The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems I: Electron-Impact H2 and Accretion Shocks

Abstract

We present deep spectroscopic observations of the classical T Tauri stars DF Tau and V4046 Sgr in order to better characterize two important sources of far-ultraviolet continuum emission in protoplanetary disks. These new Hubble Space Telescope-Cosmic Origins Spectrograph observations reveal a combination of line and continuum emission from collisionally excited H2 and emission from accretion shocks. H2 is the dominant emission in the 1400-1650 A band spectrum of V4046 Sgr, while an accretion continuum contributes strongly across the far-ultraviolet spectrum of DF Tau. We compare the spectrum of V4046 Sgr to models of electron-impact induced H2 emission to constrain the physical properties of the emitting region, after making corrections for attenuation within the disk. We find reasonable agreement with the broad spectral characteristics of the H2 model, implying N(H2) ~ 10^{18} cm^{-2}, T(H2) = 3000^{+1000}_{-500} K, and a characteristic electron energy in the range of ~ 50 - 100 eV. We propose that self-absorption and hydrocarbons provide the dominant attenuation for H2 line photons originating within the disk. For both DF Tau and V4046 Sgr, we find that a linear fit to the far-UV data can reproduce near-UV/optical accretion spectra. We discuss outstanding issues concerning how these processes operate in protostellar/protoplanetary disks, including the effective temperature and absolute strength of the radiation field in low-mass protoplanetary environments. We find that the 912-2000A continuum in low-mass systems has an effective temperature of ~10^{4} K with fluxes 10^{5-7} times the interstellar level at 1 AU.

14 pages, 8 figures, 3 tables. ApJ, accepted

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Cited by in corpus (27)

• The Cosmic Origins Spectrograph
• A Far-ultraviolet Atlas of Low-resolution Hubble Space Telescope Spectra of T Tauri Stars
• A Hubble Space Telescope Survey of H2 Emission in the Circumstellar Environments of Young Stars
• HST measures of Mass Accretion Rates in the Orion Nebula Cluster
• High-resolution Ultraviolet Radiation Fields of Classical T Tauri Stars
• Lyman alpha dominance of the Classical T Tauri FUV Radiation Field
• A survey for near-infrared H2 emission in Herbig Ae/Be stars: emission from the outer disks of HD 97048 and HD 100546
• CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk
• The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption
• HST spectra reveal accretion in MY Lupi
• Shielding by Water and OH in FUV and X-ray Irradiated Protoplanetary Disks
• Using FUV to IR Variability to Probe the Star-Disk Connection in the Transitional Disk of GM Aur
• Direct Measurement of Interstellar Extinction Toward Young Stars Using Atomic Hydrogen Lyman-$\alpha$ Absorption
• Rovibrationally resolved photodissociation of HeH+
• Characterizing CO Fourth Positive Emission in Young Circumstellar Disks
• The 1600 Angstrom Emission Bump in Protoplanetary Disks: A Spectral Signature of H${}_2$O Dissociation
• CO and H2 Absorption in the AA Tauri Circumstellar Disk
• Characterizing the AB Doradus Moving Group Via High Resolution Spectroscopy and Kinematic Traceback
• Using Multiwavelength Variability to Explore the Connection between X-ray Emission, the Far-Ultraviolet H2 Bump, and Accretion in T Tauri Stars
• A GALEX based search for the sparse young stellar population in the Taurus-Aurigae star forming region
• HST/COS Spectra of DF Tau and V4046 SGR: First Detection of Molecular Hydrogen Absorption Against the Ly$\alpha$ Emission Line
• A UV-to-NIR Study of Molecular Gas in the Dust Cavity Around RY Lupi
• Probing the Inner Regions of Protoplanetary Disks with CO Absorption Line Spectroscopy
• Diagnostics of collisions between electrons and water molecules in near-ultraviolet and visible wavelengths
• HST UV Spectroscopy of the Planet-Hosting T Tauri Star PDS 70
• Probing UV-Sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
• Residual Gas & Dust Around Transition Objects and Weak T Tauri Stars