Research Note

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

¹ Departamento de Astronomía, Universidad de Chile, Chile
e-mail: simon@das.uchile.cl
² Observatoire de Paris, LUTH and Université Denis Diderot, Place J. Janssen, 92190 Meudon, France
³ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763-0355, Santiago, Chile
⁴ European Southern Observatory, Alonso de Cordova 3107, Vitacura. Casilla 19001, 19 Santiago, Chile
⁵ Max-Planck Institut für Radioastronomie, Auf dem Hg¨el 69, 53121 Bonn, Germany
⁶ UMI-FCA, CNRS/INSU France UMI 3386, and Departamento de Astronomía, Universidad de Chile, Casilla 36- D Santiago, Chile
⁷ CNRS/UJF Grenoble 1, UMR 5274, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38041 Grenoble Cedex 9, France
⁸ Wesleyan University Department of Astronomy, Van Vleck Observatory, 96 Foss Hill Dr., Middletown, CT 06459, USA
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 22 May 2012
Accepted: 25 March 2013

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 i](2−1), but no C i 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 i.

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 i](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 4″ pointing offset. The C i 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).