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 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 H2 model, implying N(H2) ~ 1018 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 912-2000 Å continuum in low-mass systems has an effective temperature of ~104 K with fluxes 105-107 times the interstellar level at 1 AU.
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.- Publication:
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The Astrophysical Journal
- Pub Date:
- March 2011
- DOI:
- 10.1088/0004-637X/729/1/7
- arXiv:
- arXiv:1101.0165
- Bibcode:
- 2011ApJ...729....7F
- Keywords:
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- accretion;
- accretion disks;
- protoplanetary disks;
- stars: individual: DF Tau V4046 Sgr;
- stars: pre-main sequence;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 14 pages, 8 figures, 3 tables. ApJ, accepted