An IUE atlas of pre-main-sequence stars.: II.: Far-ultraviolet accretion diagnostics in T Tauri stars

We use our ultraviolet (UV) atlas of pre-main-sequence stars constructed from all useful, short-wavelength, low-resolution spectra in the International Ultraviolet Explorer (IUE) satellite Final Archive to analyze the short-wavelength UV properties of 49 T Tauri stars (TTSs). We compare the line and continuum fluxes in these TTSs with each other and with previously published parameters of these systems, including rotation rate, infrared excess, and mass accretion rate. The short-wavelength continuum in the classical TTSs (CTTSs) appears to originate in a similar to 10,000 K optically thick plasma, while in the naked TTSs (NTTSs-stars without dusty disks) the continuum appears to originate in the stellar atmosphere. We show that all of the TTSs in our sample lie in the regime of saturated magnetic activity due to their small Rossby numbers. However, while some of the TTSs show emission line surface fluxes consistent with this saturation level, many CTTSs show significantly stronger emission than predicted by saturation. In these stars, the emission line luminosity in the high ionization lines present in the spectrum between 1200 and 2000 a correlates well with the mass accretion rate. Therefore, we conclude that the bulk of the short-wavelength emission seen in CTTSs results from accretion related processes and not from dynamo-driven magnetic activity. Using CTTSs with known mass accretion rates, we calibrate the relationship between M and L-Crv to derive the mass accretion rate for some CTTSs which for various reasons have never had their mass accretion rates measured. Finally, several of the CTTSs show strong emission from molecular hydrogen. While emission from H-2 cannot form in gas at a temperature of similar to 10(5) K, the strength of the molecular hydrogen emission is nevertheless well correlated with all the other emissions displayed in the IUE short-wavelength bandpass. This suggests that the H-2 emission is in fact fluorescent emission pumped by the emission (likely Ly alpha) from hotter gas.