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Modelling methanol and hydride formation in the JWST Ice Age era

T0 review · 2 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read Five astrochemical codes find methanol ice forms predominantly (>99%) via CO hydrogenation, not the water-rich CH3+OH and C+H2O routes.

desk verdict First multi-code test of the new methanol routes under Ice Age conditions; qualitative dominance of CO hydrogenation is plausible, but the >99% 'negligible' claim needs a rate-constant sensitivity run. read the letter →

arxiv 2502.10123 v1 pith:5PPDG5ZR submitted 2025-02-14 astro-ph.GA astro-ph.SR

Izaskun Jiménez-Serra , Andrés Megías , Joseph Salaris , Herma Cuppen , Angèle Taillard , Miwha Jin , Valentine Wakelam , Anton I. Vasyunin
show 14 more authors
Paola Caselli Yvonne J. Pendleton Emmanuel Dartois Jennifer A. Noble Serena Viti Katerina Borshcheva Robin T. Garrod Thanja Lamberts Helen Fraser Gary Melnick Melissa McClure Will Rocha Maria N. Drozdovskaya Dariusz C. Lis
This is my paper · ORCID
classification astro-ph.GAastro-ph.SR
keywords interstellaricesmethanolformationCOhydrogenationgrain-surfacechemistryastrochemicalmodellingJWSTIceAgeprogramChamaeleonImolecularcloud
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

The paper asks which chemical route builds methanol ice in the dense, heavily obscured gas that JWST's Ice Age program samples toward two background stars in Chamaeleon I. By running five independent astrochemical codes over a grid of densities, dust temperatures, extinctions, and collapse times, it claims that more than 99% of $\mathrm{CH_3OH}$ ice forms by successive hydrogenation of CO on dust grains, while the recently proposed water-rich routes $\mathrm{CH_3 + OH \rightarrow CH_3OH}$ and $\mathrm{C + H_2O \rightarrow H_2CO}$ are negligible. It also claims that matching the observed ice column densities requires gas densities $\gtrsim 10^5\,\mathrm{cm^{-3}}$ and collapse times $\gtrsim 10^5\,\mathrm{yr}$, and that non-diffusive surface chemistry taking over below about 12 K may explain the nearly constant ice composition toward dense cores with very different extinctions. If true, methanol in these prestellar ices is a tracer of CO freeze-out rather than of a water-rich environment, which shifts how the 3 $\mu$m ice band and the organic budget delivered to planet-forming disks are interpreted.

What carries the argument

The load-bearing machinery is a competition between methanol formation routes on interstellar dust grains: the CO hydrogenation chain ($\mathrm{CO \rightarrow HCO \rightarrow H_2CO \rightarrow CH_3O \rightarrow CH_3OH}$) versus the two water-rich alternatives $\mathrm{CH_4 + OH \rightarrow CH_3 + H_2O}$ / $\mathrm{CH_3 + OH \rightarrow CH_3OH}$ and $\mathrm{C + H_2O \rightarrow H_2CO}$. Around these reactions the paper builds a grid of free-fall collapse models spanning gas density, dust temperature, extinction, and collapse time, executed with five codes that differ in whether chemistry is solved by rate equations or stochastically and whether non-diffusive (in-place) surface reactions are included. The kinetic Monte Carlo lattice simulation is the decisive instrument for the $\mathrm{CO_2}$ sub-question: because it follows individual surface species, it shows that both the $\mathrm{CO+O}$ channel and the HOCO-mediated $\mathrm{CO+OH}$ channel contribute comparably despite the tiny rate constant of the former, a conclusion rate-equation codes cannot reach cleanly.

What would settle it

Resolve the ice along the NIR38 and J110621 sightlines in space or in kinematic components and measure whether CH3OH ice (9.8 micron feature) appears in regions where CO has not frozen out (gas-phase depletion factor near 1). Since the paper's dominance claim requires CO on the grains for hydrogenation, a sightline or sub-region with little CO depletion but methanol ice at the observed 4-9% of water would falsify the claim that CO hydrogenation is the sole significant route.

Watch

Extended reading notes

Core claim

On the paper's own terms, the central discovery is that $\mathrm{CH_3OH}$ ice toward the highly extinguished lines of sight NIR38 and J110621 forms predominantly (>99%) via the classical grain-surface hydrogenation chain $\mathrm{CO \rightarrow HCO \rightarrow H_2CO \rightarrow CH_3O \rightarrow CH_3OH}$. The alternative formation paths proposed for water-rich ices—$\mathrm{CH_4 + OH \rightarrow CH_3 + H_2O}$ followed by $\mathrm{CH_3 + OH \rightarrow CH_3OH}$, and $\mathrm{C + H_2O \rightarrow H_2CO}$ with subsequent hydrogenation—contribute negligibly in all five codes, and even when $\mathrm{CH_4}$ is abundant (as in the Uclchem models) the $\mathrm{CH_3 + OH}$ route stays below about 1.4% of the methanol produced. The paper further reports that the models best matching the JWST ice abundances have $n_{\mathrm H} \gtrsim 2\times10^5\,\mathrm{cm^{-3}}$ and collapse times $\gtrsim 10^5\,\mathrm{yr}$; that $\mathrm{CO_2}$ ice forms via $\mathrm{CO+OH}$ or $\mathrm{CO+O}$ depending on the code, with KMC simulations showing both are efficient because atomic O is highly mobile on the surface; that $\mathrm{CH_4}$ is underproduced except in Uclchem, where extra atomic C survives the translucent phase; and that at $T_{\mathrm{dust}}<12$ K non-diffusive chemistry replaces diffusive chemistry, which is proposed as the reason ice compositions look similar across dense cores.

Load-bearing premise

The models treat each observed sightline as a single homogeneous parcel collapsing at a constant dust and gas temperature, with no grain growth, ice morphology, or line-of-sight structure, and they take the literature rate constants for the alternative methanol routes as fixed.

Editorial extensions

If this is right

  • Ice observations toward high-extinction background stars should be read as evidence of CO freeze-out and hydrogenation, not of methanol formation in water-rich ice.
  • Dense cores with $n_{\mathrm H}\gtrsim 2\times10^5\,\mathrm{cm^{-3}}$ and collapse timescales $\gtrsim 10^5\,\mathrm{yr}$ are the environments whose ices match JWST; fast, low-density collapse is ruled out.
  • At dust temperatures below about 12 K, non-diffusive surface chemistry buffers ice abundances against temperature differences, explaining the similar ice compositions seen in Chamaeleon I and other cores despite different extinctions.
  • Chemical models of $\mathrm{CO_2}$ ice should include both $\mathrm{CO+O}$ and the HOCO-mediated $\mathrm{CO+OH}$ routes, since both can be efficient even when one has a low rate constant.
  • Methanol and methane ice abundances track the availability of atomic carbon during the translucent phase, so reproducing $\mathrm{CH_4}$ requires models that carry enough atomic C into the dense phase.

Reading between the lines

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

  • If the >99% CO hydrogenation result transfers to other clouds, then the 3 $\mu$m red-wing excess toward dense sightlines may not require ammonia-hydrate mixtures; grain-growth interpretations like the one this paper cites could become the default reading.
  • A testable extension: map $\mathrm{CH_3OH}$ and CO ice absorption along resolved sub-structures in Chamaeleon I; if $\mathrm{CH_3OH}$ ice appears where CO is not depleted, the homogeneous-collapse assumption and the dominance claim would both be testable.
  • If non-diffusive chemistry indeed buffers ice composition below 12 K, then laboratory work on binding-energy distributions and in-place reaction probabilities becomes the key unknown for predicting ice inventories in cold cores.
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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

2 major / 6 minor

Summary. This paper models the ice chemistry toward two highly extincted background stars in the Chamaeleon I cloud observed by JWST, using five gas-grain codes with different architectures (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC). The models follow a translucent-cloud phase and then a free-fall collapse over a grid of final densities, collapse timescales, dust temperatures, and visual extinctions, with the chemical networks augmented by the alternative, water-rich methanol routes CH4+OH, CH3+OH, and C+H2O. A two-step median absolute error (MAE) comparison to the JWST column densities selects best-fit models for each code. The paper reports that high gas densities and long collapse timescales best match the observations, that CH3OH ice forms predominantly (>99%) via CO hydrogenation with negligible contribution from the added alternative routes, that CO2 formation proceeds via CO+OH or CO+O depending on the code, that CH4 is underproduced except in UCLCHEM, and that the difference between diffusive and non-diffusive chemistry explains the weak temperature dependence at Tdust<12 K.

Significance. The multi-code comparison is a genuine strength: five independently developed codes with different treatments of surface chemistry agree on the dominance of CO hydrogenation for methanol formation under the studied conditions, and the KMC simulations give microscopic support for considering both CO+OH and CO+O routes to CO2. The paper does not fit microphysical parameters to the JWST data, so the mechanistic conclusion is not circular; observational uncertainties are propagated into the MAE errors. The main limitations are that the quantitative >99% share claim rests on unvaried literature rate constants and that the abstract's density preference conflicts with one code's best-fit model for one source. If these points are addressed, the paper will provide a robust benchmark for ice-chemistry modelling in the JWST Ice Age era.

major comments (2)
  1. [§2.6, §4.2] The central quantitative claim that CH3OH forms predominantly (>99%) via CO hydrogenation relies on the literature rate constants adopted for the added alternative routes (CH4+OH→CH3+H2O, CH3+OH→CH3OH, and C+H2O→H2CO), but no sensitivity runs vary these values. Since the models systematically underproduce CH3OH (Fig. 5), plausible upward revisions of the CH3+OH or C+H2O rates could both raise CH3OH toward the observed values and reduce the CO-hydrogenation share below 99%. The UCLCHEM add/remove test described in §3.2.4 is a partial check, but it only shows that inclusion at the adopted rates does not change the final ice abundances; it does not test the rate values. Please add sensitivity runs, for example scaling the alternative-route rates by factors of 10 and 100 or varying them within their reported uncertainties, and report the resulting CH3OH formation share for all codes, or explicitly delimit the claim to the adopted rate set.
  2. [§3.3, Table 5, Abstract] The abstract and conclusions state that the JWST ice observations are better reproduced for gas densities ≥10^5 cm^-3 and collapse times ≥10^5 yr, but the Nautilus best-fit model for NIR38 has nH=2×10^4 cm^-3 and tcol=10^6 yr (Table 5), with an MAE of 0.54, which is lower than the MAE of 0.95 for the corresponding high-density model. This is an internal inconsistency: either the Nautilus low-density model should be treated as an exception, or the summary should be reworded to say that most codes prefer high densities or that the high-density preference is driven by the other codes and by the H2O column-density filter. Please clarify how the NIR38 Nautilus result is reconciled with the general density claim.
minor comments (6)
  1. [§4.3] The stated 'rate constant of 10^10 s^-1' for the indirect HOCO route appears to be a typo, as a bimolecular surface reaction rate of 10^10 s^-1 is implausible; please correct the value and specify units.
  2. [§3.2.5, §4.10] For the KMC finite-size check, the paper states that 'a few short test simulations' with a 100×100 grain give the same results as the 50×50 grain; please report the test conditions and the quantified comparison so readers can judge the statement.
  3. [§3.3, Appendix D] The first step of the model selection uses H2O column densities that agree 'within a factor of a few'; please state the numerical threshold used and how it was applied uniformly across codes, since it affects the set of models entering the MAE ranking.
  4. [Introduction and throughout] There are typographical artifacts such as 'ysosmolecular' and 'di fferent' that should be corrected in a careful proofreading pass.
  5. [§4.3] The claim that CO+O and CO+OH 'contributed roughly equally' to CO2 in the KMC simulations is presented without a quantitative table or run-to-run scatter; please report the flux percentages and, ideally, the standard deviation over multiple KMC realisations.
  6. [Table 5 and Figure 5] For the Nautilus NIR38 best model, the preference for the low-density model over the high-density model appears driven by the complete absence of CH4 in the latter; showing which species dominate each MAE contribution would improve transparency.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity; the central methanol-formation claim follows from explicit model runs with literature rate constants, and the JWST data are used only for grid selection.

full rationale

The paper's central claim that CH3OH ice forms predominantly via CO hydrogenation is a model output, not an input. The alternative routes CH4+OH -> CH3+H2O, CH3+OH -> CH3OH, and C+H2O -> H2CO are added with rate constants taken from external laboratory and quantum-chemistry studies (Qasim et al. 2018; Molpeceres et al. 2021; Potapov et al. 2021); the paper does not fit those rates to the JWST observations. The observed ice column densities are used only to select among a precomputed grid of physical conditions via the MAE statistic (Appendix D), so the inferred densities and collapse times are a selection, not a fitted parameter renamed as a prediction. No equation defines CO-hydrogenation dominance in terms of the target abundances, and no uniqueness theorem or self-citation chain is invoked to forbid the alternative mechanisms; the conclusion that CH3+OH and C+H2O are negligible is reported as a result of the five code runs (Sections 3.2 and 4.2). The acknowledged limitations (Section 4.8: single-parcel free-fall collapse, no grain growth, no line-of-sight structure) are caveats on the model's realism, not circular steps. Self-citations to code papers (e.g., Garrod 2019, Vasyunin & Herbst 2013, Cuppen & Herbst 2007) describe implementation details of codes that are executed and compared in this work, so they do not supply the paper's conclusions. The absence of sensitivity runs on the alternative-route rate constants is a robustness concern, not a circularity.

Assumptions & free parameters 9 free parameters · 7 assumptions · 0 invented entities

The central mechanistic claims rest on standard gas-grain model assumptions: adopted binding energies, diffusion barriers, reaction barriers, EA1 initial abundances, free-fall collapse, and the mapping of a homogeneous parcel onto the observed sightlines. No new chemical entities are postulated. The main parameters are grid choices and code-specific constants, none fitted to the JWST data, but the alternative methanol route rates and grain surface parameters are not varied in sensitivity tests.

free parameters (9)
  • Final gas density (nH) grid selection = 2e5-2e6 cm^-3 for best models
    Grid of 2e4, 2e5, and 2e6 cm^-3; best models selected by MAE against JWST data. Not a continuous fit, but the physical conditions are chosen by comparison to observations.
  • Collapse timescale (tcol) grid selection = 1e5-1e6 yr for best models
    Grid of 1e4, 1e5, and 1e6 yr, set by the acceleration parameter b in Equation 1; best models selected by MAE.
  • Dust temperature Tdust = 14 K (NIR38), 12 K (J110621), 10 K in test runs
    Taken from Herschel-based dust temperature maps, with additional 10 K runs for the best models.
  • Visual extinction AV = 60 mag (NIR38), 95 mag (J110621); revised to 34 and 47 mag by Dartois et al.
    Initial values from McClure et al. (2023); the paper notes in Section 4.7 that revised values are lower but argues the chemistry is largely unaffected.
  • Initial elemental abundances (EA1) = O=1.76e-4, C=7.30e-5, N=2.14e-5, He=1.40e-1 relative to nH
    Low-metal case from Wakelam and Herbst (2008); authors note EA2 and EA3 may be more realistic for the oxygen budget.
  • Diffusion-to-binding energy ratios = 0.35/0.55 (MAGICKAL), 0.3/0.5 (MONACO), 0.4/0.8 (Nautilus), 0.5 (UCLCHEM)
    Code-specific fixed parameters from prior calibrations; they influence whether CO2 forms via CO+OH or CO+O and how much methanol forms.
  • CO+O activation barrier and width = 1000 K and 1.27 A (MAGICKAL), 627 K and 1 A (Nautilus), 2500 K (UCLCHEM)
    Adopted from Goumans and Andersson (2010) and Minissale et al. (2013); these differences drive the code-dependent CO2 formation route.
  • KMC CO+O and HOCO rate constants = k(CO+O)=1e-10 s^-1, HOCO route 1e10 s^-1, branching 0.8/0.2
    Added for the KMC simulations in Section 4.3; central to the conclusion that both CO2 routes contribute equally.
  • UV photodesorption yield in UCLCHEM = 0.1 molecules photon^-1
    Default UCLCHEM value; the paper notes that lowering it to 0.03 makes ice growth similar to the other codes and changes the phase 0 atomic C abundance.
assumptions (7)
  • domain assumption Rate equation and KMC lattice models correctly capture grain surface chemistry at these conditions
    Sections 2.1 to 2.5; all conclusions are derived from these model architectures, and no laboratory validation is provided for the full networks.
  • domain assumption Literature binding energies, diffusion barriers, and reaction barriers are correct for interstellar ice analogues
    Sections 2.2 to 2.5 and 4.3; the methanol and CO2 conclusions depend on these values, with no sensitivity analysis.
  • domain assumption EA1 low-metal initial abundances represent the gas in Chamaeleon I
    Section 2.6 and Table 3; the authors themselves note in Section 4.5 that EA2 or EA3 may be more realistic for oxygen.
  • domain assumption Free-fall collapse with equal gas and dust temperatures represents cloud evolution
    Section 2.6; the model ignores grain growth, cloud dynamics, and line-of-sight structure, as acknowledged in Section 4.8.
  • domain assumption UCLCHEM gas-phase abundances supplied to KMC, with no grain-to-gas thermal desorption feedback, are sufficient
    Section 2.1; the authors assume thermal desorption is negligible at the low temperatures considered.
  • domain assumption Finite-size effects in the 50 by 50 KMC lattice are negligible except for CH4
    Sections 2.1 and 4.10; CH4 fluctuates between one and zero molecules on the lattice, and only short tests are reported for larger lattices.
  • domain assumption The observed ice column densities and ratios from McClure et al. (2023) are correct within the quoted errors
    Section 3 and Table 4; band-strength and extinction uncertainties are discussed in Sections 4.3 and 4.7, but the calculations adopt the published values.

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Pith. "Pith review of Modelling methanol and hydride formation in the JWST Ice Age era." pith.science (2026). https://pith.science/paper/5PPDG5ZR

@misc{pith2026250210123,
  author       = {Pith},
  title        = {Pith review of: Modelling methanol and hydride formation in the JWST Ice Age era},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5PPDG5ZR}},
  note         = {Machine review of arXiv:2502.10123}
}
read the original abstract

(Abridged) JWST observations have measured the ice composition toward two highly-extinguished field stars in the Chamaeleon I cloud. The observed extinction excess on the long-wavelength side of the H2O ice band at 3 micron has been attributed to a mixture of CH3OH with ammonia hydrates, which suggests that CH3OH ice could have formed in a water-rich environment with little CO depletion. Laboratory experiments and quantum chemical calculations suggest that CH3OH could form via the grain surface reactions CH3+OH and/or C+H2O in water-rich ices. However, no dedicated chemical modelling has been carried out thus far to test their efficiency and dependence on the astrochemical code employed. We model the ice chemistry in the Chamaeleon I cloud using a set of astrochemical codes (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC simulations) to test the effects of the different code architectures and of the assumed ice chemistry. Our models show that the JWST ice observations are better reproduced for gas densities >1e5 cm-3 and collapse times >1e5 yr. CH3OH ice forms predominantly (>99%) via CO hydrogenation. The contribution of reactions CH3+OH and C+H2O, is negligible. The CO2 ice may form either via CO+OH or CO+O depending on the code. However, KMC simulations reveal that both mechanisms are efficient despite the low rate constant of the CO+O surface reaction. CH4 is largely underproduced for all codes except for UCLCHEM, for which a higher amount of atomic C is available during the initial translucent cloud phase. Large differences in the ice abundances are found at Tdust<12 K between diffusive and non-diffusive chemistry codes. This is due to the fact that non-diffusive chemistry takes over diffusive chemistry at such low Tdust. This could explain the rather constant ice chemical composition found in Chamaeleon I and other dense cores despite the different visual extinctions probed.

Figures

Figures reproduced from arXiv: 2502.10123 by the authors.

Figure 1
Figure 1. shows the abundances with respect to H2 resulting from the phase 0 stage of the simulations, where we report the time evolution of the relevant species modelled by the different as￾trochemical codes. For the gas-phase, we only plot the relative abundances of atomic C and CO since these two species are es￾sential to understanding the amount of C that will be locked into CO, CO2, and CH4 ices during the phase 1 of the… view at source ↗
Figure 2
Figure 2. The time evolution of gas (solid) and ice (dashed) abundances in MAGICKAL during the collapse ”phase 1” stage. Models with Tdust=14 K and Av=60 mag correspond to NIR38, while models with Tdust=12 K and AV=95 mag correspond to J110621. zero mobility of CO molecules at this dust temperature. Interest￾ingly, the contribution of the surface reaction O + CO → CO2 to the formation of CO2 is minor in the MONACO model both … view at source ↗
Figure 3
Figure 3. Ice compositions predicted by MAGICKAL for NIR38 for the models listed in [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: Predicted ice compositions by MAGICKAL for J110621. Details are as for [PITH_FULL_IMAGE:figures/full_fig_p009_4.png]
Figure 5
Figure 5. Figure 5: Comparison between the ice column densities predicted by all codes for the best fit models, and with the Ice Age observed values. Dif￾ferent colours indicate different molecular species and different symbols label the distinct astrochemical codes used in this work (see…

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.