Revised models of interstellar nitrogen isotopic fractionation

Abstract

Nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of 15 N enrichments found in comets and meteorites. Chemical model calculations indicate that atom-molecular ion and ion-molecule reactions could account for most of the fractionation patterns observed. However, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. Related model calculations of dense cloud chemistry show that the revised 15 N enrichments fail to match observed values.We have investigated the effects of these reaction rate modifications on the chemical model of Wirström et al. (2012) for which there are significant physical and chemical differences with respect to other models. We have included 15 N fractionation of CN in neutral-neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry.We find that the revised fractionation rates have the effect of suppressing 15 N enrichment in ammonia at all times, while the depletion is even more pronounced, reaching 14 N/ 15 N ratios of > 2000. Taking the updated nitrogen chemistry into account, no significant enrichment occurs in HCN or HNC, contrary to observational evidence in dark clouds and comets, although the 14 N/ 15 N ratio can still be below 100 in CN itself. However, such low CN abundances are predicted that the updated model falls short of explaining the bulk 15 N enhancements observed in primitive materials. It is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed.

Nitrogen fractionation in external galaxies

“…We find values of the HCN abundance w.r.t. H 2 , and 14 N/ 15 N in HCN, very similar to those computed by Wirström & Charnley (2018): in our model, the HCN abundance rises up to ∼ 10 −9 at ∼ 10 4 yrs, and then it drops by several orders of magnitude afterwards. We also find that 14 N/ 15 N for HCN/HC 15 N is about 400, as found by Wirstöm & Charnley (2018), and thus conclude that our updated model can reproduce the most recent dark cloud models including 15 N fractionation.…”

“…We also find that 14 N/ 15 N for HCN/HC 15 N is about 400, as found by Wirstöm & Charnley (2018), and thus conclude that our updated model can reproduce the most recent dark cloud models including 15 N fractionation. We note that we have not considered the doubly substituted N 2 , as done by Wirström & Charnley (2018), because we have assumed that this species is negligible for the chemistry of extragalactic environments at large scales. Our assumption is likely correct because we are able to reproduce the results of Wirström & Charnley (2018), which indicates that the reactions involving the doubly substituted N 2 are indeed negligible.…”

“…Following the approach of Wirström & Charnley (2018), we have replicated all reactions involving 14 N species, including those in which more than one product includes nitrogen. This could lead, at high densities and long times, to an artificial increase of the values of the isotopic ratios.…”

No nitrogen fractionation on 600 au scale in the Sun progenitor analogue OMC–2 FIR4

“…However, several theoretical studies have excluded low temperature isotopic exchange reactions as the main way to enhance 15 N in molecular species (e.g. Wirström et al 2012, Roueff et al 2015, Wirström & Charnley 2018, Loison et al 2019. At odd with theory, some observations of cold prestellar and protostellar objects seem to show variations of one order of magnitude (∼ 100 − 1000) in the N isotopic ratio, which even depends on the used molecule (Womack et al 1992, Caselli & Ceccarelli 2012, Bizzocchi et al 2013, Hily-Blant et al 2013, Daniel et al 2013, Fontani et al 2015a, Guzman et al 2017, Zeng et al 2017, Redaelli et al 2018, Colzi et al 2018a, De Simone et al 2018.…”

Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation

“…The emission is centered on the systemic velocity of 2.83 km s −1 and is integrated over 5 km s −1 for HCN and 2 km s −1 for the less abundant isotopologs. tion both theoretically (Roueff et al 2015;Wirström & Charnley 2018) and observationally by measuring HCN/HC 15 N in the L1498 pre-stellar core (Magalhães et al 2018). The obtained value, 338±28, agrees very well with the present-day elemental ratio of ∼330 proposed in Paper I and thus supports the prediction that chemical fractionation is not an efficient process even in cold pre-stellar cores.…”