The thermal reactivity of HCN and NH3 in interstellar ice analogues

Noble, J. A.; Theulé, Patrice; Borget, Fabien; Danger, Grégoire; Chomat, Marion; Duvernay, Fabrice; Mispelaer, F.; Chiavassa, Thierry

doi:10.1093/mnras/sts272

Cited by 60 publications

(59 citation statements)

References 71 publications

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“…It is only seen in the low ionisation case because N-bearing species formed via gas-phase chemistry in the inner disk (< 3 AU) are able to survive to 10 6 yrs. The presence or otherwise of this large peak is dependent upon the relative binding energies of HCN and CO 2 assumed in the model; recent measurements of thermal desorption of pure HCN ice do derive a higher binding energy than both CO 2 and NH 3 (e.g., Noble et al 2013), but in a mixed ice they may be more similar.…”

Section: C/o Ratiomentioning

confidence: 99%

Setting the volatile composition of (exo)planet-building material

Eistrup

Walsh

Dishoeck

2016

A&A

169

335

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Context. The atmospheres of extrasolar planets are thought to be built largely through accretion of pebbles and planetesimals. Such pebbles are also the building blocks of comets. The chemical composition of their volatiles are usually taken to be inherited from the ices in the collapsing cloud. However, chemistry in the protoplanetary disk midplane can modify the composition of ices and gases. Aims. To investigate if and how chemical evolution affects the abundances and distributions of key volatile species in the midplane of a protoplanetary disk in the 0.2-30 AU range. Methods. A disk model used in planet population synthesis models is adopted, providing temperature, density and ionisation rate at different radial distances in the disk midplane. A full chemical network including gas-phase, gas-grain interactions and grain-surface chemistry is used to evolve chemistry in time, for 1 Myr. Both molecular (inheritance from the parent cloud) and atomic (chemical reset) initial conditions are investigated. Results. Great diversity is observed in the relative abundance ratios of the main considered species: H 2 O, CO, CO 2 , CH 4 , O 2 , NH 3 and N 2 . The choice of ionisation level, the choice of initial abundances, as well as the extent of chemical reaction types included are all factors that affect the chemical evolution. The only exception is the inheritance scenario with a low ionisation level, which results in negligible changes compared with the initial abundances, regardless of whether grain-surface chemistry is included. The grain temperature plays an important role, especially in the critical 20-28 K region where atomic H no longer sticks long enough to the surface to react, but atomic O does. Above 28 K, efficient grain-surface production of CO 2 ice is seen, as well as O 2 gas and ice under certain conditions, at the expense of H 2 O and CO. H 2 O ice is produced on grain surfaces only below 28 K. For high ionisation levels at intermediate disk radii, CH 4 gas is destroyed and converted into CO and CO 2 (in contrast with previous models), and similarly NH 3 gas is converted into N 2 . At large radii around 30 AU, CH 4 ice is enhanced leading to a low gaseous CO abundance. As a result, the overall C/O ratios for gas and ice change significantly with radius and with model assumptions. For high ionisation levels, chemical processing becomes significant after a few times 10 5 yrs. Conclusions. Chemistry in the disk midplane needs to be considered in the determination of the volatile composition of planetesimals. In the inner <30 AU disk, interstellar ice abundances are preserved only if the ionisation level is low, or if these species are included in larger bodies within 10 5 yrs.

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Section: C/o Ratiomentioning

confidence: 99%

Setting the volatile composition of (exo)planet-building material

Eistrup

Walsh

Dishoeck

2016

A&A

169

335

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“…The adsorption energies of CO, N 2 and HCN, the abundance of which we present in the following section, are set to be 1150 K, 1000 K, and 3370 K, respectively. The value for HCN is adopted from the Temperature Programmed Desorption (TPD) experiment by Noble et al (2013). The dependence of molecular abundances on the adsorption energies of CO and N 2 are presented in Aikawa et al (2015).…”

Section: Chemical Modelmentioning

confidence: 99%

Multiple Paths of Deuterium Fractionation in Protoplanetary Disks

Aikawa

Furuya

Hincelin

et al. 2018

ApJ

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We investigate deuterium chemistry coupled with the nuclear spin-state chemistry of H 2 and H + 3 in protoplanetary disks. Multiple paths of deuterium fractionation are found; exchange reactions with D atoms, such as HCO + + D, are effective in addition to those with HD. In a disk model with grain sizes appropriate for dark clouds, the freeze-out of molecules is severe in the outer midplane, while the disk surface is shielded from UV radiation. Gaseous molecules, including DCO + , thus become abundant at the disk surface, which tends to make their column density distribution relatively flat. If the dust grains have grown to millimeter size, the freeze-out rate of neutral species is reduced, and the abundances of gaseous molecules, including DCO + and N 2 D + , are enhanced in the cold midplane. Turbulent diffusion transports D atoms and radicals at the disk surface to the midplane, and stable ice species in the midplane to the disk surface. The effects of turbulence on chemistry are thus multifold; while DCO + and N 2 D + abundances increase or decrease depending on the regions, HCN and DCN in the gas and ice are much reduced at the innermost radii, compared with the model without turbulence. When cosmic rays penetrate the disk, the ortho-to-para ratio (OPR) of H 2 is found to be thermal in the disk, except in the cold ( 10 K) midplane. We also analyze the OPR of H + 3 and H 2 D + , as well as the main reactions of H 2 D + , DCO + , and N 2 D + to analytically derive their abundances in the cold midplane. Subject headings: astrochemistry -star-formation -protoplanetary disksRecent observations of deuterated species in disks, however, challenge the above scenario. Qi et al. (2015) observed DCO + in HD 163296 with a higher spatial resolution, and found that the inner edge of the DCO + ring is at 40 AU, which is much closer to the central star than derived by Mathews et al. (2013). Huang et al. (2017) observed six protoplanetary disks, spatially resolving DCO + , H 13 CO + , H 13 CN, and DCN in most of them. While the DCO + emission tends to be spatially more extended than the DCN emission, the relative distributions of DCO + and DCN vary among the disks. While the DCN emission is more

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“…It has been recently recognized that the molecules in the ice lattice may change their location as a result of thermal diffusion (Öberg et al 2009c;Fayolle et al 2011a). Meanwhile, reactions are taking place directly in ice lattice, as evidenced by the formation of different molecules in ices in the presence of radicals or other reactive species (e.g., Gerakines et al 1996;Linnartz et al 2011;Öberg et al 2011b;Noble et al 2013). This was seen in a recent astrochemical model by Garrod (2013a) that includes reactions among molecules in subsurface ice.…”

Section: Sublayer Approach In Ice Modelingmentioning

confidence: 99%

The Effect of Selective Desorption Mechanisms During Interstellar Ice Formation

Kalvāns

2015

ApJ

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Major components of ices on interstellar grains in molecular clouds-water and carbon oxides-occur at various optical depths. This implies that selective desorption mechanisms are at work. An astrochemical model of a contracting low-mass molecular cloud core is presented. Ice was treated as consisting of the surface and three subsurface layers (i.e., sublayers). Photodesorption, reactive desorption, and indirect reactive desorption were investigated. The latter manifests itself through desorption from H+H reaction on grains. Desorption of shallow subsurface species was also included. Modeling results suggest the existence of a "photon-dominated ice" during the early phases of core contraction. Subsurface ice is chemically processed by interstellar photons, which produces complex organic molecules (COMs). Desorption from the subsurface layer results in high COM gas-phase abundances at A V = 2.4-10 mag. This may contribute toward an explanation for COM observations in dark cores. It was found that photodesorption mostly governs the onset of ice accumulation onto grains. Reaction-specific reactive desorption is efficient for small molecules that form via highly exothermic atom-addition reactions. Higher reactive desorption efficiency results in lower gas-phase abundances of COMs. Indirect reactive desorption allows for closely reproducing the observed H 2 O:CO:CO 2 ratio toward a number of background stars. Presumably, this can be done by any mechanism whose efficiency fits with the sequence. After the freeze-out has ended, the three sublayers represent chemically distinct parts of the mantle. The likely A V threshold for the appearance of CO ice is 8-10.5 mag. The lower value is supported by observations.

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The thermal reactivity of HCN and NH3 in interstellar ice analogues

Cited by 60 publications

References 71 publications

Setting the volatile composition of (exo)planet-building material

Setting the volatile composition of (exo)planet-building material

Multiple Paths of Deuterium Fractionation in Protoplanetary Disks

The Effect of Selective Desorption Mechanisms During Interstellar Ice Formation

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