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Origine du gaz dans les disques de débris
Le gaz détecté dans les disques de débris est-il primordial, hérité du disque protoplanétaire, ou secondaire, libéré par les collisions de planétésimaux ?
How this review was searched
271 papers found, the 80 closest to the question screened on their abstract, 48 judged relevant, 30 included. Published from 2006 to 2026. 19 founding papers, 7 from the last three years, 0 reviews.
Published in a journal or proceedings: 25 · preprints: 5 · deposits nobody reviewed: 0. The conclusions of 12 open full texts were read too.
18 of the 18 findings are backed by a sentence found word for word in a paper. So are 8 of the 8 points of agreement, disagreement and gaps.
An exploratory synthesis, not a systematic review: the papers are read from their abstracts and the conclusions of their open full texts, and papers that OpenAlex and arXiv do not index, or that our searches word differently, can be missing.
Searches and sources
Inclusion: Include observational, theoretical, and modeling papers that assess gas origin in debris disks using its composition, abundance, distribution, dynamics, age, or replenishment mechanisms.
Key concepts: Gas in debris disks, Primordial origin, Secondary origin, Observational and theoretical diagnostics, Astrophysical settings
Queries:
- gas in debris disks primordial secondary
- remnant protoplanetary gas debris disks
- planetesimal collisions secondary gas release
- exocomet volatile outgassing debris disks
- gas line emission chemical modeling debris disks
- gas evolution β Pictoris 49 Ceti
- OpenAlex, by relevance: 115
- OpenAlex, last three years: 68
- arXiv: 24
- OpenAlex, reviews: 15
- OpenAlex, most cited: 25
- Cited by several of the closest papers: 48 (1 abstract found on Semantic Scholar or arXiv)
- Recent papers citing the founding ones: 21
A paper found by several searches is counted in each.
In short
Ces travaux ne permettent pas d’attribuer une origine unique au gaz de tous les disques de débris : une production secondaire est étayée dans plusieurs systèmes, tandis que les disques les plus riches en CO restent compatibles avec plusieurs explications. [7][8][16][23][26] La présence de CO peu protégé contre la photodissociation et associé à une ceinture de poussière soutient un renouvellement par des corps glacés, sans établir que les collisions constituent partout le seul mécanisme de libération. [8][16][24][28] Pour les grands réservoirs de CO, la protection par le carbone neutre permet aux modèles secondaires de reproduire plusieurs observations, mais les faibles quantités de carbone mesurées mettent en difficulté leurs versions stationnaires actuelles. [12][15][18][19][23][26] Des simulations publiées rendent plausible la survie d’un gaz primordial pendant plusieurs dizaines de millions d’années si les petits grains sont appauvris, mais cette possibilité dépend de conditions encore mal établies et ne constitue pas une identification observationnelle de son origine. [21][22][29] Les indices les plus discriminants semblent donc devoir combiner CO, carbone atomique, autres molécules et structure spatiale ; les prépublications et les versions publiées correspondantes ne doivent pas être comptées comme des confirmations indépendantes. [13][14][17][20][21][23][25][26][27][30]
This summary rests on the findings below, each with the sentences that support it.
Approaches compared
| Approach | Papers | Strengths | Limits |
|---|---|---|---|
| Libération secondaire et composition du gaz : vaporisation, photodésorption et évaporation | [1][2][4][5][24][28] | Cette approche relie les propriétés du gaz aux processus physiques de libération et d’évolution plutôt qu’à une simple opposition entre gaz ancien et gaz nouveau. | Les mécanismes ne sont pas tous distingués observationnellement, et l’interprétation détaillée des volatils de β Pictoris proposée dans [24] repose ici uniquement sur une prépublication non évaluée. |
| Indices spatiaux d’un renouvellement exocométaire dans des systèmes individuels | [7][8][16] | La combinaison de la localisation du gaz et de sa vulnérabilité à la photodissociation apporte des arguments physiques pour une production actuelle. | Ces résultats concernent des systèmes particuliers et ne démontrent pas que les grands réservoirs de CO des autres disques possèdent la même origine. |
| Statistiques des populations et production liée à l’évolution collisionnelle | [3][9][10][11] | Les analyses de population évaluent si une explication peut rendre compte de tendances générales plutôt que d’un seul disque. | La fréquence des détections dépend de la sélection et de l’observabilité, tandis que le modèle de poussière [3] ne constitue pas en lui-même un test de l’origine du gaz. |
| Disques secondaires protégés : accumulation de CO et devenir du carbone | [12][13][14][15][18][19] | Cette famille de modèles explique comment un gaz secondaire peut atteindre des masses autrefois considérées comme un argument en faveur d’un reliquat primordial. | L’accord sur le CO ne garantit pas l’accord sur le carbone, et le recyclage proposé pour HD 32297 reste une explication issue d’une étude de ce système plutôt qu’un mécanisme démontré dans toute la population. |
| Survie prolongée d’un réservoir primordial appauvri en petits grains | [17][21][22][29][30] | Cette approche teste explicitement la possibilité physique de conserver du gaz primordial au-delà des durées de dispersion habituellement invoquées. | Elle dépend notamment d’un appauvrissement préalable en petits grains dont l’origine reste ouverte, et son extension thermochimique la plus récente n’a pas encore été évaluée par les pairs. |
| Diagnostics combinés : répartition gaz-poussière, rapports C/CO et autres molécules | [6][20][23][25][26][27] | Les diagnostics combinés testent simultanément la quantité, la composition et la distribution du gaz, réduisant les ambiguïtés du CO seul. | Les faibles rapports C/CO peuvent signaler une physique secondaire manquante plutôt qu’un gaz primordial, et le diagnostic par l’eau froide reste une proposition théorique non validée par une détection. |
Libération secondaire et composition du gaz : vaporisation, photodésorption et évaporation
Ces travaux examinent comment les solides alimentent le gaz et comment sa composition et son état physique évoluent ensuite. Ils proposent plusieurs voies secondaires, mais montrent aussi que les abondances observées dépendent du transport, du rayonnement et des échanges avec les grains, et ne reflètent donc pas simplement la composition initiale des planétésimaux.
- Autour de β Pictoris, la stabilité du gaz pose un problème dynamique, et la vaporisation collisionnelle constitue une source quantitativement modélisée mais incertaine. [1][2]
“Previous observations of β Pic concluded that the disk gas has roughly solar abundances of elements 1 , but this poses a problem because such gas should be rapidly blown away from the star, contrary to observations of a stable gas disk in Keplerian rotation 1, 2 .” [1]
1 more supporting sentence
“For the dust distribution and collision model considered, the vaporization of dust leads to the gas production rates of the order between 0.5 × 10 12 and 2 × 10 13 g s -1 for the grains with the collisional properties close to those of silicate and ice, respectively.” [2]
- Les modèles de chauffage et de transport montrent que la température et l’enrichissement en carbone et oxygène ne suffisent pas, à eux seuls, à identifier le mode de production du gaz. [4][5]
“The gas can be heated to a temperature that is warmer than that of the dust and may in some cases reach temperature for thermal escape.” [4]
1 more supporting sentence
“We study the origin of this enrichment under two opposing hypotheses: preferential production , where the gas is produced with the observed unusual abundance (as may happen if gas is produced by photodesorption from C/O-rich icy grains), and preferential depletion , where the gas evolves to the observed state from an original solar abundance (if outgassing occurs under high-speed collisions) under a number of dynamical processes.” [5]
- Une prépublication sur β Pictoris privilégie la photodésorption de volatils, tandis qu’une étude observationnelle publiée de HD 172555 identifie un gaz interne provenant de l’évaporation de matière proche de l’étoile, ce qui élargit les mécanismes secondaires au-delà des seules collisions. [24][28]
“In the debris disk phase, these volatile are preferentially out-gassed from the dust grains, likely via photo-desorption.” [24]
1 more supporting sentence
“Our JWST Mid-InfraRed Instrument/Medium Resolution Spectroscopy (MIRI MRS) observations now reveal emission from an inner gaseous disk (<0.5 au) that arises from the evaporation of close-in material.” [28]
Indices spatiaux d’un renouvellement exocométaire dans des systèmes individuels
Les observations résolues recherchent un lien entre le gaz et les régions où se trouvent les solides susceptibles de le produire. Les cas de β Pictoris, HD 181327 et TWA 7 soutiennent une origine secondaire dans des environnements stellaires différents, avec des arguments particulièrement directs lorsque le CO ne peut survivre longtemps sans renouvellement.
- Dans β Pictoris, un amas de CO marque une région de collisions accrues, mais les observations laissent ouvertes deux interprétations de cette concentration. [7]
“This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet or from the remnants of a collision of Mars-mass planets.” [7]
- Dans HD 181327, la densité et la localisation du CO favorisent sa libération secondaire par des planétésimaux glacés de l’anneau. [8]
“The CO densities and location suggest a secondary origin, i.e. released from icy planetesimals in the ring.” [8]
- Dans TWA 7, une étude publiée conclut que le CO insuffisamment protégé doit être continuellement renouvelé par des exocomètes, étendant cet argument à une étoile de type M. [16]
“The detected CO is not well shielded against photodissociation, and must thus be continuously replenished by gas release from exocomets within the belt.” [16]
Statistiques des populations et production liée à l’évolution collisionnelle
Ces études comparent les populations de disques et modélisent la production de poussière ou de gaz à partir de ceintures de planétésimaux. Elles montrent que le gaz est fréquent parmi les disques poussiéreux de jeunes étoiles A, tout en soulignant que les tendances de détection ne déterminent pas directement son origine.
- Un modèle stationnaire collisionnel reproduit les statistiques de poussière autour des étoiles A, fournissant un cadre pour la production secondaire sans établir directement l’origine du gaz. [3]
“This paper confronts a simple analytical model for the steady state evolution of debris disks due to collisions with Spitzer observations of dust around main-sequence A stars.” [3]
- Deux enquêtes trouvent une incidence importante du gaz autour de jeunes étoiles A, mais leurs échantillons et critères diffèrent et ne justifient pas de fusionner leurs taux. [9][10]
“A census of 27 A-type debris hosts within 125 pc now shows 7/16 detections of carbon-bearing gas within the 5–50 Myr epoch, with no detections in 11 older systems.” [9]
1 more supporting sentence
“This high incidence implies that the presence of CO gas in the bright debris disks around young A-type stars is more likely the rule than the exception.” [10]
- Un modèle secondaire de CO, C et O explique qualitativement les observations disponibles en 2017, mais ses auteurs réservent déjà une origine probablement primordiale à quelques exceptions. [11]
“This model provides a good qualitative explanation of all current observations, with a few exceptional systems that likely have primordial gas.” [11]
Disques secondaires protégés : accumulation de CO et devenir du carbone
Ces travaux proposent que le carbone neutre issu de la destruction du CO protège ensuite le CO restant, permettant un gaz secondaire très abondant. Des applications à plusieurs systèmes et une synthèse de population soutiennent cette possibilité, mais HD 32297 révèle un besoin supplémentaire d’élimination ou de recyclage du carbone.
- La protection par le carbone neutre permet de réinterpréter certains disques riches en CO comme entièrement secondaires, et un modèle appliqué à 49 Cet et HD 32297 reproduit leur contenu en CO. [12][15]
“Here we show that both the dust and gas in such systems could instead be secondary, with the high CO content caused by accumulation of neutral carbon (C0) that shields CO from photodissociating; i.e., these could be “shielded secondary discs”.” [12]
1 more supporting sentence
“Adapting a recent secondary gas disk model that considers both shielding by carbon atoms and self-shielding of CO, we can explain the observed CO level in both systems.” [15]
- La reconstruction spatiale et la modélisation thermochimique de HD 131835, HD 138813 et HD 156623 favorisent une origine secondaire, mais la publication et sa prépublication représentent une seule étude. [13][14]
“The derived disk parameters are used as input for thermochemical models to test both primordial and cometary scenarios for the origin of the gas.” [13]
1 more supporting sentence
“We favor a secondary origin for the gas in these disks and find that the CO gas masses ($\sim 3\times10^{-3}$ M$_{\oplus}$) require production rates ($\sim 5\times 10^{-7}$ M$_{\oplus}$~yr$^{-1}$) similar to those estimated for the bona-fide gas rich debris disk $β$ Pic.” [14]
- Une synthèse de population obtient un bon accord avec une viscosité élevée, tandis que l’étude de HD 32297 exige dans son scénario préféré une élimination rapide du carbone et une production de CO très supérieure au broyage de poussière. [18][19]
“We find a good match with a high viscosity (α ∼ 0.1), indicating that gas is lost on time-scales ∼1–10 Myr.” [18]
1 more supporting sentence
“We find that the most likely scenario to explain both C0 and CO observations is one where the carbon gas is rapidly removed on a timescale of order a thousand years and the system maintains a very high CO production rate of ∼15 M ⊕ Myr−1, much higher than the rate of dust grind-down.” [19]
Survie prolongée d’un réservoir primordial appauvri en petits grains
Ces simulations examinent si la diminution du chauffage photoélectrique permet au gaz protoplanétaire de survivre jusqu’aux âges des disques de débris riches en gaz. Les travaux publiés soutiennent cette possibilité sous conditions, et une prépublication de 2026 ajoute un test thermochimique sans résoudre toutes les contraintes spatiales.
- La prépublication de 2020 et sa version publiée de 2021 relient l’appauvrissement en petits grains à la suppression de la photoévaporation entraînée par l’ultraviolet lointain, mais ne constituent pas deux validations indépendantes. [17][21]
“We find that the grain depletion reduces photoelectric heating, so that FUV photoevaporation is not excited.” [17]
1 more supporting sentence
“We find that grain depletion reduces photoelectric heating so that far-ultraviolet photoevaporation is not excited.” [21]
- Les modèles publiés identifient les étoiles de masse intermédiaire comme particulièrement favorables à la longévité du gaz, et les simulations de 2025 permettent aussi une survie au-delà de 10 millions d’années pour d’autres masses stellaires si le disque initial est massif et la contrainte turbulente faible. [22][29]
“In this picture, it is relatively difficult for stars outside the range of spectral types from A through early B to have such long-lived gas disks.” [22]
1 more supporting sentence
“Our results show that gas in these disks can survive beyond 10 Myr regardless of the stellar mass, provided they are initially massive ( M disk ≈ 0.1 M * ) with relatively weak turbulent stress ( α ≪ 10 −2 ).” [29]
- La seule prépublication thermochimique de 2026 trouve une origine primordiale chimiquement viable, mais conserve une émission de carbone neutre trop étendue et une dépendance des prédictions de HCO+ à l’ionisation par les rayons cosmiques. [30]
“The standard CR model overproduces HCO+, whereas the weak CR model brings its predicted luminosity within current observational limits.” [30]
Diagnostics combinés : répartition gaz-poussière, rapports C/CO et autres molécules
Ces études cherchent à départager les scénarios au moyen de la géométrie et de plusieurs espèces plutôt que du seul CO. Elles apportent des contraintes défavorables aux modèles secondaires stationnaires actuels, sans transformer automatiquement ces difficultés en preuve d’une origine primordiale, et proposent de nouveaux diagnostics.
- Pour HD 21997, une étude propose un disque hybride parce qu’une production secondaire nécessiterait des taux jugés déraisonnables et expliquerait mal la séparation spatiale entre gaz et poussière. [6]
“A secondary origin, which involves gas production from colliding or active planetesimals, would require unreasonably high gas production rates and would not explain why the gas and dust are not colocated.” [6]
- L’étude ALMA de 14 disques et l’étude HST de deux disques trouvent trop peu de carbone neutre par rapport aux prédictions secondaires actuelles, la prépublication HST et l’article publié constituant ici le même résultat et non deux confirmations. [23][25][26]
“We find that current state-of-the-art models of secondary gas production overpredict the C0 content of debris disk gas.” [23]
2 more supporting sentences
“We find that the current steady-state secondary release model cannot simultaneously reproduce the CO and CI HST-derived column densities, as it predicts larger CI/CO ratios than observed.” [25]
“We find that the current steady-state secondary release model cannot simultaneously reproduce the CO and CI HST-derived column densities, as it predicts larger CI/CO ratios than observed.” [26]
- Les non-détections moléculaires disponibles ne tranchent pas nécessairement entre les scénarios, tandis qu’une seule étude théorique propose la vapeur d’eau froide comme futur discriminant encore inaccessible aux moyens observationnels considérés. [20][27]
“The observed low line ratios of these four species to CO in the debris disc sample, therefore, do not necessarily preclude the hybrid scenario (Fig. 3).” [20] · full text
1 more supporting sentence
“We show that different hypotheses lead to different masses of water vapor.” [27]
Where the papers agree
- Plusieurs observations publiées soutiennent une alimentation secondaire dans certains disques, sans autoriser une généralisation à tous les réservoirs gazeux. [8][16]
- Les études récentes du carbone convergent sur une surestimation de C neutre par les modèles secondaires actuels, sans conclure que toute origine secondaire est exclue. [23][26]
2 supporting sentences
“This does not rule out a secondary origin, but might indicate that the models require an additional C removal process.” [23]
“Our direct UV measurement of low CI/CO ratios agrees with results derived from recent Atacama Large Millimetre/submillimetre Array findings and may point to vertical layering of CI, additional CI removal, CO shielding processes, or different gas origin scenarios.” [26]
Where they disagree
- L’attribution des grands réservoirs de CO oppose une lecture historiquement primordiale ou hybride à une réinterprétation secondaire protégée, explicitement proposée pour la même catégorie de disques riches en CO. [10][12]
- Les grands contenus en CO sont interprétés comme pouvant signaler du gaz primordial dans des disques hybrides. [10]
1 supporting sentence
“In the particularly CO-rich debris systems, the gas may have a primordial origin, which is a characteristic of a hybrid disk.” [10]
- Ces grands contenus en CO peuvent aussi résulter d’une production secondaire si le carbone neutre protège suffisamment les molécules. [12]
1 supporting sentence
“Here we show that both the dust and gas in such systems could instead be secondary, with the high CO content caused by accumulation of neutral carbon (C0) that shields CO from photodissociating; i.e., these could be “shielded secondary discs”.” [12]
- Les grands contenus en CO sont interprétés comme pouvant signaler du gaz primordial dans des disques hybrides. [10]
What they leave open
- Le devenir du carbone reste insuffisamment établi : élimination additionnelle et production transitoire sont proposées pour expliquer les écarts aux modèles secondaires. [23]
1 supporting sentence
“This does not rule out a secondary origin, but might indicate that the models require an additional C removal process.” [23]
- Le scénario primordial laisse ouverte la manière dont certains disques deviennent suffisamment pauvres en petits grains pour ralentir la photoévaporation. [22]
1 supporting sentence
“A key issue this work leaves open is how some disks become depleted in small dust so that FUV photoevaporation slows.” [22]
- Le diagnostic proposé par la vapeur d’eau froide nécessite encore des observations et une modélisation plus détaillée. [27]
1 supporting sentence
“We have chosen to adopt the simplified approach in this work as the detection of water vapor in debris disks remains to be performed.” [27] · full text
- La prépublication thermochimique la plus récente demande encore des modèles multidimensionnels cohérents, des observations résolues du carbone et des recherches plus profondes de HCO+ pour tester le scénario primordial. [30]
1 supporting sentence
“Further testing of the primordial-origin scenario will require multidimensional, self-consistent modelling, spatially resolved CI observations, and deeper searches for HCO+.” [30]
Often cited, not read
Several of the papers found cite these works, but no abstract of them could be found: they are not in the synthesis. Read them if they are not already known to you.
- Inhibition of giant-planet formation by rapid gas depletion around young stars
- Protostars and Planets IV
- Five steps in the evolution from protoplanetary to debris disk
References
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[1] Stabilization of the disk around β Pictoris by extremely carbon-rich gas
Founding mesure des abondances de carbone · observational study
Why it is here: La composition très riche en carbone explique la stabilité dynamique du gaz secondaire de β Pic.
Abstract
The edge-on disk surrounding the nearby young star β Pictoris is the archetype of the “debris disks”, which are composed of dust and gas produced by collisions and evaporation of planetesimals, analogues of Solar System comets and asteroids. These disks provide a window on the formation and early evolution of terrestrial planets. Previous observations of β Pic concluded that the disk gas has roughly solar abundances of elements 1 , but this poses a problem because such gas should be rapidly blown away from the star, contrary to observations of a stable gas disk in Keplerian rotation 1, 2 . Here we report the detection of singly and doubly ionized carbon (C II ,
-
[2] Collisional Vaporization of Dust and Production of Gas in the β Pictoris Dust Disk
Founding modèle de vaporisation collisionnelle · simulation or model
Why it is here: L’étude teste si les collisions de poussières peuvent produire le gaz observé et en maintenir les abondances.
Abstract
The need for replenishment of the stable gas observed in the β Pictoris system raises a question about the origin of the gas. Correlations between the gas and the dust distribution suggest that the source is related to the dust. Spectroscopic observations imply that the gas is rotating at Keplerian velocity: this includes also the species that, in absence of braking, would be accelerated away from the star by the radiation pressure. We examine the possibility that the gas originates from collisional vaporization of the dust in the disk and the consequences for the gas velocity distribution and the line profiles of spectral features generated by the gas. A simple model of dust distribution and a model of individual dust-dust collision are used to calculate the gas production rate in the disk. Comparing with the gas column densities derived from observations, the escape times of the atoms from the disk are estimated. For the dust distribution and collision model considered, the vaporization of dust leads to the gas production rates of the order between 0.5 × 10 12 and 2 × 10 13 g s -1 for the grains with the collisional properties close to those of silicate and ice, respectively. We point out the uncertainties in the collision models. We also found that, for the lines of sight bypassing the star, velocity distributions of gas particles released from orbiting bodies can show a peak at Keplerian velocity even in the absence of braking, despite large acceleration by radiation pressure.
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[3] Steady State Evolution of Debris Disks around A Stars
Founding modèle collisionnel des poussières · simulation or model
Why it is here: Le modèle identifie HD 172555 comme poussiéreux anormalement brillant, compatible avec un événement collisionnel susceptible de produire du gaz.
Abstract
This paper confronts a simple analytical model for the steady state evolution of debris disks due to collisions with Spitzer observations of dust around main-sequence A stars. It is assumed that every star has a planetesimal belt, the initial mass and radius of which are drawn from distributions. In the model disk mass is constant until the largest planetesimals reach collisional equilibrium, whereupon mass falls ∝ t . We find that the detection statistics and trends seen at 24 and 70 μm can be fitted well by the model. While there is no need to invoke stochastic evolution or delayed stirring to explain the statistics, a moderate rate of stochastic events is not ruled out. Potentially anomalous systems are identified by a high dust luminosity compared with the maximum permissible in the model (HD 3003, HD 38678, HD 115892, HD 172555); their planetesimals may have unusual properties (high strength or low eccentricity), or this dust could be transient. The overall success of our model, which assumes planetesimals in all belts have the same strength, eccentricity, and maximum size, suggests the outcome of planet formation is reasonably uniform. The distribution of planetesimal belt radii, once corrected for detection bias, follows N ( r ) ∝ r -0.8±0.3 for 3-120 AU. Since belt boundaries may be attributed to unseen planets, this provides a unique constraint on A star planetary systems. It is also shown that P-R drag may sculpt the inner edges of A star disks close to the Spitzer detection threshold (HD 2262, HD 19356, HD 106591, HD 115892). This model can be readily applied to the interpretation of future surveys, and predictions for the upcoming SCUBA-2 survey include that 17% of A star disks should be detectable at 850 μm.
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[4] GAS EMISSION FROM DEBRIS DISKS AROUND A AND F STARS
Founding modèle d’émission et d’ionisation · simulation or model
Why it is here: Le modèle relie composition, état physique et émission du gaz, utiles pour interpréter son origine.
Abstract
Gas has been detected in a number of debris disk systems. This gas may have arisen from grain sublimation or grain photodesorption. It interacts with the surrounding dust grains through a number of charge and heat exchanges. Studying the chemical composition and physical state of this gas can therefore reveal much about the dust component in these debris disks. We have produced a new code, ontario , to address gas emission from dusty gas-poor disks around A–F stars. This code computes the gas ionization and thermal balance self-consistently, with particular care taken of heating/cooling mechanisms. Line emission spectra are then produced for each species (up to zinc) by statistical equilibrium calculations of the atomic/ionic energy levels. For parameters that resemble the observed β Pictoris gas disk, we find that the gas is primarily heated by photoelectric emission from dust grains, and primarily cooled through the C ii 157.7 μm line emission. The gas can be heated to a temperature that is warmer than that of the dust and may in some cases reach temperature for thermal escape. The dominant cooling line, C ii 157.7 μm, should be detectable by Herschel in these disks, while the O i 63.2 μm line will be too faint. We also study the dependence of the cooling line fluxes on a variety of disk parameters, in light of the much improved sensitivity to thermal line emission in the mid/far-infrared and at submillimeter wavelengths provided by, in particular, Herschel , SOFIA , and ALMA . These new instruments will yield much new information about dusty debris disks.
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[5] ON THE UNUSUAL GAS COMPOSITION IN THE β PICTORIS DEBRIS DISK
Founding modèles d’enrichissement chimique · simulation or model
Why it is here: L’étude compare des mécanismes secondaires capables d’expliquer la composition atypique du gaz de β Pictoris.
Abstract
The metallic gas associated with the β Pic debris disk is not believed to be primordial, but arises from the destruction of dust grains. Recent observations have shown that carbon and oxygen in this gas are exceptionally overabundant compared to other elements, by some 400 times. We study the origin of this enrichment under two opposing hypotheses: preferential production , where the gas is produced with the observed unusual abundance (as may happen if gas is produced by photodesorption from C/O-rich icy grains), and preferential depletion , where the gas evolves to the observed state from an original solar abundance (if outgassing occurs under high-speed collisions) under a number of dynamical processes. We include in our study the following processes: radiative blowout of metallic elements, dynamical coupling between different species, and viscous accretion onto the star. We find that, if gas viscosity is sufficiently low (the conventional α parameter ≲ 10 −3 ), differential blowout dominates. While gas accumulates gradually in the disks, metallic elements subject to strong radiation forces, such as Na and Fe, deplete more quickly than C and O, naturally leading to the observed overabundance of C and O. On the other hand, if gas viscosity is high (α ≳ 10 −1 , as expected for this largely ionized disk), gas is continuously produced and viscously accreted toward the star. This removal process does not discriminate between elements so the observed overabundance of C and O has to be explained by a preferential production that strongly favors C and O to other metallic elements. One such candidate is photodesorption off the grains. We compare our calculation against all observed elements (∼10) in the gas disk and find a mild preference for the second scenario, based on the abundance of Si alone. If true, β Pic should still be accreting at an observable rate, well after its primordial disk has disappeared.
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[6] ALMA OBSERVATIONS OF THE MOLECULAR GAS IN THE DEBRIS DISK OF THE 30 Myr OLD STAR HD 21997
Founding observations ALMA et modélisation · observational study
Why it is here: Les isotopologues et la rotation képlérienne caractérisent le gaz de HD 21997, pertinent pour les scénarios d’origine.
Abstract
The 30 Myr old A3-type star HD 21997 is one of the two known debris dust disks having a measurable amount of cold molecular gas. With the goal of understanding the physical state, origin, and evolution of the gas in young debris disks, we obtained CO line observations with the Atacama Large Millimeter/submillimeter Array (ALMA). Here, we report on the detection of 12 CO and 13 CO in the J = 2–1 and J = 3–2 transitions and C 18 O in the J = 2–1 line. The gas exhibits a Keplerian velocity curve, one of the few direct measurements of Keplerian rotation in young debris disks. The measured CO brightness distribution could be reproduced by a simple star+disk system, whose parameters are r in < 26 AU, r out = 138 ± 20 AU, M ☉ , and i = 32 6 ± 3 1. The total CO mass, as calculated from the optically thin C 18 O line, is about (4–8) × 10 −2 M ⊕ , while the CO line ratios suggest a radiation temperature on the order of 6–9 K. Comparing our results with those obtained for the dust component of the HD 21997 disk from ALMA continuum observations by Moór et al., we conclude that comparable amounts of CO gas and dust are present in the disk. Interestingly, the gas and dust in the HD 21997 system are not colocated, indicating a dust-free inner gas disk within 55 AU of the star. We explore two possible scenarios for the origin of the gas. A secondary origin, which involves gas production from colliding or active planetesimals, would require unreasonably high gas production rates and would not explain why the gas and dust are not colocated. We propose that HD 21997 is a hybrid system where secondary debris dust and primordial gas coexist. HD 21997, whose age exceeds both the model predictions for disk clearing and the ages of the oldest T Tauri-like or transitional gas disks in the literature, may be a key object linking the primordial and the debris phases of disk evolution.
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[7] Molecular Gas Clumps from the Destruction of Icy Bodies in the β Pictoris Debris Disk
Founding imagerie du CO et dynamique · observational study
Why it is here: Le clump de CO autour de β Pictoris est interprété comme du gaz libéré par des collisions de corps glacés.
Abstract
Many stars are surrounded by disks of dusty debris formed in the collisions of asteroids, comets, and dwarf planets, but is gas also released in such events? Observations at submillimeter wavelengths of the archetypal debris disk around β Pictoris show that 0.3% of a Moon mass of carbon monoxide orbits in its debris belt. The gas distribution is highly asymmetric, with 30% found in a single clump 85 astronomical units from the star, in a plane closely aligned with the orbit of the inner planet, β Pictoris b. This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet or from the remnants of a collision of Mars-mass planets.
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[8] Exocometary gas in the HD 181327 debris ring
Founding imagerie ALMA et transfert radiatif · observational study
Why it is here: La distribution spatiale du CO dans un disque de débris fournit des contraintes utiles, sans trancher directement son origine.
Abstract
An increasing number of observations have shown that gaseous debris discs are not an exception. However, until now, we only knew of cases around A stars. Here we present the first detection of 12CO (2–1) disc emission around an F star, HD 181327, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with radiative transfer calculations, we study the dust and CO mass distribution. We find the dust is distributed in a ring with a radius of 86.0 ± 0.4 au and a radial width of 23.2 ± 1.0 au. At this frequency, the ring radius is smaller than in the optical, revealing grain size segregation expected due to radiation pressure. We also report on the detection of low-level continuum emission beyond the main ring out to ∼200 au. We model the CO emission in the non-local thermodynamic equilibrium regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging between 1.2 × 10−6 M⊕ and 2.9 × 10−6 M⊕, depending on the gas kinetic temperature and collisional partners densities. The CO densities and location suggest a secondary origin, i.e. released from icy planetesimals in the ring. We derive a CO+CO2 cometary composition that is consistent with Solar system comets. Due to the low gas densities, it is unlikely that the gas is shaping the dust distribution.
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[9] Gas and dust around A-type stars at tens of Myr: signatures of cometary breakup
Founding recensement observationnel des gaz · observational study
Why it is here: La distribution des détections selon l’âge et les indices de fragmentation éclairent l’origine secondaire du gaz.
Abstract
Discs of dusty debris around main-sequence stars indicate fragmentation of orbiting planetesimals, and for a few A-type stars, a gas component is also seen that may come from collisionally released volatiles. Here we find the sixth example of a CO-hosting disc, around the ∼30 Myr-old A0-star HD 32997. Two more of these CO-hosting stars, HD 21997 and 49 Cet, have also been imaged in dust with SCUBA-2 within the SCUBA-2 Survey of Nearby Stars project. A census of 27 A-type debris hosts within 125 pc now shows 7/16 detections of carbon-bearing gas within the 5–50 Myr epoch, with no detections in 11 older systems. Such a prolonged period of high fragmentation rates corresponds quite well to the epoch when most of the Earth was assembled from planetesimal collisions. Recent models propose that collisional products can be spatially asymmetric if they originate at one location in the disc, with CO particularly exhibiting this behaviour as it can photodissociate in less than an orbital period. Of the six CO-hosting systems, only β Pic is in clear support of this hypothesis. However, radiative transfer modelling with the ProDiMo code shows that the CO is also hard to explain in a proto-planetary disc context.
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[10] Molecular Gas in Debris Disks around Young A-type Stars
Founding relevé ALMA des isotopologues · observational study
Why it is here: Les détections et l’opacité du CO contraignent les masses de gaz, données utiles à l’interprétation de son origine.
Abstract
Abstract According to the current paradigm of circumstellar disk evolution, gas-rich primordial disks evolve into gas-poor debris disks that are composed of second-generation dust. To explore the transition between these phases, we searched for 12CO, 13CO, and C18O emission in seven dust-rich debris disks around young A-type stars, using the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6. We discovered molecular gas in three debris disks. In all of these disks, the 12CO line was optically thick, highlighting the importance of less abundant molecules in reliable mass estimates. By supplementing our target list with literature data, we compiled a volume-limited sample of dust-rich debris disks around young A-type stars within 150 pc. We obtained a CO detection rate of 11/16 above a 12CO J = 2–1 line luminosity threshold of ∼1.4 × 104 Jy km s−1pc2 in the sample. This high incidence implies that the presence of CO gas in the bright debris disks around young A-type stars is more likely the rule than the exception. Interestingly, dust-rich debris disks around young FG-type stars exhibit, with the same detectability threshold as A-type stars, a significantly lower gas incidence. While the transition from the protoplanetary phase to the debris phase is associated with a drop in the dust content, our results exhibit a large spread in the CO mass in our debris sample, with peak values that are comparable to those in the protoplanetary Herbig Ae disks. In the particularly CO-rich debris systems, the gas may have a primordial origin, which is a characteristic of a hybrid disk.
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Founding modèle de production secondaire · simulation or model
Why it is here: Le modèle prédit la production et l’évolution du CO, du carbone et de l’oxygène issus des planétésimaux.
Abstract
This paper uses observations of dusty debris discs, including a growing number of gas detections in these systems, to test our understanding of the origin and evolution of this gaseous component. It is assumed that all debris discs with icy planetesimals create second generation CO, C and O gas at some level, and the aim of this paper is to predict that level and assess its observability. We present a new semi-analytical equivalent of the numerical model of Kral et al. allowing application to large numbers of systems. That model assumes CO is produced from volatile-rich solid bodies at a rate that can be predicted from the debris discs fractional luminosity. CO photodissociates rapidly into C and O that then evolve by viscous spreading. This model provides a good qualitative explanation of all current observations, with a few exceptional systems that likely have primordial gas. The radial location of the debris and stellar luminosity explain some non-detections, e.g. close-in debris (like HD 172555) is too warm to retain CO, while high stellar luminosities (like η Tel) result in short CO lifetimes. We list the most promising targets for gas detections, predicting >15 CO detections and >30 C i detections with ALMA, and tens of C ii and O i detections with future far-IR missions. We find that CO, C i, C ii and O i gas should be modelled in non-LTE for most stars, and that CO, C i and O i lines will be optically thick for the most gas-rich systems. Finally, we find that radiation pressure, which can blow out C i around early-type stars, can be suppressed by self-shielding.
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Founding imagerie ALMA et modèle de blindage · observational study
Why it is here: Les observations et le modèle montrent comment un gaz riche en CO peut être secondaire et protégé par le carbone.
Abstract · conclusions of the full text read too
Abstract Despite being >10Myr, there are ∼10 debris discs with as much CO gas as in protoplanetary discs. Such discs have been assumed to be “hybrid”, i.e., with secondary dust but primordial gas. Here we show that both the dust and gas in such systems could instead be secondary, with the high CO content caused by accumulation of neutral carbon (C0) that shields CO from photodissociating; i.e., these could be “shielded secondary discs”. New ALMA observations are presented of HD131835 that detect ∼3 × 10−3 M⊕ of C0, the majority 40-200au from the star, in sufficient quantity to shield the previously detected CO. A simple semi-analytic model for the evolution of CO, C and O originating in a volatile-rich planetesimal belt shows how CO shielding becomes important when the viscous evolution is slow (low α parameter) and/or the CO production rate is high. Shielding by C0 may also cause the CO content to reach levels at which CO self-shields, and the gas disc may become massive enough to affect the dust evolution. Application to the HD 131835 observations shows these can be explained if α ∼ 10−3; an inner cavity in C0 and CO may also mean the system has yet to reach steady state. Application to other debris discs with high CO content finds general agreement for α = 10−3 to 0.1. The shielded secondary nature of these gas discs can be tested by searching for C0, as well as CN, N2 and CH+, which are also expected to be shielded by C0.
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[13] Modeling the Spatial Distribution and Origin of CO Gas in Debris Disks
Founding imagerie ALMA et modèles thermochimiques · method
Why it is here: La distribution du CO observé est modélisée pour comparer directement les scénarios primordial et secondaire.
Abstract
Abstract The detection of gas in debris disks raises the question of whether this gas is a remnant from the primordial protoplanetary phase, or released by the collision of secondary bodies. In this paper we analyze ALMA observations at 1″–1.″5 resolution of three debris disks where the 12CO(2–1) rotational line was detected: HD 131835, HD 138813, and HD 156623. We apply the iterative Lucy–Richardson deconvolution technique to the problem of circumstellar disks to derive disk geometries and surface brightness distributions of the gas. The derived disk parameters are used as input for thermochemical models to test both primordial and cometary scenarios for the origin of the gas. We favor a secondary origin for the gas in these disks and find that the CO gas masses ( M ) require production rates (∼5 × 10−7 M ⊕ yr−1) similar to those estimated for the bona fide gas-rich debris disk β Pic.
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[14] Modelling the Spatial Distribution and Origin of CO Gas in Debris Disks
Preprint imagerie ALMA et modèles thermochimiques · method
Why it is here: La distribution du CO observé est modélisée pour comparer directement les scénarios primordial et secondaire.
Abstract · conclusions of the full text read too
The detection of gas in debris disks raises the question of whether this gas is a remnant from the primordial protoplanetary phase, or released by the collision of secondary bodies. In this paper we analyze ALMA observations at 1-1.5" resolution of three debris disks where the $^{12}$CO(2-1) rotational line was detected: HD131835, HD138813, and HD156623. We apply the iterative Lucy-Richardson deconvolution technique to the problem of circumstellar disks to derive disk geometries and surface brightness distributions of the gas. The derived disk parameters are used as input for thermochemical models to test both primordial and cometary scenarios for the origin of the gas. We favor a secondary origin for the gas in these disks and find that the CO gas masses ($\sim 3\times10^{-3}$ M$_{\oplus}$) require production rates ($\sim 5\times 10^{-7}$ M$_{\oplus}$~yr$^{-1}$) similar to those estimated for the bona-fide gas rich debris disk $β$ Pic.
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[15] New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297
Founding mesures isotopologues et modèle · observational study
Why it is here: Les masses élevées de CO et leur interprétation par le blindage testent un scénario de production secondaire.
Abstract
Abstract Previous observations revealed the existence of CO gas at nearly protoplanetary level in several dust-rich debris disks around young A-type stars. Here we used the Atacama Large Millimeter/submillimeter Array (ALMA) 7 m Array to measure 13CO and C18O emission toward two debris disks, 49 Cet and HD 32297, and detected similarly high CO content (>0.01 M ⊕). These high CO masses imply a highly efficient shielding of CO molecules against stellar and interstellar ultraviolet photons. Adapting a recent secondary gas disk model that considers both shielding by carbon atoms and self-shielding of CO, we can explain the observed CO level in both systems. Based on the derived gas densities we suggest that, in the HD 32297 disk, dust and gas are coupled and the dynamics of small grains is affected by the gaseous component. For 49 Cet, the question of coupling remains undecided. We found that the main stellar and disk properties of 49 Cet and HD 32297 are very similar to those of previously identified debris disks with high CO content. These objects constitute together the first known representatives of shielded debris disks.
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Founding observation ALMA et modèle de dégazage · observational study
Why it is here: Le CO non protégé doit être renouvelé continuellement par les exocomètes, preuve directe en faveur d’une origine secondaire.
Abstract
Abstract Millimeter observations of CO gas in planetesimal belts show a high detection rate around A stars, but few detections for later type stars. We present the first CO detection in a planetesimal belt around an M star, TWA 7. The optically thin CO (J = 3–2) emission is colocated with previously identified dust emission from the belt, and the emission velocity structure is consistent with Keplerian rotation around the central star. The detected CO is not well shielded against photodissociation, and must thus be continuously replenished by gas release from exocomets within the belt. We analyze in detail the process of exocometary gas release and destruction around young M dwarfs and how this process compares to earlier type stars. Taking these differences into account, we find that CO generation through exocometary gas release naturally explains the increasing CO detection rates with stellar luminosity, mostly because the CO production rate from the collisional cascade is directly proportional to stellar luminosity. More luminous stars will therefore on average host more massive (and hence more easily detectable) exocometary CO disks, leading to the higher detection rates observed. The current CO detection rates are consistent with a ubiquitous release of exocometary gas in planetesimal belts, independent of spectral type.
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Preprint modèle de photoévaporation · simulation or model
Why it is here: L’étude teste si du gaz hérité peut survivre assez longtemps dans des disques appauvris en grains.
Abstract · conclusions of the full text read too
Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear; namely, it can be protoplanetary remnants and/or secondary products deriving from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for $\gtrsim10{\,\rm Myr}$. However, previous models predict $\lesssim 10{\,\rm Myr}$ lifetimes because of efficient photoevaporation at the late stage of disk evolution. In the present study, we investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled as those where radiation force is sufficiently strong to continuously blow out small grains ($\lesssim 4 {\,\rm μm}$), which are an essential component for driving photoevaporation via photoelectric heating induced by stellar far-ultraviolet (FUV). We find that the grain depletion reduces photoelectric heating, so that FUV photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of $2$--$5\times10^{-10}(Φ_{\rm EUV}/10^{41}{\,\rm s}^{-1})^{1/2}\,M_\odot\,{\rm yr}^{-1}$, where $Φ_{\rm EUV}$ is the EUV emission rate. The estimated lifetimes of the gas component are $\sim 50 (M_{\rm disk}/10^{-2}\,M_\odot)(Φ_{\rm EUV}/10^{41}\,{\rm s}^{-1})^{1/2}\,{\rm Myr}$ and depend on the ``initial'' disk mass at the point small grains have been depleted in the system. With an order estimation, we show that the gas component can survive for a much longer time around A-type stars than lower-mass stars. This trend is consistent with the higher frequency of gas-rich debris disks around A-type stars, implying the possibility of the gas component being protoplanetary remnants.
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[18] Population synthesis of exocometary gas around A stars
Founding synthèse numérique du gaz secondaire · simulation or model
Why it is here: Le modèle relie le dégazage des planétésimaux, l’évolution visqueuse et le blindage du CO aux masses observées.
Abstract
ABSTRACT The presence of CO gas around 10–50 Myr old A stars with debris discs has sparked debate on whether the gas is primordial or secondary. Since secondary gas released from planetesimals is poor in H2, it was thought that CO would quickly photodissociate never reaching the high levels observed around the majority of A stars with bright debris discs. Kral et al. showed that neutral carbon produced by CO photodissociation can effectively shield CO and potentially explain the high CO masses around 9 A stars with bright debris discs. Here, we present a new model that simulates the gas viscous evolution, accounting for carbon shielding and how the gas release rate decreases with time as the planetesimal disc loses mass. We find that the present gas mass in a system is highly dependant on its evolutionary path. Since gas is lost on long time-scales, it can retain a memory of the initial disc mass. Moreover, we find that gas levels can be out of equilibrium and quickly evolving from a shielded on to an unshielded state. With this model, we build the first population synthesis of gas around A stars, which we use to constrain the disc viscosity. We find a good match with a high viscosity (α ∼ 0.1), indicating that gas is lost on time-scales ∼1–10 Myr. Moreover, our model also shows that high CO masses are not expected around FGK stars since their planetesimal discs are born with lower masses, explaining why shielded discs are only found around A stars. Finally, we hypothesize that the observed carbon cavities could be due to radiation pressure or accreting planets.
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[19] The Surprisingly Low Carbon Mass in the Debris Disk around HD 32297
Founding observation ALMA et modèle du carbone · observational study
Why it is here: La faible masse de carbone et son retrait rapide, comparés à la production de CO, soutiennent un apport secondaire continu.
Abstract
Abstract Gas has been detected in a number of debris disks. It is likely secondary, i.e., produced by colliding solids. Here, we report ALMA Band 8 observations of neutral carbon in the CO-rich debris disk around the 15–30 Myr old A-type star HD 32297. We find that C0 is located in a ring at ∼110 au with an FWHM of ∼80 au and has a mass of (3.5 ± 0.2) × 10−3 M ⊕. Naively, such a surprisingly small mass can be accumulated from CO photodissociation in a time as short as ∼104 yr. We develop a simple model for gas production and destruction in this system, properly accounting for CO self-shielding and shielding by neutral carbon, and introducing a removal mechanism for carbon gas. We find that the most likely scenario to explain both C0 and CO observations is one where the carbon gas is rapidly removed on a timescale of order a thousand years and the system maintains a very high CO production rate of ∼15 M ⊕ Myr−1, much higher than the rate of dust grind-down. We propose a possible scenario to meet these peculiar conditions: the capture of carbon onto dust grains, followed by rapid CO re-formation and rerelease. In steady state, CO would continuously be recycled, producing a CO-rich gas ring that shows no appreciable spreading over time. This picture might be extended to explain other gas-rich debris disks.
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[20] Lack of other molecules in CO-rich debris discs: is it primordial or secondary gas?
Founding observations moléculaires comparatives · observational study
Why it is here: La présence ou l’absence de molécules traceuses est utilisée pour examiner l’origine primordiale ou secondaire du gaz.
Abstract · conclusions of the full text read too
ABSTRACT The nature of the gas in CO-rich debris discs remains poorly understood, as it could either be a remnant from the earlier Class II phase or of secondary origin, driven by the destruction of icy planetesimals. The aim of this paper was to elucidate the origin of the gas content in the debris discs via various simple molecules that are often detected in the less-evolved Class II discs. We present millimetre molecular line observations of nine circumstellar discs around A-type stars: four CO-rich debris discs (HD 21997, HD 121617, HD 131488, HD 131835) and five old Herbig Ae protoplanetary discs (HD 139614, HD 141569, HD 142666, HD 145718, HD 100453). The sources were observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in Bands 5 and 6 with 1–2 arcsec resolution. The Herbig Ae discs are detected in the CO isotopologues, CN, HCN, HCO+, C2H, and CS lines. In contrast, only CO isotopologues are detected in the debris discs, showing a similar amount of CO to that found in the Herbig Ae protoplanetary discs. Using chemical and radiative transfer modelling, we show that the abundances of molecules other than CO in debris discs are expected to be very low. We consider multiple sets of initial elemental abundances with various degrees of H2 depletion. We find that the HCO+ lines should be the second brightest after the CO lines, and that their intensities strongly depend on the overall CO/H2 ratio of the gas. However, even in the ISM-like scenario, the simulated HCO+ emission remains weak as required by our non-detections.
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Founding modèle de photoévaporation · simulation or model
Why it is here: L’étude montre comment l’appauvrissement en grains peut réduire les pertes et permettre la survie du gaz primordial.
Abstract · conclusions of the full text read too
Abstract Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear, but it could be protoplanetary remnants and/or secondary products from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for ≳ 10 Myr . However, previous models predict ≲ 10 Myr lifetimes because of efficient photoevaporation at the late stage of disk evolution. We investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled like those devoid of small grains ( ≲ 4 μ m ). We find that grain depletion reduces photoelectric heating so that far-ultraviolet photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of the order of 1 × 10 − 11 ( Φ EUV / 10 38 s − 1 ) 1 / 2 M ⊙ yr − 1 , where Φ EUV is the EUV emission rate of the host star. The estimated gas–disk lifetimes are ∼ 100 ( M disk / 10 − 3 M ⊙ ) ( Φ EUV / 10
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[22] A Primordial Origin for the Gas-rich Debris Disks around Intermediate-mass Stars
modèle d’évolution des disques · simulation or model
Why it is here: Le modèle montre comment des disques primordiaux appauvris en petits grains peuvent conserver leur gaz.
Abstract · conclusions of the full text read too
Abstract While most debris disks consist of dust with little or no gas, a fraction have significant amounts of gas detected via emission lines of CO, ionized carbon, and/or atomic oxygen. Almost all such gaseous debris disks known are around A-type stars with ages up to 50 Myr. We show, using semianalytic disk evolution modeling, that this can be understood if the gaseous debris disks are remnant protoplanetary disks that have become depleted of small grains compared to the interstellar medium. Photoelectric heating by the A stars’ far-UV (FUV) radiation is then inefficient, while the stars’ extreme-UV (EUV) and X-ray emissions are weak owing to a lack of surface convective zones capable of driving magnetic activity. In this picture, it is relatively difficult for stars outside the range of spectral types from A through early B to have such long-lived gas disks. Less-massive stars have stronger magnetic activity in the chromosphere, transition region, and corona with resulting EUV and X-ray emission, while more-massive stars have photospheres hot enough to produce strong EUV radiation. In both cases, primordial disk gas is likely to photoevaporate well before 50 Myr. These results come from 0D disk evolution models where we incorporate internal accretion stresses, MHD winds, and photoevaporation by EUV and X-ray photons with luminosities that are functions of the stellar mass and age. A key issue this work leaves open is how some disks become depleted in small dust so that FUV photoevaporation slows. Candidates include the grains’ growth, settling, radial drift, radiation force, and incorporation into planetary systems.
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[23] Primordial or Secondary? Testing Models of Debris Disk Gas with ALMA*
observations multi-espèces et modèles · observational study
Why it is here: Les abondances de CO, de carbone et d’ions sont confrontées aux modèles de gaz secondaire.
Abstract · conclusions of the full text read too
Abstract The origin and evolution of gas in debris disks are still not well understood. Secondary gas production from cometary material or a primordial origin have been proposed. So far, observations have mostly concentrated on CO, with only a few C observations available. We overview the C and CO content of debris disk gas and test state-of-the-art models. We use new and archival Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO and C i emission, complemented by C ii data from Herschel, for a sample of 14 debris disks. This expands the number of disks with ALMA measurements of both CO and C i by 10 disks. We present new detections of C i emission toward three disks: HD 21997, HD 121191, and HD 121617. We use a simple disk model to derive gas masses and column densities. We find that current state-of-the-art models of secondary gas production overpredict the C0 content of debris disk gas. This does not rule out a secondary origin, but might indicate that the models require an additional C removal process. Alternatively, the gas might be produced in transient events rather than a steady-state collisional cascade. We also test a primordial gas origin by comparing our results to a simplified thermochemical model. This yields promising results, but more detailed work is required before a conclusion can be reached. Our work demonstrates that the combination of C and CO data is a powerful tool to advance our understanding of debris disk gas.
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[24] Argon in beta Pictoris -- entrapment and release of volatile in disks
Recent Preprint modèle photoionisé des abondances · simulation or model
Why it is here: Les abondances élémentaires modélisées de β Pic servent à inférer la composition et l’histoire volatile des corps producteurs de gaz.
Abstract · conclusions of the full text read too
Chemical compositions of planets reveal much about their formation environments. Such information is well sought-after in studies of Solar System bodies and extra-solar ones. Here, we investigate the composition of planetesimals in the beta Pictoris debris disk, by way of its secondary gas disk. We are stimulated by the recent JWST detection of an ArII emission line, and aim to reproduce extensive measurements from the past four decades. Our photo-ionization model reveals that the gas has to be heavily enriched in C, N, O, and Ar (but not S and P), by a uniform factor of about 100 relative to other metals. Such an abundance pattern is both reminiscent of, and different from, that of Jupiter's atmosphere. The fact that Ar, the most volatile and therefore the hardest to capture into solids, is equally enriched as C, N, and O suggests that the planetesimals were formed in a very cold region (T < 20- 35K), possibly with the help of entrapment if water ice is over-abundant. In the debris disk phase, these volatile are preferentially out-gassed from the dust grains, likely via photo-desorption. The debris grains must be `dirty' aggregates of icy and refractory clusters. Lastly, the observed strength of the ArII line can only be explained if the star beta Pic (a young A6V star) has sizable chromospheric and coronal emissions, on par with those from the modern Sun. In summary, observations of the beta Pic gas disk rewind the clock to reveal the formation environment of planetesimals.
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[25] Low CI/CO Abundance Ratio Revealed by HST UV Spectroscopy of CO-rich Debris Disks
Recent Preprint spectroscopie UV et modèle secondaire · observational study
Why it is here: Les rapports CI/CO mesurés sont comparés à des modèles d’évolution d’un gaz secondaire.
Abstract · conclusions of the full text read too
The origin and evolution of CO gas in debris disks has been debated since its initial detection. The gas could have a primordial origin, as a remnant of the protoplanetary disk or a secondary exocometary origin. This paper investigates the origin of gas in two debris disks, HD110058 and HD131488, using HST observations of CI and CO, which play critical roles in the gas evolution. We fitted several electronic transitions of CI and CO rovibronic bands to derive column densities and temperatures for each system, revealing high CO column densities ($\sim$3-4 orders of magnitude higher than $β$ Pictoris), and low CI/CO ratios in both. Using the exogas model, we simulated the radial evolution of the gas in the debris disk assuming a secondary gas origin. We explored a wide range of CO exocometary release rates and $α$ viscosities, which are the key parameters of the model. Additionally, we incorporated photodissociation due to stellar UV to the exogas model and found that it is negligible for typical CO-rich disks and host stars, even at a few au due to the high radial optical depths in the EUV. We find that the current steady-state secondary release model cannot simultaneously reproduce the CO and CI HST-derived column densities, as it predicts larger CI/CO ratios than observed. Our direct UV measurement of low CI/CO ratios agrees with results derived from recent ALMA findings and may point to vertical layering of CI, additional CI removal, CO shielding processes, or different gas origin scenarios.
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[26] Low CI/CO abundance ratio revealed by HST UV spectroscopy of CO-rich debris discs
Recent spectroscopie UV et modèle d’évolution · observational study
Why it is here: Les rapports C I/CO et leur modélisation testent si le gaz peut être secondaire et continuellement produit.
Abstract · conclusions of the full text read too
ABSTRACT The origin and evolution of CO gas in debris discs has been debated since its initial detection. The gas could have a primordial origin, as a remnant of the protoplanetary disc or a secondary exocometary origin. This paper investigates the origin of gas in two debris discs, HD110058 and HD131488, using Hubble Space Telescope (HST) observations of CI and CO, which play critical roles in the gas evolution. We fitted several electronic transitions of CI and CO rovibronic bands to derive column densities and temperatures for each system, revealing high CO column densities (∼3–4 orders of magnitude higher than β Pictoris), and low CI/CO ratios in both. Using the exogas model, we simulated the radial evolution of the gas in the debris disc assuming a secondary gas origin. We explored a wide range of CO exocometary release rates and α viscosities, which are the key parameters of the model. Additionally, we incorporated photodissociation due to stellar ultraviolet (UV) to the exogas model and found that it is negligible for typical CO-rich discs and host stars, even at a few au due to the high radial optical depths in the Extreme ultraviolet radiation (EUV). We find that the current steady-state secondary release model cannot simultaneously reproduce the CO and CI HST-derived column densities, as it predicts larger CI/CO ratios than observed. Our direct UV measurement of low CI/CO ratios agrees with results derived from recent Atacama Large Millimetre/submillimetre Array findings and may point to vertical layering of CI, additional CI removal, CO shielding processes, or different gas origin scenarios.
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[27] Water vapor as a probe of the origin of gas in debris disks
Recent analyse comparative de vapeur d’eau · theory
Why it is here: Elle propose la vapeur d’eau froide comme indicateur permettant de distinguer les deux origines du gaz.
Abstract · conclusions of the full text read too
Context. Debris disks contain the formation and evolution histories of planetary systems. Recent detections of gas in these disks have received considerable attention, as the origin of the gas sheds light on ongoing disk evolution and the current composition of planet-forming materials. Aims. Observations of CO gas alone, however, cannot reliably differentiate between two leading and competing hypotheses: (1) that the observed gas is a leftover of protoplanetary disk gas, and (2) that the gas is the outcome of collisions between icy bodies. We propose that such a differentiation may become possible by observing cold water vapor. Methods. We performed order-of-magnitude analyses and compared these with existing observations. Results. We show that different hypotheses lead to different masses of water vapor. This occurs because, for both hypotheses, the presence of cold water vapor is attributed to photodesorption from dust particles by attenuated interstellar UV radiation. Cold water vapor cannot be observed by current astronomical facilities as most of its emission lines fall in the far-IR (FIR) range. Conclusions. This work highlights the need for a future FIR space observatory to reveal the origin of gas in debris disks and the evolution of planet-forming disks in general.
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[28] MINDS: Detection of an Inner Gas Disk Caused by Evaporating Bodies around HD 172555
Recent spectroscopie infrarouge JWST · observational study
Why it is here: La composition et l’emplacement du gaz interne relient directement son émission à l’évaporation de petits corps.
Abstract
Abstract Mechanisms such as collisions of rocky bodies or cometary activity give rise to dusty debris disks. Debris disks trace the leftover building blocks of planets, and thus also planetary composition. HD 172555, a stellar twin of β Pic, hosts a debris disk thought to have resulted from a giant collision. It is known for its extreme mid-infrared silica dust feature, indicating a warm population of silica-rich grains in the asteroid belt (∼5 au), cold CO observed by Atacama Large Millimeter/submillimeter Array (ALMA), and small bodies evaporating as they approach close to the star. Our JWST Mid-InfraRed Instrument/Medium Resolution Spectroscopy (MIRI MRS) observations now reveal emission from an inner gaseous disk (<0.5 au) that arises from the evaporation of close-in material. For the first time in a debris disk, we detect neutral atomic chlorine and sulfur, as well as ionized nickel. We recovered the neutral sulfur line in ∼20 yr old Spitzer data, showing it is long-lived and stable. Ionized iron, previously seen only in β Pic, is also detected. All lines are broadened by Keplerian rotation, pinpointing the gas location. The HD 172555 system serves as a unique laboratory to study the composition of planetesimals, asteroids, and comets beyond the solar system. The comparison to β Pic reveals that the gas in HD 172555 is hotter, closer to the star, and poor in argon—suggesting it originates from evaporating rocky bodies near the star, while β Pic’s gas may trace volatile-rich bodies from larger separations.
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[29] Secret of Longevity: Protoplanetary Disks as a Source of Gas in Debris Disks
Recent simulations d’évolution des disques · simulation or model
Why it is here: Les simulations évaluent si des disques hérités peuvent survivre au-delà de dix millions d’années.
Abstract
Abstract While protoplanetary disks (PPDs) are generally thought to disperse within several million years, recent observations have revealed gas in their older counterparts, debris disks. The origin of this gas remains uncertain, with one possibility being the unexpectedly long survival of PPDs (the primordial-origin scenario). To explore the plausibility of this scenario, we conduct 1D disk evolution simulations, varying parameters like stellar mass, disk mass, turbulent stress, and the model of magnetohydrodynamic winds, while incorporating stellar evolution to account for time-varying photoevaporation rates. Our focus is on disks where small grains are depleted, as these are potentially long lived due to reduced far-ultraviolet photoevaporation. Our results show that gas in these disks can survive beyond 10 Myr regardless of the stellar mass, provided they are initially massive ( M disk ≈ 0.1 M * ) with relatively weak turbulent stress ( α ≪ 10 −2 ). The longest lifetimes are consistently found for M * = 2 M ⊙ across a wide parameter space, with gas typically persisting at ∼10–10 3 au. Roughly estimated CO masses for these disks fall within the observed range for the most massive gas-rich debris disks around early A stars. These alignments support the plausibility of the primordial-origin scenario. Additionally, our model predicts that accretion persists for as long as the disk survives, which could explain the accretion signatures detected in old disks hosted by low-mass stars, including Peter Pan disks. Our finding also suggests that ongoing accretion may exist in gas-rich debris disks. Thus, searching for accretion signatures could be a key to determining the origins of gas in debris disks.
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[30] Thermochemical constraints on a primordial-origin of gas-rich debris disks
Recent Preprint modèles thermochimiques primordiaux · simulation or model
Why it is here: Les modèles testent si un gaz primordial peut reproduire les abondances observées et les limites de détection.
Abstract · conclusions of the full text read too
Recent observations have revealed gas-rich debris disks around intermediate-mass stars at ages of tens of Myr. The origin of this gas remains unclear: it may be primordial, retained from the protoplanetary phase, or secondary, released from volatile-rich solids. Secondary-origin models reproduce CO emission but often overpredict neutral carbon. Recent observations and disk-evolution models suggest that primordial gas may survive longer than previously assumed, motivating thermochemical tests of the primordial-remnant scenario. We test the previously unexplored possibility that primordial-origin disks satisfy the observational constraints on gas-rich debris disks. Specifically, we determine under what conditions a disk around a $2\,M_{\odot}$ star reproduces substantial CO, low CI/CO ratios, and weak HCO+ emission consistent with current non-detections. We post-processed 20-40 Myr structures from 1D disk-evolution models with Cloudy, varying irradiation geometry, dust-to-gas mass ratio (DTG), and cosmic-ray ionisation rate. In the dust-poor models (DTG $=10^{-4}$), CO remains optically thick around $R\sim100$~au. The models yield low disk-integrated CI/CO mass ratios. Our model produces CO radial intensities of the observed order of magnitude, but its CI-emitting region is more extended than observed. The standard CR model overproduces HCO+, whereas the weak CR model brings its predicted luminosity within current observational limits. These results demonstrate that a primordial origin remains chemically viable for CO-rich debris disks. The main remaining tension is the excessive radial extent of the CI emission, although it may reflect our simplified modelling. Further testing of the primordial-origin scenario will require multidimensional, self-consistent modelling, spatially resolved CI observations, and deeper searches for HCO+.