The FUV spectrum of TW Hya. I. Observations of H2 Fluorescence
Abstract
We observed the classical T Tauri star TW Hya with HST/STIS using the E140M grating, from 1150--1700 , with the E230M grating, from 2200--2900 , and with from 900--1180 . Emission in 143 Lyman-band H2 lines representing 19 progressions dominates the spectral region from 1250--1650 . The total H2 emission line flux is 1.94 × 10-12 erg cm-2 s-1, which corresponds to 1.90×10-4 L at TW Hya's distance of 56 pc. A broad stellar line photoexcites the H2 from excited rovibrational levels of the ground electronic state to excited electronic states. The C2 1335 doublet, C3 1175 \ multiplet, and C4 1550 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 11.910.16 . One H2 line is significantly weaker than predicted by this model because of C2 wind absorption. We also do not observe any H2 absorption against the stellar 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., C4, Si4, O6) 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.
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