Doubly<sup>15</sup>N-substituted diazenylium: THz laboratory spectra and fractionation models

Dore, Luca; Bizzocchi, L.; Wirström, E. S.; Esposti, C. Degli; Tamassia, Filippo; Charnley, Steven B.

doi:10.1051/0004-6361/201629725

Cited by 6 publications

(6 citation statements)

References 26 publications

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“…Although the carbon fractionation in HCN may not be well understood, chemical models emphasize the caveats of indirect measurements of isotopic ratios. Recent works have suggested that doubly substituted N 2 H + could be a potential direct probe of the nitrogen isotopic ratio (Dore et al 2017). As demonstrated in the present work, observational, theoretical, and modeling efforts must be combined to enable direct, reliable estimates of isotopic ratios.…”

Section: Discussionmentioning

confidence: 83%

Abundance of HCN and its C and N isotopologues in L1498

Magalhaes

Hily-Blant

Fauré

et al. 2018

A&A

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The isotopic ratio of nitrogen in nearby protoplanetary disks, recently measured in CN and HCN, indicates that a fractionated reservoir of volatile nitrogen is available at the earliest stage of comet formation. This reservoir also presents a 3:1 enrichment in 15 N relative to the elemental ratio of 330, identical to that between the solar system comets and the protosun, suggesting that similar processes are responsible for the fractionation in the protosolar nebula (PSN) and in these PSN analogs. However, where, when, and how the fractionation of nitrogen takes place is an open question. Previously obtained HCN/HC 15 N abundance ratios suggest that HCN may already be enriched in 15 N in prestellar cores, although doubts remain on these measurements, which rely on the double-isotopologue method. Here we present direct measurements of the HCN/H 13 CN and HCN/HC 15 N abundance ratios in the L1498 prestellar core based on spatially resolved spectra of HCN(1-0), (3-2), H 13 CN(1-0), and HC 15 N(1-0) rotational lines. We use state-of-the-art radiative transfer calculations using ALICO, a 1D radiative transfer code capable of treating hyperfine overlaps. From a multiwavelength analysis of dust emission maps of L1498, we derive a new physical structure of the L1498 cloud. We also use new, high-accuracy HCN-H 2 hyperfine collisional rates, which enable us to quantitatively reproduce all the features seen in the line profiles of HCN(1-0) and HCN(3-2), especially the anomalous hyperfine line ratios. Special attention is devoted to derive meaningful uncertainties on the abundance ratios. The obtained values, HCN/H 13 CN=45±3 and HCN/HC 15 N=338±28, indicate that carbon is heavily fractionated in HCN, but nitrogen is not. For the H 13 CN/HC 15 N abundance ratio, our detailed study validates to some extent analyses based on the single excitation temperature assumption. Comparisons with other measurements from the literature suggest significant core-to-core variability. Furthermore, the heavy 13 C enrichment we found in HCN could explain the superfractionation of nitrogen measured in solar system chondrites.

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Section: Discussionmentioning

confidence: 83%

Abundance of HCN and its C and N isotopologues in L1498

Magalhaes

Hily-Blant

Fauré

et al. 2018

A&A

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“…In that scenario, 15 N atoms are released more efficiently through selective photodissociation of 14 N 15 N around the chemical transition from atomic to molecular nitrogen and are converted efficiently to ammonia ice through hydrogenation on grains. Searching for 15 N in 15 N 15 NH + and in the various deuterated substitutes of N 2 H + , 14 N 15 ND + , 15 N 14 ND + and 15 N 15 ND + is possible thanks to detailed spectroscopic studies (Dore et al 2009, 2017) and could help to further constrain the issue.…”

Section: Resultsmentioning

confidence: 99%

A sensitive λ 3 mm line survey of L483

Agúndez

Marcelino

Cernicharo

et al. 2019

A&A

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An exhaustive chemical characterization of dense cores is mandatory to our understanding of chemical composition changes from a starless to a protostellar stage. However, only a few sources have had their molecular composition characterized in detail. Here we present a λ 3 mm line survey of L483, a dense core around a Class 0 protostar, which was observed with the IRAM 30m telescope in the 80-116 GHz frequency range. We detected 71 molecules (140 including different isotopologs), most of which are present in the cold and quiescent ambient cloud according to their narrow lines (FWHM ~0.5 km s−1) and low rotational temperatures (≲10 K). Of particular interest among the detected molecules are the cis isomer of HCOOH, the complex organic molecules HCOOCH3, CH3OCH3, and C2H5OH, a wide variety of carbon chains, nitrogen oxides like N2O, and saturated molecules like CH3SH, in addition to eight new interstellar molecules (HCCO, HCS, HSC, NCCNH+, CNCN, NCO, H2NCO+, and NS+) whose detection has already been reported. In general, fractional molecular abundances in L483 are systematically lower than in TMC-1 (especially for carbon chains), tend to be higher than in L1544 and B1-b, and are similar to those in L1527. Apart from the overabundance of carbon chains in TMC-1, we find that L483 does not have a marked chemical differentiation with respect to starless/prestellar cores like TMC-1 and L1544, although it does chemically differentiate from Class 0 hot corino sources like IRAS 16293–2422. This fact suggests that the chemical composition of the ambient cloud of some Class 0 sources could be largely inherited from the dark cloud starless/prestellar phase. We explore the use of potential chemical evolutionary indicators, such as the HNCO/C3S, SO2/C2S, and CH3SH/C2S ratios, to trace the prestellar/protostellar transition. We also derived isotopic ratios for a variety of molecules, many of which show isotopic ratios close to the values for the local interstellar medium (remarkably all those involving 34S and 33S), while there are also several isotopic anomalies like an extreme depletion in 13C for one of the two isotopologs of c-C3H2, a drastic enrichment in 18O for SO and HNCO (SO being also largely enriched in 17O), and different abundances for the two 13C substituted species of C2H and the two 15N substituted species of N2H+. We report the first detection in space of some minor isotopologs like c-C3D. The exhaustive chemical characterization of L483 presented here, together with similar studies of other prestellar and protostellar sources, should allow us to identify the main factors that regulate the chemical composition of cores along the process of formation of low-mass protostars.

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“…Mapping of these molecules may also provide insight into fractionation mechanisms. One interesting possibility is that 15 N2H + could be a significant reservoir of 15 N; recent spectroscopic data mean that searches for this molecule are now possible (Dore et al 2017). (solid lines), as compared to the W12 nitrogen reaction network (dashed lines).…”

Section: Discussionmentioning

confidence: 99%

Revised models of interstellar nitrogen isotopic fractionation

Wirström

Charnley

2017

Monthly Notices of the Royal Astronomical Society

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Nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of 15 N enrichments found in comets and meteorites. Chemical model calculations indicate that atom-molecular ion and ion-molecule reactions could account for most of the fractionation patterns observed. However, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. Related model calculations of dense cloud chemistry show that the revised 15 N enrichments fail to match observed values.We have investigated the effects of these reaction rate modifications on the chemical model of Wirström et al. (2012) for which there are significant physical and chemical differences with respect to other models. We have included 15 N fractionation of CN in neutral-neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry.We find that the revised fractionation rates have the effect of suppressing 15 N enrichment in ammonia at all times, while the depletion is even more pronounced, reaching 14 N/ 15 N ratios of > 2000. Taking the updated nitrogen chemistry into account, no significant enrichment occurs in HCN or HNC, contrary to observational evidence in dark clouds and comets, although the 14 N/ 15 N ratio can still be below 100 in CN itself. However, such low CN abundances are predicted that the updated model falls short of explaining the bulk 15 N enhancements observed in primitive materials. It is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed.

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Doubly¹⁵N-substituted diazenylium: THz laboratory spectra and fractionation models

Cited by 6 publications

References 26 publications

Abundance of HCN and its C and N isotopologues in L1498

Abundance of HCN and its C and N isotopologues in L1498

A sensitive λ 3 mm line survey of L483

Revised models of interstellar nitrogen isotopic fractionation

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Doubly15N-substituted diazenylium: THz laboratory spectra and fractionation models

Cited by 6 publications

References 26 publications

Abundance of HCN and its C and N isotopologues in L1498

Abundance of HCN and its C and N isotopologues in L1498

A sensitive λ 3 mm line survey of L483

Revised models of interstellar nitrogen isotopic fractionation

Contact Info

Product

Resources

About

Doubly¹⁵N-substituted diazenylium: THz laboratory spectra and fractionation models