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Context. The accretion process in Classical T Tauri Stars (CTTSs) can bestudied through the analysis of some UV and X-ray emission lines which tracehot gas flows and act as diagnostics of the post-shock downfalling plasma. Inthe UV band, where higher spectral resolution is available, these lines arecharacterized by rather complex profiles whose origin is still not clear.  Aims. We investigate the origin of UV and X-ray emission at impact regions ofdensity structured (fragmented) accretion streams.We study if and how thestream fragmentation and the resulting structure of the post-shock regiondetermine the observed profiles of UV and X-ray emission lines.  Methods. We model the impact of an accretion stream consisting of a series ofdense blobs onto the chromosphere of a CTTS through 2D MHD simulations. Weexplore different levels of stream fragmentation and accretion rates. From themodel results, we synthesize C IV (1550 {\AA}) and OVIII (18.97 {\AA}) lineprofiles.  Results. The impacts of accreting blobs onto the stellar chromosphere producereverse shocks propagating through the blobs and shocked upflows. Theseupflows, in turn, hit and shock the subsequent downfalling fragments. As aresult, several plasma components differing for the downfalling velocity,density, and temperature are present altoghether. The profiles of C IV doubletare characterized by two main components: one narrow and redshifted to speed$\approx$ 50 km s$^{-1}$ and the other broader and consisting of subcomponentswith redshift to speed in the range 200 $\approx$ 400 km s$^{-1}$. The profilesof OVIII lines appear more symmetric than C IV and are redshifted to speed$\approx$ 150 km s$^{-1}$.  Conclusions. Our model predicts profiles of C IV line remarkably similar tothose observed and explains their origin in a natural way as due to streamfragmentation.

Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV and X-ray emission lines