We exploit the high sensitivity and moderate spectral resolution of the HST -Cosmic Origins Spectrograph to detect far-ultraviolet 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 \pm 1 \times 10 ^ { 17 } cm ^ { -2 } and T _ { rot } ( CO ) 500 \pm 200 K for the absorbing gas .
We also detect CO A – X band emission in RECX-11 and V4046 Sgr , excited by ultraviolet line photons , predominantly H i Ly \alpha .
All three objects show emission from CO bands at \lambda > 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 Ly \alpha emission profile .
We find CO parameters in the range : N ( CO ) \sim 10 ^ { 18 - 19 } cm ^ { -2 } , T _ { rot } ( CO ) \gtrsim 300 K for the Ly \alpha -pumped emission .
We combine these results with recent work on photo- and collisionally-excited H _ { 2 } emission , concluding that the observations of ultraviolet-emitting CO and H _ { 2 } are consistent with a common spatial origin .
We suggest that the CO/H _ { 2 } ratio ( \equiv N ( CO ) / N ( H _ { 2 } ) ) in the inner disk is \sim 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 .