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 Eu/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 (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 (~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.
Highlights
In quiescent regions irradiated by cosmic– or X–rays, the oxygtoenatcohmeimc iosxtryygiesni,nfitoiramteidngbyOt+heanchdaOrgHe+t.raInnswfearrfmroemnvHir+oannmdeHnt+3s it can start with the reaction of atomic oxygen with H2(ν=0)to form OH
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)
High density and temperature photochemical models predict such enhanced OH columns at low depths (AV 1) and small spatial scales (∼1015 cm), where OH formation is driven by gas-phase endothermic reactions of atomic oxygen with molecular hydrogen
Summary
In quiescent regions irradiated by cosmic– or X–rays, the oxygtoenatcohmeimc iosxtryygiesni,nfitoiramteidngbyOt+heanchdaOrgHe+t.raInnswfearrfmroemnvHir+oannmdeHnt+3s it can start with the reaction of atomic oxygen with H2(ν=0). Most of the low-J molecular line emission arises in this extended “interclump” medium (Tielens & Hollenbach 1985; Simon et al 1997; van der Wiel et al 2009; Habart et al 2010) Another component of higher density clumps was introduced to Goicoechea et al.: OH emission towards the Orion Bar PDR. Fit the observed H2, high–J CO, CO+ and other high density and temperature tracers (Burton et al 1990; Parmar et al 1991; Storzer et al 1995; van der Werf et al 1996; Young Owl et al 2000) Owing their small filling factor this clumpy structure would allow FUV radiation to permeate the region. We present the first detection of OH towards this prototypical PDR
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