Elemental nitrogen partitioning in dense interstellar clouds

Abstract

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...

THE C(3P) + NH3REACTION IN INTERSTELLAR CHEMISTRY. II. LOW TEMPERATURE RATE CONSTANTS AND MODELING OF NH, NH2, AND NH3ABUNDANCES IN DENSE INTERSTELLAR CLOUDS

“…Our new measurements of the low temperature rates of the C + NH 3 reaction, in addition to an updated chemistry for NH, NH 2 and NH 3 coupled with recent studies of N 2 production pathways (Daranlot et al 2012;Daranlot et al 2013;Daranlot et al 2011;Loison et al 2014a;Ma et al 2012) lead to a strong disagreement between the observed and simulated abundances for NH, NH 2 and NH 3 from pure gas-phase models. In contrast, the gas-grain simulations produce good agreement with observations, a result which is essentially independent of the ortho-to-para ratio of H 2 .…”

“…N 2 itself is produced by atomic nitrogen reactions with small neutral radicals (Daranlot et al 2012;Le Gal et al 2014) which experimental and theoretical studies have shown to be less efficient than previously assumed (Daranlot et al 2012;Daranlot et al 2013;Daranlot et al 2011;Jorfi & Honvault 2009;Loison et al 2014a;Ma et al 2012 (Vigren et al 2012) which is nonetheless sufficient to make this process the major source of NH radicals in current gas-phase astrochemical models (Le Gal et al 2014). Reaction (R1) is problematic due to the endothermicity of the N + + para-H 2 (p-H 2 ) reaction (Zymak et al 2013).…”

Photodissociation of interstellar N2

“…One possible explanation is that the adopted N 2 photodissociation rate is incorrect. Even in dense cores, not all nitrogen appears to have been transformed to molecular form (Maret et al 2006;Daranlot et al 2012). Observations of HCN in the surface layers of protoplanetary disks suggest that the nitrogen chemistry is strongly affected by whether or not a star has sufficiently hard UV radiation to photodissociate N 2 (Pascucci et al 2009).…”

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

“…We did not find any CN 2 H 2 + species lying below the reagent energy level, and we consider that this reaction cannot take place. Considering only the triplet surface, the only barrierless reaction is attack on the carbon atom that leads to the ground state 3 NCN intermediate (Daranlot et al 2012;Ma et al 2012). The main exit channel is C + N 2 after isomerization of the NCN intermediate through a tight TS, and then back dissociation may be important and isotope exchange possible.…”