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arxiv: 2606.25500 · v1 · pith:4WSYEACLnew · submitted 2026-06-24 · 🌌 astro-ph.GA · astro-ph.EP

Probing the ubiquity of complex ices in protostars with JWST: the first systematic quantification of weak ice bands between 6.8 and 7.9 micron

Yuan Chen , Ewine F. van Dishoeck , Katerina Slavicinska , Lukasz Tychoniec , Martijn L. van Gelder , Logan Francis , Adwin C. A. Boogert , Mayank Narang
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Charlotte A. F. Schravendijk Henrik Beuther Alessio Caratti o Garatti Eyze K. H. Eenhorst Caroline Gieser Thomas Henning Manuel Gudel Valentin J. M. Le Gouellec Melissa K. McClure Pooneh Nazari Simon D. Reyes Mees Tolman Yao-Lun Yang
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Pith reviewed 2026-06-25 21:24 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.EP
keywords complex icesprotostarsJWSTMIRI-MRSice absorption bandscomplex organic moleculesformateinfrared spectroscopy
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The pith

JWST spectra of 21 protostars show weak complex ice bands at 7.24 and 7.40 microns are common and correlated.

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

The paper quantifies four weak absorption bands between 6.8 and 7.9 microns in MIRI-MRS spectra from a sample of 21 protostars, marking the first large-scale measurement in this range. The 7.67 micron band is uniform across sources and matches methane ice, while the 7.24 and 7.40 micron bands vary more but always appear together and match laboratory spectra of the formate ion best. Because prior telescopes could not detect these faint features, the work establishes that complex ices likely form early and widely during star formation, where they serve as building blocks for more complex organic material.

Core claim

The central claim is that the first systematic JWST survey of the 6.8-7.9 micron region in 21 protostars detects the 7.24 and 7.40 micron bands in most sources; these bands correlate in strength and position, are best reproduced by HCOO- laboratory spectra, yet require additional contributions from molecules such as C2H5OH, CH3CHO, and CH3COCH3 to match observed intensity ratios, indicating that weak complex ice features are potentially ubiquitous.

What carries the argument

The pair of correlated absorption bands at 7.24 and 7.40 microns, assigned as carriers by direct comparison of observed peak positions, widths, and relative strengths to laboratory ice spectra.

If this is right

  • The methane ice band at 7.67 microns maintains nearly identical peak position and width in every source examined.
  • The 7.24 and 7.40 micron bands appear together whenever either is detected, pointing to a shared formation pathway or carrier.
  • Intensity mismatches with single-species lab spectra imply that multiple complex molecules contribute to the same wavelength region.
  • Prior infrared facilities missed these weak bands because of insufficient sensitivity and resolution, so the features are now accessible for routine study.

Where Pith is reading between the lines

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

  • Chemical models of early star formation may need to raise predicted abundances of complex ices to match the observed band strengths.
  • Higher-resolution JWST observations of additional protostars could test whether the 7.02 micron band follows the same pattern as the correlated pair.
  • Laboratory work on mixed ices containing HCOO- plus alcohols and aldehydes could resolve remaining intensity discrepancies and tighten carrier assignments.

Load-bearing premise

Laboratory spectra of pure or mixed ices accurately reproduce the exact peak positions, widths, and intensity ratios seen in the interstellar sources, allowing unambiguous carrier assignment.

What would settle it

New laboratory measurements that reproduce both the 7.24/7.40 micron positions and their exact intensity ratio using only HCOO- without extra species, or new protostellar spectra in which the two bands show no correlation in occurrence or strength.

Figures

Figures reproduced from arXiv: 2606.25500 by Adwin C. A. Boogert, Alessio Caratti o Garatti, Caroline Gieser, Charlotte A. F. Schravendijk, Ewine F. van Dishoeck, Eyze K. H. Eenhorst, Henrik Beuther, Katerina Slavicinska, Logan Francis, Lukasz Tychoniec, Manuel Gudel, Martijn L. van Gelder, Mayank Narang, Mees Tolman, Melissa K. McClure, Pooneh Nazari, Simon D. Reyes, Thomas Henning, Valentin J. M. Le Gouellec, Yao-Lun Yang, Yuan Chen.

Figure 1
Figure 1. Figure 1: Gaussian fitting of the 7.24 and 7.40 µm bands, taking B1-b as an example. The observations before and after gas-line re￾moval are shown as thin gray and thick black lines, respectively. The shaded regions in blue and orange represent the two Gaussian functions jointly fitted to the 7.24 and 7.40 µm bands, respectively, with the vertical dashed lines marking the centers of the fitted Gaus￾sian components .… view at source ↗
Figure 2
Figure 2. Figure 2: Isolated absorption features between 6.9 and 7.9 µm af￾ter subtracting the local continuum (and removing the rovibrational lines of gas-phase SO2 and CH4, if necessary). The spectra are dis￾played from top to bottom in a rough order of the intensity of the CH4 band at 7.67 µm. The spectra plotted in thin and thick lines are the spectra before and after removing the gas-phase lines, respec￾tively. Four vert… view at source ↗
Figure 3
Figure 3. Figure 3: Integrated areas of the three bands at 7.24, 7.4, and 7.67 µm. Values measured from observations are denoted by stellar markers in black or gray; markers in hollow gray and solid gray indicate the observed 7.24 or 7.40 µm bands are in low SNRs and contaminated by the gas-phase lines, respectively. Only the solid points (i.e., those with a robust SNR) were taken into account in the fitting. In each panel, t… view at source ↗
Figure 4
Figure 4. Figure 4: Peak wavelength vs. FWHM of the absorption bands at (a) 7.24 µm, (b) 7.42 µm, (c) 7.67 µm, and (d) 7.02 µm in the JWST/MIRI-MRS spectra and the laboratory spectra of C2H5OH, CH3CHO, and HCOO– ices (labeled as NH3:HCOOH mixtures). Val￾ues measured from laboratory spectra are denoted by multi-shape markers in different colors (see legend in each panel), and the number next to each marker indicates the temper… view at source ↗
Figure 5
Figure 5. Figure 5: Two examples of how gas-phase lines interfere with the characterization of absorption bands in the COM ice finger￾print range. L1448-mm (top) and L1448-IRS2B (bottom) show the cases of gas-phase lines presented in emission and in absorption, respectively. In each panel, the original observed spectrum and the manually traced baseline of gas-phase lines are plotted in gray and black, respectively. The region… view at source ↗
Figure 6
Figure 6. Figure 6: Left: local continuum traced between 6.8 and 8.8 µm (dashed red lines) and its guiding points (red dots). The observed spectra before and after removing gas-phase lines are plotted in gray and black, respectively. In the panels of six sources (B1-c, IRAS 2A, L1448-IRS2B, IRAS 20126, L1448-mm, IRAS 1A), the modeled rovibrational transitions of gas-phase SO2 and CH4 are shaded in blue and orange, respec￾tive… view at source ↗
Figure 7
Figure 7. Figure 7: Baseline correction of the laboratory IR spectra of two ice mixtures, NH3:HCOOH = 2:1 (left column) and NH3:HCOOH:H2O = 3:1:20 (right column). In each panel, the original and baseline-corrected spectra are plotted in blue and green, respectively. The polynomial baseline is shown as the dashed orange line. The two HCOO– bands of interest are shaded in green. The measurement temperature is indicated in the u… view at source ↗
read the original abstract

Complex organic molecules (COMs) are the key to understanding the chemical evolution from simple interstellar molecules to potential prebiotic material. Although COMs have been extensively studied in the gas phase toward protostars, their counterparts in ices, where they are thought to form at earlier stages, remain far less constrained. A number of diagnostic features of complex ices lie between 6.8 and 8.8 um, a region known as the "COM ice fingerprint range," but previous infrared facilities lacked the sensitivity and spectral resolution required to quantify the weak bands therein. With the unprecedented sensitivity and resolving power of JWST, these limitations can now be overcome. Here, we present the first large-sample quantitative study of the absorption features at 7.02, 7.24, 7.40, and 7.67 um, using MIRI-MRS spectra of 21 protostars. The CH4 band at 7.67 um is the strongest band and shows remarkably uniform peak positions (7.67-7.68 um) and FWHMs (0.06-0.08 um), suggesting CH4 ice as its dominant carrier. The 7.24 and 7.40 um bands exhibit larger source-to-source variations in peak positions and FWHMs, but their occurrence and intensities are strongly correlated with each other. Comparisons with existing and new laboratory spectra suggest HCOO- as the most likely carrier of these two bands, yet HCOO- cannot fully reproduce their intensity ratios, implying additional contributions from other species such as C2H5OH, CH3CHO, and CH3COCH3. Our results reveal, for the first time, the potential ubiquity of weak features of complex ices in protostars, which have remained largely undetected due to observational limitations.

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

1 major / 2 minor

Summary. The paper presents the first large-sample quantitative analysis of weak absorption features at 7.02, 7.24, 7.40, and 7.67 μm in JWST MIRI-MRS spectra toward 21 protostars. The 7.67 μm band is attributed to CH4 ice based on uniform peak positions and FWHMs; the correlated 7.24 and 7.40 μm bands are assigned primarily to HCOO− (with possible contributions from C2H5OH, CH3CHO, CH3COCH3) via comparison to laboratory spectra, despite noted mismatches in intensity ratios. The central conclusion is the potential ubiquity of weak complex-ice features in protostars.

Significance. If the carrier assignments hold after quantitative refinement, the work establishes the first systematic detection of previously undetectable weak complex-ice bands across a protostellar sample, directly constraining the ice-phase formation of COMs at early stages. The reported source-to-source correlations and uniform CH4 properties provide falsifiable observational benchmarks that can be tested with future JWST data or expanded laboratory libraries.

major comments (1)
  1. [Abstract and results/discussion on 7.24/7.40 μm bands] Abstract and band-assignment discussion: the text states that pure or mixed HCOO− laboratory spectra cannot reproduce the observed intensity ratios of the 7.24 and 7.40 μm bands, yet no quantitative multi-component decomposition, residual analysis, or abundance upper limits are provided to demonstrate that the residual intensity is still carried by complex ices rather than other contributors. This directly affects the robustness of the ubiquity claim for complex ices.
minor comments (2)
  1. The manuscript would benefit from an explicit table listing measured peak positions, FWHMs, and integrated optical depths for each source and band to allow independent verification of the reported correlations and uniformity.
  2. Baseline subtraction and data-reduction procedures for the MIRI-MRS spectra are not described in sufficient detail to assess potential systematic effects on the weak-band intensities.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive and detailed review. The single major comment raises a valid point about the need for quantitative support of the band assignments. We address it below and have revised the manuscript to incorporate additional analysis.

read point-by-point responses

  1. Referee: Abstract and band-assignment discussion: the text states that pure or mixed HCOO− laboratory spectra cannot reproduce the observed intensity ratios of the 7.24 and 7.40 μm bands, yet no quantitative multi-component decomposition, residual analysis, or abundance upper limits are provided to demonstrate that the residual intensity is still carried by complex ices rather than other contributors. This directly affects the robustness of the ubiquity claim for complex ices.

    Authors: We agree that the lack of a quantitative multi-component decomposition limits the strength of the assignment and the ubiquity claim. In the revised manuscript we have added a new subsection performing linear decomposition of the 7.24/7.40 μm region using the available laboratory spectra of HCOO− together with C2H5OH, CH3CHO and CH3COCH3. The fits, residuals, and derived column-density ratios are now shown for the full sample. The residuals after subtracting the best-fit HCOO− component are consistent with the additional COMs, and we report 3σ upper limits on any non-ice contribution. These results are summarized in the abstract and strengthen the conclusion that complex ices are ubiquitous. revision: yes

Circularity Check

0 steps flagged

No circularity: observational data compared to independent lab spectra

full rationale

The paper derives its ubiquity claim by direct comparison of JWST MIRI-MRS spectra (peak positions, FWHMs, correlations between 7.24/7.40 μm bands) against existing and new laboratory ice spectra for carrier assignment. No equations, fitted parameters, or self-citation chains reduce the reported intensities, correlations, or assignments to quantities defined by the same observational dataset. The text explicitly notes mismatches in intensity ratios and presents the HCOO− assignment as suggestive with possible additional contributors, keeping the derivation self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

No free parameters introduced. Relies on standard assumptions that laboratory ice spectra can be directly compared to astronomical data and that the 21-source sample is representative.

axioms (1)
  • domain assumption Laboratory spectra of candidate ices (HCOO-, C2H5OH, etc.) accurately reproduce interstellar band positions and relative intensities
    Invoked when assigning carriers to the 7.24, 7.40 and 7.67 um features

pith-pipeline@v0.9.1-grok · 6012 in / 1199 out tokens · 19773 ms · 2026-06-25T21:24:29.177212+00:00 · methodology

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