Nitrogen Isotopic Fractionation of Interstellar Nitriles

Rodgers, S. D.; Charnley, Steven B.

doi:10.1086/592195

Cited by 64 publications

(71 citation statements)

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“…The nitrogen chemistry of prestellar cores has been the subject of debate in recent years because models of such regions employing recommended rate constants have been unable to reproduce observed abundances of nitrogen containing species (23)(24)(25). One of the most well-known prestellar cores is Barnard 68 (B68).…”

Section: Resultsmentioning

confidence: 99%

Elemental nitrogen partitioning in dense interstellar clouds

Daranlot

Hincelin

Bergeat

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

100

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Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N 2 , with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N 2 is difficult to detect spectroscopically through infrared or millimeterwavelength transitions. Therefore, its abundance is often inferred indirectly through its reaction product N 2 H þ . Two main formation mechanisms, each involving two radical-radical reactions, are the source of N 2 in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N þ CN reaction, down to 56 K. The measured rate constants for this reaction, and those recently determined for two other reactions implicated in N 2 formation, are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N 2 depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N 2 formation are inefficient, we argue that N 2 does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH 3 should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.astrochemistry | chemical kinetics E lemental nitrogen is present in the atmospheres of telluric planets predominantly in the form of N 2 . As a stable molecule with a high bond energy, it is hard to break N out of N 2 , so the amount of nitrogen available for the formation of complex prebiotic molecules in such environments depends on the presence of nitrogen in more labile forms such as NH 3 . Despite the high abundance of N 2 in the solar system, observations of N 2 beyond our own planetary system are scarce, as N 2 possesses no allowed rotational or vibrational transitions. Only one direct measurement (1) of N 2 in the far ultraviolet wavelength range (thereby accessing its electronic transitions) has been reported in a diffuse interstellar cloud, where low column densities allow light to pass and a star along the line of sight was used to probe the species within. In dense molecular clouds where temperatures as low as 10 K prevail, UV starlight is absorbed long before it can penetrate to the cloud core where N 2 is predicted to form efficiently (2, 3). Instead, gas-phase N 2 densities are inferred through observations of N 2 H þ , a product of the N 2 þ H 3 þ reaction (4-6). The major pathways for N 2 formation in dense clouds rely on reactions involving neutral radical species:Mechanism (II) Recent experimental and theoretical studies of reactions 1 (7) and 2 (8-10) indicate that these reactions are slower than previously thought, suggesting that current astrochemical models overestimate molecular nitrogen abundances produced by Mechanism (I) both at steady state and at specific times. The influence of Mechani...

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Section: Resultsmentioning

confidence: 99%

Elemental nitrogen partitioning in dense interstellar clouds

Daranlot

Hincelin

Bergeat

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

100

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“…Improved models (Rodgers & Charnley 2008a;Gerin et al 2009) which include additional ion/neutral and neutral/neutral reaction channels predict that assuming typical dense-core parameters, 15 N enrichment of ammonia is only moderate in the gas phase, while much stronger enrichment is expected for N 2 H + (Gerin et al 2009, Fig. 2).…”

Section: Discussionmentioning

confidence: 99%

Detection of N$^\mathsf{{15}}$NH⁺in L1544

Bizzocchi

Caselli²,

Dore

2010

A&A

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Context. Excess levels of 15 N isotopes which have been detected in primitive solar system materials are explained as a remnant of interstellar chemistry which took place in regions of the protosolar nebula. Aims. Chemical models of nitrogen fractionation in cold clouds predict an enhancement in the gas-phase abundance of 15 N-bearing molecules, thus we have searched for 15 N variants of the N 2 H + ion in L1544, which is one of the best candidate sources for detection owing to its low central core temperature and high CO depletion. Methods. With the IRAM 30 m telescope we have obtained deep integrations of the N 15 NH + (1−0) line at 91.2 GHz. Results. The N 15 NH + (1−0) line has been detected toward the dust emission peak of L1544. The 14 N/ 15 N abundance ratio in N 15 NH + resulted 446 ± 71, very close to the protosolar value of ∼ 450, higher than the terrestrial ratio of ∼270, and significantly lower than the lower limit in L1544 found by Gerin et al. (2009, ApJ, 570, L101) in the same object using ammonia isotopologues.

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“…Rodgers & Charnley (2004, 2008a used these suggested reaction-rate constants to predict nitrogen isotopic fractionation in chemical models of dense interstellar molecular cores. They specifically discussed the role of the atomic-to-molecular nitrogen ratio in the fractionation process and the possible link between nitrogen hydrides and CN containing molecules (nitriles).…”

Section: Introductionmentioning

confidence: 99%

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

Roueff

Loison

Hickson

2015

A&A

173

349

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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.

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Nitrogen Isotopic Fractionation of Interstellar Nitriles

Cited by 64 publications

References 62 publications

Elemental nitrogen partitioning in dense interstellar clouds

Elemental nitrogen partitioning in dense interstellar clouds

Detection of N$^\mathsf{{15}}$NH⁺in L1544

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

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Nitrogen Isotopic Fractionation of Interstellar Nitriles

Cited by 64 publications

References 62 publications

Elemental nitrogen partitioning in dense interstellar clouds

Elemental nitrogen partitioning in dense interstellar clouds

Detection of N$^\mathsf{{15}}$NH+in L1544

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

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Detection of N$^\mathsf{{15}}$NH⁺in L1544