Kinetic temperatures in the Orion Bar

Batrla, W.; Wilson, Thomas L.

doi:10.1051/0004-6361:20030905

Cited by 27 publications

(36 citation statements)

References 10 publications

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“…All in all, the best model is found for a source of high density (n H 10 7 cm −3 ) and warm gas temperatures (T k = 160−220 K). This temperature lies in between the ∼600 K derived in the H 2 emitting regions (Allers et al 2005) and the ∼150 K derived from NH 3 lines (Batrla & Wilson 2003) near the ridge of dense and cooler H 13 CN clumps (T k 50 K; Lis & Schilke 2003). In our simple model, the best fit is obtained for a source of small filling factor (η 10%) with a source-averaged OH column density of 10 15 cm −2 (see Appendix A).…”

Section: Oh Column Density Determinationmentioning

confidence: 57%

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

Goicoechea

Joblin²,

Contursi³

et al. 2011

A&A

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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 2 Π; 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 10 15 cm −2 . High density and temperature photochemical models predict such enhanced OH columns at low depths (A V 1) and small spatial scales (∼10 15 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/H 2 O abundance ratios (>1) much higher than those expected in equally warm regions without enhanced FUV radiation fields.

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Section: Oh Column Density Determinationmentioning

confidence: 57%

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

Goicoechea

Joblin²,

Contursi³

et al. 2011

A&A

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“…Although temperatures are known to range from 50 K in dense clumps up to 150 K in the interclump medium (Batrla & Wilson 2003;Lis & Schilke 2003), we assume a kinetic temperature of 85 K and a volume density of 10 5 cm −3 , as applicable to the extended molecular gas . Since some of the line emission may originate from dense clumps, we also list column densities estimated in the high-density (LTE) limit (Table 1).…”

Section: Resultsmentioning

confidence: 99%

Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations

Wiel

Tak

Ossenkopf

et al. 2009

A&A

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Context. Photon-dominated regions (PDRs) are expected to show a layered structure in molecular abundances and emerging line emission, which is sensitive to the physical structure of the region as well as the UV radiation illuminating it. Aims. We aim to study this layering in the Orion Bar, a prototypical nearby PDR with a favorable edge-on geometry.Methods. We present new maps of 2 × 2 fields at 14 -23 resolution toward the Orion Bar in the SO 8 8 -9 9 , H 2 CO 5 15 -4 14 , 13 CO 3-2, C 2 H 4 9/2 -3 7/2 and 4 7/2 -3 5/2 , C 18 O 2-1 and HCN 3-2 transitions. Results. The data reveal a clear chemical stratification pattern. The C 2 H emission peaks close to the ionization front, followed by H 2 CO and SO, while C 18 O, HCN and 13 CO peak deeper into the cloud. A simple PDR model reproduces the observed stratification, although the SO emission is predicted to peak much deeper into the cloud than observed while the H 2 CO peak is predicted to peak closer to the ionization front than observed. In addition, the predicted SO abundance is higher than observed while the H 2 CO abundance is lower than observed. Conclusions. The discrepancies between the models and observations indicate that more sophisticated models, including production of H 2 CO through grain surface chemistry, are needed to quantitatively match the observations of this region.

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“…The high kinetic temperatures derived from mid-J isotopomeric CO data together with the widespread C i atomic fine structure line emission, generally well correlated with 13 CO (e.g., Lis, Schilke, & Keene 1997; Keene et al 1997), have been a challenge for PDR models, even when clumping of the gas is taken into account. A high kinetic temperature in the Orion Bar region (∼150 K) has been derived recently from NH 3 observations by Batrla & Wilson (2003), who argue that ammonia molecules are released from the surfaces of dense clumps facing the Trapezium cluster. As discussed by GH02, the interiors of dense clumps are likely to be relatively well shielded from the FUV field and thus significantly cooler (∼50 K).…”

Section: Properties Of H 13 Cn Clumpsmentioning

confidence: 89%

Dense Molecular Clumps in the Orion Bar Photon-dominated Region

Lis

Schilke

2003

ApJ

135

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We present high angular resolution observations of the Orion Bar photon-dominated region (PDR) in optically thin H 13 CN and H 13 CO ϩ (1-0) lines, obtained using the IRAM Plateau de Bure interferometer. At least 10 spatially resolved molecular condensations are identified in the H 13 CN image with virial masses in the range 0.5-1.5 M , . The median value of their H 2 volume density, ∼ cm Ϫ3 , is a factor of ∼4 higher than the estimate based 6 6 # 10 on previous PDR modeling of the main isotopomers of HCN and HCO ϩ . Since optically thin H 13 CN emission is likely to trace the densest gas in the clump interiors, as compared to the main isotopomer, the H 13 CN clumps appear to be close to virial equilibrium. The H 13 CN fractional abundance is a factor of ∼8 lower than that in the Orion ridge, well shielded from the far-ultraviolet (FUV) photons (∼ ). The H 13 CN condensations can Ϫ10 1 # 10 be described in the framework of models of photoevaporating clumps exposed to an intense flux of FUV photons. The derived clump parameters are consistent with models of clumps of turbulent origin that evolve, so that their column densities are equal to the critical value determined by the incident FUV field. In this case, the column densities of the H 13 CN clumps seem high enough so that gravitational collapse can be triggered by the FUVdriven shock wave compression. The clumps may thus be collapsing to form low-mass stars. The observed H 13 CN clump parameters are also consistent with pressure-confined clump models. However, in this case the clumps would not be virialized and susceptible to gravitational collapse.

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Kinetic temperatures in the Orion Bar

Cited by 27 publications

References 10 publications

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

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

Chemical stratification in the Orion Bar: JCMT Spectral Legacy Survey observations

Dense Molecular Clumps in the Orion Bar Photon-dominated Region

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