Els Peeters, Alexander G. G. M. Tielens, Louis J. Allamandola, Mark G. Wolfire
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.
View original:
http://arxiv.org/abs/1112.3386
No comments:
Post a Comment