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arxiv: 2605.15321 · v1 · pith:DIR7GTUBnew · submitted 2026-05-14 · 🌌 astro-ph.SR · astro-ph.GA

Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. III. SiO and SiS abundances

R. Unnikrishnan , E. De Beck , L.-{\AA}. Nyman , H. Olofsson , W. H. T. Vlemmings , M. Maercker , M. Van de Sande , T. J. Millar
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T. Danilovich M. Andriantsaralaza S. B. Charnley M. G. Rawlings
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Pith reviewed 2026-05-19 16:06 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.GA
keywords SiO abundancesSiS abundancescarbon-rich AGB starscircumstellar chemistryALMA observationsradiative transfer modelingphotodissociation rates
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The pith

SiO peak abundances in most carbon-rich AGB stars are about five times higher than in the archetype IRC+10216, while SiS abundances stay similar across the sample.

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

The paper measures SiO and SiS abundances in five carbon-rich asymptotic giant branch stars by combining ALMA spatially resolved maps with single-dish spectra. Detailed 1D non-LTE radiative transfer modeling yields well-constrained radial abundance profiles and isotopic ratios for four of the five sources. The work shows that current chemical models match the observed SiO profiles but over-predict the spatial extent of SiS, pointing to an underestimated photodissociation rate for SiS. These results indicate that the well-studied star IRC+10216 is not fully representative for silicon-bearing molecules in this class of stars.

Core claim

Good fits to the SiO and SiS line profiles allow derivation of peak abundances and e-folding radii for all sources except AFGL 3068. SiS peak abundances range from 2.0×10^{-6} to 4.7×10^{-6} and remain similar across the sample, while SiO peak abundances in the other stars are a factor of roughly five larger than the value found for IRC+10216. The e-folding radii increase with gas density for both molecules. Chemical models reproduce the SiO abundance profiles but over-predict the SiS e-folding radii, leading to the conclusion that the currently assumed SiS photodissociation rate in chemical models is too low.

What carries the argument

1D non-LTE radiative transfer modeling constrained by both ALMA spatial maps and single-dish spectra to recover radial abundance profiles and e-folding radii.

Load-bearing premise

Simplified one-dimensional stellar envelope models and non-LTE radiative transfer accurately recover the true radial abundance profiles despite known limitations of such approximations.

What would settle it

A direct measurement or laboratory determination showing that the SiS photodissociation rate is not lower than the value currently used in chemical models, or new observations finding SiO abundances in other carbon stars equal to the IRC+10216 value.

Figures

Figures reproduced from arXiv: 2605.15321 by E. De Beck, H. Olofsson, L.-{\AA}. Nyman, M. Andriantsaralaza, M. G. Rawlings, M. Maercker, M. Van de Sande, R. Unnikrishnan, S. B. Charnley, T. Danilovich, T. J. Millar, W. H. T. Vlemmings.

Figure 1
Figure 1. Figure 1: Grid results for SiO RT models for four of the sample stars. The 1, 2, and 3 [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Observed (black) and modelled (red) SiO line profiles for IRAS 15194-5115. The transition quantum numbers, telescope [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Grid results for SiS RT models for four stars of the sample. As in Fig. [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Observed (black) and modelled (red) SiS line profiles for IRAS 15194-5115. The transition quantum numbers, telescope [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: SiO and SiS grid results for AFGL 3068. The 1, 2, and 3 [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Ratio of the excitation temperature (Tex) and the kinetic temperature (Tkin) for various low- and high-J SiO transitions (top), and their corresponding line-emitting regions (bottom), as functions of radius across the CSE, from the best-fit model for IRAS 15194−5115. The solid lines are from a model that takes into account IR pumping, while the dashed lines are from a model that does not, but is otherwise … view at source ↗
Figure 9
Figure 9. Figure 9: SiO and SiS peak abundances versus gas densities. The [PITH_FULL_IMAGE:figures/full_fig_p011_9.png] view at source ↗
Figure 8
Figure 8. Figure 8: Re of (a) SiO and (b) SiS abundance profiles versus the mass-loss density (ratio of the gas MLR and terminal expansion velocity). The dashed cyan lines are from chemical modelling re￾sults (Maes et al. 2023), while the other trends plotted are calcu￾lated from RT models. The red and green shaded bands represent the uncertainties on the respective lines. The magenta points are obtained from chemical modelli… view at source ↗
Figure 10
Figure 10. Figure 10: Comparison of abundance profiles obtained from RT modelling (solid lines with shaded 1 [PITH_FULL_IMAGE:figures/full_fig_p012_10.png] view at source ↗
read the original abstract

The present understanding of C-rich AGB chemistry largely relies on observations of the archetypal carbon star IRC+10216. Current molecular abundance estimates for carbon stars are based either on single-dish spectra sampling a range of excitation conditions, or on interferometric mapping of a few lines. We aim to estimate the circumstellar abundances of SiO, SiS, and their most abundant isotopologues ($^{29}$SiO, $^{30}$SiO, $^{29}$SiS, $^{30}$SiS, and Si$^{34}$S) for a sample of five carbon stars. We derived molecular abundances using detailed 1D non-local thermodynamic equilibrium (non-LTE) radiative transfer (RT) modelling, constrained by both morphological and excitation information obtained from spatially resolved ALMA maps and single-dish observations. We further compared the derived abundances to chemical modelling results. We obtain good fits to the SiO and SiS line profiles, and derived well-constrained abundance profiles and reliable isotopic ratios for all sources except AFGL 3068. While the SiS peak abundances are very similar across the sample (2.0$\times$10$^{-6}-4.7\times$10$^{-6}$), we find that the SiO peak abundances of the rest of the stars are a factor of $\sim$5 larger than that of IRC+10216. The $e$-folding radii ($R_\mathrm{e}$) are in the range 1.3$\times$10$^{16}-7.0\times$10$^{16}$ cm for SiO and 6.0$\times$10$^{15}-1.0\times$10$^{17}$ cm for SiS. The $R_\mathrm{e}$ increases with gas density for both SiO and SiS. Chemical models reproduce the derived SiO abundance profiles well, while over-predicting the SiS $R_\mathrm{e}$ values. Our models highlight the necessity of having spatially resolved observations across a broad range of excitation conditions, while also making evident the limitations inherent in 1D RT modelling using simplified (circum)stellar models. We find that the currently assumed SiS photodissociation rate in chemical models is underestimated.

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. The manuscript derives SiO, SiS, and isotopologue abundances for five carbon-rich AGB stars via 1D non-LTE radiative transfer modeling constrained by ALMA spatially resolved maps and single-dish spectra. It reports good fits and well-constrained profiles for all sources except AFGL 3068, with SiS peak abundances similar across the sample (2.0×10^{-6} to 4.7×10^{-6}), SiO peak abundances ~5 times higher than in IRC+10216 for the remaining stars, e-folding radii (R_e) of 1.3×10^{16}–7.0×10^{16} cm for SiO and 6.0×10^{15}–1.0×10^{17} cm for SiS that increase with gas density, and a conclusion that chemical models over-predict SiS R_e values, implying the assumed SiS photodissociation rate is underestimated.

Significance. If the results hold, the work provides valuable new constraints on molecular abundances and radial distributions in multiple C-rich AGB envelopes beyond the archetype IRC+10216. The combination of interferometric and single-dish data yields tighter limits on peak abundances and R_e than single-dish-only studies, and the suggestion of a revised SiS photodissociation rate would directly impact standard chemical networks for AGB chemistry. The explicit discussion of 1D modeling limitations is a strength that helps frame the reliability of the derived parameters.

major comments (2)
  1. [Abstract] Abstract: The claim that the SiS photodissociation rate is underestimated rests on chemical models over-predicting the observationally derived SiS R_e values (6.0×10^{15}–1.0×10^{17} cm). These R_e are recovered from 1D non-LTE RT fits that adopt simplified spherical density/temperature profiles. The abstract itself notes the limitations of such modeling for real envelopes; without a quantitative test of how azimuthal asymmetries, shocks, or unmodeled density gradients would shift the recovered R_e, the rate adjustment remains vulnerable to systematic bias.
  2. [Modeling section] Modeling section: The paper reports that acceptable fits could not be obtained for AFGL 3068, yet the sample-wide statements on SiS abundance similarity (2.0–4.7×10^{-6}) and the factor-of-~5 SiO enhancement relative to IRC+10216 are presented without explicit sensitivity tests excluding this source. If the poor fit for AFGL 3068 indicates that its envelope deviates strongly from the assumed 1D structure, the robustness of the cross-source trends requires clarification.
minor comments (2)
  1. [Notation] The notation for e-folding radius (R_e) should be used consistently in all figures, tables, and text to avoid reader confusion.
  2. [Results] A brief table summarizing the best-fit peak abundances, R_e, and isotopic ratios for each source would improve readability of the results.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed review. We have addressed each major comment below, revising the manuscript to improve clarity on modeling limitations and the robustness of cross-source trends while maintaining the integrity of our analysis.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The claim that the SiS photodissociation rate is underestimated rests on chemical models over-predicting the observationally derived SiS R_e values (6.0×10^{15}–1.0×10^{17} cm). These R_e are recovered from 1D non-LTE RT fits that adopt simplified spherical density/temperature profiles. The abstract itself notes the limitations of such modeling for real envelopes; without a quantitative test of how azimuthal asymmetries, shocks, or unmodeled density gradients would shift the recovered R_e, the rate adjustment remains vulnerable to systematic bias.

    Authors: We agree that the interpretation of the SiS photodissociation rate relies on 1D-derived R_e and that unquantified systematic effects from non-spherical structures could influence the comparison with chemical models. The manuscript already highlights 1D modeling limitations in the abstract and discussion. To address the concern directly, we have revised the abstract to use more cautious language, stating that the results 'suggest' the photodissociation rate may be underestimated. We have also expanded the discussion to include a qualitative assessment of how azimuthal asymmetries and shocks (referencing prior studies on AGB envelope structures) could affect recovered R_e values, while noting that a full quantitative 3D test lies outside the scope of this 1D-focused study. revision: partial

  2. Referee: [Modeling section] Modeling section: The paper reports that acceptable fits could not be obtained for AFGL 3068, yet the sample-wide statements on SiS abundance similarity (2.0–4.7×10^{-6}) and the factor-of-~5 SiO enhancement relative to IRC+10216 are presented without explicit sensitivity tests excluding this source. If the poor fit for AFGL 3068 indicates that its envelope deviates strongly from the assumed 1D structure, the robustness of the cross-source trends requires clarification.

    Authors: We concur that explicit sensitivity tests would better demonstrate the robustness of the reported trends. The original text already qualifies results by excluding AFGL 3068 from the good-fit sources and specifies the SiO enhancement applies to 'the rest of the stars.' We have now added a dedicated sensitivity analysis subsection in the results, showing SiS peak abundances and SiO enhancement factors both with and without AFGL 3068. The SiS range (2.0–4.7×10^{-6}) and the factor-of-~5 SiO increase remain unchanged when AFGL 3068 is excluded. We have further elaborated on the likely cause of the poor fit for AFGL 3068, linking it to its documented complex, asymmetric envelope that may deviate more from 1D assumptions than the other sources. revision: yes

Circularity Check

0 steps flagged

No significant circularity: abundances and rate inference derived from new observational fits

full rationale

The paper's central derivation obtains SiO and SiS abundance profiles and e-folding radii by fitting 1D non-LTE radiative transfer models directly to new ALMA spatially resolved maps plus single-dish spectra. These observationally constrained profiles are then compared against independent chemical networks; the inference that the SiS photodissociation rate is underestimated follows from the mismatch between fitted R_e values and model predictions. No step reduces by construction to prior fitted constants, self-citations, or ansatzes imported from the authors' own prior work. The derivation remains self-contained against external observational benchmarks even while acknowledging modeling limitations.

Axiom & Free-Parameter Ledger

3 free parameters · 2 axioms · 0 invented entities

Central claims rest on fitted radial abundance profiles and e-folding radii obtained by adjusting parameters in 1D radiative transfer models to match ALMA maps and spectra. Chemical model comparisons rely on standard reaction networks whose rates are taken from prior literature.

free parameters (3)
  • SiO peak abundance per star
    Fitted to reproduce observed line profiles and spatial maps for each of the five sources.
  • SiS peak abundance
    Fitted across the sample, reported in the range 2.0e-6 to 4.7e-6.
  • e-folding radius R_e for SiO and SiS
    Adjusted to match the radial extent seen in the data; values range 1.3e16-7.0e16 cm for SiO and 6.0e15-1.0e17 cm for SiS.
axioms (2)
  • domain assumption Circumstellar envelopes can be adequately described by 1D spherically symmetric density and temperature profiles
    Invoked throughout the radiative transfer modeling section.
  • domain assumption Non-LTE excitation conditions apply to the SiO and SiS transitions observed
    Basis for the detailed radiative transfer calculations.

pith-pipeline@v0.9.0 · 6036 in / 1724 out tokens · 63580 ms · 2026-05-19T16:06:30.902811+00:00 · methodology

discussion (0)

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