The 1600 Angstrom Emission Bump in Protoplanetary Disks: A Spectral Signature of H${}_2$O Dissociation

Abstract

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 A Bump". The origin of this feature remains unclear. We have assembled archival FUV spectra of 37 disk-hosting systems observed by the {\it Hubble Space Telescope}. Clear 1600 A Bump emission is observed above the smooth, underlying 1100-1800 A continuum spectrum in 19/37 Classical T Tauri disks in the HST 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 A) from the underlying FUV continuum, finding an average Bump luminosity, $L$(Bump7 x 10^{29} erg s^{-1}. We find that the 1600 A Bump is characterized by a peak wavelength of 1598.6 +/- 3.3 A. Contrary to previous studies, we find that this feature is inconsistent with models of H2 excited by electron-impact. We show that this Bump emits roughly 10-80% of the total fluorescent H2 luminosity for stars with well-defined Bump features. Energetically, this suggests that the carrier of the 1600 A Bump emission is powered by Ly-a photons. We argue that the most likely mechanism is Ly-a-driven dissociation of H2O in the inner disk, r < 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-a-driven H2O dissociation rate is 1.7x 10^{42} water molecules s^{-1}.

28 pages, 17 figures. Accepted for publication in the Astrophysical Journal

References in corpus (20)

• An Optical Spectroscopic Study of T Tauri Stars. I. Photospheric Properties
• UV excess measures of accretion onto young very low-mass stars and brown dwarfs
• The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
• Accretion Rates in Herbig Ae stars
• Spectro-astrometric imaging of molecular gas within protoplanetary disk gaps
• H2O and OH gas in the terrestrial planet-forming zones of protoplanetary disks
• VLBA determination of the distance to nearby star-forming regions I. The distance to T Tauri with 0.4% accuracy
• Tracing Slow Winds from T Tauri Stars via Low Velocity Forbidden Line Emission
• Probing the Dust and Gas in the Transitional Disk of CS Cha with Spitzer
• The X-ray soft excess in classical T Tauri stars
• The depletion of water during dispersal of planet-forming disk regions
• CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk
• Constraining the Sub-AU-Scale Distribution of Hydrogen and Carbon Monoxide Gas around Young Stars with the Keck Interferometer
• Grand challenges in protoplanetary disc modelling
• Periodic radial velocity variations in RU Lupi
• The evolution of accretion in young stellar objects: Strong accretors at 3 - 10 Myr
• CO Fundamental Emission from V836 Tau
• Molecular Hydrogen emission from disks in the eta Chamaeleontis cluster
• A "Rosetta Stone" for protoplanetary disks: The synergy of multi-wavelength observations
• Near Infrared High Resolution Spectroscopy and Spectro-astrometry of Gas in Disks around Herbig Ae/Be Stars

Cited by in corpus (12)

• Multi-epoch Ultraviolet HST Observations of Accreting Low-mass Stars
• The ODYSSEUS Survey. Motivation and First Results: Accretion, Ejection, and Disk Irradiation of CVSO 109
• OH mid-infrared emission as a diagnostic of H${}_2$O UV photodissociation. III. Application to planet-forming disks
• Using Multiwavelength Variability to Explore the Connection between X-ray Emission, the Far-Ultraviolet H2 Bump, and Accretion in T Tauri Stars
• The UV Perspective of Low-Mass Star Formation
• Probing Protoplanetary Disk Winds with C II Absorption
• Probing UV-Sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
• The Radial Distribution and Excitation of H2 around Young Stars in the HST-ULLYSES Survey
• Lyman-alpha Scattering Models Trace Accretion and Outflow Kinematics in T Tauri Systems
• UV Fluorescence Traces Gas and LyA Evolution in Protoplanetary Disks
• Detection of H${}_2$ in the TWA 7 System: A Probable Circumstellar Origin
• Evidence of a Disk-wind Origin for Fluorescent H${}_2$ in Classical T Tauri Stars