Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C
Menten, K. M.; Boulanger, F.; Falgarone, E.; Martin, P. G.; Murphy, J. A.; Maret, S.; Krełowski, J.; Godard, B.; Ceccarelli, C.; Martín-Pintado, J.; Morris, P.; Teyssier, D.; Schieder, R.; Ossenkopf, V.; Erickson, N.; Comito, C.; Schilke, P.; Lis, D. C.; Bergin, E. A.; Phillips, T. G.; Bell, T. A.; Crockett, N. R.; Wang, S.; Blake, G. A.; Caux, E.; Cernicharo, J.; Daniel, F.; Dubernet, M. -L.; Emprechtinger, M.; Encrenaz, P.; Gerin, M.; Giesen, T. F.; Goicoechea, J. R.; Goldsmith, P. F.; Gupta, H.; Herbst, E.; Joblin, C.; Johnstone, D.; Lord, S. D.; Melnick, G. J.; Müller, H. S. P.; Neufeld, D. A.; Pearson, J. C.; Pérault, M.; Plume, R.; Qin, S. -L.; Schlemmer, S.; Stutzki, J.; Trappe, N.; van der Tak, F. F. S.; Vastel, C.; Yorke, H. W.; Yu, S.; Schmidt, M.; Sonnentrucker, P.; Mookerjea, B.; Hennebelle, P.; Black, J. H.; Dartois, E.; Boogert, A.; Kooi, J.; Zaal, P.; Hily-Blant, P.; de Luca, M.; Geballe, T. R.; Gry, C.; Coutens, A.; Kołos, R.; Monje, R.; Persson, C.; Salez, M.; Zmuidzinas, J.; Karpov, A.; Kaźmierczak, M.; Maiwald, F. W.
United States, Germany, France, Sweden, Spain, Poland, Canada, India, Ireland, Netherlands
Abstract
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.