Ortho-to-para ratio of interstellar heavy water [Letter]

Vastel, C., Ceccarelli, C., Caux, E., Coutens, A., Cernicharo, J., Bottinelli, S., Demyk, K., Faure, A., Wiesenfeld, L., Scribano, Y., Bacmann, A., Hily-Blant, P., Maret, S., Walters, A., Bergin, E. A., Blake, G. A., Castets, A., Crimier, N., Dominik, C., Encrenaz, P., Gerin, M., Hennebelle, P., Kahane, C., Klotz, A., Melnick, G., Pagani, L., Parise, Berengere, Schilke, P., Wakelam, V., Baudry, A., Bell, T., Benedettini, M., Boogert, A., Cabrit, S., Caselli, P., Codella, C., Comito, C., Falgarone, E., Fuente, A., Goldsmith, P. F., Helmich, F., Henning, T., Herbst, E., Jacq, T., Kama, M., Langer, W., Lefloch, B., Lis, D., Lord, S., Lorenzani, A., Neufeld, D., Nisini, B., Pacheco, S., Pearson, J., Phillips, T., Salez, M., Saraceno, P., Schuster, K., Tielens, X., van der Tak, F., van der Wiel, M. H. D., Viti, S., Wyrowski, F., Yorke, H., Cais, P., Krieg, J. M., Olberg, M. and Ravera, L. 2010. Ortho-to-para ratio of interstellar heavy water [Letter]. Astronomy and Astrophysics 521 , L31. 10.1051/0004-6361/201015101

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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 1- 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 1 – 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.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Physics and Astronomy
Subjects:	Q Science > QB Astronomy
Uncontrolled Keywords:	astrochemistry ; ISM: molecules ; submillimeter: ISM / ISM: abundances ; molecular processes ; line: identification
Additional Information:	Pdf uploaded in accordance with publisher's policy at http://www.sherpa.ac.uk/romeo/issn/0004-6361/ (accessed 17/04/2014)
Publisher:	EDP Sciences
ISSN:	0004-6361
Date of First Compliant Deposit:	30 March 2016
Last Modified:	27 May 2023 00:54
URI:	https://orca.cardiff.ac.uk/id/eprint/52801

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