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

M. de Simone , F. Fontani , C. Codella , C. Ceccarelli (1) , B. Lefloch (1) , R. Bachiller , A. López-Sepulcre (1) , E. Caux (2) , C. Vastel (2) , J. Soldateschi

1 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble
2 IRAP - Institut de recherche en astrophysique et planétologie

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.

Mots clés: stars: formation; ISM: clouds; ISM: molecules; radio lines: ISM; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies

Domaines: Planète et Univers [physics]

M. de Simone, F. Fontani, C. Codella, C. Ceccarelli, B. Lefloch, et al.. Deuterium and ¹⁵N fractionation in N₂H⁺ during the formation of a Sun-like star. Monthly Notices of the Royal Astronomical Society, 2018, 476, pp.1982-1990. ⟨10.1093/mnras/sty353⟩. ⟨insu-03678213⟩