Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation

Hily-Blant, Pierre; Souza, V. Magalhaes de; Kastner, Joel H.; Forveille, T.

doi:10.1051/0004-6361/201936750

Cited by 32 publications

(31 citation statements)

References 30 publications

(45 reference statements)

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“…Still, for the one disk with spatially resolved emission (V4046 Sgr), an increasing HCN/HC 15 N ratio moving outwards in the disk suggests that in situ photodissociation-driven fractionation is important. A similar result is seen in the TW Hya disk by Hily-Blant et al (2019). Measurements of D and 15 N fractionation in HCN towards Class 0/I disks are still needed to evaluate whether younger disks demonstrate similar fractionation patterns, or if there is a distinct Class II fractionation chemistry.…”

Section: Introductionsupporting

confidence: 63%

“…We note that 13 C fractionation is not expected to be important for HCN in protostellar or disk environments (e.g. Roberts et al 2002;Hily-Blant et al 2019). For J1609, J1612, and J1614, C 18 O column densities are found from 13 CO assuming the local ISM 16 O/ 18 O ratio of 560±25 (Wilson & Rood 1994) and the same 12 C/ 13 C ratio as before.…”

Section: Hcn C 2 H and C 18 O Correlations Across Evolutionary Stagessupporting

confidence: 49%

“…We note that a photochemical fractionation scenario requires gas-phase HCN formation via N 2 dissociation, implying that HCN observed in the protostellar cores is at least in part formed in situ in the gas. Photochemical fractionation is also thought to be important in Class II disks based on radially resolved HCN/HC 15 N measurements (Guzmán et al 2017;Hily-Blant et al 2019). The agreement between HCN/HC 15 N ratios in the most evolved protostellar sources and the Class II sources supports that the same mechanism is at play.…”

Section: Evolution Of Hcn Fractionationmentioning

confidence: 92%

“…As mentioned in Section 4.3, 13 C fractionation is not expected to be important in protostellar or disk environments (e.g. Roberts et al 2002;Hily-Blant et al 2019). HCN/DCN and HCN/HC 15 N ratios are then found from the ratio of column densities determined in Section 3.1.7.…”

Section: Class 0/i Sourcesmentioning

confidence: 99%

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An Evolutionary Study of Volatile Chemistry in Protoplanetary Disks

Bergner

Öberg

Bergin

et al. 2020

ApJ

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The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The volatile chemistry in the disk is expected to evolve over time, though this evolution is poorly constrained observationally. We present ALMA observations of C 18 O, C 2 H, and the isotopologues H 13 CN, HC 15 N, and DCN towards five Class 0/I disk candidates. Combined with a sample of fourteen Class II disks presented in Bergner et al. (2019b), this data set offers a view of volatile chemical evolution over the disk lifetime. Our estimates of C 18 O abundances are consistent with a rapid depletion of CO in the first ∼0.5-1 Myr of the disk lifetime. We do not see evidence that C 2 H and HCN formation are enhanced by CO depletion, possibly because the gas is already quite under-abundant in CO. Further CO depletion may actually hinder their production by limiting the gas-phase carbon supply. The embedded sources show several chemical differences compared to the Class II stage, which seem to arise from shielding of radiation by the envelope (impacting C 2 H formation and HC 15 N fractionation) and sublimation of ices from infalling material (impacting HCN and C 18 O abundances). Such chemical differences between Class 0/I and Class II sources may affect the volatile composition of planet-forming material at different stages in the disk lifetime.

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Section: Introductionsupporting

confidence: 63%

Section: Hcn C 2 H and C 18 O Correlations Across Evolutionary Stagessupporting

confidence: 49%

Section: Evolution Of Hcn Fractionationmentioning

confidence: 92%

Section: Class 0/i Sourcesmentioning

confidence: 99%

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An Evolutionary Study of Volatile Chemistry in Protoplanetary Disks

Bergner

Öberg

Bergin

et al. 2020

ApJ

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“…One is isotope-selective photodissociation of N 2 (Heays et al 2014(Heays et al , 2017, which is expected to be effective in environments where UV radiation has a significant impact on chemistry, such as the upper layers of protoplanetary disks. A recent measurement of the 14 N: 15 N ratio in the CN and HCN molecules in the circumstellar disk of the 8 Myr-old T Tauri object TW Hya demonstrated that the HCN:HC 15 N ratio increases with distance from the central star (Hily-Blant et al 2019). This observational result lends support to the hypothesis that selective photodissociation of N 2 (Heays et al 2014) could be the leading fractionation process in such disks.…”

Section: Introductionmentioning

confidence: 58%

Depletion and fractionation of nitrogen in collapsing cores

Hily-Blant

Forêts

Fauré

et al. 2020

A&A

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Measurements of the nitrogen isotopic ratio in Solar System comets show a constant value, ≈140, which is three times lower than the protosolar ratio, a highly significant difference that remains unexplained. Observations of static starless cores at early stages of collapse confirm the theoretical expectation that nitrogen fractionation in interstellar conditions is marginal for most species. Yet, observed isotopic ratios in N2H+ are at variance with model predictions. These gaps in our understanding of how the isotopic reservoirs of nitrogen evolve, from interstellar clouds to comets, and, more generally, to protosolar nebulae, may have their origin in missing processes or misconceptions in the chemistry of interstellar nitrogen. So far, theoretical studies of nitrogen fractionation in starless cores have addressed the quasi-static phase of their evolution such that the effect of dynamical collapse on the isotopic ratio is not known. In this paper, we investigate the fractionation of 14N and 15N during the gravitational collapse of a pre-stellar core through gas-phase and grain adsorption and desorption reactions. The initial chemical conditions, which are obtained in steady state after typically a few Myr, show low degrees of fractionation in the gas phase, in agreement with earlier studies. However, during collapse, the differential rate of adsorption of 14N- and 15N-containing species onto grains results in enhanced 15N:14N ratios, in better agreement with the observations. Furthermore, we find differences in the behavior, with increasing density, of the isotopic ratio in different species. We find that the collapse must take place on approximately one free-fall timescale, based on the CO abundance profile in L183. Various chemical effects that bring models into better agreement with observations are considered. Thus, the observed values of 14N2H+:N15NH+ and 14N2H+:15NNH+ could be explained by different temperature dependences of the rates of dissociative recombination of these species. We also study the impact of the isotopic sensitivity of the charge-exchange reaction of N2 with He+ on the fractionation of ammonia and its singly deuterated analog and find significant depletion in the 15N variants. However, these chemical processes require further experimental and theoretical investigations, especially at low temperature. These new findings, such as the depletion-driven fractionation, may also be relevant to the dense, UV-shielded regions of protoplanetary disks.

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On the Origin and Evolution of the Material in 67P/Churyumov-Gerasimenko

et al. 2020

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Primitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth-and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semivolatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further

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Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation

Cited by 32 publications

References 30 publications

An Evolutionary Study of Volatile Chemistry in Protoplanetary Disks

An Evolutionary Study of Volatile Chemistry in Protoplanetary Disks

Depletion and fractionation of nitrogen in collapsing cores

On the Origin and Evolution of the Material in 67P/Churyumov-Gerasimenko

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