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We determine the ortho/para ratios of NH2D and NHD2 in two dense, starlesscores, where their formation is supposed to be dominated by gas-phasereactions, which, in turn, is predicted to result in deviations from thestatistical spin ratios. The Large APEX sub-Millimeter Array (LAsMA) multibeamreceiver of the Atacama Pathfinder EXperiment (APEX) telescope was used toobserve the prestellar cores H-MM1 and Oph D in Ophiuchus in the ground-statelines of ortho and para NH2D and NHD2. The fractional abundances of thesemolecules were derived employing 3D radiative transfer modelling, usingdifferent assumptions about the abundance profiles as functions of density. Wealso ran gas-grain chemistry models with different scenarios concerning protonor deuteron exchanges and chemical desorption from grains to find out if one ofthese models can reproduce the observed spin ratios. The observationallydeduced ortho/para ratios of NH2D and NHD2 are in both cores within 10% oftheir statistical values 3 and 2, respectively, and taking 3-sigma limits,deviations from these of about 20% are allowed. Of the chemistry models testedhere, the model that assumes proton hop (as opposed to full scrambling) inreactions contributing to ammonia formation, and a constant efficiency ofchemical desorption, comes nearest to the observed abundances and spin ratios.The nuclear spin ratios derived here are in contrast with spin-state chemistrymodels that assume full scrambling in proton donation and hydrogen abstractionreactions leading to deuterated ammonia. The efficiency of chemical desorptioninfluences strongly the predicted abundances of NH3, NH2D, and NHD2, but has alesser effect on their ortho/para ratios. For these the proton exchangescenario in the gas is decisive. We suggest that this is because of rapidre-processing of ammonia and related cations by gas-phase ion-moleculereactions.

Nuclear spin ratios of deuterated ammonia in prestellar cores. LAsMA observations of H-MM1 and Oph D