2013A&A...553A..64C

The molecular gas in protoplanetary disks can be traced with single-dish instruments in low J rotational lines of CO, in systems clear of coincident extended emission. Other rotational lines of CO also sample the gas-phase CO reservoir, albeit with different biases; CO(6-5) traces warmer molecular gas. A rarefied atomic gas could be traced in [Ci](2-1), but no Ci detections in disks exist. Our goals are to identify gas-rich systems that are bright in CO(6-5), for subsequent ALMA observations, and search for Ci. We follow-up a CO(3-2) survey in protoplanetary disks with APEX/CHAMP+ maps in CO(6-5) 691.4 GHz and [Ci](2-1) 809.3 GHz. We obtain one compact CO(6-5) detection in HD 142527, three upper limits, and extended CO(6-5) emission in HD 37389. Given the CO(2-1) flux, the CO(6-5) line in HD 142527 would imply a temperature of 19 K if the CO ladder was isothermal, for a common solid angle, which is close to CO freezeout. This low temperature, together with a modulated CO(6-5) line profile, can be explained by a 4^'' pointing offset. The Ci observations yield upper limits on the mass of rarefied gas phase neutral carbon. The CO(6-5) detection in HD 142527 adds to the other two southern systems with known CO(6-5) signal (HD 100546 and TW Hya). The CO(6-5)/CO(3-2) flux ratio varies from 2 to 24 among seven sources with bona-fide CO(6-5) detections, and is uncorrelated with spectral type in this sample. The upper limits on CI emission constrain the fraction of carbon in low-density (<10³/cm3) atomic gas to <4x10^–3 (3σ).