Deuterium and ${}^{15}$N fractionation in N${}_2$H${}^{+}$ during the formation of a Sun-like star

Abstract

Although chemical models predict that the deuterium fractionation in N${}_2$H${}^{+}$ 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 N${}_2$H${}^{+}$, based on a classical scenario in which ion-neutral reactions occurring in cold gas should have caused an enhancement of the abundance of N${}_2$D${}^{+}$, ${}^{15}$NNH${}^{+}$, and N${}^{15}$NH${}^{+}$. In the framework of the ASAI IRAM-30m large program, we have investigated the fractionation of deuterium and ${}^{15}$N in N${}_2$H${}^{+}$ 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 ${}^{15}$N fractionation. We do not find any evolutionary trend of the ${}^{14}$N/${}^{15}$N ratio from both the ${}^{15}$NNH${}^{+}$ and N${}^{15}$NH${}^{+}$ isotopologues. Therefore, our findings confirm that, during the formation of a Sun-like star, the core evolution is irrelevant in the fractionation of ${}^{15}$N. The independence of the ${}^{14}$N/${}^{15}$N ratio with time, found also in high-mass star-forming cores, indicates that the enrichment in ${}^{15}$N revealed in comets and protoplanetary disks is unlikely to happen at core scales. Nevertheless, we have firmly confirmed the evolutionary trend expected for the H/D ratio, with the N${}_2$H${}^{+}$/N${}_2$D${}^{+}$ 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.

9 pages, 2 figures, accepted for publication in MNRAS

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