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 fluxe s in these stars and in the Sun indicate very high levels of activity in th eir 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 dens ities, metastable levels can attain comparable populations to the ground le vel and ionization fractions can be altered as a result of the sensitivity of dielectronic recombination. In Brooks et al. we improved the treatment o f these effects using the ADAS software package, the atomic models and data of which are based on collisional-radiative theory. Here we extend the ana lysis 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) distributio n for each star in the sample. We find that the decrease in EM with increas ing 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 de nsity effects and thus must be a result of uncertainties in the atomic data , unreliabilities in the fluxes or the failure of physical assumptions in t he method. We have made a series of tests and comparisons, including examin ation 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 ambig uous. This supports and extends the earlier work of Jordan & Kuin on RU Lup i. Interestingly, we find that the EM distribution for GW Ori has values at le ast 10 times larger than those of RY Tau, which appears to have approximate ly the same electron density. A similar difference is found between CV Cha and RU Lupi. Following geometrical arguments, we suggest that the UV emissi on in GW Ori and CV Cha is formed in a more extended region than in the oth er three stars.