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. Müller , E. A. Bergin , M. Gerin , T. A. Bell , M. Emprechtinger , J. H. Black , G. A. Blake , F. Boulanger , E. Caux (1) , C. Ceccarelli (2) , J. Cernicharo , A. Coutens (1) , N. R. Crockett , F. Daniel , E. Dartois , M. de Luca , M. -L. Dubernet (3, 4) , P. Encrenaz , E. Falgarone , T. R. Geballe , B. Godard , T. F. Giesen , J. R. Goicoechea , C. Gry , H. Gupta , P. Hennebelle , P. Hily-Blant (2) , R. Kołos , J. Krełowski , C. Joblin (1) , D. Johnstone , M. Kaźmierczak , S. D. Lord , S. Maret (2) , P. G. Martin , J. Martín-Pintado , G. J. Melnick , K. M. Menten , R. Monje , B. Mookerjea , P. Morris , J. A. Murphy , V. Ossenkopf , J. C. Pearson , M. Pérault , 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 (1) , S. Wang , H. W. Yorke , S. Yu , J. Zmuidzinas , A. Boogert , N. Erickson , A. Karpov , J. Kooi , F. W. Maiwald , R. Schieder , P. Zaal

1 CESR - Centre d'étude spatiale des rayonnements
2 LAOG - Laboratoire d'Astrophysique de Grenoble
3 UPMC - Université Pierre et Marie Curie - Paris 6
4 LUTH (UMR_8102) - Laboratoire Univers et Théories

We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H_2¹⁶O and H_2¹⁸O 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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Keywords: astrochemistry; ISM: abundances; ISM: molecules; molecular processes; submillimeter: ISM; astrochemistry

Domains: Sciences of the Universe [physics]; Astrophysics [astro-ph]

D. C. Lis, T. G. Phillips, P. F. Goldsmith, D. A. Neufeld, E. Herbst, et al.. Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C. Astronomy and Astrophysics - A&A, 2010, 521, pp.L26+. ⟨10.1051/0004-6361/201015072⟩. ⟨insu-03625635⟩