Diffusion of molecules in the bulk of a low density amorphous ice from molecular dynamics simulations

Ghesquière, Pierre; Mineva, Tzonka; Talbi, Dahbia; Theulé, Patrice; Noble, J. A.; Chiavassa, Thierry

doi:10.1039/c5cp00558b

Cited by 59 publications

(65 citation statements)

References 66 publications

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“…Both the surface and the bulk are chemically active but the binding and diffusion barrier are set differently so that the reactivity in the bulk is much slower: the diffusion-to-binding energy ratio is 0.4 on the surface and 0.8 in the bulk. In addition, in the bulk, the binding energies, smaller than the one of water, are set to the one of water (except for H, H 2 , C, N and O) in agreement with the findings of Ghesquière et al (2015). Diffusion of species occurs through thermal hopping except for oxygen, which has been shown to diffuse through tunneling effects Minissale et al (2013).…”

Section: Model Descriptionsupporting

confidence: 59%

Protoplanetary discs: sensitivity of the chemical composition to various model parameters

Wakelam

Chapillon

Dutrey

et al. 2019

Monthly Notices of the Royal Astronomical Society

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Protoplanetary disks are challenging objects for astrochemical models due to strong density and temperature gradients and due to the UV photons 2D propagation. In this paper, we have studied the importance of several model parameters on the predicted column densities of observed species. We considered: 1) 2-phase (gas and homogeneous grains) or 3-phase (gas, surface, and bulk of grains) models, 2) several initial compositions, 3) grain growth and dust settling, and 4) several cosmic-ray ionization rates. Our main result is that dust settling is the most crucial parameter. Including this effect renders the computed column densities sensitive to all the other model parameters, except cosmic-ray ionization rate. In fact, we found almost no effect of this parameter for radii larger than 10 au (the minimum radius studied here) except for N 2 H + . We also compared all our models with all the column densities observed in the protoplanetary disk around DM Tau and were not able to reproduce all the observations despite the studied parameters. N 2 H + seems to be the most sensitive species. Its observation in protoplanetary disks at large radius could indicate enough N 2 in the gas-phase (inhibited by the 3-phase model, but boosted by the settling) and a low electron abundance (favored by low C and S elemental abundances).

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Section: Model Descriptionsupporting

confidence: 59%

Protoplanetary discs: sensitivity of the chemical composition to various model parameters

Wakelam

Chapillon

Dutrey

et al. 2019

Monthly Notices of the Royal Astronomical Society

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“…While this parameter is found to be of 30% for CO on water ice from Karssemeijer & Cuppen (2014), which is of the same order of the water-on-water diffusion derived experimentally (Collings et al 2003a), recent studies also point out to very small values for the CO diffusion barriers (Lauck et al 2015). However, the CO-on-CO diffusion has not been determined, but studies highlight the large differences between bulk and surface diffusion (Ghesquiere et al 2015). The diffusion parameter α sets the temperature at which CO molecules can rearrange and diffuse in the ices to form more dense ices.…”

Section: Diffusionmentioning

confidence: 92%

CO Depletion: A Microscopic Perspective

Cazaux

Martín-Doménech

Chen

et al. 2017

ApJ

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In regions where stars form, variations in density and temperature can cause gas to freeze out onto dust grains forming ice mantles, which influences the chemical composition of a cloud. The aim of this paper is to understand in detail the depletion (and desorption) of CO on (from) interstellar dust grains. Experimental simulations were performed under two different (astrophysically relevant) conditions. In parallel, Kinetic Monte Carlo simulations were used to mimic the experimental conditions. In our experiments, CO molecules accrete onto water ice at temperatures below 27 K, with a deposition rate that does not depend on the substrate temperature. During the warm-up phase, the desorption processes do exhibit subtle differences, indicating the presence of weakly bound CO molecules, therefore highlighting a low diffusion efficiency. IR measurements following the ice thickness during the TPD confirm that diffusion occurs at temperatures close to the desorption. Applied to astrophysical conditions, in a pre-stellar core, the binding energies of CO molecules, ranging between 300 and 850 K, depend on the conditions at which CO has been deposited. Because of this wide range of binding energies, the depletion of CO as a function of A V is much less important than initially thought. The weakly bound molecules, easily released into the gas phase through evaporation, change the balance between accretion and desorption, which result in a larger abundance of CO at high extinctions. In addition, weakly bound CO molecules are also more mobile, and this could increase the reactivity within interstellar ices.

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“…In this model, both the mantle and the surface are considered as chemically active. As opposed to previous models, we assume that the bulk diffusion is driven by the diffusion of water molecules in the ice, as suggested by molecular dynamics simulations and experiments (Ghesquière et al 2015). This paper is organized as follows.…”

Section: Introductionmentioning

confidence: 99%

Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model

Ruaud

Wakelam

Hersant

2016

Mon. Not. R. Astron. Soc.

266

342

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We present an extended version of the 2-phase gas-grain code NAUTILUS to the 3phase modelling of gas and grain chemistry of cold cores. In this model, both the mantle and the surface are considered as chemically active. We also take into account the competition among reaction, diffusion and evaporation. The model predictions are confronted to ice observations in the envelope of low-mass and massive young stellar objects as well as toward background stars. Modelled gas-phase abundances are compared to species observed toward TMC-1 (CP) and L134N dark clouds. We find that our model successfully reproduces the observed ice species. It is found that the reaction-diffusion competition strongly enhances reactions with barriers and more specifically reactions with H 2 , which is abundant on grains. This finding highlights the importance to have a good approach to determine the abundance of H 2 on grains. Consequently, it is found that the major N-bearing species on grains go from NH 3 to N 2 and HCN when the reaction-diffusion competition is accounted. In the gasphase and before few 10 5 yrs, we find that the 3-phase model does not have a strong impact on the observed species compared to the 2-phase model. After this time, the computed abundances dramatically decrease due to the strong accretion on dust, which is not counterbalanced by the desorption less efficient than in the 2-phase model. This strongly constrains the chemical-age of cold cores to be of the order of few 10 5 yrs.

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Diffusion of molecules in the bulk of a low density amorphous ice from molecular dynamics simulations

Cited by 59 publications

References 66 publications

Protoplanetary discs: sensitivity of the chemical composition to various model parameters

Protoplanetary discs: sensitivity of the chemical composition to various model parameters

CO Depletion: A Microscopic Perspective

Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model

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