The 1600 Å Emission Bump in Protoplanetary Disks: A Spectral Signature of H{sub 2}O Dissociation

• Published in: The Astrophysical journal 2017-08, Vol.844 (2)
• Main Authors: France, Kevin, Roueff, Evelyne, Abgrall, Hervé
• Format: Article
• Language: English
• Subjects: ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; DETECTION; DISSOCIATION; ELECTRONS; FAR ULTRAVIOLET RADIATION; FLUORESCENCE; HYDROGEN; LUMINOSITY; LYMAN LINES; MAIN SEQUENCE STARS; MOLECULES; NEAR ULTRAVIOLET RADIATION; PHOTONS; PLANETS; PROTOPLANETS; PROTOSTARS; SPACE; SPECTRA; T TAURI STARS; TELESCOPES
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/AA7CEE

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 that 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) ≈ 7 × 10{sup 29} erg s{sup −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 λ {sub o} = 1598.6 ± 3.3 Å, with FWHM = 35.8 ± 19.1 Å. Contrary to previous studies, we find that this feature is inconsistent with models of H{sub 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{sub 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 α photons. We argue that the most likely mechanism is Ly α -driven dissociation of H{sub 2}O in the inner disk, r ≲ 2 au. We demonstrate that non-thermally populated H{sub 2}O fragments can qualitatively account for the observed emission (discrete and continuum) and find that the average Ly α -driven H{sub 2}O dissociation rate is 1.7 × 10{sup 42} water molecules s{sup −1}.