Ortho-to-para ratio of interstellar heavy water
Falgarone, E.; Maret, S.; Henning, T.; Codella, C.; Ceccarelli, C.; Dominik, C.; Pagani, L.; Benedettini, M.; Comito, C.; Schilke, P.; Bergin, E. A.; Blake, G. A.; Caux, E.; Cernicharo, J.; Encrenaz, P.; Goldsmith, P. F.; Herbst, E.; Vastel, C.; Olberg, M.; Melnick, G.; Lord, S.; Klotz, A.; Schuster, K.; Fuente, A.; van der Tak, F.; Hennebelle, P.; Wyrowski, F.; Boogert, A.; Lorenzani, A.; Pearson, J.; Ravera, L.; Bell, T.; Hily-Blant, P.; Kama, M.; Pacheco, S.; van der Wiel, M. H. D.; Nisini, B.; Caselli, P.; Lefloch, B.; Lis, D.; Faure, A.; Kahane, C.; Cabrit, S.; Neufeld, D.; Saraceno, P.; Demyk, K.; Viti, S.; Coutens, A.; Salez, M.; Cais, P.; Wakelam, V.; Baudry, A.; Helmich, F.; Jacq, T.; Parise, B.; Wiesenfeld, L.; Phillips, T.; Yorke, H.; Bacmann, A.; Bottinelli, S.; Castets, A.; Crimier, N.; Walters, A.; Langer, W.; Tielens, X.; Scribano, Y.; Gérin, M.; Krieg, J. M.
France, Spain, United States, Netherlands, Germany, Italy, United Kingdom, Sweden
Abstract
Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular D/H ratios have been found in the environments of low-mass star-forming regions, and in particular the Class 0 protostar IRAS 16293-2422.
Aims: The CHESS (Chemical HErschel Surveys of Star forming regions) key program aims to study the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI instrument provide a unique opportunity to observe the fundamental 11,1-00,0 transition of the ortho-D2O molecule, which is inaccessible from the ground, and determine the ortho-to-para D2O ratio.
Methods: We detected the fundamental transition of the ortho-D2O molecule at 607.35 GHz towards IRAS 16293-2422. The line is seen in absorption with a line opacity of 0.62 ± 0.11 (1σ). From the previous ground-based observations of the fundamental 11,0-10,1 transition of para-D2O seen in absorption at 316.80 GHz, we estimate a line opacity of 0.26 ± 0.05 (1σ).
Results: We show that the observed absorption is caused by the cold gas in the envelope of the protostar. Using these new observations, we estimate for the first time the ortho-to-para D2O ratio to be lower than 2.6 at a 3σ level of uncertainty, which should be compared with the thermal equilibrium value of 2:1.