Understanding the atmospheric structure of T Tauri stars - II. UV spectroscopy of RY Tau, BP Tau, RU Lupi, GW Ori and CV Cha

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 by density effects and thus must be a result of uncertainties in the atomic data, unreliabilities in the fluxes or the failure of physical assumptions in the method. We have made a series of tests and comparisons, including examination of opacity effects, and these clearly favour the latter explanation. They also lead us to suggest the presence of two separate components in the UV emission for each of the TTS, although the case of CV Cha is more ambiguous. This supports and extends the earlier work of Jordan & Kuin on RU Lupi. Interestingly, we find that the EM distribution for GW Ori has values at least 10 times larger than those of RY Tau, which appears to have approximately the same electron density. A similar difference is found between CV Cha and RU Lupi. Following geometrical arguments, we suggest that the UV emission in GW Ori and CV Cha is formed in a more extended region than in the other three stars.