Letter to the Editor

Evolution of interstellar dust with Herschel. First results in the photodissociation regions of NGC 7023^*

¹ Institut d'Astrophysique Spatiale, UMR 8617, CNRS/Université Paris-Sud 11, 91405 Orsay, France e-mail: alain.abergel@ias.u-psud.fr
² 
Canadian Institute for Theoretical Astrophysics, Toronto, Ontario, M5S 3H8, Canada
³ 
Department of Physics and Astronomy, Cardiff University, Cardiff, UK
⁴ 
Laboratoire d'Astrophysique de Marseille (UMR 6110 CNRS & Université de Provence), 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
⁵ 
CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France
⁶ 
CESR, Université de Toulouse (UPS), CNRS, UMR 5187, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
⁷ 
CNRS/INSU, Laboratoire d'Astrophysique de Bordeaux, UMR 5804, BP 89, 33271 Floirac Cedex, France
⁸ 
University of California, Radio Astronomy Laboratory, Berkeley, 601 Campbell Hall, US Berkeley CA 94720-3411, USA
⁹ 
Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France
¹⁰ 
National Astronomical Observatories (China)
¹¹ 
The Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK
¹² Blue Sky Spectrosocpy Inc, Lethbridge, Canada
¹³ 
Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Canada
¹⁴ 
Istituto di Fisica dello Spazio Interplanetario, INAF, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁵ NASA - Goddard SFC, USA
¹⁶ 
Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK

Received: 31 March 2010
Accepted: 10 May 2010

Abstract

Context. In photodissociation regions (PDRs), the physical conditions and the excitation evolve on short spatial scales as a function of depth within the cloud, providing a unique opportunity to study how the dust and gas populations evolve with the excitation and physical conditions. The mapping of the PDRs in NGC 7023 performed during the science demonstration phase of Herschel is part of the “Evolution of interstellar dust” key program. The goal of this project is to build a coherent database on interstellar dust emission from diffuse clouds to the sites of star formation.

Aims. We study the far-infrared/submillimeter emission of the PDRs and their fainter surrounding regions. We combine the Herschel and Spitzer maps to derive at each position the full emission spectrum of all dust components, which we compare to dust and radiative transfer models in order to learn about the spatial variations in both the excitation conditions and the dust properties.

Methods. We adjust the emission spectra derived from PACS and SPIRE maps using modified black bodies to derive the temperature and the emissivity index β of the dust in thermal equilibrium with the radiation field. We present a first modeling of the NGC 7023-E PDR with standard dust properties and abundances.

Results. At the peak positions, a value of β equal to 2 is compatible with the data. The detected spectra and the spatial structures are strongly influenced by radiative transfer effects. We are able to reproduce the spectra at the peak positions deduced from Herschel maps and emitted by dust particles at thermal equilibrium, and also the evolution of the spatial structures observed from the near infrared to the submillimeter. On the other hand, the emission of the stochastically heated smaller particles is overestimated by a factor ~2.