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16 April 2024 |
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Deuteration of ammonia in the starless core Ophiuchus/H-MM1 | | Jorma Harju
; Fabien Daniel
; Olli Sipilä
; Paola Caselli
; Jaime E. Pineda
; Rachel K. Friesen
; Anna Punanova Rolf Güsten
; Laurent Wiesenfeld
; Philip C. Myers
; Alexandre Faure
; Pierre Hily-Blant
; Claire Rist
; Erik Rosolowsky
; Stephan Schlemmer
; Yancy L. Shirley
; | | Date: |
19 Apr 2016 | | Abstract: | Ammonia and its deuterated isotopologues probe physical conditions in dense
molecular cloud cores. With the aim of testing the current understanding of the
spin-state chemistry of these molecules, we observed spectral lines of NH3,
NH2D, NHD2 , ND3 , and N2D+ 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.
High deuterium fractionation ratios with NH2D/NH3=0.5, NHD2/NH2D=0.2, and
ND3/NHD2=0.07 were 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 a
clearly better agreement with the observations is found using the assumption of
statistical fractionation and spin ratios, which implies NH2D/NH3 = 3 NHD2/NH2D
= 9 ND3/NHD2. The longevity of N2H+ and NH3 in dense gas, which is prerequisite
to their strong deuteration, can be attributed to the chemical inertia of N2 on
grain surfaces. 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. | | Source: | arXiv, 1604.5602 | | Services: | Forum | Review | PDF | Favorites | |
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