Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C
Lis, D.C. and Phillips , T.G. and Goldsmith, P.F. and Neufeld, D.A. and Herbst, E. and Comito, C. and Schilke, P. and Muller, H.S.P. and Bergin, E.A. and Gerin, M. and Bell, T.A. and Emprechtinger, M. and Black, J.H. and Blake, G.A. and Boulanger, F. and Caux, E. and Ceccarelli, C. and Cernicharo, J. and Coutens, A. and Crockett, N.R. and Daniel, F. and Dartois, E. and De Luca, M. and Dubernet, M.-L. and Encrenaz, P. and Falgarone, E. and Geballe, T.R. and Godard, B. and Giesen, T.F. and Goicoechea, J.R. and Gry, C. and Gupta, H. and Hennebelle, P. and Hily-Blant, P. and Kolos, R. and Krelowski, J. and Joblin, C. and Johnstone, D. and Kazmierczak, M. and Lord, S.D. and Maret, S. and Martin, P.G. and Martin-Pintado, J. and Melnick, G.J. and Menten, K.M. and Monjel, R. and Mookerjea, B. and Morris, P. and Murphy, J.A. and Ossenkopf, V. and Pearson, J.C. and Perault, M. and Persson, C. and Plume, R. and Qin, S.-L. and Salez, M. and Schlemmer, S. and Schmidt, M. and Sonnentrucker, P. and Stutzki, J. and Teyssier, D. and Trappe, N. and van der Tak, F.F.S. and Vastel, C. and Wang, S. and Yorke, H.W. and Yu, S. and Zmuidzinas, J. and Boogert, A. and Erickson, N. and Karpov, A. and Kooi, J. and Maiwald, F.W. and Schieder, R. and Zaal, P. (2010) Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C. Astronomy & Astrophysics, 521 (L26). pp. 1-5. ISSN 0004-6361
We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H16 2 O and H18 2 O in absorption towards Sagittarius B2(M) and W31C. The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3, within the observational uncertainties. However, somewhat unexpectedly, we derive a low ortho/para ratio of 2.35±0.35, corresponding to a spin temperature of ∼27 K, towards Sagittarius B2(M) at velocities of the expanding molecular ring. Water molecules in this region appear to have formed with, or relaxed to, an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust.
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