Astronomy and Astrophysics, volume 594A, 93-93 (2016/10-1)
Impacts of fragmented accretion streams onto classical T Tauri stars: UV and X-ray emission lines.
COLOMBO S., ORLANDO S., PERES G., ARGIROFFI C. and REALE F.
Abstract (from CDS):
Context. The accretion process in classical T Tauri stars (CTTSs) can be studied through the analysis of some UV and X-ray emission lines which trace hot gas flows and act as diagnostics of the post-shock downfalling plasma. In the UV-band, where higher spectral resolution is available, these lines are characterized by rather complex profiles whose origin is still not clear. Aims. We investigate the origin of UV and X-ray emission at impact regions of density structured (fragmented) accretion streams. We study if and how the stream fragmentation and the resulting structure of the post-shock region determine the observed profiles of UV and X-ray emission lines. Methods. We modeled the impact of an accretion stream consisting of a series of dense blobs onto the chromosphere of a CTTS through two-dimensional (2D) magnetohydrodynamic (MHD) simulations. We explored different levels of stream fragmentation and accretion rates. From the model results, we synthesize CIV (1550Å) and OVIII (18.97Å) line profiles. Results. The impacts of accreting blobs onto the stellar chromosphere produce reverse shocks propagating through the blobs and shocked upflows. These upflows, in turn, hit and shock the subsequent downfalling fragments. As a result, several plasma components differing for the downfalling velocity, density, and temperature are present altoghether. The profiles of CIV doublet are characterized by two main components: one narrow and redshifted to speed ~=50km/s and the other broader and consisting of subcomponents with redshift to speed in the range 200-400km/s. The profiles of OVIII lines appear more symmetric than CIV and are redshifted to speed ~=150km/s. Conclusions. Our model predicts profiles of CIV line remarkably similar to those observed and explains their origin in a natural way as due to stream fragmentation.