Letter to the Editor

Herschel/HIFI observations of high-J CO transitions in the protoplanetary nebula CRL 618^*

¹ Observatorio Astronómico Nacional (IGN), Ap 112, 28803 Alcalá de Henares, Spain e-mail: v.bujarrabal@oan.es
² Observatorio Astronómico Nacional (IGN), Alfonso XII N°3, 28014 Madrid, Spain
³ Joint ALMA Observatory, El Golf 40, Las Condes, Santiago, Chile
⁴ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁵ CAB, INTA-CSIC, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁷ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
⁸ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
⁹ Onsala Space Obseravtory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden
¹⁰ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
¹¹ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
¹² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ The Johns Hopkins University, 3400 North Charles St, Baltimore, MD 21218, USA
¹⁴ Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
¹⁵ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
¹⁶ Instituto de Radioastronomía Milimétrica (IRAM), Avda. Divina Pastora 7, 18012 Granada, Spain
¹⁷ Observatorio Astronómico Nacional (IGN), Centro Astronómico de Yebes, Apartado 148, 19080 Guadalajara, Spain
¹⁸ SRON, Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁹ Sterrewacht Leiden, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁰ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 28 May 2010
Accepted: 22 June 2010

Abstract

Aims. We aim to study the physical conditions, particularly the excitation state, of the intermediate-temperature gas components in the protoplanetary nebula CRL 618. These components are particularly important for understanding the evolution of the nebula.

Methods. We performed Herschel/HIFI observations of several CO lines in the far-infrared/sub-mm in the protoplanetary nebula CRL 618. The high spectral resolution provided by HIFI allows measurement of the line profiles. Since the dynamics and structure of the nebula is well known from mm-wave interferometric maps, it is possible to identify the contributions of the different nebular components (fast bipolar outflows, double shells, compact slow shell) to the line profiles. The observation of these relatively high-energy transitions allows an accurate study of the excitation conditions in these components, particularly in the warm ones, which cannot be properly studied from the low-energy lines.

Results. The ¹²CO J = 16–15, 10–9, and 6–5 lines are easily detected in this source. Both ¹³CO J = 10–9 and 6–5 are also detected. Wide profiles showing spectacular line wings have been found, particularly in ¹²CO J = 16–15. Other lines observed simultaneously with CO are also shown. Our analysis of the CO high-J transitions, when compared with the existing models, confirms the very low expansion velocity of the central, dense component, which probably indicates that the shells ejected during the last AGB phases were driven by radiation pressure under a regime of maximum transfer of momentum. No contribution of the diffuse halo found from mm-wave data is identified in our spectra, because of its low temperature. We find that the fast bipolar outflow is quite hot, much hotter than previously estimated; for instance, gas flowing at 100 km s^-1 must have a temperature higher than ~200 K. Probably, this very fast outflow, with a kinematic age <100 yr, has been accelerated by a shock and has not yet cooled down. The double empty shell found from mm-wave mapping must also be relatively hot, in agreement with the previous estimate.