Lyα DOMINANCE OF THE CLASSICAL T TAURI FAR-ULTRAVIOLET RADIATION FIELD

Abstract

ABSTRACT 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. H2 fluorescence, 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 H2 progression 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.