Isotopic fractionation of carbon, deuterium, and nitrogen: a full chemical study

Abstract

Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. The 14 N/ 15 N ratio is difficult to measure directly for molecules containing carbon. Aims. Using a time-dependent gas-phase chemical model, we check the underlying hypothesis that the 13 C/ 12 C ratio of nitriles and isonitriles is equal to the elemental value. Methods. We built a chemical network that contains D, 13 C, and 15 N molecular species after a careful check of the possible fractionation reactions at work in the gas phase. Results. Model results obtained for two different physical conditions that correspond to a moderately dense cloud in an early evolutionary stage and a dense, depleted prestellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in 15 N, which agrees with observations. The 14 N/ 15 N ratio in N 2 H + is found to be close to the elemental value, in contrast to previous models that obtain a significant enrichment, because we found that the fractionation reaction between 15 N and N 2 H + has a barrier in the entrance channel. The high values of the N 2 H + / 15 NNH + and N 2 H + /N 15 NH + ratios derived in L1544 cannot be reproduced in our model. Finally, we find that nitriles and isonitriles are in fact significantly depleted in 13 C, thereby challenging previous interpretations of observed C 15 N, HC 15 N, and H 15 NC abundances from 13 C containing isotopologues.

Cometary Isotopic Measurements

“…However, very recently, Roueff et al (2015) published a study which re-assessed several of the ion-molecule fractionation pathways proposed by Terzieva and Herbst (2000), quantified some of the neutralneutral reactions suggested by , and considered new fractionation reactions for 13 C. Quantum-chemical calculations presented by Roueff et al show that, in conflict with Terzieva and Herbst (2000), reactions (R9), (R10a), (R10b) and (R12) in Table 3 each possess activation energy barriers that will lead to negligible rates at around 10 K. Model calculations incorporating these new data indicate that, as expected, the calculated 14 N/ 15 N ratios in HCN and HNC are much less enriched in 15 N than predicted in earlier models (e.g., and is observed (see Table 3). The calculations of Roueff et al also predict that the ( 14 N/ 15 N ratio in N 2 H + should be close to elemental (≈ 400), in contrast to the depleted ratios observed in L1544 (∼ 1000, Bizzocchi et al 2013) and the enrichments predicted by previous theoretical models (e.g., Wirström et al 2012).…”

How Dense Is Your Gas? On the Recoverability of LVG Model Parameters

“…Furthermore, isotopologue selective chemical effects such as selective photodissociation and isotope charge exchange reactions can lead to isotope fractionation. When coupled with real variations in the elemental isotope abundances, such as in accreted primordial gas or cosmic-ray-dominated star formation paradigms, can lead to large variations in the isotopologue abundance ratios from species to species and galaxy to galaxy (Henkel et al 2010(Henkel et al , 2014Papadopoulos 2010;Papadopoulos et al 2011;Ritchey et al 2011;Roueff et al 2015). We therefore explore the effects of assuming an incorrect abundance ratio on the recovered T k and n H 2 , using HCN.…”

Modeling the Molecular Gas in NGC 6240

“…The theory of 12 C/ 13 C isotope fractionation rests heavily, although not exclusively, upon isotope charge exchange (ICE) reactions, such as Watson et al 1976;Langer et al 1978Langer et al , 1984Roueff et al 2015). Since these forward reactions are slightly exothermic, in cold molecular gas these reactions can lead isotope fractionation.…”

“…The most recent, state-of-the-art chemical modeling study of isotope fractionation was conducted by Roueff et al (2015) Loison et al 2014), which couples HCN back into the chemical network post-production, allowing it to be fractionated at any time that the 13 C pool is depleted. In addition, Roueff et al (2015) use an updated ΔE/k B for Equation (2) of 17 K, c.f.…”

“…In addition, Roueff et al (2015) use an updated ΔE/k B for Equation (2) of 17 K, c.f. 9 K in Langer et al (1984;Lohr 1998;Mladenović & Roueff 2014).…”