Ortho-to-para ratio of interstellar heavy water
Abstract
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. The CHESS (Chemical HErschel Surveys of Star forming regions) Key Program aims at studying the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the HIFI instrument provide a unique opportunity to observe the fundamental 1,1,1 - 0,0,0 transition of the ortho-D2O molecule, inaccessible from the ground, and to determine the ortho-to-para D2O ratio. We have 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 (1 sigma). From the previous ground-based observations of the fundamental 1,1,0 - 1,0,1 transition of para-D2O seen in absorption at 316.80 GHz we estimate a line opacity of 0.26 +/- 0.05 (1 sigma). 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 3 sigma level of uncertainty, to be compared with the thermal equilibrium value of 2:1.
Keywords
Cite
@article{arxiv.1007.4410,
title = {Ortho-to-para ratio of interstellar heavy water},
author = {C. Vastel and C. Ceccarelli and E. Caux and A. Coutens and J. Cernicharo and S. Bottinelli and K. Demyk and A. Faure and L. Wiesenfeld and Y. Scribano and A. Bacmann and P. Hily-Blant and S. Maret and A. Walters and E. A. Bergin and G. A. Blake and A. Castets and N. Crimier and C. Dominik and P. Encrenaz and M. Gérin and P. Hennebelle and C. Kahane and A. Klotz and G. Melnick and L. Pagani and B. Parise and P. Schilke and V. Wakelam and A. Baudry and T. Bell and M. Benedettini and A. Boogert and S. Cabrit and P. Caselli and C. Codella and C. Comito and E. Falgarone and A. Fuente and P. F. Goldsmith and F. Helmich and T. Henning and E. Herbst and T. Jacq and M. Kama and W. Langer and B. Lefloch and D. Lis and S. Lord and A. Lorenzani and D. Neufeld and B. Nisini and S. Pacheco and J. Pearson and T. Phillips and M. Salez and P. Saraceno and K. Schuster and X. Tielens and F. van der Tak and M. H. D. van der Wiel and S. Viti and F. Wyrowski and H. Yorke and P. Cais and J. M. Krieg and M. Olberg and L. Ravera},
journal= {arXiv preprint arXiv:1007.4410},
year = {2015}
}
Comments
5 pages, 5 figures, accepted the A&A HIFI Special Issue as a letter