The 1600 Å Emission Bump in Protoplanetary Disks: A Spectral Signature of H2O Dissociation

The FUV continuum spectrum of many accreting pre-main sequence stars, Classical T Tauri Stars (CTTSs), does not continue smoothly from the well-studied Balmer continuum emission in the NUV, suggesting that additional processes contribute to the short-wavelength emission in these objects. The most notable spectral feature in the FUV continuum of some CTTSs is a broad emission approximately centered at 1600 angstrom, which has been referred to as the 1600 angstrom Bump. The origin of this feature remains unclear. In an effort to better understand the molecular properties of planet-forming disks and the UV spectral properties of accreting protostars, we have assembled archival FUV spectra of 37 disk-hosting systems observed by the Hubble Space Telescope-Cosmic Origins Spectrograph. Clear 1600 angstrom Bump emission is observed above the smooth, underlying 1100-1800 angstrom continuum spectrum in 19/37 Classical T Tauri disks in the HST-COS sample, with the detection rate in transition disks (8/8) being much higher than that in primordial or non-transition sources (11/29). We describe a spectral deconvolution analysis to separate the Bump (spanning 1490-1690 angstrom) from the underlying FUV continuum, finding an average Bump luminosity L(Bump) approximate to 7 x 10(29) erg s(-1). Parameterizing the Bump with a combination of Gaussian and polynomial components, we find that the 1600 angstrom Bump is characterized by a peak wavelength lambda(0) =. 1598.6 +/- 3.3 angstrom, with FWHM = 35.8 +/- 19.1 angstrom. Contrary to previous studies, we find that this feature is inconsistent with models of H-2 excited by electron -impact. We show that this Bump makes up between 5%-50% of the total FUV continuum emission in the 1490-1690 angstrom band and emits roughly 10%-80% of the total fluorescent H-2 luminosity for stars with well-defined Bump features. Energetically, this suggests that the carrier of the 1600 angstrom Bump emission is powered by Ly alpha photons. We argue that the most likely mechanism is Ly alpha-driven dissociation of H2O in the inner disk, r less than or similar to 2 au. We demonstrate that non-thermally populated H2O fragments can qualitatively account for the observed emission (discrete and continuum) and find that the average Ly alpha-driven H2O dissociation rate is 1.7 x 10(42) water molecules s(-1).