Depletion and fractionation of nitrogen in collapsing cores

Abstract

Measurements of the nitrogen isotopic ratio in Solar System comets show a constant value, ≈140, which is three times lower than the protosolar ratio, a highly significant difference that remains unexplained. Observations of static starless cores at early stages of collapse confirm the theoretical expectation that nitrogen fractionation in interstellar conditions is marginal for most species. Yet, observed isotopic ratios in N2H+ are at variance with model predictions. These gaps in our understanding of how the isotopic reservoirs of nitrogen evolve, from interstellar clouds to comets, and, more generally, to protosolar nebulae, may have their origin in missing processes or misconceptions in the chemistry of interstellar nitrogen. So far, theoretical studies of nitrogen fractionation in starless cores have addressed the quasi-static phase of their evolution such that the effect of dynamical collapse on the isotopic ratio is not known. In this paper, we investigate the fractionation of 14N and 15N during the gravitational collapse of a pre-stellar core through gas-phase and grain adsorption and desorption reactions. The initial chemical conditions, which are obtained in steady state after typically a few Myr, show low degrees of fractionation in the gas phase, in agreement with earlier studies. However, during collapse, the differential rate of adsorption of 14N- and 15N-containing species onto grains results in enhanced 15N:14N ratios, in better agreement with the observations. Furthermore, we find differences in the behavior, with increasing density, of the isotopic ratio in different species. We find that the collapse must take place on approximately one free-fall timescale, based on the CO abundance profile in L183. Various chemical effects that bring models into better agreement with observations are considered. Thus, the observed values of 14N2H+:N15NH+ and 14N2H+:15NNH+ could be explained by different temperature dependences of the rates of dissociative recombination of these species. We also study the impact of the isotopic sensitivity of the charge-exchange reaction of N2 with He+ on the fractionation of ammonia and its singly deuterated analog and find significant depletion in the 15N variants. However, these chemical processes require further experimental and theoretical investigations, especially at low temperature. These new findings, such as the depletion-driven fractionation, may also be relevant to the dense, UV-shielded regions of protoplanetary disks.

Abundance of HCN and its C and N isotopologues in L1498

“…For the double-isotopologue method, the values of 12 C/ 13 C were measured for all carbon atoms in either HC 3 N or HC 5 N (Taniguchi et al 2016), making the determination more robust than would be otherwise expected. However, as pointed out recently in the case of HC 3 N in the L1544 prestellar core, different analyses of the same data can lead to ratios that may differ by almost a factor of 2 (Hily-Blant et al 2018). Isotopic ratios obtained in HCN and HNC toward prestellar and protostellar cores are summarized in Fig.…”

“…In some instances, however, hyperfine splitting of the rotational lines may provide optically thin transitions, as in CN, which can be used to directly infer the total column density (Adande & Ziurys 2012), although departures of the hyperfine intensity ratios from single excitation temperature predictions can perturb the analysis. For nitriles such as HC 3 N and HC 5 N, abundances are intrisically low and the 14 N/ 15 N ratio can be measured directly Saito 2017 andHily-Blant et al 2018). Interestingly, the HC 5 N/HC 15 5 N abundance ratio was measured both directly and indirectly toward the cyanopolyyne peak of TMC-1, leading to 14 N/ 15 N = 344 ± 53 and 14 N/ 15 N = 323 ± 80, respectively, in harmony with the elemental ratio of 330.…”

The initial magnetic criticality of prestellar cores

“…Brünken et al 2014;Lin et al 2020) return values 1 Myr, much greater than the typical free-fall times, and thus also suggestive of a subcritical mode of star formation. Other studies find ages closer to a few 10 5 yr (Pagani et al 2007(Pagani et al , 2009(Pagani et al , 2013Hily-Blant et al 2020;Caselli et al 2022), but these cannot be considered conclusive evidence in either direction: our D4L3-SUB model, for example, has a similar lifetime (0.4 versus 0.3 Myr; Table 1) to the corresponding supercritical model core, and a low inferred age could simply indicate a subcritical core seen at an early evolutionary stage. The ages obtained from molecular line observations are also highly sensitive to the details of the model employed, such as whether the density profile is taken to be static or allowed to evolve (Sipilä & Caselli 2018).…”

ALMA-IRDC II. First high-angular resolution measurements of the 14N/15N ratio in a large sample of infrared-dark cloud cores

“…Overall, the fact that we find either no or little correlation between 14 N/ 15 N and core properties confirm the chemical models of Roueff et al (2015) and Loison et al (2019), in which enrichment in 15 N is always very low or even negligible for all species formed in the gas phase like N 2 H + . In Loison et al (2019), a significant 15 N enrichment is possible if the gas temperature is below ∼ 10 K, due to a possibly different exothermicity of the formation reactions of N 2 H + and N 15 NH + , not yet experimentally measured (see also Hily-Blant et al 2020). However, the temperature of our cores at the scales studied by us is always above ∼ 10 K, in agreement with the lack of 15 N enrichment, although the temperature measurement is taken over an angular scale (i.e.…”

“…In fact, in low-mass pre-stellar cores the 14 N/ 15 N ratio measured from N 2 H + varies from ∼ 330 (Daniel et al 2013(Daniel et al , 2016 to ∼ 1000 (Bizzocchi et al 2013, Redaelli et al 2018, De Simone et al 2018, namely up to more than twice the PSN value, indicating a depletion rather than an enrichment of 15 N. This depletion has been proposed to be due to the transition from atomic to molecular nitrogen in the earlier evolutionary stage of the cores by , although their models are not able to reach the antifractionation levels measured in pre-stellar cores. Some works (Loison et al 2019, Hily-Blant et al 2020 propose that only a different dissociative recombination rate for N 2 H + and N 15 NH + could solve the problem and reconcile observations with chemical models, although it is hard to understand what could cause the different rate for the two species. On the other hand, in CN and HCN the 14 N/ 15 N ratio seems more consistent with the PSN value (Hily-Blant et al 2013a, 2013b.…”