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 V4046Sgr 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 ≲ λ ≲ 1650 band spectrum of V4046Sgr, while an accretion continuum contributes strongly across the far-ultraviolet spectrum of DF Tau. We compare the spectrum of V4046Sgr to models of electron-impact-induced H 2 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 H 2 model, implying N(H2) 1018cm-2, T(H2) = 3000+1000-500K, and a characteristic electron energy in the range of 50-100eV. We propose that self-absorption and hydrocarbons provide the dominant attenuation for H2 line photons originating within the disk. For both DF Tau and V4046Sgr, 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 continuum in low-mass systems has an effective temperature of 104K with fluxes 105-10 7 times the interstellar level at 1AU. © 2011. The American Astronomical Society. All rights reserved.