CO(6−5) and [C i](2−1) pointed observations of five protoplanetary disks: Warm gas in HD 142527

Abstract

Context. 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 [C](2−1), but no C detections in disks exist. Aims. Our goals are to identify gas-rich systems that are bright in CO(6−5), for subsequent ALMA observations, and search for C. Methods. We follow-up a CO(3−2) survey in protoplanetary disks with APEX/CHAMP+ maps in CO(6−5) 691.4 GHz and [C](2−1) 809.3 GHz. Results. We obtain one compact CO(6−5) detection in HD 142 527, 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 400 pointing offset. The C observations yield upper limits on the mass of rarefied gas phase neutral carbon. Conclusions. 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 cm−3 ) atomic gas to <4 × 10−3 (3σ).