The Far-Ultraviolet Spectrum of TW Hydrae. I. Observations of H₂ Fluorescence^*

We observed the classical T Tauri star TW Hya with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST) using the E140M grating, from 1150 to 1700 Å, with the E230M grating, from 2200 to 2900 Å, and with the Far Ultraviolet Spectroscopic Explorer from 900 to 1180 Å. Emission in 146 Lyman-band H₂ lines, representing 19 progressions, dominates the spectral region from 1250 to 1650 Å. The total H₂ emission line flux is 1.94 × 10^-12 ergs cm^-2 s^-1, which corresponds to 1.90 × 10^-4 L_☉ at TW Hya's distance of 56 pc. A broad stellar Lyα line photoexcites the H₂ from excited rovibrational levels of the ground electronic state to excited electronic states. The C II λ1335 doublet, C III λ1175 multiplet, and C IV λ1550 doublet also electronically excite H₂. The velocity shift of the H₂ lines is consistent with the photospheric radial velocity of TW Hya, and the emission is not spatially extended beyond the 005 resolution of HST. The H₂ lines have an intrinsic FWHM of 11.91 ± 0.16 km s^-1. One H₂ line is significantly weaker than predicted by this model because of C II wind absorption. We also do not observe any H₂ absorption against the stellar Lyα profile. From these results we conclude that the H₂ emission is more consistent with an origin in a disk rather than in an outflow or circumstellar shell. We also analyze the hot accretion region lines (e.g., C IV, Si IV, O VI) of TW Hya, which are formed at the accretion shock, and discuss some reasons why Si lines appear significantly weaker than other TR region lines.