Letter to the Editor

Gas morphology and energetics at the surface of PDRs: New insights with Herschel observations of NGC 7023 ^*

¹ Université de Toulouse, UPS, CESR, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France e-mail: christine.joblin@cesr.fr
² CNRS, UMR 5187, 31028 Toulouse, France
³ Observatorio Astronómico Nacional (OAN), Apdo. 112, 28803 Alcalá de Henares (Madrid), Spain
⁴ LERMA, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁵ Leiden Observatory, Universiteit Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
⁷ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁸ Observatoire de Paris, LUTH and Université Denis Diderot, Place J. Janssen, 92190 Meudon, France
⁹ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
¹⁰ Centro de Astrobiología, CSIC-INTA, 28850 Madrid, Spain
¹¹ Institute for Astronomy, ETH Zürich, 8093 Zürich, Switzerland
¹² Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
¹³ Department of Astronomy and Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
¹⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany
¹⁵ Astronomy Department, University of Maryland, College Park, MD 20742, USA
¹⁶ Instituto de Radio Astronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
¹⁷ IPAC/Caltech, MS 100-22, Pasadena, CA 91125, USA
¹⁸ Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Mumbai 400005, India
¹⁹ California Institute of Technology, 320-47, Pasadena, CA 91125-4700, USA
²⁰ Kapteyn Astronomical Institute, University of Groningen, PO box 800, 9700 AV Groningen, The Netherlands
²¹ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA
²² MPI für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany

Received: 1 June 2010
Accepted: 19 July 2010

Abstract

Context. We investigate the physics and chemistry of the gas and dust in dense photon-dominated regions (PDRs), along with their dependence on the illuminating UV field.

Aims. Using Herschel/HIFI observations, we study the gas energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023 is the prototype of a PDR illuminated by a B2V star and is one of the key targets of Herschel.

Methods. Our approach consists in determining the energetics of the region by combining the information carried by the mid-IR spectrum (extinction by classical grains, emission from very small dust particles) with that of the main gas coolant lines. In this letter, we discuss more specifically the intensity and line profile of the 158 μm (1901 GHz) [C ii] line measured by HIFI and provide information on the emitting gas.

Results. We show that both the [C ii] emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) arise from the regions located in the transition zone between atomic and molecular gas. Using the Meudon PDR code and a simple transfer model, we find good agreement between the calculated and observed [C ii] intensities.

Conclusions. HIFI observations of NGC 7023 provide the opportunity to constrain the energetics at the surface of PDRs. Future work will include analysis of the main coolant line [O i] and use of a new PDR model that includes PAH-related species.