The FUV spectrum of TW Hya. I. Observations of H${}_2$ Fluorescence

Abstract

We observed the classical T Tauri star TW Hya with \textit{HST}/STIS using the E140M grating, from 1150--1700 Å, with the E230M grating, from 2200--2900 Å, and with \FUSE from 900--1180 Å. Emission in 143 Lyman-band H${}_2$ lines representing 19 progressions dominates the spectral region from 1250--1650 Å. The total H${}_2$ emission line flux is $1.94\times 10^{-12}$ erg cm${}^{-2}$ s${}^{-1}$, which corresponds to $1.90\times 10^{-4}$ $L_{\odot }$ at TW Hya's distance of 56 pc. A broad stellar \Lya line photoexcites the H${}_2$ from excited rovibrational levels of the ground electronic state to excited electronic states. The \ion{C}{2} 1335 Ådoublet, \ion{C}{3} 1175 Å multiplet, and \ion{C}{4} 1550 Ådoublet also electronically excite H${}_2$. The velocity shift of the H${}_2$ lines is consistent with the photospheric radial velocity of TW Hya, and the emission is not spatially extended beyond the $0\farcs05$ resolution of \textit{HST}. The H${}_2$ lines have an intrinsic FWHM of $11.91\pm 0.16$ \kms. One H${}_2$ line is significantly weaker than predicted by this model because of \ion{C}{2} wind absorption. We also do not observe any H${}_2$ absorption against the stellar \Lya profile. From these results, we conclude that the H${}_2$ 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., \ion{C}{4}, \ion{Si}{4}, \ion{O}{6}) 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.

accepted by ApJ, 42 pages -- 20 text, 11 figures

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