Physics
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50511t&link_type=abstract
Spitzer Proposal ID #50511
Physics
Scientific paper
The mid-IR spectra of photodissociation regions (PDRs) are dominated by the diagnostic lines of molecular hydrogen as well as the 3.3, 6.2, 7.7, 11.3, and 12.7 um emission features due to polycyclic aromatic hydrocarbon molecules (PAHs). The molecular hydrogen lines are key diagnostics of the temperature and column density of warm molecular gas in the PDR, while the relative strength of the IR emission features probe the molecular edge structure and degree of ionization of the emitting PAHs. Previous Spitzer studies of the warm and dense southern PDR shell in the reflection nebula NGC2023 have demonstrated that the detailed characteristics of the IR emission features vary between the atomic H surface layer of this region and the deeper molecular H2 layers. We attribute these variations to H abstraction by reactive atomic H in this surface layer forming H2. We propose to use the superb sensitivity and high spatial resolution of the IRS on Spitzer to spectroscopically map a lower density region in this reflection nebula with the goal of quantifying the dependence of the IR emission characteristics of the emitting PAHs to the local physical conditions. Specifically, we will address the role of atomic H in the loss of peripheral aromatic H on interstellar PAHs. The selected location has a well-determined gas density and temperatureand incident UV field, which ultimately drive the physical and chemical characteristics of the emitting PAHs and hence provide a unique opportunity to quantitatively study the evolution of PAHs in space as well as the role of PAHs in the formation of molecular hydrogen.
Allamandola Louis
Peeters Els
Tielens Alexander
Wolfire Mark
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