Accretion shocks in T Tauri stars: diagnosis via semiforbidden ultraviolet line ratios

Abstract

Numerical calculations of the structure of accretion shocks in T Tauri stars (TTSs) indicate that the C III]1909, O III]1661+1666 and Si III]1892 ultraviolet lines should have comparable intensities. We show how the density and the velocity of the accreted gas can be derived from these line ratios. We also indicate how these parameters can be used, together with other less reliable parameters such as the distance and the extinction, to derive the accretion rate and the accretion luminosity. It is shown that the line ratios as well as their absolute fluxes [as measured with the International Ultraviolet Explorer (IUE)] are in agreement with the predictions of the accretion-shock model. However, this method is best suited for the analysis of high-resolution data [such as those obtained with the Hubble Space Telescope (HST)] from which accurate fluxes can be determined and blended components (stellar atmosphere, winds) can also be properly subtracted out.