Mapping PAH sizes in NGC 7023 with SOFIA

¹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
e-mail: croiset@strw.leidenuniv.nl
² Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
³ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex, France

Received: 9 November 2015
Accepted: 29 February 2016

Abstract

Context. NGC 7023 is a well-studied reflection nebula, which shows strong emission from polycyclic aromatic hydrocarbon (PAH) molecules in the form of aromatic infrared bands (AIBs). The spectral variations of the AIBs in this region are connected to the chemical evolution of the PAH molecules which, in turn, depends on the local physical conditions.

Aims. Our goal is to map PAH sizes in NGC 7023 with respect to the location of the star. We focus on the north west (NW) photo-dissociation region (PDR) and the south PDR of NGC 7023 to understand the photochemical evolution of PAHs, using size as a proxy.

Methods. We use the unique capabilities of the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe a 3.2′ × 3.4′ region of NGC 7023 at wavelengths that we observe with high spatial resolution (2.7′′) at 3.3 and 11.2 μm. We compare the SOFIA images with existing images of the PAH emission at 8.0 μm (Spitzer), emission from evaporating very small grains (eVSG) extracted from Spitzer-IRS spectral cubes, the extended red emission (Hubble Space Telescope and Canadian French Hawaiian Telescope), and H₂ (2.12 μm). We create maps of the 11.2/3.3 μm ratio to probe the morphology of the PAH size distribution and the 8.0/11.2 μm ratio to probe the PAH ionization. We make use of an emission model and of vibrational spectra from the NASA Ames PAH database to translate the 11.2/3.3 μm ratio to PAH sizes.

Results. The 11.2/3.3 μm ratio map shows the smallest PAH concentrate on the PDR surface (H₂ and extended red emission) in the NW and south PDR. We estimated that PAHs in the NW PDR bear, on average, a number of carbon atoms (N_c) of ~70 in the PDR cavity and ~50 at the PDR surface. In the entire nebula, the results reveal a factor of 2 variation in the size of the PAH. We relate these size variations to several models for the evolution of the PAH families when they traverse from the molecular cloud to the PDR.

Conclusions. The high-resolution PAH size map enables us to follow the photochemical evolution of PAHs in NGC 7023. Small PAHs result from the photo-evaporation of VSGs as they reach the PDR surface. Inside the PDR cavity, the PAH abundance drops as the smallest PAH are broken down. The average PAH size increases in the cavity where only the largest species survive or are converted into C₆₀ by photochemical processing.