Hyperfine excitation of N2H+ by H2: towards a revision of N2H+ abundance in cold molecular clouds

Lique, François; Daniel, F.; Pagani, L.; Feautrier, N.

doi:10.1093/mnras/stu2188

Cited by 39 publications

(53 citation statements)

References 42 publications

(44 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The availability of new collisional rate coefficients for the N 2 H + -H 2 system (Lique et al 2015) may, however, modify these conclusions. We also discarded the 15 N + HCNH + and 15 N + NO exchange reactions through similar arguments.…”

Section: Discussionmentioning

confidence: 99%

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

Roueff

Loison

Hickson

2015

A&A

171

349

View full text Add to dashboard Cite

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.

show abstract

Section: Discussionmentioning

confidence: 99%

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

Roueff

Loison

Hickson

2015

A&A

171

349

View full text Add to dashboard Cite

show abstract

“…Collisional rate coefficients for the N 2 H + -H 2 (J = 0) 1 system have been published recently by Lique et al (2015). We calculated hyperfine-structure-resolved excitation rate coefficients that are based on a new potential energy surface (PES) obtained from highly correlated ab initio calculations (Spielfiedel et al 2015) for temperatures ranging from 5 K to 70 K. The new rate coefficients are significantly higher than the N 2 H + -He rate coefficients previously published (Daniel et al 2005).…”

Section: N 2 H + and N 15 Nh + Collisional Rate Coefficientsmentioning

confidence: 95%

“…We therefore performed close-coupling calculations (Arthurs & Dalgarno 1960) for the pure rotational excitation cross-sections using the MOLSCAT program (Hutson & Green 1994), as in Lique et al (2015) for the main isotopolog. The N 15 NH + rotational energy levels were computed using the rotational constants of Dore et al (2009).…”

Section: N 2 H + and N 15 Nh + Collisional Rate Coefficientsmentioning

confidence: 99%

N₂H⁺and N¹⁵NH⁺toward the prestellar core 16293E in L1689N

Daniel

Fauré

Pagani

et al. 2016

A&A

View full text Add to dashboard Cite

Context. Understanding the processes that could lead to an enrichment of molecules in 15 N atoms is of particular interest because this may shed light on the relatively strong variations observed in the 14 N/ 15 N ratio in various solar system environments. Aims. The sample of molecular clouds where 14 N/ 15 N ratios have been measured currently is small and has to be enlarged to allow statistically significant studies. In particular, the N 2 H + molecule currently shows the broadest spread of 14 N/ 15 N ratios in high-mass star-forming regions. However, the 14 N/ 15 N ratio in N 2 H + was obtained in only two low-mass star-forming regions (L1544 and B1b). We here extend this sample to a third dark cloud. Methods. We targeted the 16293E prestellar core, where the N 15 NH + J = 1−0 line was detected. Using a model previously developed for the physical structure of the source, we solved the molecular excitation with a nonlocal radiative transfer code. For this purpose, we computed specific collisional rate coefficients for the N 15 NH + -H 2 collisional system. As a first step of the analysis, the N 2 H + abundance profile was constrained by reproducing the N 2 H + J = 1−0 and 3−2 maps. A scaling factor was then applied to this profile to match the N 15 NH + J = 1−0 spectrum. Results. We derive a column density ratio N 2 H + /N 15 NH + = 330 +170 −100 . Conclusions. We performed a detailed analysis of the excitation of N 2 H + and N 15 NH + in the direction of the 16293E core with modern models that solve the radiative transfer and with the most accurate collisional rate coefficients available to date. We obtained the third estimate of the N 2 H + /N 15 NH + column density ratio in the direction of a cold prestellar core. The current estimate ∼330 agrees with the typical value of the elemental isotopic ratio in the local interstellar medium. It is lower than in some other cores, however, where values as high as 1300 have been reported.

show abstract

“…The collisional rate coefficients for NH 2 D were adopted from Daniel et al (2014), and for NHD 2 and ND 3 we used the newly calculated coefficients from Daniel et al (2016b). For N 2 D + we have used the collisional rate coefficients for N 2 H + from the recent work of Lique et al (2015).…”

Section: Average Abundances From Line Modellingmentioning

confidence: 99%

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Harju

Daniel

Sipilä

et al. 2017

A&A

View full text Add to dashboard Cite

Context. Ammonia and its deuterated isotopologues probe physical conditions in dense molecular cloud cores. The time-dependence of deuterium fractionation and the relative abundances of different nuclear spin modifications are supposed to provide means of determining the evolutionary stages of these objects. Aims. We aim to test the current understanding of spin-state chemistry of deuterated species by determining the abundances and spin ratios of NH 2 D, NHD 2 , and ND 3 in a quiescent, dense cloud. Methods. Spectral lines of NH 3 , NH 2 D, NHD 2 , ND 3 , and N 2 D + were observed towards a dense, starless core in Ophiuchus with the APEX, GBT, and IRAM 30-m telescopes. The observations were interpreted using a gas-grain chemistry model combined with radiative transfer calculations. The chemistry model distinguishes between the different nuclear spin states of light hydrogen molecules, ammonia, and their deuterated forms. Different desorption schemes can be considered. Results. High deuterium fractionation ratios with NH 2 D/NH 3 ∼ 0.4, NHD 2 /NH 2 D ∼ 0.2, and ND 3 /NHD 2 ∼ 0.06 are found in the core. The observed ortho/para ratios of NH 2 D and NHD 2 are close to the corresponding nuclear spin statistical weights. The chemistry model can approximately reproduce the observed abundances, but predicts uniformly too low ortho/para-NH 2 D, and too large ortho/para-NHD 2 ratios. The longevity of N 2 H + and NH 3 in dense gas, which is prerequisite to their strong deuteration, can be attributed to the chemical inertia of N 2 on grain surfaces. Conclusions. The discrepancies between the chemistry model and the observations are likely to be caused by the fact that the model assumes complete scrambling in principal gas-phase deuteration reactions of ammonia, which means that all the nuclei are mixed in reactive collisions. If, instead, these reactions occur through proton hop/hydrogen abstraction processes, statistical spin ratios are to be expected. The present results suggest that while the deuteration of ammonia changes with physical conditions and time, the nuclear spin ratios of ammonia isotopologues do not probe the evolutionary stage of a cloud.

show abstract

Hyperfine excitation of N2H+ by H2: towards a revision of N2H+ abundance in cold molecular clouds

Cited by 39 publications

References 42 publications

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

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

N₂H⁺and N¹⁵NH⁺toward the prestellar core 16293E in L1689N

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Contact Info

Product

Resources

About

Hyperfine excitation of N2H+ by H2: towards a revision of N2H+ abundance in cold molecular clouds

Cited by 39 publications

References 42 publications

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

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

N2H+and N15NH+toward the prestellar core 16293E in L1689N

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

Contact Info

Product

Resources

About

N₂H⁺and N¹⁵NH⁺toward the prestellar core 16293E in L1689N