AnIUEAtlas of Pre–Main‐Sequence Stars. II. Far‐Ultraviolet Accretion Diagnostics in T Tauri Stars

Abstract

We use our ultraviolet (UV) atlas of premain-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 —uxes 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 contin- uum in the classical TTSs (CTTSs) appears to originate in a D10,000 K optically thick plasma, while in the naked TTSs (NTTSsstars 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 ii saturated ˇˇ magnetic activity due to their small Rossby numbers. However, while some of the TTSs show emission line surface —uxes consistent with this saturation level, many CTTSs show signi—cantly stronger emission than pre- dicted by saturation. In these stars, the emission line luminosity in the high ionization lines present in the spectrum between 1200 and 2000 correlates well with the mass accretion rate. Therefore, we con- Ae clude that the bulk of the short-wavelength emission seen in CTTSs results from accretion related pro- cesses and not from dynamo-driven magnetic activity. Using CTTSs with known mass accretion rates, we calibrate the relationship between and to derive the mass accretion rate for some CTTSs M0 L CIV 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 cannot form in gas at a H 2 temperature of D105 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 —uorescent emission pumped by the emission (likely Lya) from hotter gas. Subject headings: accretion, accretion diskscircumstellar matterstars: activity ¨ stars: premain-sequence