Temporal Changes in the Infrared Spectra of Magellanic Carbon Stars

A.A. Zijlstra; B. Aringer; E. Lagadec; E. Montiel; G.C. Sloan; I. McDonald; J. Cami; K.E. Kraemer; K. Eriksson; M. Matsuura

REVIEW 2 major objections 6 minor 20 references

Temporal Changes in the Infrared Spectra of Magellanic Carbon Stars

T0 review · 2 major / 6 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read JWST spectra show that the molecular absorption bands of the LMC carbon star WBP 29 changed markedly between the 2005 Spitzer and 2024 MRS epochs, with C3/CO at 5 µm halved, the 7.5 µm acetylene band nearly gone, and a stronger unknown 10…

desk verdict New and worth a referee, but the WBP29 result is asserted from visual comparison and needs quantitative band strengths and fringe treatment before it hardens into a secure claim. read the letter →

arxiv 2411.12842 v1 pith:W3TMMJ3C submitted 2024-11-19 astro-ph.SR

G.C. Sloan , K.E. Kraemer , B. Aringer , J. Cami , K. Eriksson , S. Hoefner , E. Lagadec , M. Matsuura

show 4 more authors

I. McDonald E. Montiel R. Sahai A.A. Zijlstra

This is my paper · ORCID

classification astro-ph.SR

keywords carbonstarsasymptoticgiantbranchMagellanicCloudsinfraredspectroscopymolecularabsorptionbandsstellarpulsationJWSTMIRISpitzerIRS

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

This paper reports the first three spectra from a JWST MRS program targeting nine carbon stars in the Large Magellanic Cloud and compares them with Spitzer IRS spectra taken 16–19 years earlier. Two of the three stars have changed: the long-period Mira MSX LMC 736 is brighter and shows warmer dust, consistent with being caught near pulsation maximum, while the short-period Mira WBP 29 shows large changes in molecular band strengths. In WBP 29 the $\mathrm{C_3}$ and CO absorption at 5 µm is cut roughly in half, the 7.5 µm acetylene band nearly vanishes, and a stronger absorption from an unknown carrier appears near 10 µm. The semi-regular variable J050629 changed little. The point of the paper is that the molecular chemistry of carbon stars is time-variable on decade timescales, so single-epoch spectra cannot be treated as static: whether the WBP 29 changes are pulsation-phase effects or genuine evolution remains open.

What carries the argument

The argument rests on two-epoch spectral comparison: JWST MRS spectra at resolving power 2000–3000, resampled onto the Spitzer IRS wavelength grid, are compared with IRS spectra obtained 16–19 years earlier. The molecular bands that carry the comparison are $\mathrm{C_3}$ and CO near 5 µm, the 7.5 µm acetylene band, the 13.7 µm acetylene Q branch, and an unidentified broad carrier near 10 µm, with SiC emission at 11.5 µm as a secondary check. Multi-epoch WISE and IRAC photometry provides light curves that assign pulsation periods and phase estimates for the two observing epochs. The paper also leans on the roughly 2% spectrophotometric agreement between MRS and IRS to argue that the differences are astrophysical rather than instrumental.

What would settle it

Observe WBP 29 with MRS across a full 246-day cycle, applying a residual-fringe correction, and compare band strengths at the same pulsation phase as the 2005 IRS spectrum; if the 5 µm $\mathrm{C_3}$/CO and 7.5 µm $\mathrm{C_2H_2}$ bands recover their 2005 depths at that phase, the changes are pulsation-phase effects rather than evolution. If instead the bands stay at their 2024 depths at every phase, the chemical change is real and possibly evolutionary.

Watch

Extended reading notes

Core claim

On the paper's own terms, the discovery is that re-observing three LMC carbon stars with the MRS on JWST reveals clear temporal changes in their infrared molecular bands over roughly 16–19 years. For WBP 29 in particular, the 5 µm $\mathrm{C_3}$/CO absorption is cut in half, the 7.5 µm $\mathrm{C_2H_2}$ band has nearly vanished, and a much stronger absorption band from an unknown carrier centered at 10 µm has appeared. The changes are so large that the authors conclude the molecular chemistry has clearly changed, while leaving open whether pulsation or evolution is responsible. The blue [3.6]–[4.5] color of WBP 29 for a Mira suggests it may be transitioning from a semi-regular variable to a Mira, a shift associated with increased amorphous-carbon dust production.

Load-bearing premise

The load-bearing premise is that the spectral differences between the MRS and IRS data are astrophysical and not artifacts: residual fringing in the default-pipeline MRS spectra and limited IRS signal-to-noise beyond 14 µm are assumed not to mimic the reported band changes.

Editorial extensions

If this is right

Carbon-star spectra must be treated as time-resolved data; single-epoch molecular band strengths are not fixed.
WBP 29's blue color and changing bands make it a candidate caught in the semi-regular-to-Mira transition, with the 10 µm carrier possibly tracing the onset of amorphous-carbon dust production.
For the long-period Mira MSX LMC 736, the observed brightening and warmer dust are consistent with its 690-day pulsation, validating phase assignment from light curves as a way to correct for variability.
High-resolution MRS line structure can now be used to disentangle the molecules responsible for blended bands and to model the temperature and density of the absorbing gas.
If the WBP 29 changes are pulsational, they imply that molecular band depths swing strongly over a 246-day cycle, which would affect any pulsation-averaged abundance or dust-production analysis.

Reading between the lines

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

The cleanest test is to monitor WBP 29 through a full 246-day pulsation cycle with MRS: if the 5 µm and 7.5 µm bands return to their 2005 strengths at the same phase, the changes are cyclic, not evolutionary.
The same two-epoch comparison applied to the remaining six program stars could show whether band-flip events correlate with [3.6]–[4.5] color or dust-production rate, making the 10 µm carrier a tracer of the semi-regular-to-Mira shift.
A quantitative re-reduction of the MRS data with residual-fringe correction, combined with a formal uncertainty budget, would test whether the apparent WBP 29 changes survive a stricter comparison to IRS.
Because J050629 shows a weak 10 µm feature like the strong one in WBP 29, the carrier may be a common, previously unnoticed carbon-star molecule whose visibility is phase- or temperature-dependent.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

I'll be direct: this paper reports something genuinely new, and the central plot is credible. The first MRS-versus-IRS temporal comparison for Magellanic carbon stars shows WBP29's molecular spectrum changed dramatically over 18.6 years—C3/CO at 5 um roughly halved, the 7.5 um C2H2 band nearly gone, a stronger unknown band at 10 um. The displayed spectra support that qualitative reading. J050629, the semi-regular, has changed little, and MSX LMC 736's brightening is plausibly pulsation. That is a useful observational result.

The paper is honest about its own limits. It flags residual fringing, uncertain phases for WBP29, and low S/N in the IRS data beyond 14 um. It does not pretend to know whether pulsation or evolution causes the changes. For a short report, that restraint is good.

The soft spot is the one the stress test flags: the WBP29 comparison is visual. Band strengths are not measured, no error bars are placed on the feature depths, and the MRS spectra went through the default pipeline with no residual-fringe correction. The ~2% calibration agreement with IRS is quoted but not propagated. Residual fringing is quasi-periodic and can reach several percent in band-depth units, so in principle it could mimic or exaggerate exactly the reported changes. That said, the reported changes are large—halved, nearly vanished—and the two spectra look qualitatively different, so I don't think fringing is likely to explain all of it. But 'likely' is not 'shown.' The fix is straightforward: compute equivalent widths or band depths after fringe removal, and show differences exceed the propagated uncertainty. That would convert a suggestive plot into a secure measurement.

The phase fitting for WBP29 is the other soft spot. They fit a period and zero-phase to the same WISE photometry and then use it to argue about pulsation, while conceding the fit is inconclusive. That's fine as speculation, but it should not carry the interpretation.

Who is this for? People working on AGB mass loss, carbon-star chemistry, and MRS calibration. It's a short paper, but the observation is new and the authors are appropriately cautious. I would send it to peer review with a request for quantitative band strengths and a fringe treatment, not because the result is dead but because the central claim deserves a firmer evidentiary base.

Referee Report

2 major / 6 minor

Summary. The paper compares JWST/MRS mid-infrared spectra of three carbon stars in the Large Magellanic Cloud with Spitzer/IRS spectra obtained 16-19 years earlier. Two of the three targets show significant spectral differences. J050629 (a semi-regular variable) has changed little between epochs. MSX LMC 736 (a long-period Mira) is brighter in the MRS epoch and shows spectral changes that the authors interpret as consistent with pulsation phase, with the IRS epoch near minimum and the MRS epoch near maximum. WBP 29 (a short-period Mira) is reported to show dramatic changes: the 5 micron C3/CO absorption is said to be cut in half, the 7.5 micron acetylene band nearly absent, and a stronger absorption band near 10 microns present. The authors are careful that the cause for WBP29 (pulsation versus evolution) is unclear, but they conclude that the molecular chemistry has clearly changed.

Significance. If the WBP29 result is secure, the paper demonstrates that molecular band strengths in at least some carbon stars can change substantially on decadal timescales, which would be an important constraint on AGB pulsation and envelope chemistry. The paper has real strengths: it uses public JWST and Spitzer archival data, the comparison is a direct observational one that does not depend on the authors' own model quantities, and the authors are appropriately cautious about the unknown cause and about the unreliability of some pulsation-phase estimates. The main weakness is that the central WBP29 claim is assessed visually rather than measured: no equivalent widths, band-depth ratios, or uncertainty budgets are provided, and the MRS spectra retain residual fringes from the default pipeline. The paper is a timely short contribution, but the headline claim needs quantitative support before it can be accepted at face value.

major comments (2)

[Section 5.3 and Figure 2] The load-bearing claim that WBP29's molecular bands changed dramatically between the IRS and MRS epochs is not secured by the analysis as presented. The MRS spectrum used for the comparison retains residual fringes, as acknowledged in Section 3, and the comparison is made visually against a low-resolution IRS spectrum. No equivalent width, band-depth ratio, or continuum-normalized statistic is given for the 5 micron C3/CO feature, the 7.5 micron C2H2 feature, or the 10 micron band, and the Law et al. (2024) ~2% calibration agreement is not translated into a wavelength-dependent uncertainty for these band measurements. Because residual MRS fringing is quasi-periodic in wavenumber and can be several percent in amplitude, an uncorrected fringe could mimic or exaggerate the reported halving of the 5 micron band or the near-disappearance of the 7.5 micron band when the continuum is estimated locally. I recommend a quantitative comparison of the two epochs, for example equivalent widths or band-depth ratios measured on fringe-corrected (or fringe-toleranced) MRS spectra and on the IRS spectra, so that the concluding sentence 'the molecular chemistry has clearly changed' is directly supported.
[Section 4 and Section 5.2] The pulsation-phase interpretation for MSX LMC 736 rests on a period and zero-phase epoch fitted to the same WISE/IRAC photometry shown in Figure 3, and the statement that the spectral changes are 'consistent with its pulsation cycle' is qualitative. There is independent support for the period from earlier determinations (Sloan et al. 2016; Groenewegen & Sloan 2018; Groenewegen et al. 2020), so this is not circular, but the spectral comparison would be much stronger if the authors stated the fitted phase difference between the IRS and MRS epochs and gave a quantitative expectation for how much spectral variation would accompany the observed photometric amplitude. Please add the fitted phases (with uncertainties) and, if possible, a simple check of whether the 5-12 micron excess relative to the scaled IRS spectrum is consistent with the star's known luminosity and temperature variation near maximum.

minor comments (6)

[Abstract and Section 3] The instrument is called the 'Medium-Resolution Spectrometer' in the abstract and the 'Medium-Resolution Spectrograph' in Section 3; please use a consistent name.
[Section 2] The sentence 'All of the photometry in Figure 1 are from the Infrared Array Camera...' has a subject-verb agreement problem; consider 'All photometric measurements in Figure 1 are from...'.
[Section 4] The phrase 'the supposedly corrected IRAC photometry' is informal and vague; please specify the color-correction procedure and quantify how the 0.2 magnitude offset affects the fitted phases for J050629 and WBP29.
[Section 5.3] The text describes a 10 micron absorption band from an unknown carrier in WBP29 but does not state whether this is the same carrier as the 10 micron band seen in J050629; please clarify whether 'stronger' means stronger than in the earlier WBP29 spectrum or stronger than the analogous band in J050629.
[Figure 2 caption] The caption says the MRS data are 'downsampled to the IRS wavelength grid' without defining the resampling method; please specify whether this is simple binning, interpolation, or convolution, since it affects the apparent band strengths.
[References] The Fazio et al. reference is missing the publication year (ApJS, 154, 10).

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the reported spectral changes are direct comparisons of independent MRS and IRS observations, not derived from fitted parameters or self-citations.

full rationale

The paper's central claim is an observational comparison between two independent spectroscopic datasets (JWST/MRS and Spitzer/IRS) for three LMC carbon stars. The reported changes in molecular band strengths, especially for WBP29, are presented as direct visual and qualitative comparisons of observed spectra; no equation, model, or fitted quantity is used to construct those changes. The only fitted parameters in the paper are pulsation periods and light-curve phases, derived from multi-epoch WISE/IRAC photometry. These are used solely to place the spectra in the context of the pulsation cycle and do not feed back into the spectral comparison. For MSX LMC 736, the 690-day period is obtained from photometry independent of the spectra and is consistent with prior determinations, so the pulsation-phase interpretation is not circular. For J050629 and WBP29, the paper explicitly cautions that the phase information is unreliable or inconclusive, weakening rather than forcing any phase-related interpretation. The self-citations to Sloan et al. (2016) provide calibration corrections and a period-fitting algorithm; they are contextual and not load-bearing for the detection of spectral variability. The concern that uncorrected MRS fringing or calibration residuals could mimic the changes is a legitimate correctness/robustness issue, but it is not a circularity issue: the comparison is not equivalent to its inputs by construction. The paper contains no fitted input renamed as a prediction, no uniqueness theorem imported from the authors' prior work, and no ansatz smuggled in via citation. Therefore no circular steps are identified.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The central observational comparison is essentially model-free. Free parameters appear only in the light-curve fits used to place the two epochs in pulsation phase, and those fits are acknowledged to be uncertain for J050629 and WBP29. The main spectral-change claim depends on external calibration agreement and prior band identifications, not on new fitted constants. No new physical entities are postulated; the broad 10 um feature is explicitly attributed to an unknown carrier.

free parameters (3)

MSX LMC 736 pulsation period = 690 d
Fitted to WISE and IRAC light curves using the minimization algorithm from Sloan et al. (2016); the fitted period is used to assign IRS to minimum and MRS to maximum, underpinning the pulsation-cycle interpretation.
J050629 zero-phase epoch and amplitude = Period adopted as 154 d from OGLE III; sine amplitude and phase fitted to WISE photometry
Used to estimate pulsation phases for the IRS and MRS epochs; the paper notes a two-day period shift could reverse the phases, so this interpretation is explicitly uncertain.
WBP29 zero-phase epoch and amplitude = Period adopted as 246 d from MACHO/OGLE averages; sine amplitude and phase fitted to WISE photometry
Used to estimate pulsation phases, but the light curve is not conclusive; the paper states the phase information may not be reliable, which limits the pulsation-versus-evolution interpretation.

assumptions (4)

domain assumption The three targets are carbon stars with C/O > 1, so their infrared spectra are dominated by carbon molecule absorption and carbon-rich dust features.
Invoked in the introduction and Section 5 when interpreting C3, CO, C2H2, and SiC features; based on prior spectral classification and photometry.
domain assumption The absolute spectrophotometric calibration of JWST MRS agrees with Spitzer IRS to about 2% or better (Law et al. 2024).
Section 3 uses this agreement to justify comparing MRS and IRS spectra as astrophysical differences rather than instrumental offsets; if the wavelength-dependent calibration mismatch is larger, the band strength changes could be artifacts.
ad hoc to paper The fitted sinusoidal light curves correctly trace the pulsation phase of MSX LMC 736, and approximately for WBP29.
Section 5.2 uses the well-fitted 690-day light curve to conclude that MRS caught MSX LMC 736 near maximum and IRS near minimum; Section 5.3 admits the WBP29 phase is not conclusive, so this assumption is load-bearing only for the pulsation interpretation.
domain assumption Molecular band identifications (C3 at about 5 um, C2H2 at 7.5 and 13.7 um, SiC at 11.5 um, and an unidentified 10 um carrier) are correct.
Rely on prior laboratory and astronomical spectroscopy; the 10 um band is explicitly unidentified, so its interpretation as a single absorption carrier is provisional.

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Cite this review

Pith. "Pith review of Temporal Changes in the Infrared Spectra of Magellanic Carbon Stars." pith.science (2026). https://pith.science/paper/W3TMMJ3C

@misc{pith2026241112842,
  author       = {Pith},
  title        = {Pith review of: Temporal Changes in the Infrared Spectra of Magellanic Carbon Stars},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/W3TMMJ3C}},
  note         = {Machine review of arXiv:2411.12842}
}

read the original abstract

The Medium-Resolution Spectrometer on the Mid-Infrared Instrument on JWST obtained spectra of three carbon stars in the Large Magellanic Cloud. Two of the spectra differ significantly from spectra obtained ~16-19 years earlier with the Infrared Spectrograph on the Spitzer Space Telescope. The one semi-regular variable among the three has changed little. The long-period Mira variable in the sample shows changes consistent with its pulsation cycle. The short-period Mira shows dramatic changes in the strength of its molecular absorption bands, with some bands growing weaker and some stronger. Whether these variations result from its pulsation cycle or its evolution is not clear.

Figures

Figures reproduced from arXiv: 2411.12842 by the authors.

**Figure 1.** shows the full sample of nine carbon stars in our program and the three stars observed with the MRS between 2023 November and 2024 March [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗

**Figure 2.** [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** As explained in the next section, the estimated phases for the MRS and IRS epochs are not reliable for J050629 and are uncertain for WBP 29 [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

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Pith tools

Reviewed August 12, 2026 · model on record in the stance chip above.

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