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arxiv: 2604.25915 · v1 · submitted 2026-04-28 · 🌌 astro-ph.GA

JOYS+ analyses of OCN^-, N₂O, NO, and complex cyanides in ices -- Thermal processing results in modest enhancement of OCN^- ice

P. Nazari , N. Brunken , Y. Chen , K. Slavicinska , E. F. van Dishoeck , W. R. M. Rocha , A. C. A. Boogert , M. G. Navarro
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V. J. M. Le Gouellec L. Francis \L. Tychoniec A. Caratti o Garatti C. Gieser T. P. Greene P. J. Kavanagh
This is my paper

Pith reviewed 2026-05-07 15:30 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords OCN-interstellar icesJWSTnitrogen chemistrythermal processingClass 0 and I objectsice formationNO abundances
0
0 comments X p. Extension

The pith

Thermal processing only modestly enhances OCN- abundances in interstellar ices

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

The paper examines JWST spectra of ices toward 19 young stellar objects to trace how nitrogen-bearing molecules form in cold interstellar environments. It reports firm detections of OCN- in every source and shows that the OCN- to CO2 ratio rises by a factor of roughly two to three only in those ices that display the double-peaked CO2 absorption feature taken as a sign of heating. This limited boost implies that the mere presence of OCN- cannot be read as proof that ices have been warmed. Column-density relations with visual extinction further suggest OCN- appears after water and CO2 ices have already formed. Upper limits on NO ice fall below earlier model predictions for mature disks, pointing instead to other precursor molecules as the likely source of observed gas-phase NO.

Core claim

Through JOYS+ JWST observations of eight Class 0 and eleven Class I objects, the authors find OCN- present in all targets with OCN-/CO2 ratios larger by a factor of ~2-3 precisely where the CO2 feature shows the double peak diagnostic of thermal processing. Relations of H2O, CO2, and OCN- column densities with AV indicate that OCN- forms at a later stage than the other two ices. Tentative detections of CH3CN, C2H5CN, and N2O yield ratios to OCN- that stay within one order of magnitude across the sample, while NO upper limits lie an order of magnitude below prior disk-model predictions.

What carries the argument

The OCN-/CO2 column-density ratio measured separately in ices that do and do not exhibit the double-peaked CO2 absorption band, treated as a direct marker of thermal processing.

If this is right

  • OCN- detection by itself does not demonstrate that ices have experienced heating.
  • OCN- production is enhanced only modestly by temperature, so other formation routes must dominate in cold ices.
  • OCN- likely appears after H2O and CO2 ices have already accumulated.
  • CH3CN, C2H5CN, and N2O share similar ice environments with OCN- across the observed sources.
  • Observed gas-phase NO in mature disks probably arises from a different ice precursor such as N2O rather than from direct NO ice.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Future observations of pre-stellar cores could map the earliest nitrogen-ice chemistry before any heating begins.
  • The modest temperature dependence suggests radiation or density effects may also control OCN- formation in models.
  • Constant ratios among the complex species imply linked nitrogen pathways that can be tested once more firm detections become available.
  • Disk chemistry models may need to shift emphasis from direct NO ice desorption to N2O or other routes to match observed gas-phase NO.

Load-bearing premise

That the CO2 double peak reliably flags thermal processing and that AV values cleanly trace the relative formation order of OCN- versus H2O and CO2 without major selection or line-of-sight biases.

What would settle it

A large survey of pre-stellar cores lacking any CO2 double peak yet returning OCN-/CO2 ratios comparable to those seen in processed ices would falsify the claim of only modest thermal enhancement.

Figures

Figures reproduced from arXiv: 2604.25915 by A. C. A. Boogert, A. Caratti o Garatti, C. Gieser, E. F. van Dishoeck, K. Slavicinska, L. Francis, \L. Tychoniec, M. G. Navarro, N. Brunken, P. J. Kavanagh, P. Nazari, T. P. Greene, V. J. M. Le Gouellec, W. R. M. Rocha, Y. Chen.

Figure 1
Figure 1. Figure 1: Example of spline fitting for two of our objects. Gray view at source ↗
Figure 2
Figure 2. Figure 2: Our fits to the OCN− and CO absorption features toward three of the studied objects. As explained in the text, OCN− is fitted simultaneously with CO ice. The orange shaded area shows the fit to the OCN− feature. Shaded blue, green, and pink areas show the Lorentzian and Gaussian components fitted to the CO feature. The total fit is presented with solid lime green line. The black solid line shows the spline… view at source ↗
Figure 3
Figure 3. Figure 3: Final fits to the cyanide stretch for objects showing tentative detections of complex cyanides and N view at source ↗
Figure 4
Figure 4. Figure 4: Ice column density ratios of nitrogen-bearing molecules considered here. Green represents Class 0 objects and pink Class I view at source ↗
Figure 5
Figure 5. Figure 5: Histogram of Class 0 (smooth green) and I (hatched view at source ↗
Figure 6
Figure 6. Figure 6: Effect of heating on OCN− /CO2 ratio. The objects are di￾vided into two groups based on the shape of the CO2 band. Those with double-peaked CO2 are shown in red (thermally processed ices) while those with a single-peaked CO2 are shown in blue (no thermal processing). The dashed lines show the median of each group and the shaded areas show the range where the data falls in between the upper and lower quarti… view at source ↗
Figure 7
Figure 7. Figure 7: Relation between AV and column densities of H2O (green), CO2 (blue), and OCN− (red). The lower panel shows a zoomed-in version of the upper panel for OCN− . The two data points of OCN− shown with a white marker are omitted when fitting the trend. The x-intercept of each molecule is printed in its respective color. The CO2 column densities are taken from Brunken et al. (2024b). H2O, CO2, and CO in molecular… view at source ↗
read the original abstract

Nitrogen-bearing molecules are more difficult to observe than oxygen-bearing ones, mainly due to the lower abundance of nitrogen in the interstellar medium. Therefore, the formation pathways of many of these species is still under debate. Studies prior to the launch of the JWST did not have the sensitivity to observe ices toward the youngest and most deeply embedded Class 0 objects. Here we will focus on OCN$^-$, CH$_3$CN, C$_2$H$_5$CN, NO, and N$_2$O in ices to better understand their formation. We use the data from the JOYS+ program to study 8 Class 0 and 11 Class I objects with JWST. We firmly detect OCN$^-$ in ices for all these objects, tentatively detect CH$_3$CN, C$_2$H$_5$CN, and N$_2$O toward three sources, and find upper limits on the NO abundance in ices. The OCN$^-$/CO$_2$ ratios are found to be larger by a factor of ~2-3 for the objects that have a visible CO$_2$ double peak (a sign of ice thermal processing) pointing to the moderate effect of temperature on OCN$^-$ production. Relation of H$_2$O, CO$_2$, and OCN$^-$ with $A_{\rm V}$ indicates that OCN$^-$ may tentatively form at a later stage than H$_2$O and CO$_2$. We find that the ratios of CH$_3$CN, C$_2$H$_5$CN, and N$_2$O with respect to OCN$^-$ are relatively constant within one order of magnitude across our objects, likely suggesting that they have similar ice environments. The upper limit abundances of NO are ~1 order of magnitude lower than what was previously predicted in ices of a mature protoplanetary disk. This indicates that the detected gas-phase NO in that disk may be a product of another molecule (e.g. N$_2$O) in the ices. We conclude that OCN$^-$ can get enhanced at higher temperatures by only a factor of ~2-3 and thus OCN$^-$ detection alone does not imply ice heating. Large-sample studies of OCN$^-$ toward pre-stellar cores will be useful to further confirm the formation timeline of this molecule.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. This manuscript reports JWST/JOYS+ observations of ice absorption features toward 19 embedded protostars (8 Class 0, 11 Class I). Firm detections of OCN^- are reported in all sources, with tentative detections of CH3CN, C2H5CN, and N2O in three objects and upper limits for NO. The key finding is that OCN^-/CO2 column density ratios are higher by a factor of approximately 2-3 in sources exhibiting the CO2 ice double peak feature, which the authors interpret as evidence for thermal processing. Correlations with visual extinction A_V are used to suggest that OCN^- forms after H2O and CO2. Other species show roughly constant ratios to OCN^-, and NO upper limits are lower than model predictions for a protoplanetary disk, suggesting alternative formation routes. The authors conclude that temperature causes only modest enhancement of OCN^- and that its detection does not necessarily indicate heated ices.

Significance. If the interpretation of the CO2 double-peak morphology as a thermal processing indicator holds and the A_V correlations are not significantly biased, this work provides valuable empirical constraints on the formation timelines and temperature dependence of nitrogen-bearing ices in the earliest stages of star formation. The firm OCN^- detections across a sample including the youngest Class 0 objects represent a strength, extending previous studies. The comparison to prior model predictions for NO and the suggestion of constant ratios for complex cyanides add to the understanding of ice chemistry networks. However, the modest sample size and tentative nature of some detections limit the robustness of broader conclusions.

major comments (2)
  1. [Abstract and Results] The claim that the visible CO2 double peak indicates ice thermal processing, leading to the OCN^-/CO2 ratio being larger by ~2-3, is central to concluding modest temperature enhancement. The paper should address whether this morphology could arise from non-thermal effects such as ice composition variations or UV processing, as this assumption underpins the interpretation that OCN^- detection alone does not imply heating.
  2. [A_V correlation analysis] The suggestion that OCN^- forms later than H2O and CO2 based on relations with A_V may be influenced by line-of-sight effects or source selection biases in the sample of 8 Class 0 and 11 Class I objects. A more quantitative assessment of these potential confounders would be needed to support the formation timeline conclusions.
minor comments (2)
  1. [Abstract] The abstract states 'large-sample studies of OCN^- toward pre-stellar cores will be useful'; a specific reference to ongoing or planned surveys would strengthen this forward-looking statement.
  2. [Throughout] Ensure consistent use of subscripts in chemical formulas (e.g., OCN^-, N_2O) in all figure labels and tables for clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive comments on our manuscript. We have addressed each of the major comments point by point below. We believe these revisions will strengthen the paper and clarify the interpretations of our JWST observations.

read point-by-point responses

  1. Referee: The claim that the visible CO2 double peak indicates ice thermal processing, leading to the OCN^-/CO2 ratio being larger by ~2-3, is central to concluding modest temperature enhancement. The paper should address whether this morphology could arise from non-thermal effects such as ice composition variations or UV processing, as this assumption underpins the interpretation that OCN^- detection alone does not imply heating.

    Authors: We acknowledge that the CO2 double-peak morphology could potentially arise from non-thermal effects such as ice composition variations or UV processing, and we will revise the manuscript to explicitly discuss these alternatives in the context of our observations. While previous studies have associated the double peak with thermal processing, we will add caveats and note that the correlation with higher OCN^-/CO2 ratios in our sample provides supporting evidence for the thermal interpretation. This will be incorporated as a partial revision to the discussion section, without altering the primary conclusions. revision: partial

  2. Referee: The suggestion that OCN^- forms later than H2O and CO2 based on relations with A_V may be influenced by line-of-sight effects or source selection biases in the sample of 8 Class 0 and 11 Class I objects. A more quantitative assessment of these potential confounders would be needed to support the formation timeline conclusions.

    Authors: We agree that line-of-sight effects and source selection biases could affect the A_V correlations. In the revised manuscript, we will provide a more quantitative assessment by analyzing the A_V distributions for Class 0 and Class I sources separately and discussing potential biases. We will also reinforce the tentative nature of the formation timeline conclusions. This revision will be made to the relevant sections of the results and discussion. revision: yes

Circularity Check

0 steps flagged

No circularity in observational ratio comparisons

full rationale

The paper reports direct JWST spectral detections of OCN^- in all 19 sources and empirical comparisons of OCN^-/CO2 ratios split by the presence/absence of the CO2 double-peak feature. No equations, model derivations, parameter fits, or self-citations are used to generate the factor-of-2-3 result; the conclusion follows from the observed data split and literature association of the double peak with thermal processing. AV correlations and upper limits are likewise presented as raw observational constraints without reduction to the paper's own inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on standard spectral feature interpretations and assumptions about ice evolution in star-forming regions rather than new postulates.

axioms (2)
  • domain assumption The double-peaked CO2 ice absorption feature reliably indicates thermal processing of ices
    Invoked to classify sources and link to the observed OCN-/CO2 ratio increase
  • domain assumption Abundance trends with visual extinction AV reflect relative formation timelines of ice species
    Used to infer that OCN- forms later than H2O and CO2

pith-pipeline@v0.9.0 · 5863 in / 1491 out tokens · 78974 ms · 2026-05-07T15:30:18.705523+00:00 · methodology

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Reference graph

Works this paper leans on

2 extracted references · 1 canonical work pages

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    on top of the total ice fits from Rocha et al. (2024) and Chen et al. (2024) in pink for IRAS 2A (top) and B1-c (bottom). Article number, page 18 of 19 P. Nazari et al.: JOYS+: Analyses of OCN −, N2O, NO, and complex cyanides in ices 5.30 5.35 5.40 5.45 [ m] 0.06 0.04 0.02 0.00 0.02 0.04 0.06 Optical depth N < 4.3 × 1016cm 2 B1-a-N 5.30 5.35 5.40 5.45 [ m...

    2024