Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

D. C. Lis , T. G. Phillips , P. F. Goldsmith , D. A. Neufeld , E. Herbst , C. Comito , P. Schilke , H. S. P. M\"uller

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E. A. Bergin M. Gerin T. A. Bell M. Emprechtinger J. H. Black G. A. Blake F. Boulanger S. Cabrit E. Caux C. Ceccarelli J. Cernicharo A. Coutens N. R. Crockett F. Daniel E. Dartois M. De Luca M.-L. Dubernet P. Encrenaz E. Falgarone T. R. Geballe B. Godard T. F. Giesen J. R. Goicoechea C. Gry H. Gupta P. Hennebelle P. Hily-Blant R. Ko{\l}os J. Kre{\l}owski C. Joblin D. Johnstone M. Ka\'zmierczak S. D. Lord S. Maret P. G. Martin J. Mart\'in-Pintado G. J. Melnick K. M. Menten R. Monje B. Mookerjea P. Morris J. A. Murphy V. Ossenkopf L. Pagani J. C. Pearson M. P\'erault C. Persson R. Plume S.-L. Qin M. Salez S. Schlemmer M. Schmidt P. Sonnentrucker J. Stutzki D. Teyssier N. Trappe F. F. S. van der Tak C. Vastel S. Wang H. W. Yorke S. Yu J. Zmuidzinas A. Boogert N. Erickson A. Karpov J. Kooi F. W. Maiwald R. Schieder P. Zaal

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keywords ratioorthoparatowardssagittariuswatercorrespondingherschel

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We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H$_2^{16}$O and H$_2^{18}$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 $2.35 \pm 0.35$, corresponding to a spin temperature of $\sim$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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Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

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