Elemental nitrogen partitioning in dense interstellar clouds (original) (raw)
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Nitrogen chemistry in the interstellar medium
1995
We report the first detection of interstellar nitric oxide (NO) in a cold dark cloud, L134N, and confirm the subsequent detection of NO emission towards TMC-1 by Gerin, Viala and Casoli (1993). Nitric oxide was observed by means of its two 2ni/2, J = 3/2^1/2, F = 5/2^3 /2, rotational transition hyperfine components at 150.2 and 150.5 GHz, which occur because of A-doubling. The inferred total column density for L134N is Nno^4x10^^cm'^towards the position of peak SO emission in that cloud. The inferred total column density for TMC-1 is Nno~2 x 10^'* cm~^towards the position of peak ammonia emission. These values corresponds to a fractional abundance relative to molecular hydrogen of fNo~5 x 10-^a nd fNo~2x 10"^t owards L134N and TMC-1, respectively, and are in good agreement with predictions of quiescent cloud ion-molecule chemistry. NO was not detected towards the position of peak cyanopolyyne emission in TMC-1 to an upper limit of fNo < 1 x 10~^.
Chemical Physics, 2017
Nitrogen content of the interstellar regions is undergoing a re-evaluation based on some recent comet data and some excellent researches in this area. Finding of secondary channels for the formation of N 2 will definitely have significant effect in accounting the elemental nitrogen partitioning and also the active nitrogen chemistry of the interstellar medium. In this work, computational calculations of potential energy surface for the reaction between NS and NSi has been carried out. We were able to locate many dissociation channels leading to the formation of N 2 from some of these isomers, both in singlet as well as the triplet potential energy surfaces. Based on the analysis of the dissociation paths, it has been argued that such dissociation reactions leading to the formation of N 2 will be possible not only in hot-cores, but also in the cold interstellar clouds of the interstellar medium.
Review of important reactions for the nitrogen chemistry in the interstellar medium
Predictions of astrochemical models depend strongly on the reaction rate coefficients used in the simulations. We reviewed a number of key reactions for the chemistry of nitrogen-bearing species in the dense interstellar medium and proposed new reaction rate coefficients for those reactions. The details of the reviews are given in the form of a datasheet associated with each reaction. The new recommended rate coefficients are given with an uncertainty and a temperature range of validity and will be included in KIDA (http://kida.obs.u-bordeaux1.fr).
Monthly Notices of the Royal Astronomical Society, 2000
The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral±neutral reactions thought to be important in the formation of HNC and HCN, respectively ± C 1 NH 2 3 HNC 1 H, and N 1 CH 2 3 HCN 1 HX We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds.
We experimentally show that the reaction between ground state nitrogen atoms N(4 S) and acetonitrile CH 3 CN can lead to two distinct chemical pathways that are both thermally activated at very low temperatures. First is CH 3 CN isomerization which produces CH 3 NC and H 2 CCNH. Second is CH 3 CN decomposition which produces HNC and CH 3 CNH + CN − fragments, with the possible release of H 2. Our results reveal that the mobility of N(4 S)-atoms is stimulated in the 3-11 K temperature range, and that its subsequent encounter with one acetonitrile molecule is sufficient for the aforementioned reactions to occur without the need for additional energy to be supplied to the CH 3 CN + N(4 S) system. These findings shed more light on the nitrogen chemistry that can possibly take place in dense molecular clouds, which until now was thought to only involve high-energy processes and therefore be unlikely to occur in such cold and dark interstellar regions. The reaction pathways we propose in this study have very important astrochemical implications, as it was shown recently that the atomic nitrogen might be more abundant, in many interstellar icy grain mantles, than previously thought. Also, these reaction pathways can now be considered within dense molecular clouds, and possibly affect the branching ratios for N-bearing molecules computed in astrochemical modelling.
The Excitation of N 2 H + in Interstellar Molecular Clouds. II. Observations
The Astrophysical Journal, 2007
We present observations of the J ¼ 1Y0, 2Y1, and 3Y2 rotational transitions of N 2 H + and N 2 D + toward a sample of prototypical dark clouds. The data have been interpreted using nonlocal radiative transfer models. For all sources previously studied through millimeter-continuum observations, we find a good agreement between the volume density estimated from our N 2 H + data and that estimated from the dust emission. This confirms that N 2 H + depletion is not very efficient in dark clouds for densities as large as 10 6 cm À3 , and also points out that a simultaneous analysis based on millimeter-continuum, N 2 H + and N 2 D + observations should lead to reliable estimates for the temperature and density structure of cold dark clouds. From multiline modeling of N 2 H + and N 2 D + , we derive the deuterium enrichment in the observed clouds. Our estimates are similar or higher than previous ones. The differences can be explained by the assumptions made on the cloud density profile and by the chemical fractionation occurring in the clouds. For two of the observed objects, L183 and TMC 2, multiposition observations have allowed us to derive the variation of the N 2 D þ /N 2 H þ abundance ratio with the radius. We have found that it decreases by an order of magnitude for radii greater than a few 0.01 pc (i.e., outside the central cores). Inside the dense condensations, the fractionation is efficient and, compared to the abundance ratio expected from statistical considerations based on the cosmic D/H ratio, the deuterium enrichment is estimated to be ' 0:1Y 0:5 ð Þ; 10 5 .
NITROGEN ISOTOPIC FRACTIONATION IN INTERSTELLAR AMMONIA
The Astrophysical Journal, 2010
Using the Green Bank Telescope (GBT), we have obtained accurate measurements of the 14 N/ 15 N isotopic ratio in ammonia in two nearby cold, dense molecular clouds, Barnard 1 and NGC 1333. The 14 N/ 15 N ratio in Barnard 1, 334 ± 50 (3σ), is particularly well constrained and falls in between the local interstellar medium/proto-solar value of ∼450 and the terrestrial atmospheric value of 272. The NGC 1333 measurement is consistent with the Barnard 1 result, but has a larger uncertainty. We do not see evidence for the very high 15 N enhancements seen in cometary CN. Sensitive observations of a larger, carefully selected sample of prestellar cores with varying temperatures and gas densities can significantly improve our understanding of the nitrogen fractionation in the local interstellar medium and its relation to the isotopic ratios measured in various solar system reservoirs.
Potential Variations in the Interstellar N i Abundance
The Astrophysical Journal, 2003
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph observations of the weak interstellar N I λ1160 doublet toward 17 high-density sightlines [N(H tot) ≥ 10 21 cm −2 ]. When combined with published data, our results reveal variations in the fractional N I abundance showing a systematic deficiency at large N(H tot). At the FUSE resolution (∼ 20 km s −1) the effects of unresolved saturation cannot be conclusively ruled out, although O I λ1356 shows little evidence for saturation. We investigated the possibility that the N I variability is due to the formation of N 2 in our mostly dense regions. The 0-0 band of the c ′ 4 1 Σ + u − X 1 Σ + g transition of N 2 at 958Å should be easily detected in our F USE data; for 10 of the denser sightlines N 2 is not observed at a sensitivity level of a few times 10 14 cm −2. The observed N I variations are suggestive of an incomplete understanding of nitrogen chemistry.
The Journal of Chemical Physics, 2016
N2 is a diatomic molecule with complex electronic structure. Interstate crossings are prominent in the high energy domain, introducing significant perturbations to the system. Nitrogen mainly photodissociates in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum through both direct and indirect predissociation. Due to the complexity introduced by these perturbations, the nitrogen isotopic fractionation in N2 photodissociation is extremely hard to calculate and an experimental approach is required. Here we present new data of N-isotopic fractionation in N2 photodissociation at low temperature (80K), which shows a distinctly different 15 N enrichment profile compared to that at relatively higher temperatures (200 and 300 K). The new data is discussed in light of the knowledge of N2 photochemistry and calculated photoabsorption cross-sections in the VUV. This data, important to understanding the N-isotopic compositions measured in meteorites and other planetary bodies, is discussed in light of the knowledge of N2 photochemistry and calculated photoabsorption cross-sections in the VUV. I. INTRODUCTION Nitrogen is the fifth most abundant element in the universe and an essential component as a prebiotic molecular building block. In interstellar clouds, nitrogen exists in atomic and molecular 2 forms (N and N2) in gas-phase reservoirs. Atomic nitrogen is very reactive and takes part in chemical reactions leading to ammonia, nitriles, and other nitrogen compounds. No such reactions occur when nitrogen is in N2 form. Nitrogen isotopic analyses of meteorites, terrestrial planets, atmospheres of giant planets and their moons, solar wind, comets, and interplanetary dust particles advance understanding of volatile chemistry and prebiotic processes in the early solar system 1. Direct astronomical observations of N2 is difficult because of the absence of strong pure rotational or vibrational lines. It is well studied through the electronic transitions at ultraviolet wavelengths 2, 3. The isotopic inventory of nitrogen in astronomical environments is also reasonably well known 1, 4. The solar system was formed with an initial 15 N/ 14 N ratio acquired from parent molecular clouds from the interstellar medium (ISM). The near-identical compositions measured in the solar wind and the Jovian atmosphere (δ 15 N ~-400 ‰) may indicate the formation of the gas giant with the initial solar system materials of the same N-isotopic composition 5, 6. Bulk meteorite analysis exhibits a variation in the range of few hundred permil in δ 15 N (wrt to air-N2) 7-9 with occasional exceptionally high values (as well as a range of variation) in some carbonaceous