Mapping PAH sizes in NGC 7023 with SOFIA

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) photodissociation 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 H 2 (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 (H 2 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 C 60 by photochemical processing.

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

“…Combining Spitzer and SOFIA observations, Croiset et al (2016) derived 3.3/11.3 ratios in NGC 7023. This ratio is expected to serve as a proxy for PAH size.…”

“…The 3.3/11.3 ratio varies slightly in our spectra due to the changes in the ionization state of the molecules. Croiset et al (2016) report values of 0.5 to 0.3 when going from the surface of the PDR to 52 arcsec away from the star. According to our estimates, PAHs of intermediate size can reproduce such values.…”

“…While the PAH abundance decreases rapidly into the cavity, the fullerene abundance actually increases (Berné & Tielens 2012). The average size of the PAHs at the PDR front is smaller than the sizes of PAHs deeper in the PDR, as well as the PAHs in the cavity (Croiset et al 2016). All of these effects have been attributed to photoprocessing of PAHs, driving dehydrogenation, graphene formation and isomerization to cages and fullerenes (Berné & Tielens 2012;Berné et al 2015).…”

“…Because of their similarity in molecular structure, their physical and spectroscopic properties show a global trend (Ricca et al 2012). The sizes of these three species span the range of astrophysical relevant PAHs Croiset et al 2016). Also, given their compact structure, the coronene family is expected to be among the most stable PAHs in the ISM (Ricca et al 2012).…”

“…Both ratios remain relatively constant, which shows that they are not sensitive to the ionization state of the molecules under these conditions. Instead, they do vary with PAH size (Bauschlicher et al 2008;Ricca et al 2012;Croiset et al 2016). …”