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Sipilae, O., Harju, J., Caselli, P. and Schlemmer, S. (2015). Spin-state chemistry of deuterated ammonia. Astron. Astrophys., 581. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

Aims. We aim to develop a chemical model that contains a consistent description of spin-state chemistry in reactions involving chemical species with multiple deuterons. We apply the model to the specific case of deuterated ammonia, to derive values for the various spin-state ratios. Methods. We applied symmetry rules in the context of the complete scrambling assumption to calculate branching ratio tables for reactions between chemical species that include multiple protons and/or deuterons. New reaction sets for both gas-phase and grain-surface chemistry were generated using an automated routine that forms all possible spin-state variants of any given reaction with up to six H/D atoms, using the predetermined branching ratios. Both a single-point and a modified Bonnor-Ebert model were considered to study the density and temperature dependence of ammonia and its isotopologs, and the associated spin-state ratios. Results. We find that the spin-state ratios of the ammonia isotopologs are, at late times, very different from their statistical values. The ratios are rather insensitive to variations in the density, but present strong temperature dependence. We derive high peak values(similar to 0.1) for the deuterium fraction in ammonia, in agreement with previous (gas-phase) models. The deuterium fractionation is strongest at high density, corresponding to a high degree of depletion, and also presents temperature dependence. We find that in the temperature range 5 K to 20 K, the deuterium fractionation peaks at similar to 15K, while most of the ortho/para (and meta/para for ND3) ratios present a minimum at 10 K (ortho/para NH2D has instead a maximum at this temperature). Conclusions. Owing to the density and temperature dependence found in the abundances and spin-state ratios of ammonia and its isotopologs, it is evident that observations of ammonia and its deuterated forms can provide important constraints on the physical structure of molecular clouds.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Sipilae, O. UNSPECIFIED UNSPECIFIED UNSPECIFIED Harju, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Caselli, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Schlemmer, S. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-393795
DOI:	10.1051/0004-6361/201526468
Journal or Publication Title:	Astron. Astrophys.
Volume:	581
Date:	2015
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 TO-PARA RATIO; SELECTION-RULES; NITROGEN HYDRIDES; CLOUD CORES; INTERSTELLAR; FRACTIONATION; ABUNDANCES; ORTHO-H-2; DEUTERIUM; STABILITY Multiple languages Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/39379