OH emission from warm and dense gas in the Orion Bar PDR

Abstract

As part of a far-infrared (FIR) spectral scan with Herschel/PACS, we present the first detection of the hydroxyl radical (OH) towards the Orion Bar photodissociation region (PDR). Five OH (X ²Π; ν = 0) rotational Λ-doublets involving energy levels out to E_u/k ~ 511 K have been detected (at ~65, ~79, ~84, ~119 and ~163 μm). The total intensity of the OH lines is ∑ I(OH) ≃ 5 × 10^-4 erg s^-1 cm^-2 sr^-1. 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 (but apparently not with water vapour), pointing towards a common origin. 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 (T_k ≃ 160-220 K) and dense (n_H ≃ 10^6-7 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 (A_V ≲ 1) and small spatial scales (~10¹⁵ cm), 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 far-ultraviolet (FUV) radiation near the Bar edge (photoevaporating clumps or filaments) and not from the lower density "interclump" medium. Photodissociation leads to OH/H₂O abundance ratios (>1) much higher than those expected in equally warm regions without enhanced FUV radiation fields.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendix B is available in electronic form at http://www.aanda.org