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

被引：48

作者：

Goicoechea, J. R. ^{[1
]}

Joblin, C. ^{[2
,3
]}

Contursi, A. ^{[4
]}

Berne, O. ^{[5
]}

Cernicharo, J. ^{[1
]}

Gerin, M. ^{[6
]}

Le Bourlot, J. ^{[7
,8
]}

Bergin, E. A. ^{[9
]}

Bell, T. A. ^{[1
]}

Roellig, M. ^{[10
]}

机构：

[1] Ctr Astrobiol CSIC INTA, Madrid 28850, Spain

[2] Univ Toulouse, UPS OMP, IRAP, Toulouse, France

[3] CNRS, IRAP, F-31028 Toulouse 4, France

[4] Max Planck Inst Extraterr Phys, D-85741 Garching, Germany

[5] Leiden Univ, Leiden Observ, NL-2300 RA Leiden, Netherlands

[6] Ecole Normale Super, Observ Paris, CNRS, LERMA,UMR 8112, Paris, France

[7] Observ Paris, LUTH, F-92190 Meudon, France

[8] Univ Paris 07, F-92190 Meudon, France

[9] Univ Michigan, Dept Astron, Ann Arbor, MI 48109 USA

[10] Univ Cologne, Inst Phys 1, D-50937 Cologne, Germany

来源：

ASTRONOMY & ASTROPHYSICS | 2011年 / 530卷

关键词：

astrochemistry; infrared: ISM; ISM: abundances; ISM: molecules; PHOTODISSOCIATION REGIONS; CHEMICAL STRATIFICATION; HERSCHEL OBSERVATIONS; INTERSTELLAR-OH; H-2; CHEMISTRY; CLUMPS; WATER;

D O I：

10.1051/0004-6361/201116977

中图分类号：

P1 [天文学];

学科分类号：

0704 ;

摘要：

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)Pi; nu = 0) rotational.-doublets involving energy levels out to E-u/k similar to 511 K have been detected (at similar to 65, similar to 79, similar to 84, similar to 119 and similar to 163 mu m). The total intensity of the OH lines is Sigma I(OH) similar or equal to 5x10(-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 similar or equal to 160-220 K) and dense (n(H) similar or equal to 106-7 cm(-3)) gas with source-averaged OH column densities of greater than or similar to 10(15) cm(-2). High density and temperature photochemical models predict such enhanced OH columns at low depths (AV less than or similar to 1) and small spatial scales (similar to 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/H2O abundance ratios (>1) much higher than those expected in equally warm regions without enhanced FUV radiation fields.

引用

页数：6

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