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 Å , which has been referred to as the “ 1600 Å 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 Å Bump emission is observed above the smooth , underlying 1100 – 1800 Å 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 in primordial or non-transition sources ( 11/29 ) .
We describe a spectral deconvolution analysis to separate the Bump ( spanning 1490 – 1690 Å ) from the underlying FUV continuum , finding an average Bump luminosity , L ( Bump ) \approx 7 \times 10 ^ { 29 } erg s ^ { -1 } .
Parameterizing the Bump with a combination of Gaussian and polynomial components , we find that the 1600 Å Bump is characterized by a peak wavelength \lambda _ { o } = 1598.6 \pm 3.3 Å , with FWHM = 35.8 \pm 19.1 Å .

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 Å 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 Å Bump emission is powered by Ly \alpha photons .
We argue that the most likely mechanism is Ly \alpha -driven dissociation of H _ { 2 } O in the inner disk , r \lesssim 2 AU .
We demonstrate that non-thermally populated H _ { 2 } O fragments can qualitatively account for the observed emission ( discrete and continuum ) , and find that the average Ly \alpha -driven H _ { 2 } O dissociation rate is 1.7 \times 10 ^ { 42 } water molecules s ^ { -1 } .