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REVIEW 3 major objections 5 minor 72 references

Observational studies on S-bearing molecules in massive star forming regions

T0 review · 3 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read In 50 massive star-forming hot cores, the abundances of H2CS and HCS+ track each other almost exactly, with slope 1.00 and r = 0.94.

desk verdict A valuable 51-source survey of sulfur chemistry in hot cores whose headline H2CS–HCS+ correlation should be viewed with a common-denominator grain of salt. read the letter →

arxiv 2412.08390 v1 pith:JFOXMAKY submitted 2024-12-11 astro-ph.GA

R. Luo , J. Z. Wang , X. Zhang , D. H. Quan , X. J. Jiang , J. Li , Q. Gou , Y.Q. Li
show 4 more authors
Y.N. Xu S.Q. Zheng C. Ou Y.J. Liu
This is my paper · ORCID
classification astro-ph.GA
keywords ISM:moleculesabundanceshotcoresmassivestar-formingregionsH2CSHCS+H2Sshockchemistry
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 single-dish millimetre observations of the S-bearing molecules H2S, H2CS, HCS+, and SiO toward 51 late-stage massive star-forming regions, with detections in 50 sources. The central finding is that the beam-averaged abundances of H2CS and HCS+ are tightly and linearly correlated across the sample, with $r = 0.94$ and slope $1.00$ when normalized by H2; the other pairs (H2S–H2CS, H2S–HCS+) correlate more weakly ($r \approx 0.76$–$0.77$). The authors read this, together with nearly equal line widths, as evidence that the three molecules are chemically linked, with HCS+ and H2CS sharing the closest connection. They further argue that a three-phase chemical model can reproduce the observed abundances of all three molecules at a single age of $2$–$3\times10^{5}$ yr for almost every source, and that the increase of S-bearing abundances with SiO indicates an important role for shock chemistry. If the paper is right, H2CS and HCS+ form a tightly coupled abundance pair that can serve as a diagnostic of gas-phase sulfur chemistry in hot cores.

What carries the argument

The load-bearing observational quantity is the across-source Pearson correlation and least-squares slope between beam-averaged abundances, computed with H2S column densities corrected through the H234S isotopologue and the galactocentric 32S/34S ratio relation, H2 column densities from C18O via the standard conversion, and a common LTE excitation temperature of $18.75$ K. The model comparison is carried out with a three-phase chemical code including gas, grain surface, and icy mantle phases, using the paper's stated initial abundances and reaction network, run on a physical model with a $10$ K collapse phase to $n_{\rm H} = 1.6\times10^{7}$ cm$^{-3}$ followed by warm-up to 60, 100, or 150 K; the age of $2$–$3\times10^{5}$ yr is inferred from where the observed abundances intersect the model curves of H2S, H2CS, and HCS+.

What would settle it

Observe H2CS and HCS+ (together with H2S) toward a mixed sample whose evolutionary stages are independently known to differ, such as pre-stellar cores, young and evolved hot cores, and ultra-compact HII regions, and test whether the near-unity slope and $r \approx 0.94$ correlation survive. If the tight correlation is specific to a narrow evolutionary window, it should weaken or vanish when stages are mixed. Independently, derive ages for the same sources using other molecular clocks and check whether they indeed all lie near $2$–$3\times10^{5}$ yr; if not, the model-age match is an artifact. A cheaper check is to recompute column densities with excitation temperatures between 10 K and 37 K and see whether the rank correlation and slopes remain close to unity.

Watch

Extended reading notes

Core claim

The paper's central claim is that the abundances of H2CS and HCS+ in late-stage hot cores are linearly related with a slope indistinguishable from unity (1.00 when normalized by H2, 1.09 when normalized by H2S) and a Pearson coefficient of 0.94, the tightest of all pairwise relations among H2S, H2CS, and HCS+. The line widths of H234S, H2CS, HCS+, and HC3N are also nearly equal within each source, which the authors take to mean that the transitions trace the same gas. On this basis they conclude that H2S, H2CS, and HCS+ are chemically linked, with HCS+ and H2CS the most correlated pair; they further propose that a three-phase chemical model (gas, grain surface, icy mantle) with a collapse followed by warm-up reproduces the observed abundances of all three molecules at a single time of $2$–$3\times10^{5}$ yr for almost all sources, and that the increase of S-bearing abundances with SiO indicates an important role for shock chemistry.

Load-bearing premise

The claim that a single model age of $2$–$3\times10^{5}$ yr fits every source assumes that all 50 detected sources are at the same evolutionary stage; if the sources span different stages, picking the 'more reasonable' of several possible age intersections per source is arbitrary and the single-age conclusion is not supported.

Editorial extensions

If this is right

  • H2CS and HCS+ abundances can be used as a paired diagnostic of gas-phase sulfur chemistry in hot cores, with a predicted near-unity slope; sources that deviate from the line would flag unusual physical conditions.
  • If the single matching age of $2$–$3\times10^{5}$ yr is real, late-stage hot cores in this sample are observed at a common chemical phase, making S-bearing molecules a clock for massive star formation.
  • The observed increase of H2S, H2CS, and HCS+ abundances with SiO abundance implies that shock chemistry must be included in models of sulfur-bearing molecules in hot cores, not just thermal warm-up.
  • The model's overprediction of H2S at a maximum temperature of 60 K indicates missing gas-phase destruction pathways for H2S at lower temperatures, a specific target for future chemical networks.
  • The approach of correcting H2S through H234S and the galactocentric 32S/34S ratio provides a template for deriving reliable H2S column densities from optically thick lines.

Reading between the lines

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

  • The near-unity slope of the H2CS–HCS+ relation suggests a shared gas-phase precursor, most plausibly CS reacting with H3+, HCO+, or HCNH+; if so, the correlation should also hold in sources where CS is abundant, and would be worth testing with CS observations in the same fields.
  • If shock chemistry is genuinely important, sources with stronger shocks (higher SiO) should show enhanced S-bearing abundances even after controlling for line-of-sight column density; the reported correlations with SiO ($r \approx 0.6$) are moderate, so the causal link remains tentative.
  • The single-age conclusion may be an artifact of the coarse grid of maximum temperatures (60, 100, 150 K); a model with a continuous temperature ladder would likely spread the inferred ages, and the comparison should be redone.
  • Extending the same abundance-pair analysis to SO and SO2, which the paper lists as additional S-bearing shock tracers, would test whether the H2CS–HCS+ pair remains tightly correlated when more of the sulfur network is included.
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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

3 major / 5 minor

Summary. The paper reports IRAM 30-m single-dish observations of H2S 1_10-1_01, H2^34S 1_10-1_01, H2CS 5_14-4_14, HCS+ 4-3, SiO 4-3, HC3N 19-18, and C18O 1-0 toward 51 late-stage massive star-forming regions. Beam-averaged column densities are derived under an assumed LTE excitation temperature of 18.75 K, with H2 column densities from C18O and H2S column densities from H2^34S using a galactocentric 32S/34S relation. The paper reports that the three S-bearing molecules are detected in 50 of 51 sources, that their abundances show strong positive correlations, especially H2CS and HCS+ (Pearson r = 0.94 with H2 normalization and r = 0.87 with H2S normalization, slopes near unity), and that their line widths are similar. A three-phase NAUTILUS chemical model is compared with the observed abundances, and the paper claims a single possible evolutionary time of 2-3e5 yr for most sources. Positive correlations with SiO are interpreted as evidence for shock-related chemistry.

Significance. The survey value of the paper is real: a homogeneous, fairly large sample of hot-core S-bearing species observed with a single telescope and reduced in a uniform manner, with line parameters and column densities made publicly available via Zenodo, is a useful observational contribution. The similar line widths of H2CS and HCS+ (Fig. 5) are an independent and robust indication that the two molecules trace common gas, which is not affected by ratio-correlation artifacts. If the claimed correlation strengths survive appropriate controls, the paper would provide meaningful constraints on sulfur chemistry in massive star-forming regions. However, the headline conclusion that HCS+ and H2CS are the most tightly chemically linked rests on correlation coefficients that may be inflated by common normalization denominators, and the model age claim is a fitted outcome rather than a parameter-free prediction. The central observational and interpretive claims therefore need additional supporting analysis before the paper's conclusions can be accepted at face value.

major comments (3)
  1. [§3.3, Figs. 2-3] The headline correlation between H2CS and HCS+ abundances (r = 0.94 for H2-normalized values, r = 0.87 for H2S-normalized values) may be substantially inflated by the common-denominator effect. N(H2) spans 1.13e22 to 4.80e23 cm^-2 across the sample (Table B.1), so dividing two measured column densities by the same source-varying N(H2) induces a positive correlation even if the numerators are independent. The same issue applies, with additional force, to the H2S-normalized comparison, because N(H2S) itself varies by ~2 orders of magnitude and is strongly correlated with both H2CS and HCS+ (r ~ 0.77 and 0.76 in Fig. 2a,b). The paper should report partial correlations, rank-based correlations, or null-model tests in which independent H2CS and HCS+ column densities are rescaled by the observed denominators. The line-width correlation (Fig. 5, r = 0.98) supports co-spatiality, but it does not by itself establish a preferential chemical link between H2CS and HCS+ over H2S; the strength of the chemical-connection claim is overstated without the denominator controls.
  2. [§4.2, Fig. 7] The statement that 'there is one possible time (2-3e5 yr) for each source' is a fitted statement, not a model prediction. For each source, the evolutionary time is chosen as the intersection of the observed abundance with the model curve, and the agreement criterion is loose (within one order of magnitude). Figure 7 shows that multiple intersections exist for several molecules and that the selection of filled versus transparent circles is justified only by the assertion that 'the samples are in the same evolutionary stage.' The paper should quantify how many sources have a unique intersection, how many have multiple intersections, and how many are actually reproduced within the stated tolerance for all three molecules simultaneously. As written, the claim of a single 2-3e5 yr epoch for the whole sample is not supported by the presented evidence.
  3. [§2.2, Eq. (2), Table B.1] The absolute abundances used for the model comparison rest on several strong assumptions: a single LTE excitation temperature of 18.75 K for all species, optically thin emission, unity beam filling, and the fixed conversion N(H2) = 4.37e6 N(C18O). The authors acknowledge that the choice of Tex changes the derived H2^34S column density by factors of 1.32, 1.05, and 1.55 for Tex = 37.5, 25, and 10 K, respectively, but H2CS and HCS+ will respond differently to a change in Tex because their partition functions and upper-level energies differ. Since the model-age conclusion depends on matching absolute abundance levels, the paper should propagate a reasonable range of Tex values through all three abundances and show that the claimed 2-3e5 yr intersection survives. Without this, the model comparison is only valid for the specific, unverified Tex assumption.
minor comments (5)
  1. [Fig. 2 caption] The caption for panels (a) and (b) both describe 'the relationship between HCS+ and H2S abundances'; panel (b) should read 'the relationship between H2CS and H2S abundances' to match the plotted quantities.
  2. [Table E.1] The entry for G031.28+00.06 lists [H2S/HCS+] = 405 ± 2.62, which is inconsistent with the values in Table D.1 (N(H2S)/N(H2) ≈ 5.52e-9 and N(HCS+)/N(H2) ≈ 8.65e-11, giving a ratio of ~64). This appears to be a typographical error and should be corrected.
  3. [§3.1, Tables B.1 and D.1] The text says H2S, H2^34S, H2CS, HCS+, and HC3N were 'detected in all sources except for G012.90-00.24,' but Table B.1 and Table D.1 list values for G012.90-00.26 and the text later uses G012.90-00.26; please clarify which source is meant and whether the non-detection is in G012.90-00.24 or G012.90-00.26.
  4. [§3.3] The interpretation that slopes close to 1 imply close chemical relatedness should be stated more carefully: a slope of unity in log-log space only establishes a power-law index of one between the two abundance ratios; it does not by itself indicate the same formation route.
  5. [§3.3, Fig. 4] The correlations with SiO are modest (r = 0.57-0.68) and the conclusion that 'shock chemistry may be important' is reasonable as a suggestion, but the paper should avoid presenting these moderate correlations as strong evidence without also discussing possible alternative explanations (e.g., common dependence on density or temperature).

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: the central correlation claim and the model comparison are self-contained, with only a minor non-load-bearing self-citation to the authors' prior physical model.

full rationale

The paper's main claim is an observational correlation between measured column-density ratios, not a derived quantity that is equivalent to its own inputs by construction. The H2- and H2S-normalized abundances are computed from independent column densities, and the reported Pearson coefficients and slopes are descriptive statistics rather than fitted parameters dressed as predictions. The H2-normalized correlation may be partly influenced by the shared N(H2) denominator, and the H2S-normalized correlation shares N(H2S) as a common denominator, but this is a statistical robustness concern, not a circularity of the derivation chain; the line-width comparison (r = 0.98) independently supports the co-spatiality interpretation. The chemical-model comparison is also not circular: the NAUTILUS code and the collapse/warm-up physical model are external inputs, and identifying a time at which model abundances match observed values is a fit rather than a parameter-free prediction, but the paper does not mislabel this as a prediction. The only self-referential element is the use of the physical model from Zhang et al. (2023), on which some authors overlap; however, its physical parameters are also attributed to independent references (Garrod & Herbst 2006; Bonfand et al. 2019; Coutens et al. 2018), so this self-citation is minor and not load-bearing. No step reduces by construction to its own inputs.

Assumptions & free parameters 2 free parameters · 6 assumptions · 0 invented entities

The central observational correlations rest on standard LTE radiative transfer assumptions, a fixed excitation temperature, a fixed C18O-to-H2 conversion factor, and a galactocentric 32S/34S relation. The model comparison adds the assumption of a common evolutionary stage and a permissive matching tolerance. No new entities are introduced.

free parameters (2)
  • Evolutionary time per source = 2-3 x 10^5 yr
    The model comparison varies time until the modeled H2S, H2CS and HCS+ abundances fall within one order of magnitude of the observed values; the resulting 'one possible time' is a fit, not a prediction (Section 4.2, Fig. 7).
  • Excitation temperature Tex = 18.75 K
    Adopted for all species in the LTE column density calculation; the paper notes molecules are sub-thermal and that changing Tex to 37.5 K shifts column densities by a factor ~1.32, so the choice affects all derived abundances (Section 2.2).
assumptions (6)
  • domain assumption LTE with a single excitation temperature of 18.75 K for H2S, H2CS, HCS+, SiO and C18O
    Invoked in Section 2.2 to convert integrated intensities to column densities; real excitation temperatures likely vary with source and species.
  • domain assumption H2 column density from C18O via N(H2)=4.37e6 N(C18O)
    Adopted from Frerking et al. (1982) in Section 2.2; the conversion factor may not be universal across hot cores.
  • domain assumption H2S column density derived from H234S using the galactocentric 32S/34S relation of Yan et al. (2023)
    Section 2.2; errors in the 32S/34S gradient propagate directly into H2S abundances and all H2S-normalized ratios.
  • domain assumption All 50 detected sources are at the same evolutionary stage
    Used in Section 4.2 to select the 'more reasonable' model intersection among multiple possible times; if false, the single-age claim is unsupported.
  • domain assumption The chemical network of Vidal et al. (2017) and initial abundances of Vidal and Wakelam (2018) are complete enough for sulfur chemistry in hot cores
    The NAUTILUS model inherits these networks; the paper itself notes missing gas-phase H2S destruction may explain the 60 K overprediction (Section 4.2).
  • domain assumption The selected lines are optically thin, except H2S which is corrected via H234S
    Assumed in Section 2.2 for column density calculation; optical depth corrections and self-absorption affect complex-profile sources.

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Cite this review

Pith. "Pith review of Observational studies on S-bearing molecules in massive star forming regions." pith.science (2026). https://pith.science/paper/JFOXMAKY

@misc{pith2026241208390,
  author       = {Pith},
  title        = {Pith review of: Observational studies on S-bearing molecules in massive star forming regions},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JFOXMAKY}},
  note         = {Machine review of arXiv:2412.08390}
}
abstract

Aims. We present observational results of H$_{2}$S 1$_{10}$-1$_{01}$, H$_{2}$$^{34}$S 1$_{10}$-1$_{01}$, H$_{2}$CS 5$_{14}$-4$_{14}$, HCS$^{+}$ 4-3, SiO 4-3, HC$_{3}$N 19-18 and C$^{18}$O 1-0 toward a sample of 51 late-stage massive star-forming regions, to study relationships among H$_{2}$S, H$_{2}$CS, HCS$^{+}$ and SiO in hot cores. Chemical connections of these S-bearing molecules are discussed based on the relations between relative abundances in sources. Results. H$_{2}$S 1$_{10}$-1$_{01}$, H$_{2}$$^{34}$S 1$_{10}$-1$_{01}$, H$_{2}$CS 5$_{14}$-4$_{14}$, HCS$^{+}$ 4-3 and HC$_{3}$N 19-18 were detected in 50 of the 51 sources, while SiO 4-3 was detected in 46 sources. C$^{18}$O 1-0 was detected in all sources. The Pearson correlation coefficients between H$_{2}$CS and HCS$^+$ normalized by H$_{2}$ and H$_{2}$S are 0.94 and 0.87, respectively, and a tight linear relationship is found between them with slope of 1.00 and 1.09, while they are 0.77 and 0.98 between H$_2$S and H$_2$CS, respectively, and 0.76 and 0.97 between H$_2$S and HCS$^+$. The values of full width at half maxima (FWHM) of them in each source are similar to each other, which indicate that they can trace similar regions. Comparing the observed abundance with model results, there is one possible time (2-3$\times$10$^{5}$ yr) for each source in the model. The abundances of these molecules increase with the increment of SiO abundance in these sources, which implies that shock chemistry may be important for them. Conclusions. Close abundance relation of H$_2$S, H$_2$CS and HCS$^+$ molecules and similar line widths in observational results indicate that these three molecules could be chemically linked, with HCS$^+$ and H$_2$CS the most correlated. The comparison of the observational results with chemical models shows that the abundances can be reproduced for almost all the sources at a specific time. The observational results, including abundances in these sources need to be considered in further modeling H$_{2}$S, H$_{2}$CS and HCS$^{+}$ in hot cores with shock chemistry.

Figures

Figures reproduced from arXiv: 2412.08390 by the authors.

Figure 1
Figure 1. The spectra of H2S 110-101, H2 34S 110-101, H2CS 514-414 detected with IRAM 30-m for three sources are shown in (a), (b) and (c), while HCS+ 4-3, SiO 4-3 lines are shown in (d), (e) and (f). All of the observed spectra are aligned to the peak with HC3N 19-18. In (a), (b) and (c), red lines are H2S 110-101, blue lines are H2 34S 110-101, green lines are H2CS 514-414. In (d), (e) and (f), HCS+ 4-3 lines are marked in … view at source ↗
Figure 2
Figure 2. The relationship among H2S, H2CS and HCS+ molecules normalized by H2. (a) shows the relationship between HCS+ and H2S abundances. And (b) shows relationship between the HCS+ and H2S abundances, while (c) prints the relationship between HCS+ and H2CS abundances. The fitting lines are marked in red, using the least square method. G011.49-01.48 and G0168.06+00.82, which have large errors, are also marked, where H2 34S … view at source ↗
Figure 3
Figure 3. The distribution of H2CS and HCS+ molecular abundances in each source. Two sources (G011.49-01.48 and G0168.06+00.82) with large errors due to weak emission of H2 34S lines. The molecular abundance ratios from Fontani et al. (2023) are also presented. 4. Discussion 4.1. Possible chemical connections between H2S, H2CS, and HCS+ in hot cores? S-bearing species are good tracers of hot cores because they are particularl… view at source ↗
Figures from the paper (4 more)
Figure 4
Figure 4. Figure 4: The relationship between the three molecules and SiO, which use H [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]
Figure 5
Figure 5. Figure 5: Comparison among the lines FWHMs of the observed species. In all plots, the two blue points with red circles that are far [PITH_FULL_IMAGE:figures/full_fig_p006_5.png]
Figure 6
Figure 6. Figure 6: The density, temperature, and AV profiles as functions of time for collapse and warm-up stages in hot core models. The left panel represents the collapse stage, during the stage, the gas density gradually increases from 3×103 cm−3 to 1.6×107 cm−3 over a period of 1×106…
Figure 7
Figure 7. Figure 7: Chemical model including includes the gas, the dust grain surface and the icy mantle. The solid lines correspond to the gas [PITH_FULL_IMAGE:figures/full_fig_p007_7.png]

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Pith tools

Reviewed August 11, 2026 · model on record in the stance chip above.