  In classical T Tauri stars, X-rays are produced by two plasma components: a
hot low-density plasma, with frequent flaring activity, and a high-density
lower temperature plasma. The former is coronal plasma related to the stellar
magnetic activity. The latter component, never observed in non-accreting stars,
could be plasma heated by the shock formed by the accretion process. However
its nature is still being debated. Our aim is to probe the soft X-ray emission
from the high-density plasma component in classical T Tauri stars to check
whether this is plasma heated in the accretion shock or whether it is coronal
plasma. High-resolution X-ray spectroscopy allows us to measure individual line
fluxes. We analyze X-ray spectra of the classical T Tauri stars MP Muscae and
TW Hydrae. Our aim is to evaluate line ratios to search for optical depth
effects, which are expected in the accretion-driven scenario. We also derive
the plasma emission measure distributions EMD, to investigate whether and how
the EMD of accreting and non accreting young stars differ. The results are
compared to those obtained for the non-accreting weak-line T Tauri star TWA 5.
We find evidence of resonance scattering in the strongest lines of MP Mus,
supporting the idea that soft X-rays are produced by plasma heated in the
accretion shock. We also find that the EMD of MP Mus has two peaks: a cool peak
at temperatures expected for plasma heated in the accretion shock, and a hot
peak typical of coronal plasma. The shape of the EMD of MP Mus appears to be
the superposition of the EMD of a pure coronal source, like TWA 5, and an EMD
alike that of TW Hydrae, which is instead dominated by shock-heated plasma.
