Universität zu Köln

Harju, J., Daniel, F., Sipilae, O., Caselli, P., Pineda, J. E., Friesen, R. K., Punanova, A., Guesten, R., Wiesenfeld, L., Myers, P. C., Faure, A., Hily-Blant, P., Rist, C., Rosolowsky, E., Schlemmer, S. and Shirley, Y. L. (2017). Deuteration of ammonia in the starless core Ophiuchus/ H-MM1. Astron. Astrophys., 600. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

Context. Ammonia and its deuterated isotopologues probe physical conditions in dense molecular cloud cores. The time-dependence of deuterium fractionation and the relative abundances of different nuclear spin modifications are supposed to provide a means of determining the evolutionary stages of these objects. Aims. We aim to test the current understanding of spin-state chemistry of deuterated species by determining the abundances and spin ratios of NH2D, NHD2 and ND3 in a quiescent, dense cloud. Methods. Spectral lines of NH3, NH2D, NHD2, ND3 and N2D+ were observed towards a dense, starless core in Ophiuchus with the APEX, GBT and IRAM 30-m telescopes. The observations were interpreted using a gas-grain chemistry model combined with radiative transfer calculations. The chemistry model distinguishes between the different nuclear spin states of light hydrogen molecules, ammonia and their deuterated forms. Different desorption schemes can be considered. Results. High deuterium fractionation ratios with NH2D = NH3 similar to 0 : 4, NHD2 = NH2D similar to 0 : 2 and ND3 = NHD2 similar to 0 : 06 are found in the core. The observed ortho/para ratios of NH2D and NHD2 are close to the corresponding nuclear spin statistical weights. The chemistry model can approximately reproduce the observed abundances, but consistently predicts too low ortho/para-NH2D, and too large ortho/para-NHD2 ratios. The longevity of N2H+ and NH3 in dense gas, which is prerequisite to their strong deuteration, can be attributed to the chemical inertia of N-2 on grain surfaces. Conclusions. The discrepancies between the chemistry model and the observations are likely to be caused by the fact that the model assumes complete scrambling in principal gas-phase deuteration reactions of ammonia, which means that all the nuclei are mixed in reactive collisions. If, instead, these reactions occur through proton hop/hydrogen abstraction processes, statistical spin ratios are to be expected. The present results suggest that while the deuteration of ammonia changes with physical conditions and time, the nuclear spin ratios of ammonia isotopologues do not probe the evolutionary stage of a cloud.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Harju, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Daniel, F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Sipilae, O. UNSPECIFIED UNSPECIFIED UNSPECIFIED Caselli, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Pineda, J. E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Friesen, R. K. UNSPECIFIED UNSPECIFIED UNSPECIFIED Punanova, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Guesten, R. UNSPECIFIED UNSPECIFIED UNSPECIFIED Wiesenfeld, L. UNSPECIFIED UNSPECIFIED UNSPECIFIED Myers, P. C. UNSPECIFIED UNSPECIFIED UNSPECIFIED Faure, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Hily-Blant, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Rist, C. UNSPECIFIED UNSPECIFIED UNSPECIFIED Rosolowsky, E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Schlemmer, S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Shirley, Y. L. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-234663
DOI:	10.1051/0004-6361/201628463
Journal or Publication Title:	Astron. Astrophys.
Volume:	600
Date:	2017
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language RADIO-ASTRONOMICAL SPECTROSCOPY; PRE-PROTOSTELLAR COLLAPSE; BONNOR-EBERT SPHERES; GOULD BELT SURVEY; HYPERFINE-STRUCTURE; PRESTELLAR CORES; MOLECULAR CLOUD; DARK CLOUDS; NITROGEN HYDRIDES; DEUTERIUM FRACTIONATION Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/23466