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Published June 10, 2011 | Published
Journal Article Open

The Far-ultraviolet "Continuum" in Protoplanetary Disk Systems. II. Carbon Monoxide Fourth Positive Emission and Absorption


We exploit the high sensitivity and moderate spectral resolution of the Hubble Space Telescope Cosmic Origins Spectrograph to detect far-ultraviolet (UV) spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. The CO absorption most likely arises in warm inner disk gas. We measure a CO column density and rotational excitation temperature of N(CO) = (2 ± 1) × 10^(17) cm^(–2) and T_(rot)(CO) 500 ± 200 K for the absorbing gas. We also detect CO A-X band emission in RECX-11 and V4046 Sgr, excited by UV line photons, predominantly H I Lyα. All three objects show emission from CO bands at λ > 1560 Å, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H_2, and photo-excited H_2, all of which appeared as a "continuum" whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar Lyα emission profile. We find CO parameters in the range: N(CO) ~ 10^(18)-10^(19) cm^(–2), T_(rot)(CO) gsim 300 K for the Lyα-pumped emission. We combine these results with recent work on photo-excited and collisionally excited H_2 emission, concluding that the observations of UV-emitting CO and H_2 are consistent with a common spatial origin. We suggest that the CO/H_2 ratio (≡ N(CO)/N(H_2)) in the inner disk is ~1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.

Additional Information

© 2011 The American Astronomical Society. Received 2011 January 17; accepted 2011 April 3; published 2011 May 23. 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. K.F. thanks Roxana Lupu for enjoyable discussions regarding the far-UV spectrum of CO. This work was support by NASA grants NNX08AC146 and NAS5-98043 to the University of Colorado at Boulder and STScI grants to program GO-11616.

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