Other
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280e..70g&link_type=abstract
The Molecular Universe, Proceedings of the 280th Symposium of the International Astronomical Union held in Toledo, Spain, May 30
Other
Scientific paper
Due to its much improved angular resolution and sensitivity compared to previous far-IR space missions, Herschel observations are providing a new perspective of the chemistry and thermal balance in photodissociation regions (PDRs; the Orion Bar, the Horsehead, etc.). As part of a far-IR spectral scan with Herschel/PACS, we present the first detection of OH, a key molecule in the oxygen-network, towards the Orion Bar. Five OH rotational Λ-doublets involving energy levels out to Eu/k 511 K have been detected (at 65, 79, 84, 119 and 163um). The observed emission of rotationally excited OH lines is extended and correlates well with the high-J CO and CH+ J=3-2 line emission also detected by PACS, thus pointing towards a common origin, but apparently does not correlate with the observed water vapour emission. Nonlocal, non-LTE radiative transfer models including excitation by the ambient FIR radiation field suggest that OH arises in a small filling factor component of warm (Tk~160-220 K) and dense (nH~106-7 cm-3) gas with source-averaged OH column densities of ~1015 cm-2. High density and temperature photochemical models predict such enhanced OH columns at low depths (AV~1) and small spatial scales, where OH formation is driven by gas-phase endothermic reactions of atomic oxygen with molecular hydrogen. We interpret the extended OH emission as coming from unresolved structures exposed to FUV radiation near the Bar edge (photoevaporating clumps or filaments) and not from the lower density ``interclump'' medium. FUV photodissociation leads to OH/H2O abundance ratios (>1) much higher than those expected in equally warm regions without enhanced FUV radiation fields.
Bell Thomas A.
Bergin Edwin A.
Berné Olivier
Cernicharo Jose
Contursi Alessandra
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