Spatial variation of the cooling lines in the reflection nebula NGC 7023^⋆

¹ Department of Physical SciencesThe Open University, MK7 6AA Milton Keynes, UK
e-mail: jeronimo.bernard-salas@open.ac.uk
² Institut d’Astrophysique Spatiale, Paris-Sud 11, 91405 Orsay, France
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Laboratoire AIM, CEA/DSM-CNRS-Universite Paris Diderot, DAPNIA/Service d’Astrophysique, Saclay, France
⁵ Laboratoire d’Astrophysique de Marseille, CNRS/INSU Université de Provence, 13388 Marseille, France
⁶ Institute for Space Imaging Science, Department of Physics & Astronomy, University of Lethbridge, Lethbridge AB T1K 3M4, Canada
⁷ Space Science & Technology Division, The Rutherford Appleton Laboratory, Chilton, Didcot OX11 0NL, UK
⁸ Wolfson Brain Imaging Centre, Addenbrookeś Hospital, University of Cambridge, Cambridge CB2 0QQ, UK

Received: 24 February 2014
Accepted: 4 December 2014

Abstract

Context. The north-west photo-dissociation region (PDR) in the reflection nebula NGC 7023 displays a complex structure. Filament-like condensations at the edge of the cloud can be traced via the emission of the main cooling lines, offering a great opportunity to study the link between the morphology and energetics of these regions.

Aims. We study the spatial variation of the far-infrared fine-structure lines of [C ii] (158 μm) and [O i] (63 and 145 μm). These lines trace the local gas conditions across the PDR. We also compare their emission with molecular tracers including rotational and ro-vibrational lines of H₂ and high-rotational lines of CO.

Methods. We used observations from the Herschel/PACS instrument to map the spatial distribution of these fine-structure lines. The observed region covers a square area of about 110″ × 110″ with an angular resolution that varies from 4′′ to 11′′. We compared this emission with ground-based and Spitzer observations of H₂ lines, Herschel/SPIRE observations of CO lines, and Spitzer/IRAC 3.6 μm images that trace the emission of polycyclic aromatic hydrocarbons. We used a PDR code to model the [O i]145 μm line and infer the physical conditions in the region.

Results. The [C ii] (158 μm) and [O i] (63 and 145 μm) lines arise from the warm cloud surface where the PDR is located and the gas is warm, cooling the region. We find that although the relative contribution to the cooling budget over the observed region is dominated by [O i]63 μm (>30%), H₂ contributes significantly in the PDR (~35%), as does [C ii]158 μm outside the PDR (30%). Other species contribute little to the cooling ([O i]145 μm 9%, and CO 4%). Enhanced emission of these far-infrared atomic lines trace the presence of condensations, where high-excitation CO rotational lines and dust emission in the submillimetre are detected as well. The [O i] maps resolve these condensations into two structures and show that the peak of [O i] is slightly displaced from the molecular H₂ emission. The size of these structures is about 8″ (0.015 pc) and in surface cover about 9% of the PDR emission. We have tested whether the density profile and peak densities that were derived in previous studies to model the dust and molecular emission can predict the [O i]145 μm emission. We find that the model with a peak density of 10⁶ cm^-3, and 2 × 10⁴⁻⁵ cm^-3 in the oxygen emitting region predicts an [O i]145 μm line that is only 30% lower than the observed emission. Finally, we did not detect emission from [N ii]122 μm, suggesting that the cavity is mostly filled with non-ionised gas.