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Origins of Gas in Debris Discs
Is the gas detected in debris discs primordial, left over from the protoplanetary disc, or secondary, released by colliding planetesimals?
How this review was searched
296 papers found, the 80 closest to the question screened on their abstract, 47 judged relevant, 30 included. Published from 2001 to 2026. 22 founding papers, 5 from the last three years, 2 reviews.
Published in a journal or proceedings: 29 · preprints: 1 · deposits nobody reviewed: 0. The conclusions of 13 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 10 of the 10 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 and theoretical studies that assess whether gas in debris discs is primordial or secondary, including proposed production and retention mechanisms, chemical and isotopic diagnostics, spectroscopy, and gas dynamics across different stellar and disc settings.
Key concepts: Debris discs, Primordial gas, Secondary gas, Gas-origin diagnostics
Queries:
- debris disc gas origin
- primordial residual protoplanetary gas
- secondary exocometary collisional gas
- planetesimal collisions volatile release
- gas abundances isotopic ratios debris
- debris disc gas spectroscopy kinematics
- OpenAlex, by relevance: 126
- OpenAlex, last three years: 79
- arXiv: 22
- OpenAlex, reviews: 15
- OpenAlex, most cited: 25
- Cited by several of the closest papers: 50 (2 abstracts found on Semantic Scholar or arXiv)
- Recent papers citing the founding ones: 22
A paper found by several searches is counted in each.
In short
The papers support secondary gas production in several debris discs, but do not establish a single origin for all systems, especially those with unusually large CO reservoirs. [8][9][25][28] Evidence for secondary gas includes CO associated with planetesimal belts, excitation inconsistent with long-term H2 shielding, and, in one recent study, a directly constrained high CO/H2 ratio in HD 110058. [8][9][28] Large CO masses alone do not distinguish primordial remnants from secondary gas, because carbon shielding and CO self-shielding can allow gas released by planetesimals to accumulate. [16][18][19] However, secondary models often predict more neutral carbon than observed, while primordial-retention models remain plausible but incompletely tested; the latest thermochemical support includes an unreviewed preprint. [24][25][26][30]
This summary rests on the findings below, each with the sentences that support it.
Approaches compared
| Approach | Papers | Strengths | Limits |
|---|---|---|---|
| Surveys, stellar demographics, and hot versus cold gas | [2][6][10][17][20][23] | Surveys establish how frequently different gas components are detectable and identify stellar and disc populations in which competing origin models must operate. | Detection rates and optical absorptions do not independently determine gas origin, and sensitivity, viewing geometry, and interstellar contamination complicate comparisons. |
| Secondary gas tested through location, excitation, and composition | [5][8][9][15][27][28] | Excitation and composition test the H2-rich primordial interpretation more directly than CO detection or mass alone. | The strongest conclusions are system-specific, and spatial associations or abundance patterns do not uniquely identify the detailed release mechanism. |
| Secondary gas evolution, shielding, and viscous spreading | [7][11][16][18][19][29] | This approach links gas mass, chemistry, stellar demographics, and spatial extent within an evolutionary framework. | Its explanations depend on production histories, shielding, and transport parameters, so agreement with CO alone does not uniquely establish secondary origin. |
| Hybrid discs and long-lived primordial reservoirs | [3][12][24][30] | Primordial-retention models address both the unusually old gas reservoirs and the low carbon-to-CO ratios that challenge secondary models. | Survival and chemical viability are not direct demonstrations of primordial origin, and the newest thermochemical result is a preprint rather than peer-reviewed evidence. |
| Carbon budgets, transients, and molecular tests of gas origin | [13][21][22][25][26] | Combining atomic carbon, CO, and other molecular constraints tests the material and chemical budgets underlying competing explanations. | The observed discrepancies do not uniquely distinguish missing secondary-gas physics from primordial gas, and proposed removal or recycling mechanisms remain tentative. |
| Gas–dust dynamics and the ambiguity of disc morphology | [1][4][14] | Dynamical modelling explains how even limited gas can alter observable dust and guards against overly simple morphological interpretations. | These structural results establish possible effects of gas, not whether the gas was retained from the protoplanetary phase or released later. |
Surveys, stellar demographics, and hot versus cold gas
Surveys changed the picture of debris discs from uniformly gas-poor systems to a population in which detectable CO is common among bright, young A-star discs. Detections around later-type stars and comparisons between optical absorption and millimetre emission broaden this picture, but show that incidence depends on sample selection, sensitivity, and which gas component is measured.
- A volume-limited survey found a high CO detection rate in bright young A-star debris discs, whereas detection around the G-type star HD 129590 established that gas-bearing debris belts are not restricted to A stars. [10][20]
“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.” [10]
1 more supporting sentence
“A crucial finding of our paper is that we detect CO for the first time around the young 10–16 Myr old G1V star HD 129590, similar to our early Sun.” [20]
- Discovery observations did not resolve the gas origin in either HD 21997 or HD 131835, showing that detecting CO is not itself an origin diagnostic. [2][6]
“From our results, neither primordial origin nor steady secondary production from icy planetesimals can unequivocally explain the presence of CO gas in the disk of HD21997.” [2]
1 more supporting sentence
“Our current data cannot conclude on the origin of gas in HD 131835.” [6]
- Although optical absorption studies suggested that hot and cold gas commonly coexist, a subsequent targeted survey found that hot-gas detections could not reliably predict detectable cold CO at its sensitivity. [17][23]
“Although an ISM origin cannot categorically be excluded, the results suggest that the stable and variable absorptions arise from relatively hot gas located in the CS close-in environment of the stars.” [17]
1 more supporting sentence
“This means at this level of sensitivity, we cannot rely on hot gas as an indicator for cold gas in debris discs, and deeper studies are needed.” [23] · full text
Secondary gas tested through location, excitation, and composition
These studies seek evidence that gas is being supplied by solid bodies rather than surviving unchanged from the protoplanetary phase. Belt-associated CO, short survival times, excitation constraints, and abundance measurements support secondary production in particular systems, while inner-disc spectroscopy also reveals gas attributed to evaporating rocky material.
- Resolved CO associated with the dust belt in HD 181327 and a strong gas clump in β Pictoris support release from planetesimals, although the original β Pictoris study allowed more than one dynamical cause for the enhanced collisions. [5][8]
“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.” [5]
1 more supporting sentence
“The CO densities and location suggest a secondary origin, i.e. released from icy planetesimals in the ring.” [8]
- For β Pictoris, multitransition CO analysis argues that H2 cannot provide lifetime-long shielding, while a separate nitrogen measurement supports preferential production of carbon- and oxygen-rich gas over enrichment through selective removal. [9][15]
“As shown for β Pictoris, subthermal (NLTE) CO excitation is symptomatic of H2 densities that are insufficient to shield CO from photodissociation over the system's lifetime.” [9]
1 more supporting sentence
“The solar nitrogen abundance supports the preferential production hypothesis, in which the composition of gas in β Pictoris is the result of photodesorption from icy grains that are rich in C and O or collisional vaporisation of C- and O-rich dust in the disc.” [15]
- A single 2026 study provides especially direct compositional evidence for secondary gas in the CO-rich HD 110058 belt, but its H2 constraint does not formally exclude primordial gas in HD 131488. [28]
“These constraints demonstrate that, at least for HD 110058, the exocometary gas is compositionally distinct and significantly H 2 -poor compared to the <10 −4 CO/H 2 ratios typical of protoplanetary discs.” [28]
- One JWST study identifies a distinct secondary-gas setting in HD 172555: an inner gas disc attributed to evaporation of nearby material rather than solely to an outer icy belt. [27]
“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.” [27]
Secondary gas evolution, shielding, and viscous spreading
These models connect volatile release from planetesimals to CO photodissociation, carbon and oxygen production, and radial gas transport. Adding neutral-carbon shielding and CO self-shielding extends the secondary interpretation to CO-rich discs that had previously appeared to require primordial gas, with observational tests from carbon measurements and resolved CO distributions.
- Secondary-evolution models explain gas as CO released by solids and converted into atomic gas that spreads viscously, with population predictions broadly matching many observations but originally retaining some likely primordial exceptions. [7][11]
“While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside.” [7]
1 more supporting sentence
“This model provides a good qualitative explanation of all current observations, with a few exceptional systems that likely have primordial gas.” [11]
- Carbon shielding offers a secondary explanation for high CO content in HD 131835, 49 Ceti, and HD 32297, so a protoplanetary-scale CO reservoir is not sufficient evidence of primordial origin. [16][18]
“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”.” [16]
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.” [18]
- Time-dependent population modelling finds that gas retains a memory of disc evolution, and high-resolution ARKS observations show extended CO distributions that would require effective shielding and spreading under a secondary interpretation. [19][29]
“We find that the present gas mass in a system is highly dependant on its evolutionary path.” [19]
1 more supporting sentence
“In a second-generation scenario, this would require significant shielding of CO that would allow it to viscously spread to the observed widths.” [29]
Hybrid discs and long-lived primordial reservoirs
The hybrid interpretation combines secondary dust with gas retained from the protoplanetary disc, motivated particularly by large CO masses and gas–dust separation in HD 21997. Disc-dispersal work supplies the broader lifetime problem, while newer models propose that small-grain depletion and weak high-energy irradiation around intermediate-mass stars can permit unusually long gas survival.
- The HD 21997 ALMA study favoured a hybrid disc because secondary production required excessive rates and did not explain the observed gas–dust separation. [3]
“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.” [3]
- Although the dispersal review describes typical disc lifetimes of a few million years followed by rapid clearing, a later semianalytic study argues that small-grain-depleted primordial remnants can account for gas-bearing A-star systems lasting tens of millions of years. [12][24]
“The relatively long disc lifetime (a few Myr) but short disc dispersal timescale ($\sim 10^{5}$ yr) can be explained by: i) photoevaporation taking over long-lived viscously evolving discs; ii) MHD winds if the inner disc can retain most of its magnetic flux while the outer disc loses it during evolution; or iii) a combination of the two, where a thermally driven wind, threaded by magnetic lines, exerts a torque on the gas remaining in the disk and drives accretion.” [12] · full text
1 more supporting sentence
“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.” [24]
- An unreviewed 2026 preprint finds primordial models chemically viable for substantial CO and low CI/CO, but requires weak cosmic-ray ionisation to satisfy HCO+ limits and still predicts overly extended CI emission. [30]
“The standard CR model overproduces HCO+, whereas the weak CR model brings its predicted luminosity within current observational limits.” [30]
Carbon budgets, transients, and molecular tests of gas origin
Joint carbon and CO observations expose limitations in steady secondary-release models that are less apparent when CO is considered alone. These studies investigate recent production events, carbon removal, recycling, and alternative origins, while molecular non-detections show why chemical simplicity does not automatically demonstrate H2-poor gas.
- Independent ALMA and HST analyses find less neutral carbon relative to CO than current secondary models predict, challenging those models without excluding secondary production itself. [25][26]
“We find that current state-of-the-art models of secondary gas production overpredict the C0 content of debris disk gas.” [25]
1 more supporting sentence
“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]
- Individual carbon studies motivate different revisions to secondary evolution: recent production in β Pictoris and rapid carbon removal with possible CO recycling in HD 32297. [13][21]
“No evidence is seen for an atomic accretion disk inward of the CO belt, perhaps because the gas did not yet have time to spread radially.” [13]
1 more supporting sentence
“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.” [21]
- A comparative molecular survey found only CO isotopologues in its debris-disc sample, but concluded that low abundances of several other molecules do not rule out hybrid discs because optically thin dust leaves them vulnerable to photodissociation. [22]
“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.” [22]
Gas–dust dynamics and the ambiguity of disc morphology
Gas can influence the dust structures used to infer planetary architecture and interpret disc evolution. Migration and hydrodynamic models produce rings without planets, while the broader debris-disc review stresses that multiwavelength observations may trace different grain populations rather than one common distribution.
- Simulations show that gas–dust interactions can create rings and eccentric structures otherwise attributed to planets, so dust morphology alone is not a unique indicator of the underlying dynamical explanation. [1][4]
“Dust migration can result in observation of a ring or a bimodal radial dust distribution, possibly very similar to the ones produced by gap-opening planets embedded in the disk, or shepherding it from inside or outside.” [1]
1 more supporting sentence
“We find a robust clumping instability that organizes the dust into narrow, eccentric rings, similar to the Fomalhaut debris disk.” [4]
- The review cautions that scattered-light and thermal images may probe physically distinct dust populations, complicating interpretations based on a single dust morphology. [14]
“Comparison of high-resolution thermal emission and scattered light observations may also reveal that each method is probing a physically distinct dust population (see Figure 3), contrary to standard assumptions.” [14] · full text
Where the papers agree
- CO observations alone are insufficient to settle the origin of CO-rich debris-disc gas; carbon measurements and constraints on H2 provide more discriminating tests. [25][28]
2 supporting sentences
“Our work demonstrates that the combination of C and CO data is a powerful tool to advance our understanding of debris disk gas.” [25]
“Overall, a high CO/H 2 ratio for HD 110058 indicates that the gas in this CO-rich belt is most likely not primordial in composition, supporting the presence of exocometary gas.” [28]
- Recent emission and absorption studies agree that present steady secondary-gas models overpredict neutral carbon relative to CO, although this does not by itself disprove secondary origin. [25][26]
2 supporting sentences
“This does not rule out a secondary origin, but might indicate that the models require an additional C removal process.” [25]
“Hence, the vertically well-mixed, second-generation gas release scenario is missing some important physics or chemistry, one of our assumptions is wrong, or the gas is not of second generation around these systems.” [26] · full text
Where they disagree
- The papers offer competing explanations for the CO-rich debris-disc population: long-lived primordial remnants versus shielded gas produced by planetesimals. [24][16]
- Primordial-retention modelling argues that depleted small-grain populations permit remnant protoplanetary gas to survive around intermediate-mass stars. [24]
1 supporting sentence
“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.” [24]
- Shielded-secondary modelling argues that high CO reservoirs need not be primordial because accumulated neutral carbon protects newly released CO. [16]
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”.” [16]
- Primordial-retention modelling argues that depleted small-grain populations permit remnant protoplanetary gas to survive around intermediate-mass stars. [24]
- Successive β Pictoris studies disagree about whether the observed gas clump supports a migrating planet trapping planetesimals in resonance. [9][13]
- CO mapping favoured resonant migration over a giant impact because the clump is radially broad. [9]
1 supporting sentence
“We find that (1) the CO clump is radially broad, favouring the resonant migration over the giant impact scenario for its dynamical origin, (2) the CO disc is vertically tilted compared to the main dust disc, at an angle consistent with the scattered light warp.” [9]
- Later carbon imaging argued against that resonant interpretation because carbon and CO share the same asymmetry. [13]
1 supporting sentence
“The fact that C and CO share the same asymmetry argues against a previously proposed scenario where the clump is due to an outward-migrating planet trapping planetesimals in a resonance, nor can the observations be explained by an eccentric planetesimal belt secularly forced by a planet.” [13]
- CO mapping favoured resonant migration over a giant impact because the clump is radially broad. [9]
What they leave open
- The physical process that creates the small-grain depletion required by the primordial-survival model remains unresolved. [24]
1 supporting sentence
“A key issue this work leaves open is how some disks become depleted in small dust so that FUV photoevaporation slows.” [24]
- Carbon removal, CO re-formation, vertical structure, and transient production remain alternative explanations for low CI/CO rather than established solutions. [25][26]
2 supporting sentences
“Alternatively, the gas might be produced in transient events rather than a steady-state collisional cascade.” [25]
“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]
- Primordial thermochemical models still require more realistic spatial modelling and stronger observational tests, a limitation explicitly acknowledged by the unreviewed 2026 preprint. [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]
- Direct H2 constraints have not yet resolved the origin even for both CO-rich belts examined in the 2026 absorption study. [28]
1 supporting sentence
“For HD 131488, we further compared the CO photodissociation timescale to the age of the system through simple shielding arguments, and find that we cannot formally rule out a primordial origin; however, we suggest that a more realistic model of CO survival likely supports a secondary origin for this system as well.” [28]
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.
- Two families of exocomets in the β Pictoris system
- Five steps in the evolution from protoplanetary to debris disk
- Inhibition of giant-planet formation by rapid gas depletion around young stars
References
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[1] Dust Migration and Morphology in Optically Thin Circumstellar Gas Disks
Founding gas-dust dynamics modeling · simulation or model
Why it is here: It models gas retention and dust migration in transitional disks, informing how gas can persist into debris-like stages.
Abstract
We analyze the dynamics of gas-dust coupling in the presence of stellar radiation pressure in circumstellar disks, which are in a transitional stage between the gas-dominated, optically thick, primordial nebulae, and the dust-dominated, optically thin Vega-type disks. Dust grains undergo radial migration, either leaving the disk owing to a strong radiation pressure or seeking a stable equilibrium orbit in corotation with gas. In our models of A-type stars surrounded by a total gas mass from a fraction to dozens of Earth masses, the outward migration speed of dust is comparable with the gas sound speed. Equilibrium orbits are circular, with exception of those significantly affected by radiation pressure, which can be strongly elliptic with apocenters extending beyond the bulk of the gas disk. The migration of dust gives rise to radial fractionation of dust and creates a variety of possible observed disk morphologies, which we compute by considering the equilibrium between the dust production and the dust-dust collisions removing particles from their equilibrium orbits. Large grains (typically ≳200 μm) are distributed throughout most of the gas disk. Smaller grains (in the range of 10-200 μm) concentrate in a prominent ring structure in the outer region of the gas disk (presumably at radius ~100 AU), where gas density is rapidly declining with radius. The width and density, as well as density contrast of the dust ring with respect to the inner dust disk, depend on the distribution of gas and the mechanical strength of the particles. Our results open the prospect for deducing the distribution of gas in circumstellar disks by observing their dust. We have qualitatively compared our models with two observed transitional disks around HR 4796A and HD 141569A. Dust migration can result in observation of a ring or a bimodal radial dust distribution, possibly very similar to the ones produced by gap-opening planets embedded in the disk, or shepherding it from inside or outside. We conclude that a convincing planet detection via dust imaging should include specific nonaxisymmetric structure (spiral waves, streamers, resonant arcs) following from the dynamical simulations of perturbed disks.
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[2] MOLECULAR GAS IN YOUNG DEBRIS DISKS
Founding CO survey of debris discs · observational study
Why it is here: Discovers CO around HD 21997 and motivates investigation of gas evolution and its relationship to primordial discs.
Abstract · conclusions of the full text read too
Gas-rich primordial disks and tenuous gas-poor debris disks are usually considered as two distinct evolutionary phases of the circumstellar matter. Interestingly, the debris disk around the young main-sequence star 49 Ceti possesses a substantial amount of molecular gas and possibly represents the missing link between the two phases. Motivated to understand the evolution of the gas component in circumstellar disks via finding more 49 Ceti-like systems, we carried out a CO J = 3−2 survey with the Atacama Pathfinder EXperiment, targeting 20 infrared-luminous debris disks. These systems fill the gap between primordial and old tenuous debris disks in terms of fractional luminosity. Here we report on the discovery of a second 49 Ceti-like disk around the 30 Myr old A3-type star HD21997, a member of the Columba Association. This system was also detected in the CO(2−1) transition, and the reliable age determination makes it an even clearer example of an old gas-bearing disk than 49 Ceti. While the fractional luminosities of HD21997 and 49 Ceti are not particularly high, these objects seem to harbor the most extended disks within our sample. The double-peaked profiles of HD21997 were reproduced by a Keplerian disk model combined with the LIME radiative transfer code. Based on their similarities, 49 Ceti and HD21997 may be the first representatives of a so far undefined new class of relatively old (≳8 Myr), gaseous dust disks. From our results, neither primordial origin nor steady secondary production from icy planetesimals can unequivocally explain the presence of CO gas in the disk of HD21997.
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[3] ALMA OBSERVATIONS OF THE MOLECULAR GAS IN THE DEBRIS DISK OF THE 30 Myr OLD STAR HD 21997
Founding CO isotopologue observations · observational study
Why it is here: Characterizes CO mass and Keplerian structure in HD 21997, providing data used in assessments of its unusual gas origin.
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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[4] Formation of sharp eccentric rings in debris disks with gas but without planets
Founding dust-gas instability modelling · simulation or model
Why it is here: Shows how gas-dust interactions can shape debris discs, a relevant dynamical mechanism but not a direct origin test.
Abstract
‘Debris disks’ around young stars (analogues of the Kuiper Belt in our Solar System) show a variety of non-trivial structures attributed to planetary perturbations and used to constrain the properties of those planets. However, these analyses have largely ignored the fact that some debris disks are found to contain small quantities of gas, a component that all such disks should contain at some level. Several debris disks have been measured with a dust-to-gas ratio of about unity, at which the effect of hydrodynamics on the structure of the disk cannot be ignored. Here we report linear and nonlinear modelling that shows that dust–gas interactions can produce some of the key patterns attributed to planets. We find a robust clumping instability that organizes the dust into narrow, eccentric rings, similar to the Fomalhaut debris disk. The conclusion that such disks might contain planets is not necessarily required to explain these systems.
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[5] Molecular Gas Clumps from the Destruction of Icy Bodies in the β Pictoris Debris Disk
Founding CO clump observations · observational study
Why it is here: Interprets asymmetric CO as evidence for gas released by collisions or exocomets in a debris belt.
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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[6] DISCOVERY OF MOLECULAR GAS AROUND HD 131835 IN AN APEX MOLECULAR LINE SURVEY OF BRIGHT DEBRIS DISKS
Founding molecular line survey · observational study
Why it is here: Adds a CO-bearing debris-disc system, contributing observational evidence on gas occurrence and properties.
Abstract
Debris disks are considered to be gas-poor, but recent observations revealed molecular or atomic gas in several 10–40 Myr old systems. We used the APEX and IRAM 30 m radio telescopes to search for CO gas in 20 bright debris disks. In one case, around the 16 Myr old A-type star HD 131835, we discovered a new gas-bearing debris disk, where the CO 3–2 transition was successfully detected. No other individual system exhibited a measurable CO signal. Our Herschel Space Observatory far-infrared images of HD 131835 marginally resolved the disk at both 70 and 100 μ m, with a characteristic radius of ∼170 AU. While in stellar properties HD 131835 resembles β Pic, its dust disk properties are similar to those of the most massive young debris disks. With the detection of gas in HD 131835 the number of known debris disks with CO content has increased to four, all of them encircling young (≤40 Myr) A-type stars. Based on statistics within 125 pc, we suggest that the presence of a detectable amount of gas in the most massive debris disks around young A-type stars is a common phenomenon. Our current data cannot conclude on the origin of gas in HD 131835. If the gas is secondary, arising from the disruption of planetesimals, then HD 131835 is a comparably young, and in terms of its disk, more massive analog of the β Pic system. However, it is also possible that this system, similar to HD 21997, possesses a hybrid disk, where the gas material is predominantly primordial, while the dust grains are mostly derived from planetesimals.
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[7] A self-consistent model for the evolution of the gas produced in the debris disc of β Pictoris
Founding coupled gas evolution modeling · simulation or model
Why it is here: It models CO released from solids and the subsequent viscous evolution of secondary atomic gas.
Abstract · conclusions of the full text read too
This paper presents a self-consistent model for the evolution of gas produced in the debris disc of β Pictoris. Our model proposes that atomic carbon and oxygen are created from the photodissociation of CO, which is itself released from volatile-rich bodies in the debris disc due to grain–grain collisions or photodesorption. While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The temperature, ionization fraction and population levels of carbon and oxygen are followed with the photodissociation region model cloudy, which is coupled to a dynamical viscous α model. We present new gas observations of β Pic, of C i observed with Atacama Pathfinder EXperiment and O i observed with Herschel, and show that these along with published C ii and CO observations can all be explained with this new model. Our model requires a viscosity α > 0.1, similar to that found in sufficiently ionized discs of other astronomical objects; we propose that the magnetorotational instability is at play in this highly ionized and dilute medium. This new model can be tested from its predictions for high-resolution ALMA observations of C i. We also constrain the water content of the planetesimals in β Pic. The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C, O or CO detections.
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[8] Exocometary gas in the HD 181327 debris ring
Founding resolved CO observations · observational study
Why it is here: Detects and characterizes exocometary CO around an F star, extending the stellar settings in which secondary gas is considered.
Abstract · conclusions of the full text read too
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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Founding ALMA CO structure analysis · observational study
Why it is here: Maps CO structure and derives gas properties in beta Pictoris, informing models of secondary production and gas dynamics.
Abstract
Recent ALMA observations unveiled the structure of CO gas in the 23 Myr old β Pictoris planetary system, a component that has been discovered in many similarly young debris discs. We here present ALMA CO J = 2−1 observations, at an improved spectro-spatial resolution and sensitivity compared to previous CO J = 3−2 observations. We find that (1) the CO clump is radially broad, favouring the resonant migration over the giant impact scenario for its dynamical origin, (2) the CO disc is vertically tilted compared to the main dust disc, at an angle consistent with the scattered light warp. We then use position-velocity diagrams to trace Keplerian radii in the orbital plane of the disc. Assuming a perfectly edge-on geometry, this shows a CO scaleheight increasing with radius as R0.75, and an electron density [derived from CO line ratios through non-local thermodynamic equilibrium (NLTE) analysis] in agreement with thermodynamical models. Furthermore, we show how observations of optically thin line ratios can solve the primordial versus secondary origin dichotomy in gas-bearing debris discs. As shown for β Pictoris, subthermal (NLTE) CO excitation is symptomatic of H2 densities that are insufficient to shield CO from photodissociation over the system's lifetime. This means that replenishment from exocometary volatiles must be taking place, proving the secondary origin of the disc. In this scenario, assuming steady state production/destruction of CO gas, we derive the CO+CO2 ice abundance by mass in β Pic's exocomets to be at most ∼6 per cent, consistent with comets in our own Solar system and in the coeval HD181327 system.
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[10] Molecular Gas in Debris Disks around Young A-type Stars
Founding CO isotopologue survey · observational study
Why it is here: Measures CO in young A-star debris discs and highlights optical-depth issues that affect gas mass and origin inference.
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 secondary gas evolution model · simulation or model
Why it is here: Predicts secondary CO, C and O production and evolution, providing observability tests against debris-disc gas detections.
Abstract · conclusions of the full text read too
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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[12] The dispersal of planet-forming discs: theory confronts observations
Review disc evolution review · review
Why it is here: It reviews gas-rich to debris-disk evolution and provides context for distinguishing retained from secondary gas.
Abstract · conclusions of the full text read too
Discs of gas and dust around million-year-old stars are a by-product of the star formation process and provide the raw material to form planets. Hence, their evolution and dispersal directly impact what type of planets can form and affect the final architecture of planetary systems. Here, we review empirical constraints on disc evolution and dispersal with special emphasis on transition discs, a subset of discs that appear to be caught in the act of clearing out planet-forming material. Along with observations, we summarize theoretical models that build our physical understanding of how discs evolve and disperse and discuss their significance in the context of the formation and evolution of planetary systems. By confronting theoretical predictions with observations, we also identify the most promising areas for future progress.
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[13] ALMA Resolves C i Emission from the β Pictoris Debris Disk
Founding ALMA atomic carbon imaging · observational study
Why it is here: It compares C and CO distributions and uses their masses and spatial structure to infer recent secondary gas production.
Abstract
Abstract The debris disk around β Pictoris is known to contain gas. Previous ALMA observations revealed a CO belt at ∼85 au with a distinct clump, interpreted as a location of enhanced gas production. Photodissociation converts CO into C and O within ∼50 a. We resolve C i emission at 492 GHz using ALMA and study its spatial distribution. C i shows the same clump as seen for CO. This is surprising, as C is expected to quickly spread in azimuth. We derive a low C mass (between 5 × 10−4 and 3.1 × 10−3 ), indicating that gas production started only recently (within ∼5000 a). No evidence is seen for an atomic accretion disk inward of the CO belt, perhaps because the gas did not yet have time to spread radially. The fact that C and CO share the same asymmetry argues against a previously proposed scenario where the clump is due to an outward-migrating planet trapping planetesimals in a resonance, nor can the observations be explained by an eccentric planetesimal belt secularly forced by a planet. Instead, we suggest that the dust and gas disks should be eccentric. Such a configuration, we further speculate, might be produced by a recent tidal disruption event. Assuming that the disrupted body has had a CO mass fraction of 10%, its total mass would be ≳3 M Moon.
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[14] Debris Disks: Structure, Composition, and Variability
Founding Review debris disk structure review · review
Why it is here: It reviews debris-disk observations and composition, providing context for gas-bearing systems and their evolution.
Abstract · conclusions of the full text read too
Debris disks are tenuous, dust-dominated disks commonly observed around stars over a wide range of ages. Those around main sequence stars are analogous to the Solar System's Kuiper Belt and zodiacal light. The dust in debris disks is believed to be continuously regenerated, originating primarily with collisions of planetesimals. Observations of debris disks provide insight into the evolution of planetary systems; and the composition of dust, comets, and planetesimals outside the Solar System; as well as placing constraints on the orbital architecture and potentially the masses of exoplanets that are not otherwise detectable. This review highlights recent advances in multiwavelength, high-resolution scattered light and thermal imaging that have revealed a complex and intricate diversity of structures in debris disks and discusses how modeling methods are evolving with the breadth and depth of the available observations. Two rapidly advancing subfields highlighted in this review include observations of atomic and molecular gas around main sequence stars and variations in emission from debris disks on very short (days to years) timescales, providing evidence of non-steady-state collisional evolution particularly in young debris disks.
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[15] Detection of nitrogen gas in the β Pictoris circumstellar disc
Founding nitrogen abundance spectroscopy · observational study
Why it is here: It uses nitrogen abundance as a diagnostic to distinguish preferential gas production from radiation-driven depletion.
Abstract
Context. The gas composition of the debris disc surrounding β Pictoris is rich in carbon and oxygen relative to solar abundances. Two possible scenarios have been proposed to explain this enrichment. The preferential production scenario suggests that the produced gas may be naturally rich in carbon and oxygen, while the alternative preferential depletion scenario states that the enrichment has evolved to the current state from a gas with solar-like abundances. In the latter case, the radiation pressure from the star expels the gas outwards, leaving behind species that are less sensitive to stellar radiation such as C and O. Nitrogen is not sensitive to radiation pressure either as a result of its low oscillator strength, which would make it also overabundant under the preferential depletion scenario. The abundance of nitrogen in the disc may therefore provide clues to why C and O are overabundant. Aims. We aim to measure the nitrogen column density in the direction of β Pictoris (including contributions by the interstellar medium and circumstellar disc), and use this information to distinguish these different scenarios to explain the C and O overabundance. Methods. Using far-UV spectroscopic data collected by the Hubble Space Telescope’s Cosmic Origins Spectrograph (COS) instrument, we analysed the spectrum and characterised the NI triplet by modelling the absorption lines. Results. We measure the nitrogen column density in the direction of β Pictoris for the first time, and find it to be log(NNI/1 cm2) = 14.9 ± 0.7. The nitrogen gas is found to be consistent with solar abundances and Halley dust. We also measure an upper limit for the column density of MnII in the disc at log(NMnII/1 cm2)CS = 12.7+0.1 and calculate the column density of SIII** in the disc to be log(NSIII★★/1 cm2)CSX = 14.2 ± 0.1. Both results are in good agreement with previous studies. Conclusions. The solar nitrogen abundance supports the preferential production hypothesis, in which the composition of gas in β Pictoris is the result of photodesorption from icy grains that are rich in C and O or collisional vaporisation of C- and O-rich dust in the disc. It does not support the hypothesis that C and O are overabundant because C and O are insensitive to radiation pressure, which would cause them to accumulate in the disc.
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Founding carbon shielding and evolution model · observational study
Why it is here: Combines a carbon detection and modelling to show how apparently primordial CO levels may arise from shielded secondary gas.
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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[17] The co-existence of hot and cold gas in debris discs
Founding optical spectroscopy survey · observational study
Why it is here: Searches for hot gas in systems with cold gas, providing complementary evidence about gas properties and possible origins.
Abstract · conclusions of the full text read too
Context. Debris discs have often been described as gas-poor discs as the gas-to-dust ratio is expected to be considerably lower than in primordial, protoplanetary discs. However, recent observations have confirmed the presence of a non-negligible amount of cold gas in the circumstellar (CS) debris discs around young main-sequence stars. This cold gas has been suggested to be related to the outgassing of planetesimals and cometary-like objects. Aims. The goal of this paper is to investigate the presence of hot gas in the immediate surroundings of the cold-gas-bearing debris-disc central stars. Methods. High-resolution optical spectra of all currently known cold-gas-bearing debris-disc systems, with the exception of β Pic and Fomalhaut, have been obtained from La Palma (Spain), La Silla (Chile), and La Luz (Mexico) observatories. To verify the presence of hot gas around the sample of stars, we have analysed the Ca II H&K and the Na I D lines searching for non-photospheric absorptions of CS origin, usually attributed to cometary-like activity. Results. Narrow, stable Ca II and/or Na I absorption features have been detected superimposed to the photospheric lines in 10 out of the 15 observed cold-gas-bearing debris-disc stars. Features are found at the radial velocity of the stars, or slightly blue- or red-shifted, and/or at the velocity of the local interstellar medium (ISM). Some stars also present transient variable events or absorptions extended towards red wavelengths (red wings). These are the first detections of such Ca II features in 7 out of the 15 observed stars. Although an ISM origin cannot categorically be excluded, the results suggest that the stable and variable absorptions arise from relatively hot gas located in the CS close-in environment of the stars. This hot gas is detected in at least ~80%, of edge-on cold-gas-bearing debris discs, while in only ~10% of the discs seen close to face-on. We interpret this result as a geometrical effect, and suggest that the non-detection of hot gas absorptions in some face-on systems is due to the disc inclination and likely not to the absence of the hot-gas component. This gas is likely released in physical processes related in some way to the evaporation of exocomets, evaporation of dust grains, or grain-grain collisions close to the central star.
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[18] New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297
Founding isotopologue gas observations · observational study
Why it is here: It measures CO isotopologues and interprets high CO content using secondary production and shielding models.
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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[19] Population synthesis of exocometary gas around A stars
Founding exocometary gas population synthesis · simulation or model
Why it is here: Models secondary gas production, carbon shielding and viscous evolution to explain observed CO around A stars.
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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[20] Survey of planetesimal belts with ALMA: gas detected around the Sun-like star HD 129590
Founding ALMA CO survey · observational study
Why it is here: It detects CO in a young debris system and frames the finding in the context of secondary gas.
Abstract · conclusions of the full text read too
ABSTRACT Gas detection around main-sequence stars is becoming more common with around 20 systems showing the presence of CO. However, more detections are needed, especially around later spectral type stars to better understand the origin of this gas and refine our models. To do so, we carried out a survey of 10 stars with predicted high likelihoods of secondary CO detection using ALMA in band 6. We looked for continuum emission of mm-dust as well as gas emission (CO and CN transitions). The continuum emission was detected in 9/10 systems for which we derived the discs’ dust masses and geometrical properties, providing the first mm-wave detection of the disc around HD 106906, the first mm-wave radius for HD 114082, 117214, HD 15745, HD 191089, and the first radius at all for HD 121191. A crucial finding of our paper is that we detect CO for the first time around the young 10–16 Myr old G1V star HD 129590, similar to our early Sun. The gas seems colocated with its planetesimal belt and its total mass is likely in the range of (2–10) × 10−5 M⊕. This first gas detection around a G-type main-sequence star raises questions as to whether gas may have been released in the Solar system as well in its youth, which could potentially have affected planet formation. We also detected CO gas around HD 121191 at a higher signal-to-noise ratio than previously and find that the CO lies much closer-in than the planetesimals in the system, which could be evidence for the previously suspected CO viscous spreading owing to shielding preventing its photodissociation. Finally, we make estimates for the CO content in planetesimals and the HCN/CO outgassing rate (from CN upper limits), which we find are below the level seen in Solar system comets in some systems.
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[21] The Surprisingly Low Carbon Mass in the Debris Disk around HD 32297
Founding carbon and CO gas modeling · observational study
Why it is here: It combines carbon and CO measurements with production, shielding, and removal modeling to test secondary-gas scenarios.
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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[22] Lack of other molecules in CO-rich debris discs: is it primordial or secondary gas?
Founding molecular line observations · observational study
Why it is here: Compares molecular abundances in CO-rich debris and Herbig Ae discs to test whether their gas resembles primordial material.
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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[23] The search for gas in debris discs: ALMA detection of CO gas in HD 36546
Founding ALMA CO survey · observational study
Why it is here: Adds a CO detection and upper limits in debris discs, informing the occurrence and distribution of gas relevant to origin studies.
Abstract · conclusions of the full text read too
ABSTRACT Debris discs represent the last stages of planet formation and as such are expected to be depleted of primordial gas. None the less, in the last few years the presence of cold gas has been reported in ∼20 debris discs from far-infrared to (sub-)mm observations and hot gas has been observed in the optical spectra of debris discs for decades. While the origin of this gas is still uncertain, most pieces of evidence point towards a secondary origin, as a result of collisions and evaporation of small bodies in the disc. In this paper, we present ALMA observations aimed at the detection of CO gas in a sample of eight debris discs with optical gas detections. We report the detection of CO (12CO and 13CO) gas in HD 36546, the brightest and youngest disc in our sample, and provide upper limits to the presence of gas in the remaining seven discs.
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[24] A Primordial Origin for the Gas-rich Debris Disks around Intermediate-mass Stars
primordial disc evolution model · simulation or model
Why it is here: Proposes that long-lived primordial gas can explain gas-rich debris discs around intermediate-mass stars and identifies retention mechanisms.
Abstract
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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[25] Primordial or Secondary? Testing Models of Debris Disk Gas with ALMA*
ALMA gas-model comparison · observational study
Why it is here: Compares observed C and CO with secondary-production models to assess whether the gas can be explained as secondary.
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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[26] Low CI/CO abundance ratio revealed by HST UV spectroscopy of CO-rich debris discs
Recent UV spectroscopy and gas modelling · observational study
Why it is here: Uses low CI/CO ratios and secondary-gas simulations to assess whether two CO-rich discs can have a secondary origin.
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] MINDS: Detection of an Inner Gas Disk Caused by Evaporating Bodies around HD 172555
Recent infrared gas spectroscopy · observational study
Why it is here: It detects inner-disk gas attributed to evaporating close-in bodies, supporting secondary gas production.
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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[28] High CO/H 2 ratio supports an exocometary origin for a CO-rich debris disc
Recent infrared absorption spectroscopy · observational study
Why it is here: Directly measures H2 relative to CO in CO-rich belts, testing the H2-poor secondary versus H2-rich primordial predictions.
Abstract
Context . Over 20 exocometary belts host detectable circumstellar gas, mostly in the form of CO. Two competing theories for its origin have emerged, positing that the gas is either primordial or secondary. Primordial gas survives from the belt’s parent protoplanetary disc and is therefore H 2 -rich. Secondary gas is outgassed in situ by exocomets and is relatively H 2 -poor. Discriminating between these scenarios has not been possible for belts that host unexpectedly large quantities of CO. Aims . We aim to break this gas origin dichotomy through direct measurement of H 2 column densities in two edge-on, CO-rich exocometary belts around ∼15 Myr-old A-type stars, constraining the CO/H 2 ratio and CO gas lifetimes. Observing edge-on belts enables rovibrational absorption spectroscopy against the stellar background. Methods . We present near-IR CRIRES+ spectra of HD 110058 and HD 131488, which provide the first direct probe of H 2 gas in CO-rich exocometary belts. We targeted the H 2 (v=1-0 S(0)) line at 2223.3 nm and the 12 CO v = 2 → 0 rovibrational lines in the range 2333.8-2335.5 nm and derived constraints on column densities along the line of sight to the stars. Results . We detect 12 CO strongly, but not H 2 , in the CRIRES+ spectra. This allows us to place 3σ lower limits on the CO/H 2 ratios of >1.35 × 10 −3 and >3.09 × 10 −5 for HD 110058 and HD 131488, respectively. These constraints demonstrate that, at least for HD 110058, the exocometary gas is compositionally distinct and significantly H 2 -poor compared to the <10 −4 CO/H 2 ratios typical of protoplanetary discs. For HD 131488, we further compared the CO photodissociation timescale to the age of the system through simple shielding arguments, and find that we cannot formally rule out a primordial origin; however, we suggest that a more realistic model of CO survival likely supports a secondary origin for this system as well. Overall, a high CO/H 2 ratio for HD 110058 indicates that the gas in this CO-rich belt is most likely not primordial in composition, supporting the presence of exocometary gas.
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[29] The ALMA survey to Resolve exoKuiper belt Substructures (ARKS)
Recent resolved CO line survey · observational study
Why it is here: It measures CO distributions and masses in debris belts, evidence relevant to gas production and retention.
Abstract
Context . CO gas is detected in a significant number (~20) of debris discs (exoKuiper belts), but understanding its origin and evolution remains elusive. Crucial pieces of evidence are its mass and spectro-spatial distribution, which are coupled through optical depth and have only been analysed at low to moderate resolution so far. The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) is the first ALMA large program to target debris discs at high spectro-spatial resolution. Aims . We used 12 CO and 13 CO J=3-2 line data of 18 debris belts observed by ARKS, 5 of which were already known to be gas-bearing, in order to analyse the spectro-spatial distribution of CO and constrain the gas mass in discs that were known to host gas previously, and to search for gas in the remaining 13 discs without previous CO detections. Methods . We developed a line-imaging pipeline for ARKS CO data with a high spectro-spatial resolution. Using this tool, we produced line cubes for each of the ARKS targets, with a spatial resolution down to about 70 mas and a spectral resolution of 26 m s −1 . We used spectro-spatial shifting and stacking techniques to produce a gallery of maps with the highest possible signal-to-noise ratio (S/N) and with radial and spectral profiles that reveal the distribution and kinematics of gas in five gas-bearing discs at unprecedented detail. Results . For each of the five gas-bearing discs (HD 9672/49 Ceti, HD 32297, HD 121617, HD 131488, and HD 131835), we constrained the inner radius of the 12 CO ( r min ~ 3-68 au), and we found that the radial brightness profile of CO peaked interior to the dust ring, but that CO was also more radially extended than the dust. In a second-generation scenario, this would require significant shielding of CO that would allow it to viscously spread to the observed widths. We present the first radially resolved 12 CO/ 13 CO isotopologue flux ratios in five gas-bearing debris discs and found them to be constant with radius for the majority (four out of five) of systems. This indicates that 12 CO and 13 CO are both optically thick or optically thin throughout the discs. We report CO line fluxes or upper limits for all systems and optical depth dependant masses for the five systems with detected CO. Finally, we analysed the 12 CO J=3-2 line luminosities for a range of ARKS debris discs and for debris discs from the literature. We confirm that gas is mostly detected in young systems. However, the high scatter seen i
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[30] Thermochemical constraints on a primordial-origin of gas-rich debris disks
Recent Preprint primordial thermochemical modelling · simulation or model
Why it is here: Tests whether primordial remnants reproduce observed CO, carbon and HCO+ constraints that challenge secondary-gas models.
Abstract
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+.