6 – 5 observations of TW Hya with the SMA

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 2D Monte Carlo radiative transfer, we find that additional surface heating is needed to fit simultaneously 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 χ analysis, and the prediction of this model yields CO J=6–5 emission consistent with the observations. Subject headings: stars: individual (TW Hya) —stars: circumstellar matter — planetary systems: protoplanetary disks —radio lines: stars —ISM: molecules Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA; [email protected], [email protected], [email protected], [email protected], [email protected] Divisions of Geological & Planetary Sciences and Chemistry & Chemical Engineering, California Institute of Technology, MS150–21, Pasadena, CA 91125, USA; [email protected]. Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands; [email protected]. Department of Astronomy, University of Michigan, 825 Dennison Building, 500 Church Street, Ann Arbor, MI 48109; [email protected], [email protected]. Academia Sinica Institute of Astronomy & Astrophysics, P.O. Box 23-141, Taipei, Taiwan, 106, R.O.C.; [email protected].