Observational constraints on CO sequestration and disk mass in the planet-hosting PDS 70 disk

M. Leemker; S. Facchini; F. Zagaria; K. Doi; J. Bae; M. Benisty; H. B. Liu; C. Longarini; V. Pezzotta; L. Rampinelli; L. M. Stapper; R. Teague

AI summary · arXiv: 2611.14632v1 · astro-ph.EP

Observational constraints on CO sequestration and disk mass in the planet-hosting PDS 70 disk

M. Leemker, S. Facchini, F. Zagaria, K. Doi, J. Bae, M. Benisty, H. B. Liu, C. Longarini,

and 4 more authors

V. Pezzotta, L. Rampinelli, L. M. Stapper, R. Teague

Original paper Submitted to arXiv on 10 Nov. 2026 Accepted for publication in A&A License: CC BY 4.0 Also in: fr

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The essentials

PDS 70’s disk has a low inferred CO abundance, and including C17O substantially lowers its estimated mass, suggesting CO sequestration may affect the atmospheres of its planets.

Question

How much CO is present in the PDS 70 disk’s molecular layers, and what disk mass is supported by its molecular-line and continuum observations? The study also asks whether the planets may accrete material from CO-sequestered gas.

Method

The authors present ALMA detections of N2H+ 1–0 and C17O 3–2, and compare integrated fluxes for N2H+, CO isotopologues, and continuum with an existing grid of thermochemical models. They report a disk-mass estimate of 0.039 solar masses before including C17O and a roughly 3.4-fold decrease when its flux is included; inferred peak CO abundances span 4.8×10−6 to 17×10−6 relative to H2 under alternative assumptions about N2H+ emission.

Findings

Result The study reports the first detections of N2H+ and C17O in the PDS 70 disk.
See the passage in the paper
We present the first detection of N2H+ and C17O in this disk.
Result C17O emission has marginal statistical evidence of an azimuthal asymmetry that trails the asymmetry previously observed in continuum emission.
See the passage in the paper
The C17O emission shows marginal statistical evidence for an azimuthal asymmetry trailing that previously seen in the continuum emission.
Result Including C17O line flux in the fit lowers the derived disk mass by a factor of about 3.4.
See the passage in the paper
We find that the derived disk mass drops by a factor of ∼ 3.4 from 0.039+0.067 −0.006 M⊙ if the C17O line flux is included in the fitting procedure; the quoted intervals do not include systematic uncertainties.
Result The inferred effective CO abundance is below the interstellar-medium reference abundance in the probed disk layers, after accounting for freeze-out and isotope-selective photodissociation.
See the passage in the paper
CO on the other hand is sequestered with an effective abundance of (4.8+4.3−2.7 − 17+8−8) × 10−6 relative to H2 in the probed disk layers after accounting for CO freeze-out and isotope selective CO photodissociation for the adopted chemical network and ionization prescription.
Authors’ interpretation The authors suggest that PDS 70’s planets could be accreting their final atmospheric material from an outer-disk molecular layer with sequestered CO.
See the passage in the paper
Therefore, the PDS 70 planets are possibly accreting the final portion of their atmospheres from the outer disk molecular layer where CO is sequestered.

Limitations

The disk-mass intervals do not include systematic uncertainties.
See the passage in the paper
the quoted intervals do not include systematic uncertainties.
The peak CO abundance depends on uncertainty in N2H+ emission and on whether that emission is assumed to be optically thin or thick.
See the passage in the paper
The range in peak CO abundances reflects the uncertainty in the N2H+ emission, assuming either optically thin emission at 21 K or optically thick emission.

Why it matters

The authors argue that CO sequestration should be considered when interpreting planetary-atmosphere chemistry. Their analysis also supplies disk-mass and CO-abundance constraints for comparison with other disks studied using the same framework.

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: PDS 70 protoplanetary disk carbon monoxide depletion N2H+ disk mass

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Comprehensive Summary
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Original abstract

The paper investigates the amount of carbon monoxide (CO) remaining in the gas of the PDS 70 protoplanetary disk and the disk mass inferred from its molecular and dust emission. PDS 70 is a particularly valuable system for studying planet formation because it is the first disk in which two accreting planets have been directly confirmed. The amount of volatile material available to growing planets may influence their atmospheric composition. However, CO emission from many disks around young stars is weaker than expected based on interstellar-medium abundances. Freeze-out, dust settling, chemical conversion, and selective photodissociation can all reduce the observable gas-phase CO. Using ALMA observations, the authors report the first detections of the N₂H⁺ 1–0 and C¹⁷O 3–2 lines in PDS 70. They combine these measurements with previously reported CO-isotopologue line fluxes and continuum measurements. N₂H⁺ helps constrain CO abundance because gas-phase CO destroys N₂H⁺, while C¹⁷O provides a useful tracer of the disk’s gas. The observations have limited spectral resolution, preventing the N₂H⁺ hyperfine components from being separated. The authors image the lines and continuum, measure integrated fluxes, and compare the results with an existing grid of thermochemical disk models. This analysis estimates the disk mass and the peak CO abundance in the molecular layers probed, accounting for CO freeze-out and isotope-selective photodissociation. The measured integrated fluxes are 76 ± 14 mJy km/s for N₂H⁺ 1–0 and 390 ± 45 mJy km/s for C¹⁷O 3–2. The C¹⁷O map shows marginal statistical evidence of an azimuthal asymmetry trailing an asymmetry previously observed in the dust continuum, but the evidence is insufficient to establish a clear physical association. Including the C¹⁷O flux in the model comparison substantially changes the inferred disk mass. The estimate decreases by a factor of about 3.4, from 0.039 solar masses (+0.067/−0.018) to 0.011 solar masses (+0.011/−0.006). These uncertainty ranges do not include systematic errors. The inferred effective peak CO abundance is far below the canonical interstellar value of about 10⁻⁴ relative to molecular hydrogen. Depending on how the N₂H⁺ emission is treated, the estimated abundance ranges from 4.8 × 10⁻⁶ (+4.3/−2.7 × 10⁻⁶) to about 17 × 10⁻⁶ (±8 × 10⁻⁶). This range reflects uncertainty over whether the N₂H⁺ emission is optically thin at an assumed temperature of 21 K or optically thick. The results indicate substantial CO sequestration in the observed disk layers, accounting for freeze-out and selective photodissociation under the adopted chemical network and ionization prescription. The authors conclude that planets in PDS 70 may be accreting the final portion of their atmospheres from an outer-disk molecular layer depleted in CO. Interpreting planetary atmospheric compositions may therefore require accounting for CO sequestration in the natal disk. These conclusions depend on the adopted thermochemical models and chemical assumptions, and the reported mass intervals omit systematic errors. The ambiguity in N₂H⁺ optical depth also limits the precision of the CO abundance estimate.

What Is Left in the PDS 70 Disk?

The young star PDS 70 hosts a particularly important laboratory for studying planet formation: it is the first known protoplanetary disk in which two accreting planets have been directly confirmed. A new study, “Observational constraints on CO sequestration and disk mass in the planet-hosting PDS 70 disk,” examines how much carbon monoxide remains in the disk’s gas and what that tells us about the material available to the planets.

The research, by M. Leemker, S. Facchini, F. Zagaria, K. Doi, J. Bae, M. Benisty et al., combines new observations with thermochemical models. Its findings suggest that the disk’s observable molecular layers contain far less carbon monoxide than expected from the standard interstellar abundance—and that including a new gas tracer substantially revises the estimated disk mass.

Why Carbon Monoxide Matters

CO is commonly used to estimate the amount of gas in a protoplanetary disk. But measurements of many disks around young stars show weaker CO emission than expected. Several processes may explain the difference: CO can freeze onto dust grains in cold regions, dust can settle toward the disk midplane, chemical reactions can convert CO into other molecules or solids, and ultraviolet radiation can selectively break apart certain CO isotopes.

This matters beyond measuring the disk itself. The volatile material available to growing planets may influence the composition of their atmospheres. If planets accrete their final atmospheric material from gas with depleted CO, their chemical makeup may differ from what estimates based on standard interstellar abundances would suggest.

Two Molecular Tracers, Two Clues

The team used observations from ALMA to report the first detections of two molecular lines in PDS 70: N₂H⁺ 1–0 and C¹⁷O 3–2. They also included previously reported CO-isotopologue lines and continuum measurements in their analysis.

These molecules offer complementary information. Gas-phase CO destroys N₂H⁺, so the amount of N₂H⁺ can help constrain how much CO remains in the gas. C¹⁷O, meanwhile, provides another way to trace the disk’s gas. The researchers imaged the lines and dust continuum, measured their integrated fluxes, and compared the results with an existing grid of thermochemical disk models.

The observations did not have enough spectral resolution to separate the individual hyperfine components of the N₂H⁺ line. That limitation is important: interpreting the line as optically thin or optically thick leads to different estimates of the CO abundance.

What the Observations Reveal

The measured integrated fluxes were 76 ± 14 mJy km/s for N₂H⁺ 1–0 and 390 ± 45 mJy km/s for C¹⁷O 3–2. The C¹⁷O map also showed marginal statistical evidence for an azimuthal asymmetry trailing an asymmetry previously seen in the dust continuum. The evidence is not strong enough to establish a clear physical connection between the two features.

Including the new C¹⁷O measurement substantially changes the inferred disk mass. The estimate falls by about a factor of 3.4, from 0.039 solar masses, with a reported range of +0.067/−0.018, to 0.011 solar masses, with a range of +0.011/−0.006. These intervals do not include systematic errors, so they do not capture every source of uncertainty.

The models also point to a low effective peak CO abundance in the molecular layers probed. Depending on how the N₂H⁺ emission is treated, the estimated abundance ranges from 4.8 × 10⁻⁶, with uncertainties of +4.3/−2.7 × 10⁻⁶, to about 17 × 10⁻⁶, with an uncertainty of ±8 × 10⁻⁶. Both estimates are well below the canonical interstellar value of about 10⁻⁴ relative to molecular hydrogen.

Implications for the Growing Planets

The results support the idea that CO is substantially sequestered in the observed disk layers, with the models accounting for freeze-out and isotope-selective photodissociation. Under the study’s adopted chemical network and ionization prescription, the gas from which planets may acquire their final atmospheric material could therefore be depleted in CO.

For PDS 70’s planets, this raises an important consideration: interpreting their atmospheric compositions may require accounting for the chemical state of the natal disk, rather than assuming that its gas retained typical interstellar proportions of CO.

Important Caveats

The conclusions depend on the adopted thermochemical models and chemical assumptions. The disk-mass ranges omit systematic errors, and the uncertainty about whether the N₂H⁺ emission is optically thin or thick limits the precision of the CO abundance estimate. The new observations offer valuable constraints, but they do not remove every ambiguity about the disk’s composition.

Even with these caveats, the study shows why multiple tracers matter. A single molecular line can give an incomplete picture of a disk; combining CO isotopologues, N₂H⁺, and dust emission produces a more informative—and substantially revised—view of the material surrounding a planet-hosting star.

Created on 13 Nov. 2026

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Observational constraints on CO sequestration and disk mass in the planet-hosting PDS 70 disk

The essentials

Question

Method

Findings

Limitations

Why it matters

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Definitions

What Is Left in the PDS 70 Disk?

Why Carbon Monoxide Matters

Two Molecular Tracers, Two Clues

What the Observations Reveal

Implications for the Growing Planets

Important Caveats

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