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

Abstract

Although chemical models predict that the deuterium fractionation in N<SUB>2</SUB>H<SUP>+</SUP> 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<SUB>2</SUB>H<SUP>+</SUP>, based on a classical scenario in which ion-neutral reactions occurring in cold gas should have caused an enhancement of the abundance of N<SUB>2</SUB>D<SUP>+, 15</SUP>NNH<SUP>+</SUP>, and N<SUP>15</SUP>NH<SUP>+</SUP>. In the framework of the ASAI IRAM-30m large program, we have investigated the fractionation of deuterium and <SUP>15</SUP>N in N<SUB>2</SUB>H<SUP>+</SUP> 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 <SUP>15</SUP>N fractionation. We do not find any evolutionary trend of the <SUP>14</SUP>N/<SUP>15</SUP>N ratio from both the <SUP>15</SUP>NNH<SUP>+</SUP> and N<SUP>15</SUP>NH<SUP>+</SUP> isotopologues. Therefore, our findings confirm that, during the formation of a Sun-like star, the core evolution is irrelevant in the fractionation of <SUP>15</SUP>N. The independence of the <SUP>14</SUP>N/<SUP>15</SUP>N ratio with time, found also in high-mass star-forming cores, indicates that the enrichment in <SUP>15</SUP>N 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 N<SUB>2</SUB>H<SUP>+</SUP>/N<SUB>2</SUB>D<SUP>+</SUP> 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.