THE 15–20 μm EMISSION IN THE REFLECTION NEBULA NGC 2023

Abstract

We present 15-20 um spectral maps towards the reflection nebula NGC2023 obtained with the Infrared Spectrograph in short-wavelength, high-resolution mode on board the Spitzer Space Telescope. These spectra reveal emission from PAHs, C60, and H2 superposed on a dust continuum. These emission components exhibit distinct spatial distributions: with increasing distance from the illuminating star, we observe the PAH emission followed by the dust continuum emission and the H2 emission. The C60 emission is located closest to the illuminating star in the south while in the north, it seems to be associated with the H/H2 transition. Emission from PAHs and PAH-related species produce features at 15.8, 16.4, 17.4, and 17.8 um and the 15-18 um plateau. These different PAH features show distinct spatial distributions. The 15.8 um band and 15-18 um plateau correlate with the 11.2 um PAH band and with each other, and are attributed to large, neutral PAHs. Conversely, the 16.4 um feature correlates with the 12.7 um PAH band, suggesting that both arise from species that are favored by the same conditions that favor PAH cations. The PAH contribution to the 17.4 um band is displaced towards the illuminating star with respect to the 11.2 and 12.7 um emission and is assigned to doubly ionized PAHs and/or a subset of cationic PAHs. The spatial distribution of the 17.8 um band suggests it arises from both neutral and cationic PAHs. In contrast to their intensities, the profiles of the PAH bands and the 15-18 um plateau do not vary spatially. Consequently, we conclude that the carrier of the 15-18 um plateau is distinct from that of the PAH bands.