2012ApJ...756L..23S

Far-ultraviolet (FUV) radiation plays an important role in determining chemical abundances in protoplanetary disks. H I Lyman α (Lyα) is suspected to be the dominant component of the FUV emission from Classical T Tauri Stars (CTTSs), but is difficult to measure directly due to circumstellar and interstellar H I absorption. To better characterize the intrinsic Lyα radiation, we present FUV spectra of 14 CTTSs taken with the Hubble Space Telescope Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph instruments. H₂fluorescence, commonly seen in the spectra of CTTSs, is excited by Lyα photons, providing an indirect measure of the Lyα flux incident upon the warm disk surface. We use observed H₂progression fluxes to reconstruct the CTTS Lyα profiles. The Lyα flux correlates with total measured FUV flux, in agreement with an accretion-related source of FUV emission. With a geometry-independent analysis, we confirm that in accreting T Tauri systems Lyα radiation dominates the FUV flux (∼1150 Å -1700 Å). In the systems surveyed this one line comprises 70%-90% of the total FUV flux.