Upper limits on gaseous CO at Pluto and Triton from high-resolution near-IR spectroscopy

We observed Pluto and Triton with the CSHELL echelle spectrograph on the Infrared Telescope Facility in April and July 1996, in an effort to detect the R(2), R(3), and R(4) rotational lines of the 2-0 vibrational transition of gaseous CO. As no lines were detected, we derived 3-σ upper limits on the average widths of these three lines of 0.040 cm^-1 for Pluto and 0.028 cm^-1 for Triton. The corresponding upper limits on the gaseous CO mole fractions depend on the assumed profiles of temperature and pressure in the atmospheres of these bodies. If Triton's atmosphere in 1996 resembles that measured by stellar occultation in 1997, we find a 3-σ upper limit to the CO mole fraction of 59%. If Pluto's atmosphere resembles the tropospheric model of J. A. Stansberry, J. I. Lunine, W. B. Hubbard, R. V. Yelle, and D. M. Hunten (1994), Icarus 11, 503-513, we find a 3-σ upper limit to the CO mole fraction of 6%. For Pluto, this limit to the gaseous mole fraction argues against intimate mixtures (e.g., "salt-and-pepper" mixtures, as opposed to solid solutions) of surface CO and N₂ frost.