The PAH emission characteristics of the reflection nebula NGC2023
  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.
