Astronomy and Astrophysics – Astronomy
Scientific paper
May 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21642412s&link_type=abstract
American Astronomical Society, AAS Meeting #216, #424.12
Astronomy and Astrophysics
Astronomy
Scientific paper
We use the IRS on Spitzer for spectral mapping of a small sample of photo-dissociation regions (PDRs) in star-forming clouds or in bubbles around exciting stars. The spectra exhibit emission lines from atomic ionized species and H2 molecules, as well as emission bands from PAH molecules. The spatial distribution of the emission depends on various parameters such as the PDR's ionization structure, radiation field strength, and gas temperature and density. The best spectral coverage of H2 line emission is obtained toward NGC 2023, the reflection nebula around HD 37903, where five pure-rotation lines from J' = 25 and 7 are detected. Specifically, we map the southern ridge in NGC 2023, which is located some 80" (or 0.2 pc) from its photo-exciting source. This target had previously provided the first indication of non-standard (i.e., not equal to 3) ortho-H2 to para-H2 ratio (OPR) in the ISM, based on single-position spectroscopy of vibration-rotation (v'>0) emission. We show that non-standard OPR values are exhibited also by the pure-rotation (v'=0) emission lines, and that in addition, these values vary with position over the field of view. Such spatial variations should reflect local changes in physical conditions and chemistry of the gas, which affect H2 formation and destruction processes in the PDR. These processes can be constrained by comparing with model results.
Galliano Frederic
Sheffer Yaron
Wolfire Mark G.
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