2011ApJ...729....7F

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 Å ≲ λ ≲ 1650 Å 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 H₂model, implying N(H₂) ∼ 10¹⁸/cm2, T(H₂) = 3000⁺¹⁰⁰⁰_–500K, 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 912-2000 Å continuum in low-mass systems has an effective temperature of ∼10⁴ K with fluxes 10⁵-10⁷ times the interstellar level at 1 AU.