The excitation of NH2 in the interstellar medium

Bouhafs, Nezha; Bacmann, A.; Fauré, Alexandre; Lique, François

doi:10.1093/mnras/stz2586

Cited by 6 publications

(5 citation statements)

References 32 publications

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“…The present spectra concern para-NH 2 , so that the nuclear spin hfs reduces to that of nitrogen. A study of NH 2 excitation was recently presented by Bouhafs et al (2019). The critical density of the transition observed by us is on the order of 10 8 -10 9 cm −3 so that in our case the excitation temperature remains close to the CMB temperature.…”

Section: Nh 2 As a Reference To Unlock The Abundances Of Nitrogen Hydrides In Our Datasupporting

confidence: 62%

Detection of deuterated molecules, but not of lithium hydride, in the z = 0.89 absorber toward PKS 1830−211

Müller

Roueff

Black

et al. 2020

A&A

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Deuterium and lithium are light elements of high cosmological and astrophysical importance. In this work we report the first detection of deuterated molecules and a search for lithium hydride, 7LiH, at redshift z = 0.89 in the spiral galaxy intercepting the line of sight to the quasar PKS 1830−211. We used ALMA to observe several submillimeter lines of ND, NH2D, and HDO, and their related isotopomers NH2, NH3, and H218O, in absorption against the southwest image of the quasar, allowing us to derive XD/XH abundance ratios. The absorption spectra mainly consist of two distinct narrow velocity components for which we find remarkable differences. One velocity component shows XD/XH abundances that is about 10 times larger than the primordial elemental D/H ratio, and no variability of the absorption profile during the time span of our observations. In contrast, the other component shows a stronger deuterium fractionation. Compared to the first component, this second component has XD/XH abundances that are 100 times larger than the primordial D/H ratio, a deepening of the absorption by a factor of two within a few months, and a rich chemical composition, with relative enhancements of N2H+, CH3OH, SO2 and complex organic molecules. We therefore speculate that this component is associated with the analog of a Galactic dark cloud, while the first component is likely more diffuse. Our search for the 7LiH (1–0) line was unsuccessful and we derive an upper limit 7LiH/H2 = 4 × 10−13 (3σ) in the z = 0.89 absorber toward PKS 1830−211. Besides, with ALMA archival data, we could not confirm the previous tentative detections of this line in the z = 0.68 absorber toward B 0218+357; we derive an upper limit 7LiH/H2 = 5 × 10−11 (3σ), although this is less constraining than our limit toward PKS 1830−211. We conclude that, as in the Milky Way, only a tiny fraction of lithium nuclei are possibly bound in LiH in these absorbers at intermediate redshift.

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Section: Nh 2 As a Reference To Unlock The Abundances Of Nitrogen Hydrides In Our Datasupporting

confidence: 62%

Detection of deuterated molecules, but not of lithium hydride, in the z = 0.89 absorber toward PKS 1830−211

Müller

Roueff

Black

et al. 2020

A&A

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“…We note that the critical densities associated with the detected transitions are 10 6 cm −3 assuming collisional rate coefficients for NHD and ND 2 of a few 10 −11 cm 3 s −1 (see Bouhafs et al 2019, for the main isotopologue NH 2 ). Excitation temperatures are therefore expected to be lower than the kinetic temperature in the protostellar envelope where T kin ∼ 10 K and n ∼ 10 5 cm −3 , but will depend on the exact physical conditions of the absorbing molecules.…”

Section: Column Density Estimatesmentioning

confidence: 97%

“…The species NH, ND and NH 2 were all seen in absorption against a background emitted by the warm dust. This is partly due to the very high critical densities of the fundamental rotational transitions of nitrogen hydrides, especially NH 2 and NH (around 10 7 cm −3 , see Dumouchel et al 2012;Bouhafs et al 2019, for the derivation of the collisional rate coefficients), and the presence of the species in the lower density envelope. Indeed, the frequency of the fundamental rotational transition of ND is a factor of 2 smaller than for NH, and therefore the critical density a factor of ∼ 10 smaller, making ND easier to be seen in emission than its hydrogenated counterpart.…”

Section: Observationsmentioning

confidence: 99%

Deuterium fractionation of nitrogen hydrides: detections of NHD and ND$_2$

Bacmann,

Faure,

Hily-Blant

et al. 2020

Preprint

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Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routinely observed in the dense interstellar medium, with the ratio between deuterated molecules and the main isotopologue enhanced by several orders of magnitude with respect to the elemental D/H ratio. In the case of ammonia, the detection of its triply deuterated isotopologue hints at high abundances of the deuterated intermediate nitrogen radicals, ND, NHD and ND 2 . So far however, only ND has been detected in the interstellar medium. In this paper, to constrain the formation of ammonia, we aim at determining the NHD/NH 2 and ND 2 /NHD abundance ratios, and compare them with the predictions of both pure gas-phase and grain-surface chemical models. We searched for the fundamental rotational transitions of NHD and ND 2 towards the class 0 protostar IRAS16293-2422, towards which NH, NH 2 and ND had been previously detected. Both NHD and ND 2 are detected in absorption towards the source. The relative abundance ratios NH 2 : NHD : ND 2 are close to 8 : 4 : 1. These ratios can be reproduced by our gas-phase chemical model within a factor of two-three. Statistical ratios as expected from grain-surface chemistry are also consistent with our data. Further investigations of the ortho-to-para ratio in ND 2 , both theoretical and observational, could bring new constraints to better understand nitrogen hydride chemistry.

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“…The first type of planned updates are species which the database currently does not cover at all. This category includes the fine structure lines of S-He [106] and Si-He [106]; the noble gas species ArH + -H [107], ArH + -e [108], and HeH + -H [109]; the diatomic ions SH + -e [76] and CH + -He [110,111]; the neutral diatomics H 2 -H [112], NH-He [113], PN-He [114], and PO-He [115]; the triaomics NH 2 -H 2 [116] and C 3 -He [117]; and the 'large' species HOCO + -He [118], and CH 3 OCHO-He [119,120]. We also regard SiS-H 2 [121] in this category, as its rates were obtained by scaling SiO data.…”

Section: Planned Updates Of Lamdamentioning

confidence: 99%

“…Non-radiative transitions are assumed to have negligible collisional rate coefficients, which is of course a simplification. For certain cases, this scaling does not work at all: NH 2 is an example [116].…”

Section: What To Do If Collisional Data Are Missingmentioning

confidence: 99%

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

Tak

Lique

Fauré

et al. 2020

Atoms

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The Leiden Atomic and Molecular Database (LAMDA) collects spectroscopic information and collisional rate coefficients for molecules, atoms, and ions of astrophysical and astrochemical interest. We describe the developments of the database since its inception in 2005, and outline our plans for the near future. Such a database is constrained both by the nature of its uses and by the availability of accurate data: we suggest ways to improve the synergies among users and suppliers of data. We summarize some recent developments in computation of collisional cross sections and rate coefficients. We consider atomic and molecular data that are needed to support astrophysics and astrochemistry with upcoming instruments that operate in the mid- and far-infrared parts of the spectrum.

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The excitation of NH2 in the interstellar medium

Cited by 6 publications

References 32 publications

Detection of deuterated molecules, but not of lithium hydride, in the z = 0.89 absorber toward PKS 1830−211

Detection of deuterated molecules, but not of lithium hydride, in the z = 0.89 absorber toward PKS 1830−211

Deuterium fractionation of nitrogen hydrides: detections of NHD and ND$_2$

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

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