2006ApJ...636L.157Q

We present the first images of the 691.473 GHz CO J=6-5 line in a protoplanetary disk, obtained along with the 690 GHz dust continuum, toward the classical T Tauri star TW Hya, using the Submillimeter Array. Imaging in the CO J=6-5 line reveals a rotating disk, consistent with previous observations of CO J=3-2 and 2-1 lines. Using an irradiated accretion disk model and two-dimensional Monte Carlo radiative transfer, we find that additional surface heating is needed to simultaneously fit the absolute and relative intensities of the CO J=6-5, 3-2, and 2-1 lines. In particular, the vertical gas temperature gradient in the disk must be steeper than that of the dust, mostly likely because the CO emission lines probe nearer to the surface of the disk. We have used an idealized X-ray heating model to fit the line profiles of CO J=2-1 and 3-2 with χ² analysis, and the prediction of this model yields CO J=6-5 emission consistent with the observations.