Deuterium and 15N fractionation in N2H+ during the formation of a Sun-like star

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+, (NNH+)-N-15, and (NNH+)-N-15. In the framework of the ASAI IRAM-30m large program, we have investigated the fractionation of deuterium and N-15 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 N-15 fractionation. We do not find any evolutionary trend of the N-14/N-15 ratio from both the (NNH)-N-15+ and (NNH+)-N-15 isotopologues. Therefore, our findings confirm that, during the formation of a Sun-like star, the core evolution is irrelevant in the fractionation of N-15. The independence of the N-14/N-15 ratio with time, found also in high-mass star-forming cores, indicates that the enrichment in N-15 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.