Up in smoke: carbon-destruction in the terrestrial planet-forming region of a young Class I disk

F. J. Ballieux; M. K. McClure; E. A. Bergin; M. L. R. Van 't Hoff; Ł. Tychoniec; L. Francis; E. F. Van Dishoeck

doi:10.3847/2041-8213/aebc81

AI summary · arXiv: 2611.04557v1 · astro-ph.SR

Up in smoke: carbon-destruction in the terrestrial planet-forming region of a young Class I disk

F. J. Ballieux, M. K. McClure, E. A. Bergin, M. L. R. Van 't Hoff, Ł. Tychoniec, L. Francis, E. F. Van Dishoeck

Original paper · DOI 10.3847/2041-8213/aebc81 Submitted to arXiv on 03 Nov. 2026 10 pages, 3 figures, 1 table. Accepted to the Astrophysical Journal Letters License: CC BY-NC-SA 4.0 Also in: fr

How to read this summary? Illustrated guide

The essentials

JWST and ALMA observations of HOPS-68 reveal warm carbon-bearing gas in a disk wind and colder carbon chains near outflow cavity walls, motivating a proposed route for carbon redistribution and chain formation.

Question

What is the origin and location of the diverse hydrocarbons observed around the young Class I protostar HOPS-68? The authors assess whether thermal release of carbon in the inner disk, followed by transport and chemistry in outflows, could help explain terrestrial carbon depletion and the observed carbon chains.

Method

The authors analyzed JWST/MIRI absorption bands with LTE slab models to estimate molecular temperatures, column densities, and velocities, using Bayesian fitting; the JWST exposure totaled 222 seconds. They also examined archival ALMA band 6 C2H observations at 262.065 GHz to map its emission relative to the H2 jet and outflow cavity.

Findings

Result The slab fits yield warm HCN and C2H2, with temperatures of 319 K and 432 K and column densities of 4.5 × 10^17 cm−2 and 1.1 × 10^17 cm−2, respectively.
See the passage in the paper
The best-fitting slab model for HCN returns a temperature of $319_{-24}^{+22}$ K and a column density of $4.5_{-0.7}^{+1.1}\times 10^{17}\,\rm{cm^{-2}}$. The $\rm{C_{2}H_{2}}$ is best fit by a slab model with a temperature of $432_{-16}^{+15}$ K and a column density of $1.1_{-0.1}^{+0.1}\times 10^{17}\,\rm{cm^{-2}}$.
Authors’ interpretation The warm HCN and C2H2 are strongly blueshifted, and the authors associate them with the same warm molecular layer as oxygen-bearing species at the base of the disk wind.
See the passage in the paper
The temperatures of HCN and $\rm{C_{2}H_{2}}$ (between $319$ and $432\,\rm{K}$) and blue-shifted velocities (by between $53$ and $39\,\rm{km\,s^{-1}}$ with respect to the source velocity), are similar to those found in paper A for the oxygen-bearing species, so we conclude that these hydrocarbons are in the same molecular layer.
Result CH3, C4H2, and CS have fitted temperatures near 40–54 K, while C6H2 is detected but cannot be reliably fitted with the slab models.
See the passage in the paper
With temperatures of $44_{-7}^{+7}\,\rm{K}$ and $38_{-11}^{+12}\,\rm{K}$ respectively, these hydrocarbons are relatively cold.
Authors’ interpretation ALMA C2H emission brackets the H2 jet and is interpreted as tracing outflowing material along the cavity walls rather than infalling material.
See the passage in the paper
These clearly bracket the JWST/MIRI observations of the $\rm{H_{2}\,S(3)}$ jet, indicating that $\rm{C_{2}H}$ traces a wider component than the jet, likely the outflow cavity walls. Furthermore, the blue-shift of the $\rm{C_{2}H}$ emission follows that of the jet, indicating that it traces outflowing, rather than infalling, material.
Authors’ interpretation The authors propose that carbon released from grains in an inner-disk destruction zone is carried outward by the disk wind, while UV-driven reactions in the cavity walls can form longer carbon chains.
See the passage in the paper
We suggest that the warm simple hydrocarbons such as $\rm{C_{2}H_{2}}$ sublimate from dust grains in a "refractory grain destruction zone" in the inner disk, and are subsequently entrained in the disk wind.

Limitations

The study does not quantify how much carbon the disk wind removes or how much this could contribute to Earth's carbon depletion; that analysis is left for future work.
See the passage in the paper
To constrain the amount of carbon carried off in the wind, and to estimate its contribution to the abundances observed on Earth, we would have to quantitatively constrain the amount of gas in the disk wind, using e.g. the IR $\rm{H_{2}}$ lines or mm CO observations. Such an analysis is outside of the scope of this letter, but will be done in future work.
The proposed cavity-wall carbon-chain formation pathway is not confirmed, and the authors call for larger-scale observations and chemical modeling.
See the passage in the paper
To determine whether the observed carbon chain species are indeed formed through UV-induced chemistry in the outer disk or envelope, we would need larger-scale (sub)-mm observations of carbon-chain species, to see if their location coincides with the observed $\rm{C_{2}H}$ emission.
The CH4 fit is not robust because overlapping features and continuum choices allow a wide range of fitted temperatures.
See the passage in the paper
Varying the continuum subtraction allowed us to fit a large range of temperatures (between $\sim$ 10 and 1000 K) to the band, indicating that the fitting results from this band are not robust.

Why it matters

The authors argue that inner-disk thermal processing and winds may provide a mechanism contributing to carbon depletion in terrestrial planet-forming material. They also propose that some carbon-chain chemistry attributed to infalling-envelope WCCC may instead occur in outflow cavity walls, with possible relevance to aliphatic organics in meteorites and asteroids.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: protoplanetary disks carbon chemistry disk winds JWST spectroscopy warm carbon-chain chemistry

Read further

Comprehensive Summary
Layman's Summary
Blog article
Original abstract

The paper investigates whether thermal processing of carbon-rich dust in a young protoplanetary disk can release carbon into the gas, potentially helping explain why Earth is so carbon-poor relative to the Sun. The authors study HOPS-68, a Class I protostar whose inner disk is thought to reach temperatures high enough to evaporate refractory material. They also explore how the observed hydrocarbons might form and whether carbon-chain chemistry usually attributed to warm carbon-chain chemistry (WCCC) could instead be linked to outflows. The team analyzes JWST Mid-Infrared Instrument spectra with local thermodynamic equilibrium absorption-slab models to estimate molecular temperatures, column densities, and velocities. They fit several bands individually or in combination, including features that overlap with water absorption. They also examine archival ALMA observations of C₂H, a molecule expected to form through ultraviolet processing of acetylene. Their interpretation builds on earlier work identifying a roughly 1,400 K inner-disk “thermostat” region and a warmer molecular layer associated with the base of a disk wind. The spectra show warm HCN and acetylene, with fitted temperatures of about 319 K and 432 K, respectively, and column densities of approximately 4.5 × 10¹⁷ and 1.1 × 10¹⁷ molecules per square centimeter. Their strongly blueshifted velocities—roughly 39–53 kilometers per second relative to the source—support an origin in the outflowing molecular layer. A second acetylene band yields a similar temperature, although some of its line parameters were fixed during fitting. The authors also report ¹³CCH₂, but the band is too faint for reliable modeling. Other detected species appear colder. The fitted temperatures are about 44 K for CH₃, 38 K for C₄H₂, and 54 K for CS. These molecules may be located in the upper outer disk, envelope, or outflow-cavity walls, but the data do not establish their locations. The study also reports a feature near 16.1 micrometers attributed to C₆H₂, described as the first detection of this species in a protostellar system. However, the authors could not model it because the necessary molecular data were unavailable. The identification is uncertain, as longer polyynes can have similar bending-mode wavelengths. A methane feature is also detected, but its inferred temperature is highly sensitive to continuum choices because it overlaps with other gas and ice features. The authors therefore do not rely on the fitted methane temperature. ALMA maps show C₂H emission extending along the outflow-cavity walls, with velocity structure consistent with outflowing rather than infalling gas. The authors propose that carbon-bearing material released from dust between roughly 500 K—the soot line—and about 1,200 K, where amorphous carbon can evaporate, is carried outward by a disk wind. In the cavity, ultraviolet light could convert acetylene into C₂H, which might then react with acetylene to form longer chains such as C₄H₂ and C₆H₂. The authors suggest that this process could contribute both to carbon loss from terrestrial planet-forming regions and to the aliphatic organic material found in meteorites and asteroids. They also propose that some polycyclic aromatic hydrocarbons (PAHs) might form through carbon combustion in the hot region, although they do not detect PAH emission in the on-source spectrum. These interpretations remain suggestive rather than established. The paper does not measure the wind’s total gas mass or quantify how much carbon it removes, and the proposed chemistry has not been confirmed through detailed modeling or spatially resolved observations of the longer carbon chains.

Can a young star’s disk send carbon up in smoke?

Earth is strikingly poor in carbon compared with the Sun. One possible explanation is that carbon-rich dust was heated and chemically transformed in the region where rocky planets formed, then carried away before it could become part of a planet. In “Up in smoke: carbon-destruction in the terrestrial planet-forming region of a young Class I disk,” F. J. Ballieux, M. K. McClure, E. A. Bergin, M. L. R. Van 't Hoff, Ł. Tychoniec, L. Francis et al. investigate whether this process may be taking place around the young protostar HOPS-68.

The study focuses on a potential route for moving carbon: heat releases carbon from dust grains near the inner disk, and a disk wind transports it outward. There, ultraviolet light may help turn simple carbon molecules into longer chains. The observations offer clues in support of this picture, but do not yet show how much carbon the wind carries or prove the proposed chemistry.

Looking for carbon in warm gas

HOPS-68 is a Class I protostar, a young object surrounded by a disk and envelope. Earlier work suggested that its inner disk includes a region near 1,400 kelvin, where conditions may allow refractory material to evaporate. The researchers also considered a warmer molecular layer near the base of a disk wind.

To investigate the gas, the team analyzed mid-infrared spectra from JWST’s Mid-Infrared Instrument. They used absorption-slab models, assuming local thermodynamic equilibrium, to estimate molecules’ temperatures, column densities, and velocities. Because some molecular bands overlap with water absorption and other features, the team fitted different spectral regions separately or together, depending on the case.

The clearest warm-gas detections were hydrogen cyanide (HCN) and acetylene. Their fitted temperatures were about 319 kelvin and 432 kelvin, with column densities of approximately 4.5 × 1017 and 1.1 × 1017 molecules per square centimeter, respectively. Both showed strong blueshifts—about 39 to 53 kilometers per second relative to the source. That motion points toward gas flowing outward, rather than sitting quietly in the disk. A second acetylene band gave a similar temperature, though some parameters had to be held fixed during its fit.

The researchers also report a faint feature attributed to ¹³CCH₂, but it was too weak for reliable modeling. Methane appears in the spectra too, yet its inferred temperature changes substantially depending on how the surrounding continuum is chosen. Since it overlaps other gas and ice features, the authors avoid drawing conclusions from that temperature.

Cooler molecules and a possible first

Several other detected molecules appear much colder: the fitted temperatures are about 44 kelvin for CH₃, 38 kelvin for C₄H₂, and 54 kelvin for CS. They could come from the upper outer disk, the envelope, or the walls of the outflow cavity. The available spectra do not reveal their locations precisely.

A feature near 16.1 micrometers is tentatively attributed to C₆H₂. If confirmed, it would be the first detection of this molecule in a protostellar system. However, the identification remains uncertain: other long carbon-chain molecules can have similar bending-mode wavelengths. The team could not model the feature because the molecular data needed for that analysis were unavailable.

Clues from the outflow cavity

The study also revisits archival observations from ALMA, which map emission from C₂H, a carbon-bearing molecule expected to form when ultraviolet radiation processes acetylene. The emission extends along the walls of HOPS-68’s outflow cavity, and its velocity structure is consistent with outward-moving gas rather than infall.

Together, the infrared and ALMA observations motivate a possible sequence. Carbon-bearing material could be released from dust between roughly 500 kelvin—the “soot line”—and about 1,200 kelvin, where amorphous carbon can evaporate. A disk wind might then carry that material outward. In the cavity, ultraviolet light could convert acetylene into C₂H; reactions between C₂H and acetylene might build longer chains such as C₄H₂ and C₆H₂.

Heat may free carbon from dust in the inner disk.
A disk wind may transport some of that carbon into the outflow cavity.
Ultraviolet-driven reactions may produce small hydrocarbons and longer carbon chains.

A promising idea, not yet a carbon budget

If this pathway operates, it could help explain why terrestrial planet-forming regions—and ultimately rocky planets—end up depleted in carbon. The authors also suggest that related processing might contribute to aliphatic organic material found in meteorites and asteroids. They raise another possibility: some polycyclic aromatic hydrocarbons, or PAHs, could form through carbon combustion in the hot region. However, the researchers did not detect PAH emission in the on-source spectrum.

Important uncertainties remain. The study does not measure the total gas mass in the wind or calculate how much carbon it removes. The proposed reaction sequence has not been tested with detailed chemical modeling, and longer carbon chains have not been mapped at the spatial resolution needed to establish where they form. The observations therefore provide suggestive evidence for carbon processing and transport—not a complete account of carbon loss.

Even so, HOPS-68 offers a compelling place to investigate how young planetary systems sort their carbon. Future spatially resolved observations and chemical models could determine whether the apparent trail from hot inner dust to outflowing hydrocarbons is a major part of the story.

Created on 04 Nov. 2026

Was this synthesis useful?

Score: 0

Ask questions about this paper to our AI assistant

How to chat with this paper? Illustrated guide

Compare with the PDF

You can also chat with multiple papers at once here.

AI assistant instructions?

Tools for this paper

Explain it differently 3 free rewrites left today

Save to a collection Group papers in collections to compare them, chat with them and export them. Create a free account Log in

Export BibTeX · RIS (Zotero, Mendeley, EndNote) · Obsidian (Markdown) · Notion (Markdown)

Advanced tools Premium

For researchers who follow a field closely: citation alerts, journal-club slides, version comparison and Zotero export. Discover Premium

Zotero

Citation alerts Citation alerts guide

Who works on it? The people and institutions who publish on its topic

Journal club Premium+ Slides to present it Propose for the journal club

Versions What changed between versions

Talk about it in public Premium+ Press kit: release, questions, hype check Press kit guide

Papers around this one

See them on a map

Ranked by how close their title and abstract are to this paper’s. Select some to read them together.

Closest papers

Close Same question
Carbon Depletion in the Early Solar System
Fabian Binkert, Til Birnstiel · 2023 · Earth and Planetary Astrophysics · cited 11 times · AI summary available

Carbon Depletion in the Early Solar System models how refractory carbon is released and transported in disks to explain carbon depletion in rocky bodies.

Abstract
Earth and other rocky objects in the inner Solar System are depleted in carbon compared to objects in the outer Solar System, the Sun, or the ISM. It is believed that this is a result of the selective removal of refractory carbon from primordial circumstellar material. In this work, we study the irreversible release of carbon into the gaseous environment via photolysis and pyrolysis of refractory carbonaceous material during the disk phase of the early Solar System. We analytically solve the one-dimensional advection equation and derive an explicit expression that describes the depletion of carbonaceous material in solids under the influence of radial and vertical transport. We find both dep…
Close Broader context
A Deep Search for Five Molecules in the 49 Ceti Debris Disk
Jessica Klusmeyer, A. Meredith Hughes, Luca Matra et al. · 2021 · Solar and Stellar Astrophysics · cited 5 times · AI summary available

A Deep Search for Five Molecules in the 49 Ceti Debris Disk searches for molecular gas in a debris disk, rather than hydrocarbons around a young protostar.

In short
A deep ALMA search detected none of five molecules beyond CO in 49 Ceti, supporting selective carbon shielding rather than necessarily unusual exocometary ice composition.
Close Broader context
Thermochemical constraints on a primordial-origin of gas-rich debris disks
H. Mitani, W. Ooyama, R. Nakatani et al. · 2026 · arXiv (Cornell University) preprint · AI summary available

Thermochemical constraints on a primordial-origin of gas-rich debris disks tests the origin of gas in evolved debris disks, not carbon processing in a young disk.

In short
Thermochemical models support a primordial origin for CO-rich debris disks under dust-poor, weak-ionisation conditions, but predict neutral-carbon emission more extended than observed.
Close Broader context
Thermochemical constraints on a primordial-origin of gas-rich debris disks
H. Mitani, W. Ooyama, R. Nakatani et al. · 2026 · Earth and Planetary Astrophysics · AI summary available

Thermochemical constraints on a primordial-origin of gas-rich debris disks tests the origin of gas in evolved debris disks, not carbon processing in a young disk.

In short
Dust-poor primordial-remnant disks can reproduce substantial CO, low neutral-carbon/CO ratios, and weak HCO+ under suppressed cosmic-ray ionisation, but their predicted neutral-carbon emission is too extended.
Close Broader context
JWST-TST High Contrast: Medium-resolution spectroscopy reveals a carbon-rich circumplanetary disk around the young accreting exoplanet Delorme 1 AB b
Mathilde Mâlin, Kimberly Ward-Duong, Sierra L. Grant et al. · 2025 · Earth and Planetary Astrophysics · cited 6 times · AI summary available

JWST-TST High Contrast uses JWST/MIRI spectroscopy to study carbon-rich material around a young accreting planet, rather than a protostellar disk.

Abstract
Young accreting planetary-mass objects are thought to draw material from a circumplanetary disk (CPD) composed of gas and dust. While the gas within the disk is expected to disperse within the first million years, strong accretion has nonetheless been detected in older systems, including the 30--45 Myr-old planetary-mass companion Delorme 1 AB b. We conducted spectroscopic observations with JWST/MIRI to investigate the presence of circumplanetary material around this young, accreting planet, and to characterize the planet's atmospheric properties and composition. We perform forward modeling using atmospheric models to characterize the planet's atmosphere. Beyond 10 microns, the SED becomes d…
Close Broader context
Carbon monoxide gas produced by a giant impact in the inner region of a young system
Tajana Schneiderman, Luca Matrà, Alan P. Jackson et al. · 2021 · Nature 598, 425-428 (2021) · cited 12 times · Not summarized yet

Carbon monoxide gas produced by a giant impact studies gas from a planetary collision in a young system, not carbon chemistry in a protostellar outflow.

Abstract
Models of terrestrial planet formation predict that the final stages of planetary assembly, lasting tens of millions of years beyond the dispersal of young protoplanetary disks, are dominated by planetary collisions. It is through these giant impacts that planets like the young Earth grow to their final mass and achieve long-term stable orbital configurations. A key prediction is that these impacts produce debris. To date, the most compelling observational evidence for post-impact debris comes from the planetary system around the nearby 23 Myr-old A star HD 172555. This system shows large amounts of fine dust with an unusually steep size distribution and atypical dust composition, previously…
Close Broader context
Planet formation in the PDS 70 system: Constraining the atmospheric chemistry of PDS 70b and c
A. J. Cridland, S. Facchini, E. F. van Dishoeck et al. · 2023 · A&A 674, A211 (2023) · cited 5 times · Not summarized yet

Planet formation in the PDS 70 system models C2H and CO in a protoplanetary disk to relate disk chemistry to planetary atmospheres.

Abstract
Understanding the chemical link between protoplanetary disks and planetary atmospheres is complicated by the fact that the popular targets in the study of disks and planets are widely separated both in space and time. The 5 Myr PDS 70 systems offers a unique opportunity to directly compare the chemistry of a giant planet's atmosphere to the chemistry of its natal disk. To that end, we derive our current best physical and chemical model for the PDS 70 disk through forward modelling of the $^{12}$CO, C$^{18}$O, and C$_2$H emission radial profiles with the thermochemical code DALI and find a volatile C/O ratio above unity in the outer disk. Using what we know of the PDS 70 disk today, we analyt…
Close Broader context
The hybrid disks: a search and study to better understand evolution of disks
J. Pericaud, E. Di Folco, A. Dutrey et al. · 2016 · A&A 600, A62 (2017) · cited 33 times · AI summary available

The hybrid disks surveys gas-rich debris disks to investigate disk evolution, rather than carbon release and hydrocarbon chemistry in a young disk.

Abstract
The increased sensitivity of millimeter-wave facilities now makes possible the detection of low amounts of gas in debris disks. Some of the gas-rich debris disks harbor peculiar properties, with possible pristine gas and secondary generated dust. The origin of the gas in these hybrid disks is strongly debated and the current sample is too sparse to understand this phenomenon. More detections are necessary to increase the statistics on this population. Lying at the final stages of evolution of proto-planetary disks and at the beginning of the debris disk phase, these objects could provide new insight into the processes involved in the making of planetary systems. We carried out a deep survey …

Go further

How are these papers found?

We compare the title and abstract of this paper with those of every paper summarized on this site, and with the papers Semantic Scholar recommends, those it cites and those citing it (for a much-cited paper, its most recent citations and, from OpenAlex, the most cited ones). Among the prior work and the follow-ups, the papers that use each other in depth, a method it uses and the much-cited follow-ups come a little higher. The levels (very close, close, related) come from the similarity of the texts, not from a judgment of quality; * marks a paper without an abstract, compared by its title. The sentence under a paper is written by an AI from the two abstracts: check it in the papers. A paper marked not relevant by three readers leaves the list.

Updated on 4 November 2026.

Disclaimer: The AI-based summarization tool and virtual assistant provided on this website may not always provide accurate and complete summaries or responses. We encourage you to carefully review and evaluate the generated content to ensure its quality and relevance to your needs.

Up in smoke: carbon-destruction in the terrestrial planet-forming region of a young Class I disk

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Can a young star’s disk send carbon up in smoke?

Looking for carbon in warm gas

Cooler molecules and a possible first

Clues from the outflow cavity

A promising idea, not yet a carbon budget

Ask questions about this paper to our AI assistant

Tools for this paper

Advanced tools Premium

Papers around this one

Closest papers

Go further

Cite this paper (BibTeX)