VLT-CRIRES Survey of Rovibrational CO Emission from Protoplanetary Disks

Abstract: We present a large, comprehensive survey of rovibrational CO line emission at 4.7 micron from 69 protoplanetary disks, obtained with CRIRES on the ESO Very Large Telescope at the highest available spectral resolving power (R=95,000, v=3.2 km/s). The CO fundamental band (Delta v=1) is a well-known tracer of warm gas in the inner, planet-forming regions of gas-rich disks around young stars, with the lines formed in the super-heated surfaces of the disks at radii of 0.1-10 AU. Our high spectral resolution data provide new insight into the kinematics of the inner disk gas. Pure double-peaked Keplerian profiles are surprisingly uncommon, beyond the frequency expected based on disk inclination. The majority of the profiles are consistent with emission from a disk plus a slow (few km/s) molecular disk wind. This is evidenced by analysis of different categories as well as an overall tendency for line profiles to have excess blue emission. Weak emission lines from isotopologues and vibrationally excited levels are readily detected. In general, 13CO lines trace cooler gas than the bulk 12CO emission and may arise from further out in the disk, as indicated by narrower line profiles. A high fraction of the sources show vibrationally excited emission (~50%) which is correlated with accretion luminosity, consistent with ultra-violet (UV) fluorescent excitation. Disks around early-type Herbig AeBe stars have narrower lines, on average, than their lower-mass late-type counterparts, due to their increased luminosity. Evolutionary changes in CO are also seen. Removal of the protostellar envelope between class I and II results in the disappearance of the strong absorption lines and CO ice feature characteristic of class I spectra. However, CO emission from class I and II objects is similar in detection frequency, excitation and line shape, indicating that inner disk characteristics are established early.