A non-equilibrium ortho-to-para ratio of water in the Orion PDR

被引：19

作者：

Choi, Y. ^{[1
,2
]}

van der Tak, F. F. S. ^{[1
,2
]}

Bergin, E. A. ^{[3
]}

Plume, R. ^{[4
]}

机构：

[1] Univ Groningen, Kapteyn Astron Inst, NL-9700 AV Groningen, Netherlands

[2] SRON Netherlands Inst Space Res, NL-9700 AV Groningen, Netherlands

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

[4] Univ Calgary, Dept Phys & Astron, Calgary, AB T2N 1N4, Canada

来源：

ASTRONOMY & ASTROPHYSICS | 2014年 / 572卷

关键词：

ISM: molecules; ISM: individual objects: Orion Bar; ISM: individual objects: Orion S; INTERSTELLAR WATER; LINE; BAR; VAPOR; CHEMISTRY; OMC-1; ICE; EXCITATION; EMISSION; STATE;

D O I：

10.1051/0004-6361/201424007

中图分类号：

P1 [天文学];

学科分类号：

0704 ;

摘要：

Context. The ortho-to-para ratio (OPR) of H2O is thought to be sensitive to the temperature of water formation. The OPR of H2O is thus useful for studying the formation mechanism of water. Aims. We investigate the OPR of water in the Orion PDR (photon-dominated region), at the Orion Bar and Orion S positions, using data from Herschel/HIFI. Methods. We detect the ground-state lines of ortho-and para-(H2O)-O-18 in the Orion Bar and Orion S and estimate the column densities using local thermodynamical equilibrium (LTE) and non-LTE methods. Results. Based on our calculations, the OPR in the Orion Bar is 0.1-0.5, which is unexpectedly low given the gas temperature of similar to 85 K, and also lower than the values measured for other interstellar clouds and protoplanetary disks. Toward Orion S, our OPR estimate is below 2. Conclusions. This low OPR at 2 positions in the Orion PDR is inconsistent with gas phase formation and with thermal evaporation from dust grains, but it may be explained by photodesorption.

引用

页数：6

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