Understanding the atmospheric structure of T Tauri stars – II. UV

We report results from our study of International Ultraviolet Explorer (IUE) data of a group of T Tauri stars (TTS). Comparisons between UV-line fluxes in these stars and in the Sun indicate very high levels of activity in their atmospheres and comparatively higher electron densities. Spectroscopic diagnostic line ratios indicate densities over an order of magnitude higher than in the ‘quiet ’ Sun at ‘transition region ’ temperatures. At these densities, metastable levels can attain comparable populations to the ground level and ionization fractions can be altered as a result of the sensitivity of dielectronic recombination. In Brooks et al. we improved the treatment of these effects using the ADAS software package, the atomic models and data of which are based on collisional-radiative theory. Here we extend the analysis to a sample of five TTS: RY Tau, BP Tau, RU Lupi, GW Ori and CV Cha. Using these models and data we derive the emission measure (EM) distribution for each star in the sample. We find that the decrease in EM with increasing temperature appears to be sharper than that found in previous work. In comparison with the Sun, the results suggest that the UV emission is formed in a region with a steeper density or volume gradient. We find mismatches between the theoretical and observed fluxes which cannot be explained b