Nitrogen fractionation in ammonia and its insights into nitrogen chemistry

Context. Observations of the nitrogen isotopic ratio 14N/15N in the interstellar medium are becoming more frequent thanks to increased telescope capabilities. However, interpreting these data is still puzzling. In particular, measurements of 14N/15N in diazenylium have revealed high levels of anti-fractionation in cold cores, which is challenging to explain. Aims. By using astrophysical simulations coupled with a gas-grain chemical code, it has been suggested that the 15N-depletion in prestellar cores could be inherited from the initial stages, when 14N15N is selectively photodissociated and 15N atoms deplete onto the dust grain, forming ammonia ices. Our aim is to test this hypothesis. Methods. We targeted three sources (the prestellar core L1544, the protostellar envelope IRAS4A, and the shocked region L1157-B1) with distinct degrees of desorption or sputtering of the ammonia ices. We observed the ammonia isotopologues with the Green Bank Telescope, and we inferred the ammonia 14N/15N via spectral fitting of the observed inversion transitions. Results.15NH3 (1,1) is detected in L1544 and IRAS4A, whilst only upper limits are deduced in L1157-B1. The NH3 isotopic ratio is significantly lower towards the protostar (14N/15N = 210 ± 50) than at the centre of L1544 (14N/15N = 390 ± 40), where it is consistent with the elemental value. We also present the first spatially resolved map of NH3 nitrogen isotopic ratio towards L1544. Conclusions. Our results are in agreement with the hypothesis that ammonia ices are enriched in 15N, leading to a decrease in the 14N/15N ratio when the ices are sublimated back into the gas phase for instance due to the temperature rise in protostellar envelopes. The ammonia 14N/15N value at the centre of L1544 is a factor of 2 lower than that of N2H+, which can be explained if a significant fraction of nitrogen remains in atomic form and if the ammonia formed on the dust grains is released in the gas phase via non-thermal desorption.