have examined the effect on the
X-ray emission of a realistic degree of field complexity. We find densities
consistent with estimates from modelling of individual flares. A simple dipole
field in contrast gives densities typically an order of magnitude less. We
suggest that T Tauri stars have coronal fields that are slightly more extended
than their main sequence counterparts, but not as extended as a purely dipolar
fields.
