  The cTTS TW Hya has been observed with high-resolution X-ray spectrometers.
Previously found high densities inferred from He-like f/i triplets strongly
suggested the detected X-ray emission to be dominated by an accretion shock.
Because of their radiation field dependence He-like f/i ratios do not provide
unambiguous density diagnostics. Here we present additional evidence for high
densities from ratios of Fe XVII lines. Key Fe XVII line ratios in TW Hya
deviate from theoretical expectations at low densities as well as from the same
measurements in a large sample of stellar coronae. However, a quantitative
assessment of densities is difficult because of atomic physics uncertainties.
In addition, estimates of low optical depth in line ratios sensitive to
resonance scattering effects also support a high-density emission scenario in
the X-ray emitting regions of cTTS.
