Modelling the molecular composition and nuclear-spin chemistryof collapsing pre-stellar sources★

Hily-Blant, Pierre; Fauré, Alexandre; Rist, Claire; Forêts, G. Pineau des; Flower, D. R.

doi:10.1093/mnras/sty881

Cited by 33 publications

(40 citation statements)

References 50 publications

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“…This is partially justified because when we calculated the rate for the para and ortho species at 10 and 100 K, we systematically found that the ortho rate was orders of magnitude higher than the para ones. Nevertheless we note that, as some studies show (Hily-Blant et al 2018;Furuya et al 2015) in some environments para H 2 may be dominant. We have therefore performed a further test where we use the rate for para-H 2 instead of the one for the ortho H 2 , essentially assuming in this way that all the molecular hydrogen is in the para form.…”

supporting

confidence: 48%

Nitrogen fractionation in external galaxies

Viti

Fontani

Jiménez-Serra

et al. 2019

Monthly Notices of the Royal Astronomical Society

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In star forming regions in our own Galaxy, the 14 N/ 15 N ratio is found to vary from ∼ 100 in meteorites, comets and protoplanetary disks up to ∼ 1000 in pre-stellar and star forming cores, while in external galaxies the very few single-dish large scale measurements of this ratio lead to values of 100-450. The extent of the contribution of isotopic fractionation to these variations is, to date, unknown. In this paper we present a theoretical chemical study of nitrogen fractionation in external galaxies in order to determine the physical conditions that may lead to a spread of the 14 N/ 15 N ratio from the solar value of ∼ 440 and hence evaluate the contribution of chemical reactions in the ISM to nitrogen fractionation. We find that the main cause of ISM enrichment of nitrogen fractionation is high gas densities, aided by high fluxes of cosmic rays.

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supporting

confidence: 48%

Nitrogen fractionation in external galaxies

Viti

Fontani

Jiménez-Serra

et al. 2019

Monthly Notices of the Royal Astronomical Society

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“…The other reason for the contrasting distributions of NH D 2 and CH OH 3 is that deuterium fractionation starts in earnest at high densities where CO has largely disappeared from the gas, whereas CH OH 3 production (through desorption) is probably most active further out in a starless core, where CO is not severely depleted (see Section 6). In the gas phase, deuterated ammonia is mainly produced in reactions between NH 3 and deuterated ions (Rodgers & Charnley 2001;Sipilä et al 2015;Hily-Blant et al 2018). Deuterated ions are enhanced as a consequence of the freeze-out of CO, which first leads to a rapid increase of + H 3 .…”

Section: Distributionmentioning

confidence: 99%

Efficient Methanol Production on the Dark Side of a Prestellar Core

Harju

Pineda

Vasyunin

et al. 2020

ApJ

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We present Atacama Large Millimeter/submillimeter Array maps of the starless molecular cloud core Ophiuchus/ H-MM1 in the lines of deuterated ammonia (ortho-NH D 2), methanol (CH OH 3), and sulfur monoxide (SO). The dense core is seen in NH D 2 emission, whereas the CH OH 3 and SO distributions form a halo surrounding the core. Because methanol is formed on grain surfaces, its emission highlights regions where desorption from grains is particularly efficient. Methanol and sulfur monoxide are most abundant in a narrow zone that follows the eastern side of the core. This side is sheltered from the stronger external radiation field coming from the west. We show that photodissociation on the illuminated side can give rise to an asymmetric methanol distribution but that the stark contrast observed in H-MM1 is hard to explain without assuming enhanced desorption on the shaded side. The region of the brightest emission has a wavy structure that rolls up at one end. This is the signature of Kelvin-Helmholtz instability occurring in sheared flows. We suggest that in this zone, methanol and sulfur are released as a result of grain-grain collisions induced by shear vorticity.

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“…In Sipilä et al (2017) we presented a new set of rate coefficients for the H + 3 + H 2 reacting system, taking rotational excitation into account in the derivation of the rate coefficients (see Hily-Blant et al 2018 for a recent similar work). We found that the inclusion of excited states leads in general to reduced deuteration levels.…”

Section: Effect Of Species-to-species Rate Coefficientsmentioning

confidence: 99%

Modeling deuterium chemistry in starless cores: full scrambling versus proton hop

Sipilä

Caselli

Harju

2019

A&A

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We constructed two new models for deuterium and spin-state chemistry for the purpose of modeling the low-temperature environment prevailing in starless and pre-stellar cores. The fundamental difference between the two models is in the treatment of ion-molecule proton-donation reactions of the form XH + + Y −→ X + YH + , which are allowed to proceed either via full scrambling or via direct proton hop, i.e., disregarding proton exchange. The choice of the reaction mechanism affects both deuterium and spin-state chemistry, and in this work our main interest is on the effect on deuterated ammonia. We applied the new models to the starless core H-MM1, where several deuterated forms of ammonia have been observed. Our investigation slightly favors the proton hop mechanism over full scrambling because the ammonia D/H ratios are better fit by the former model, although neither model can reproduce the observed NH 2 D ortho-to-para ratio of 3 (the models predict a value of ∼2). Extending the proton hop scenario to hydrogen atom abstraction reactions yields a good agreement for the spin-state abundance ratios, but greatly overestimates the deuterium fractions of ammonia. However, one can find a reasonably good agreement with the observations with this model by increasing the cosmic-ray ionization rate over the commonly-adopted value of ∼10 −17 s −1 . We also find that the deuterium fractions of several other species, such as H 2 CO, H 2 O, and CH 3 , are sensitive to the adopted proton-donation reaction mechanism. Whether the full scrambling or proton hop mechanism dominates may be dependent on the reacting system, and new laboratory and theoretical studies for various reacting systems are needed to constrain chemical models.

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Modelling the molecular composition and nuclear-spin chemistryof collapsing pre-stellar sources★

Cited by 33 publications

References 50 publications

Nitrogen fractionation in external galaxies

Nitrogen fractionation in external galaxies

Efficient Methanol Production on the Dark Side of a Prestellar Core

Modeling deuterium chemistry in starless cores: full scrambling versus proton hop

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