X-ray emission from T Tauri stars attributable to an accretion shock wave

We consider the X-ray emission of an accretion shock wave for classical T Tauri stars. The absorption of the emission by cold preshock gas is shown to make its spectrum harder than that in the formation zone. However, since the effect is not too large, the shock wave can be responsible only for the soft component of the observed X-ray emission. The observed emission with E > 1 keV is assumed to be produced by a coronal plasma with T similar to 10(7) K, which is confined by closed field lines of the large-scale stellar magnetic field near the magnetic equator. The contribution of this region to the ultraviolet and optical emission is discussed. We also show that, in contrast to the optical emission, the intensity of the X-ray emission generated by a shock wave depends only slightly on the accreted-gas density. This is consistent with the lack of correlation between the optical and X-ray variability detected in BP Tau.