Nitrogen fractionation in ammonia and its insights into nitrogen chemistry
E. Redaelli, L. Bizzocchi, P. Caselli, J. E. Pineda
Abstract
Context. Observations of the nitrogen isotopic ratio 14 N/ 15 N in the interstellar medium are becoming more frequent thanks to increased telescope capabilities. However, interpreting these data is still puzzling. In particular, measurements of 14 N/ 15 N 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 15 N-depletion in prestellar cores could be inherited from the initial stages, when 14 N 15 N is selectively photodissociated and 15 N 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 14 N/ 15 N via spectral fitting of the observed inversion transitions. Results. 15 NH 3 (1,1) is detected in L1544 and IRAS4A, whilst only upper limits are deduced in L1157-B1. The NH 3 isotopic ratio is significantly lower towards the protostar ( 14 N/ 15 N = 210 ± 50) than at the centre of L1544 ( 14 N/ 15 N = 390 ± 40), where it is consistent with the elemental value. We also present the first spatially resolved map of NH 3 nitrogen isotopic ratio towards L1544. Conclusions. Our results are in agreement with the hypothesis that ammonia ices are enriched in 15 N, leading to a decrease in the 14 N/ 15 N ratio when the ices are sublimated back into the gas phase for instance due to the temperature rise in protostellar envelopes. The ammonia 14 N/ 15 N value at the centre of L1544 is a factor of 2 lower than that of N 2 H + , 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.