First sample of N₂H⁺ nitrogen isotopic ratio measurements in low-mass protostars

Abstract

Context. The nitrogen isotopic ratio is considered an important diagnostic tool of the star formation process, and N2H+ is particularly important because it is directly linked to molecular nitrogen N2. However, theoretical models still do not provide an exhaustive explanation for the observed 14N/15N values. Aims. Recent theoretical works suggest that the 14N/15N behaviour is dominated by two competing reactions that destroy N2H+: dissociative recombination and reaction with CO. When CO is depleted from the gas phase, if the N2H+ recombination rate is lower with respect to that for N15NH+, the rarer isotopologue is destroyed more quickly. In prestellar cores, due to a combination of low temperatures and high densities, most CO is frozen in ices onto the dust grains, leading to high levels of depletion. On the contrary, in protostellar cores, where temperature are higher, CO ices evaporate back to the gas phase. This implies that the N2H+ isotopic ratio in protostellar cores should be lower than that in prestellar cores, and consistent with the elemental value of ≈440. We aim to test this hypothesis, producing the first sample of N2H+∕N15NH+ measurements in low-mass protostars. Methods. We observe the N2H+ and N15NH+ lowest rotational transition towards six young stellar objects in the Perseus and Taurus molecular clouds. We model the spectra with a custom python code using a constant Tex approach to fit the observations. We discuss in the Appendix the validity of this hypothesis. The derived column densities are used to compute the nitrogen isotopic ratios. Results. Our analysis yields an average of 14N/15N|pro = 420 ± 15 in the protostellar sample. This is consistent with the protosolar value of 440, and significantly lower than the average value previously obtained in a sample of prestellar objects. Conclusions. Our results are in agreement with the hypothesis that, when CO is depleted from the gas-phase, dissociative recombinations with free electrons destroy N15NH+ faster than N2H+, leading to high isotopic ratios in prestellar cores where carbon monoxide is frozen onto dust grains.

Chemical nitrogen fractionation in dense molecular clouds

“…The N 15 N depletion is partly conserved in the dense cloud with large Av but their model cannot reproduce the various observations. Indeed, the calculated N2H + / 15 NNH + and N2H + /N 15 NH + ratios are around 410, far smaller than the high N2H + depletion levels observed in L1544 and L429 (Redaelli et al 2018). In their model, nitrogen atoms are rapidly removed from the gas phase and transformed into s-NH3 (on grains), which is mostly retained on the surface.…”

“…In the translucent part of the molecular clouds, N2 becomes 15 N depleted while CN, HCN, HNC and NH3 become 15 N enriched. The N 15 N depletion is partly conserved in dense molecular cloud but their model cannot reproduce the high 15 N depletion of N2H + observed in L1544 and L429 (Redaelli et al 2018). Moreover, the 15 N enrichment is not conserved in the gas phase of dense molecular clouds because N atoms are efficiently transformed into s-NH3 on ices, which becomes the main 15 N reservoir.…”

Nitrogen fractionation towards a pre-stellar core traces isotope-selective photodissociation

“…Isotope-selective photodissociation is expected to be effective only at very low extinction (A V = 1-3 mag, Heays et al 2014), and it is not considered in chemical models for the fractionation of nitrogen in dense cores. Its effects are only considered as inherited from the parent cloud (Furuya & Aikawa 2018;, and they still cannot reproduce the "anti-fractionation" of nitrogen measured in N 2 H + (Redaelli et al 2020).…”

“…The 14 N/ 15 N ratio in N 2 H + instead varies from 180 to 1000 (Fontani et al 2015;Bizzocchi et al 2013;Redaelli et al 2018;Colzi et al 2019). State-of-the-art reaction networks fail to reproduce the 14 N/ 15 N variation in N 2 H + (Roueff et al 2015;Wirström & Charnley 2018), although a faster recombination with electrons of the 15 N isotopologues in L1544 with respect to the normal species might explain the depletion of N 2 H + in 15 N (Loison et al 2019;Hily-Blant et al 2020;Redaelli et al 2020). The use of 13 C isotopologues to derive the column densities of CN, HCN, and HNC is suggested to be done with caution by Roueff et al (2015) because of the possible depletion of 13 C with respect to the ISM 12 C/ 13 C ratio, and due to the interdependence of the 13 C and 15 N chemistry (Colzi et al 2020).…”

Molecular Cloud Cores with High Deuterium Fractions: Nobeyama Mapping Survey

“…Moreover, the early-type molecules (e.g., CCS) are abundant in starless cores, whereas the late-type molecules (e.g., NH 3 , N 2 H + ) are abundant in star-forming cores (Suzuki et al 1992;Hirahara et al 1992;Benson et al 1998;Ohashi et al 2014Ohashi et al , 2016. The ortho-to-para ratio in H 2 D + and D 2 H + (Pagani et al 2013;Brünken et al 2014), the CO depletion (Crapsi et al 2005;Pagani et al 2013;Hily-Blant et al 2020;Feng et al 2020), and the 14 N/ 15 N ratio in the molecule (Redaelli et al 2020;Hily-Blant et al 2020) are also used as chemical evolution tracers.…”