2018MNRAS.476.1982D

Although chemical models predict that the deuterium fractionation in N₂H⁺ is a good evolutionary tracer in the star formation process, the fractionation of nitrogen is still a poorly understood process. Recent models have questioned the similar evolutionary trend expected for the two fractionation mechanisms in N₂H⁺, based on a classical scenario in which ion-neutral reactions occurring in cold gas should have caused an enhancement of the abundance of N₂D^+, 15^NNH+^, and N¹⁵NH⁺. In the framework of the ASAI IRAM-30m large program, we have investigated the fractionation of deuterium and ¹⁵N in N₂H⁺ in the best known representatives of the different evolutionary stages of the Sun-like star formation process. The goal is to ultimately confirm (or deny) the classical 'ion-neutral reactions' scenario that predicts a similar trend for D and ¹⁵N fractionation. We do not find any evolutionary trend of the ¹⁴N/¹⁵N ratio from both the ¹⁵NNH⁺ and N¹⁵NH⁺ isotopologues. Therefore, our findings confirm that, during the formation of a Sun-like star, the core evolution is irrelevant in the fractionation of ¹⁵N. The independence of the ¹⁴N/¹⁵N ratio with time, found also in high-mass star-forming cores, indicates that the enrichment in ¹⁵N revealed in comets and protoplanetary discs is unlikely to happen at core scales. Nevertheless, we have firmly confirmed the evolutionary trend expected for the H/D ratio, with the N₂H⁺/N₂D⁺ ratio decreasing before the pre-stellar core phase, and increasing monotonically during the protostellar phase. We have also confirmed clearly that the two fractionation mechanisms are not related.