The PAH emission characteristics of the reflection nebula NGC2023

Abstract

We present 5-20 micron spectral maps of the reflection nebula NGC2023 obtained with the Infrared Spectrograph SL and SH modes on board the Spitzer Space Telescope which reveal emission from polycyclic aromatic hydrocarbons (PAHs), C60, and H2 superposed on a dust continuum. We show that several PAH emission bands correlate with each other and exhibit distinct spatial distributions revealing a spatial sequence with distance from the illuminating star. We explore the distinct morphology of the 6.2, 7.7 and 8.6 micron PAH bands and find that at least two spatially distinct components contribute to the 7--9 micron PAH emission in NGC2023. We report that the PAH features behave independently of the underlying plateaus. We present spectra of compact oval PAHs ranging in size from C_66 to C_210, determined computationally using density functional theory, and investigate trends in the band positions and relative intensities as a function of PAH size, charge and geometry. Based on the NASA Ames PAH database, we discuss the 7--9 micron components in terms of band assignments and relative intensities. We assign the plateau emission to very small grains with possible contributions from PAH clusters and identify components in the 7--9 micron emission that likely originates in these structures. Based on the assignments and the observed spatial sequence, we discuss the photochemical evolution of the interstellar PAH family as they are more and more exposed to the radiation field of the central star in the evaporative flows associated with the PDRs in NGC2023.

42 pages, 25 figures, 2 animations, accepted for ApJ

References in corpus (11)

• Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
• Spectral Mapping Reconstruction of Extended Sources
• On-line database of the spectral properties of polycyclic aromatic hydrocarbons
• Dust processing in photodissociation regions - Mid-IR emission modelling
• Spitzer Mapping of PAHs and H2 in Photodissociation Regions
• H2 formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen
• Carriers of the mid-IR emission bands in PNe reanalysed. Evidence of a link between circumstellar and interstellar aromatic dust
• Diagnostics for specific PAHs in the far-IR: searching neutral naphthalene and anthracene in the Red Rectangle
• Polycyclic Aromatic Hydrocarbons with armchair edges and the 12.7 μm band
• The 11.2 $\mu$m emission of PAHs in astrophysical objects
• The mid-infrared appearance of the Galactic Mini-Starburst W49A

Cited by in corpus (3)

• The Polycyclic Aromatic Hydrocarbon Mass Fraction on a 10 pc scale in the Magellanic Clouds
• Polycyclic Aromatic Hydrocarbon emission in Spitzer/IRS maps II: A direct link between band profiles and the radiation field strength
• Physical properties of the fluorine and neutron-capture element-rich PN Jonckheere 900