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Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks

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Pith's one-line read This paper finds that CO gas abundance in planet-forming disks varies by at least an order of magnitude with radius, so constant-abundance CO-based disk mass and radius estimates are biased.

desk verdict First radially resolved CO abundance profiles in four disks show order-of-magnitude variation, but the size of the claim depends on an untested gas surface density shape and an unfinished plot caption. read the letter →

arxiv 1908.03267 v1 pith:DOQVZA7N submitted 2019-08-08 astro-ph.EP astro-ph.SR

Ke Zhang , Edwin A. Bergin , Kamber R. Schwarz , Sebastiaan Krijt , Fred Ciesla This is my paper · ORCID
classification astro-ph.EPastro-ph.SR
keywords COabundanceprotoplanetarydisksisotopologuesradialvariationdustevolutionpebbledriftastrochemistrydiskgasmass
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Carbon monoxide (CO) is the standard tracer for gas in planet-forming disks, but its abundance is usually assumed constant at the interstellar ratio. This study compares spatially resolved CO-isotopologue images of four disks with thermo-chemical models to derive how the CO gas abundance changes with distance from the star. It finds a variation of at least an order of magnitude within each disk, with a common pattern of strong depletion in an intermediate zone and a recovery farther out. It also shows that chemistry alone puts the depletion too deep to explain the faint lines, while dust growth and the inward drift of icy grains naturally produce the observed radial shape. If correct, constant-abundance CO measurements of disk mass, radius, and gas-to-dust ratio are systematically biased.

What carries the argument

The method is an abundance-rescaling comparison: for each disk a two-dimensional thermo-chemical model with time-dependent chemistry and line radiative transfer is calibrated to known disk and stellar parameters, then the model's CO abundance is multiplied by a radius-independent rescaling factor for each annulus until the predicted C18O line surface brightness matches the observed deprojected radial profile; that factor defines the depletion. The interpretive machinery is pebble transport: CO freezes outside the mid-plane snowline, icy grains grow, settle, and drift inward, releasing vapor inside the snowline and leaving a depleted intermediate zone that recovers in the outer disk. A second candidate, cosmic-ray-driven chemical conversion, is tested but fails because the depletion occurs too deep.

What would settle it

Resolved maps of a rarer CO isotopologue such as C17O, using a known isotopologue ratio, would settle the claim: if C17O emission falls as steeply as the rescaled C18O models require, the abundance variation is real; if C17O tracks a constant interstellar ratio, the depletion is an optical-depth artifact. Alternatively, resolved HD line observations of true gas surface density at the same annuli would directly compare gas column to CO-derived column.

Watch

Extended reading notes

Core claim

The central claim is that the CO gas abundance in the warm molecular layer is not constant but varies strongly with radius in all four disks: DM Tau, TW Hya, HD 163296, and IM Lup. The derived profiles show an order-of-magnitude range within each disk, with TW Hya's outer disk depleted by factors of 100-300. The paper compares these profiles with two candidate mechanisms. Chemical processing with an interstellar cosmic-ray ionization rate can convert enough CO, but mostly in the deep disk, leaving the observable layer nearly unchanged. Dust evolution, in which CO-ice-coated grains grow, settle, and drift inward, predicts the right radial pattern - modest enhancement inside the snowline, peak depletion just beyond it, and recovery outward - but underestimates the depletion magnitude. The paper concludes that a coupled chemical-dynamical treatment is needed and that the CO abundance profile is a record of volatile transport.

Load-bearing premise

The derivation assumes that C18O emission (and TW Hya 13CO beyond 70 AU) is optically thin and that multiplying the model's CO abundance by a constant factor at each radius accurately represents the true abundance; if optical depth or isotope-selective photodissociation contributes to the faint emission, the derived order-of-magnitude radial variation could be partly a brightness effect.

Editorial extensions

If this is right

  • Disk masses and radii derived from CO lines under a constant abundance will carry a radius-dependent systematic error, not just a uniform offset.
  • Radial CO depletion profiles become a practical probe of dust evolution: the location and depth of the depletion zone locate where pebble growth and inward drift have stripped volatile ice.
  • Predictions for planet-forming volatile inventories must couple chemistry with the dynamical transport of icy grains, since either alone underpredicts the depletion.
  • The extreme outer CO deficit in TW Hya implies either severe CO removal or actual gas depletion beyond 100 au, which changes interpretations of its outer disk.
  • In HD 163296 the CO enhancement just inside its snowline is a signpost for icy pebbles crossing the snowline and vaporizing there.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the derived profiles are correct, resolved images of a rarer isotopologue such as C17O should show the same radial swing; if not, the C18O depletion would be partly an optical-depth artifact.
  • A testable extension is that disks with more evolved dust, inferred from stronger millimeter continuum or larger grain sizes, should show deeper CO depletion in the intermediate zone with the depletion peak shifting outward.
  • Comparing other carbon-bearing molecules, such as C2H, at the same radii could reveal whether the pattern reflects a general elemental carbon depletion rather than a CO-specific process.
  • Resolved hydrogen-deuteride gas surface density measurements would convert brightness-derived depletion factors into true abundances.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The paper uses spatially resolved ALMA C18O (and, for TW Hya, 13CO) observations of four protoplanetary disks — DM Tau, TW Hya, HD 163296, and IM Lup — together with the RAC2D thermo-chemical code and parametric disk structures to infer how the CO gas abundance in the warm molecular layer varies with radius. For each disk, the authors compute a standard constant-abundance chemical model, generate synthetic line images, and then rescale the model CO abundance by a radius-dependent constant factor to match the observed radial line brightness profiles. They report that CO is depleted by an order of magnitude or more relative to the interstellar ratio, that the depletion varies by at least an order of magnitude within individual disks, and that the radial depletion patterns in three of the four disks qualitatively resemble predictions from dust-evolution models that include pebble formation, settling, and drift. They also argue that chemical processing alone, even with a high cosmic-ray ionization rate, cannot explain the weak CO isotopologue emission because the depletion occurs too deep in the disk.

Significance. If the central claim holds, it would be an important result: constant-abundance CO-based estimates of disk mass, disk radius, and gas-to-dust ratio would be systematically biased, and the derived radial depletion profiles would provide a new observational constraint on dust evolution and volatile transport. The paper has notable strengths: a homogeneous four-disk framework, detailed physical-chemical modeling, explicit tests of the cosmic-ray ionization rate and of the surface-density power-law index, and a clear statement that the total gas mass normalization is not the dominant uncertainty. The use of optically thin C18O and 13CO lines is appropriate in principle, and the comparison to Krijt et al. (2018) is useful even if qualitative. However, the derivation rests on several assumptions — the shape of the gas surface density profile, optically thin emission, and fixed isotopologue ratios — that are not fully stress-tested, and the key Figure 8 presents depletion factors without propagated uncertainties. These issues are addressable, but they are load-bearing for the order-of-magnitude claim.

major comments (3)
  1. [§4.4, eq. (1), Table 4] The radial CO abundance profile is derived by comparing observed line brightness to a single model per disk with a fixed parametric gas surface density shape. The paper varies only the power-law index γ by ±0.2 (§4.3) and keeps the characteristic radius R_c and the outer exponential taper fixed. This is a concern because the inversion compares I_CO to the model's assumed gas column: any mismatch in the assumed density shape is absorbed into the inferred abundance variation. The concern is concrete for DM Tau, where R_c = 270 AU is set 'based on the extent of C18O (3-2) emission' (Table 4). If the true gas surface density has a smaller R_c or a steeper outer taper, the constant-abundance model is fainter in the outer disk, and the inferred rise of CO abundance with radius would be reduced or disappear. The statement that total gas mass normalization is not crucial does not address the shape of the profile. Please vary R_c and the outer taper (or justify them with independent constraints) and quantify how much of the inferred order-of-magnitude variation in Figure 8 survives.
  2. [§3.3 and §4.4] The inversion assumes that the observed C18O surface brightness is proportional to the CO column in the emitting layer, and that fixed interstellar isotopologue ratios (CO/13CO = 69, CO/C18O = 570) apply at all radii. If C18O emission is not fully optically thin in the regions where depletion factors are derived, or if isotope-selective photodissociation changes the local C18O/CO ratio, the derived abundance variation would partly be a brightness variation. The text cites previous work supporting the optical thinness of TW Hya 13CO beyond 70 AU, but for the other disks the C18O optical depth is asserted rather than demonstrated. Please include a per-radius check — for example, model optical depth profiles or a comparison of C18O J = 2-1 and 3-2 where available — to verify that I_line ∝ N_CO holds at the radii used in Figure 8.
  3. [Figure 8 and §4.4] The central quantitative claim — at least an order-of-magnitude radial variation in CO abundance — is presented without propagated uncertainties. The observed radial profiles in Figure 7 have 1σ error bars and the flux calibration uncertainty is stated as 10%, but the depletion factors in Figure 8 have no error bars or shaded ranges. The γ test in §4.3 shows that line brightness changes by less than 30%, but a 30% brightness change can translate into a substantially larger change in depletion factor because the mapping is nonlinear and because the depletion factor is normalized to a model that itself depends on other disk parameters. Please add uncertainty ranges to the four panels of Figure 8, or otherwise quantify how robust the radial gradient is to noise, calibration, and the adopted disk structure parameters.
minor comments (5)
  1. [Figure 8 caption] The caption contains placeholder text ('Lorem ipsum') and a stray fragment ('ebea'); these should be removed before publication.
  2. [Abstract and §5.1.3] There are typos: 'V ariations' in the title line should be 'Variations', and 'Shakura-Sunyeav' in §5.1.3 should be 'Shakura-Sunyaev'.
  3. [Table 4] The row labeled 'H100 (AU)' lists no numerical values; the adopted gas scale heights at 100 AU should be given explicitly for all four disks.
  4. [§3.3] The text says 'φ from 1 to 1.3' when describing the flaring-parameter grid; this should be ψ to match eq. (3).
  5. [§5.1.2] The statement that observed depletions are 'a factor of few higher' than the Krijt et al. (2018) simulations is vague; please give the actual ranges (for example, factor of 10 or larger observed versus factor of 2-6 predicted).

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: CO abundance profiles are observational inferences; comparison models are independent and not used to fit.

full rationale

The central inference (Section 4.4) is a data-constrained derivation, not a prediction forced by inputs. Observed C18O/13CO radial brightness profiles are compared to thermo-chemical models that assume a constant CO abundance, and the CO abundance is then rescaled at each radius by the factor needed to match the observed surface brightness. This is a model-to-data inference: the output abundance profile is a residual of observed emission relative to a constant-abundance model, so the order-of-magnitude radial variation is not an input assumption. The paper explicitly tests the sensitivity of the brightness profiles to ±0.2 changes in the gas surface-density exponent and finds <30% changes (Section 4.3), showing the inferred variations are not trivially set by the density slope. The R_c and flaring choices are taken from independent literature constraints (scattered light, SED, continuum); the DM Tau R_c is admittedly based on the C18O emission extent, which is a limitation for the absolute pattern at the outermost radii, but it is not a redefinition of the abundance result. The comparison to Krijt et al. (2018) and Schwarz et al. (2018) involves co-authors and is qualitative; these models are not used to fit the observed CO profiles, and the paper reports that the predicted depletion is a factor of a few lower than observed, so the conclusion is not imported by self-citation. The thermo-chemical code RAC2D and radiative transfer code RADMC3D are independent numerical tools. No equation or parameter is defined in terms of the claimed radial abundance variation, and no fitted parameter is renamed as a prediction. Hence no circular step can be exhibited.

Assumptions & free parameters 5 free parameters · 5 assumptions · 0 invented entities

The central claim rests on a model-dependent inversion: adopted disk density structures, dust settling choices, chemical initial conditions, isotopologue ratios, and the optically thin assumption. No new physical entities are postulated.

free parameters (5)
  • h0 (gas scale height at 100 AU) = DM Tau 6 AU; TW Hya 6 AU; HD 163296 8.5 AU; IM Lup 8 AU
    Fitted to SEDs via a grid search over h0 and psi in Section 3.3; adopted in the disk models that set the line brightness baseline.
  • psi (flaring index) = DM Tau 1.25; TW Hya 1.25; HD 163296 1.08; IM Lup 1.2
    Fitted to SEDs in the same grid search; strongly affects the temperature and vertical structure of the disk.
  • Small-to-large dust mass split = 20% small / 80% large; IM Lup 1% small / 99% large
    Chosen from previous studies, not fitted here; affects chemical processing rates and UV penetration in the models.
  • Large grain settling parameter chi_mm = 0.2
    Adopted value for the settled large-grain population; changes the vertical distribution of dust and the thermal structure.
  • Radial CO depletion factor f_dep(R) = Best-fit per radius, shown in Fig. 8; ranges roughly 0.1 to 300 across disks
    Derived by matching model C18O/13CO brightness to observed radial profiles. This is the central output of the paper rather than an input, but the central claim is built on these fitted numbers.
assumptions (5)
  • domain assumption Initial carbon is entirely in CO at abundance 1.4e-4 relative to H atoms
    Section 3.3 initialization; sets the baseline of uniform CO abundance that is later rescaled to match observations.
  • domain assumption No CO isotopologue fractionation; fixed ISM ratios CO/13CO=69 and CO/C18O=570
    Section 3.3; if C18O is selectively photodissociated in the outer disk, part of the inferred radial depletion could be an artifact of the fixed ratio.
  • domain assumption C18O low-J emission is optically thin beyond about 30 AU, and TW Hya 13CO (3-2) is optically thin beyond 70 AU
    Sections 1 and 3.3; the rescaling method assumes surface brightness is proportional to CO column in the emitting layer.
  • domain assumption Model disk density structures and stellar parameters adopted from the literature are accurate enough for the radial abundance inference
    Section 3.2 and Table 4; wrong surface density profiles would change the derived depletion factors, though gamma-index tests suggest modest sensitivity.
  • domain assumption Static density structure with no vertical mixing during chemical evolution
    Section 3.3; the conclusion that chemistry alone cannot deplete the surface layer is a direct consequence, and vertical mixing could alter it.

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Cite this review

Pith. "Pith review of Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks." pith.science (2026). https://pith.science/paper/DOQVZA7N

@misc{pith2026190803267,
  author       = {Pith},
  title        = {Pith review of: Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/DOQVZA7N}},
  note         = {Machine review of arXiv:1908.03267}
}
read the original abstract

CO is the most widely used gas tracer of protoplanetary disks. Its abundance is usually assumed to be an interstellar ratio throughout the warm molecular layer of the disk. But recent observations of low CO gas abundance in many protoplanetary disks challenge our understanding of physical and chemical evolutions in disks. Here we investigate the CO abundance structures in four well-studied disks and compare their structures with predictions of chemical processing of CO and transport of CO ice-coated dust grains in disks. We use spatially resolved CO isotopologue line observations and detailed thermo-chemical models to derive CO abundance structures. We find that the CO abundance varies with radius by an order of magnitude in these disks. We show that although chemical processes can efficiently reduce the total column of CO gas within 1 Myr under an ISM level of cosmic-ray ionization rate, the depletion mostly occurs at the deep region of a disk. Without sufficient vertical mixing, the surface layer is not depleted enough to reproduce weak CO emissions observed. The radial profiles of CO depletion in three disks are qualitatively consistent with predictions of pebble formation, settling, and drifting in disks. But the dust evolution alone cannot fully explain the high depletion observed in some disks. These results suggest that dust evolution may play a significant role in transporting volatile materials and a coupled chemical-dynamical study is necessary to understand what raw materials are available for planet formation at different distances from the central star.

Figures

Figures reproduced from arXiv: 1908.03267 by the authors.

Figure 1
Figure 1. Left column: Moment zero map (integrated intensity) of CO isotopologue line observations (continuum subtracted). The axes are labeled with angular offsets from the disk center, and the synthesized beam is shown in the lower-left corner of each panel. RIght column: deprojected radial line intensity profiles of each line. The shaded areas indicate a 1σ uncertainty. The deprojections used the disk geometric parameters … view at source ↗
Figure 2
Figure 2. An outline of the modeling processes. The output of each step is highlighted in red. lar velocity. For the large grain population, we use γmm=1, Rmm c =135 AU, and an inner radius of 20 AU based on the model of Andrews et al. (2011), which matches the 880µm continuum ob￾servations of DM Tau. • TW Hya: we adopt the general gas surface den￾sity profile of TW Hya derived from scattered light images from van Boekel et a… view at source ↗
Figure 3
Figure 3. The best-fitting SED models of the four disks. The photometry data are black dots and the best-fittings models are overlaid as solid grey lines. 3.2.2. Stellar parameters We adopt the effective temperatures and luminosi￾ties of the central stars from the literature and rescale the stellar luminosities using the latest Gaia 2 measure￾ments. The inclination and position angles are adopted from previous analysis of spa… view at source ↗
Figures from the paper (5 more)
Figure 4
Figure 4. Figure 4: Chemical structures of the standard models (ζCR=1.36×10−18 s −1 ). First column: CO gas abundance; second column: gas temperature. The 20K contours are plotted to guide eyes for a general expectation of CO freeze-out location, but the actually freeze-out level depends …
Figure 5
Figure 5. Figure 5: Comparison between high cosmic-ray ionization models and standard models (ζCR = 1.36×10−17 s −1 vs. 1.36×10−18 s −1 ). Top: CO gas column density in high ζCR (blue dash line) and standard models (black solid line). Bottom: C18O line surface brightness profiles in the m…
Figure 6
Figure 6. Figure 6: Top row: The total gas surface density profiles for models with different γ. Middle row: the CO gas column density profiles after 1 Myr chemical evolution and a ζCR of 1.36×10−18 s −1 . Bottom row: model C18O line brightness distributions vs. observations (grey line) …
Figure 7
Figure 7. Figure 7: Comparison of observed radial intensity profiles to models with different CO depletion factors. The observed profiles and their 1σ uncertainties are in light blue and the models are in black. 2016). The intermediate region beyond the CO mid￾plane snowline has shorter t…
Figure 8
Figure 8. Figure 8: (a-d) The radial profiles of CO depletion factor in four disks. The black arrow indicates the location of CO snowline at the disk mid-plane and the grey shaded areas indicate the beam size of the observations. (e): the radial profile of CO depletion of the warm molecul…

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Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

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