{"id":"031f918f-a0df-4320-9489-3585d7c470ee","arxiv_id":"1908.04247","paper_version":1,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":5.0,"correctness_risk":"low","formal_verification":"none","parameter_count":2,"one_line_summary":"Neither HCCSH nor H2CCS was detected in any of 14 surveyed sources, with abundance upper limits of about 1e-9 to 1e-10 relative to hydrogen.","lead":"Astronomers searched 14 interstellar clouds and star-forming regions for two sulfur-bearing molecules, HCCSH and H2CCS, using newly measured laboratory frequencies, and found neither. The resulting upper limits show these species are not major reservoirs of sulfur in these environments.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"No significant objection identified: the adopted excitation/source-size parameters are the least certain input, but the upper limits have enough headroom to the sulfur budget that the central claim is robust.","rationale":"The reader's weakest_assumption correctly identifies the adopted excitation temperature and source size as the principal modeling uncertainty. I agree that this is the least secure input in the analysis. Unlike a fatal flaw, though, this uncertainty does not propagate into the central conclusion. The upper limits are presented relative to H2 and are orders of magnitude below the cosmic sulfur abundance, so even a factor of 10-30 error in column density leaves the 'not a major reservoir' statement intact. For cold sources, lower Tex moves the population into lower-J lines, which are available in several of the surveys (e.g., the 241.6 GHz HCCSH transition), so the non-detection does not simply disappear at low excitation temperature. The paper's treatment of frequency extrapolation is candid: the Orion-KL and Sgr B2(N) limits are derived both from extrapolated strong lines and from weaker lines within the laboratory coverage, with consistent results. The conclusion is carefully scoped to the surveyed environments and does not overclaim for other regions or for ices. I therefore see no load-bearing concern that would change the ACCEPT verdict; the recommendation is unchanged, with the caveat that a check of the extreme parameter cases would make the robustness quantitative rather than qualitative.","tokens_in":17843,"tokens_out":22569,"duration_ms":245268,"concrete_test":"Recompute the two least favorable upper limits (HCCSH toward Sgr B2(N) from Herschel and HCCSH/H2CCS toward TMC1 from IRAM) with source size reduced by a factor of 3 and Tex halved, using the strongest line at the lower Tex; if the resulting abundances remain below 10% of S/H2 ~ 2.6x10^-5, the conclusion is robust to the assumed physical parameters.","verdict_should_be":"UNCHANGED","load_bearing_attack":"No significant objection identified. The central claim requires that the quoted abundance upper limits be low enough that neither HCCSH nor H2CCS could hold a major share of interstellar sulfur. The least secure inputs are the Tex and source-size values adopted from chemically similar molecules (Tables A1/A2), since these are not measured for the target species. However, this uncertainty does not threaten the conclusion: the least favorable upper limit in the sample (HCCSH toward Sgr B2(N), X ~ 4x10^-7) is still only about 1.5% of the total sulfur abundance relative to H2 (S/H2 ~ 2.6x10^-5) if the solar sulfur abundance is representative. Most limits are 10^-9 to 10^-10, four to five orders of magnitude below the sulfur budget. Making the limits markedly worse would require both a much smaller emitting region and a much lower Tex than assumed for the same lines; in cold gas the low-J lines that become brighter at low Tex (e.g., the 241.6 GHz HCCSH line in TMC1) would provide the better constraint. The paper also transparently separates limits from extrapolated rest frequencies and gives secondary limits from laboratory-covered lines, so the non-detections are not an artifact of frequency prediction. The conclusion is further scoped to the environments studied, which is appropriate.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports targeted searches for the rotational lines of the sulfur-bearing isomers HCCSH and H2CCS using archival and new line surveys covering dark clouds, isolated protostars, and high-mass star-forming regions. Building on new laboratory rest frequencies for HCCSH and existing data for H2CCS, the authors compute upper limits on column density for each source using a single-excitation-temperature radiative transfer model, with partition functions that include vibrational corrections. No convincing emission or absorption is found, and abundance upper limits relative to H2 are derived. The authors conclude that neither isomer is a major reservoir of interstellar sulfur in the studied environments.","tokens_in":18123,"tokens_out":11709,"duration_ms":116755,"significance":"The paper provides a clean, negative result that rules out two plausible sulfur carriers as major reservoirs across a wide range of environments. Its method is standard and transparent: it uses laboratory rest frequencies, an explicit radiative-transfer formula with vibrational corrections to the partition function, and separate limits for extrapolated and laboratory-covered lines. The central conclusion is robust to the main modeling assumption (borrowed excitation temperatures and source sizes) because even the least favorable upper limit (HCCSH toward Sgr B2(N), X~4e-7) is about two orders of magnitude below the total sulfur abundance relative to H2 (~2.6e-5), leaving ample headroom. The paper also honestly scopes its conclusion to the environments studied and points to avenues for improvement via laboratory spectroscopy.","major_comments":[],"minor_comments":[{"comment":"The equation as typeset is garbled, so the optical-depth correction term cannot be verified; please ensure the final typeset version is correct.","section":"Section 3, Eq. (1)"},{"comment":"The limits are described as '1σ upper limits'; because an upper limit conventionally implies a confidence level, I recommend either reporting 3σ upper limits or explicitly labeling these as 1σ sensitivity limits.","section":"Section 3 and Tables 2/3"},{"comment":"The statement that 'typical upper limits ... are 10^-9-10^-10' is not representative of the high-mass star-forming regions in Table 2 (e.g., Sgr B2(N), X~4e-7); please qualify the statement.","section":"Abstract and Section 5"},{"comment":"The molecule is once misspelled as 'HCSSH'; it should be 'HCCSH'.","section":"Section 3, second paragraph"},{"comment":"The transition label appears to be '441,43−41,42', which is missing a digit; it should be '441,43−431,42' to match Table 3.","section":"Figure B2, Orion-KL panel"},{"comment":"No N(H2) or X(H2) is given for this source; if this quantity cannot be reliably estimated, the omission should be noted.","section":"Table 2, NGC 6334I row"}],"recommendation":"minor_revision","confidential_remarks":"I agree with the previous reader's assessment. The paper is sound; the only issues are presentational. The non-standard use of 1σ upper limits deserves editorial attention, but since even the 3σ values would not alter the conclusion, I do not see it as a blocker."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a clean, well-scoped null result. The first astronomical searches for HCCSH and H2CCS, enabled by the new lab spectrum of HCCSH, yield non-detections across 14 sources and upper limits of 10^-9 to 10^-10 relative to H2. The conclusion that neither isomer is a major sulfur reservoir is supported by the data.\n\nWhat's actually new: the lab work (Lee et al. 2018) made the search possible, and this paper is the observational counterpart. The analysis is standard single-excitation-temperature radiative transfer, but applied carefully. They use a simulated spectrum to pick the strongest line, report limits from both the strongest predicted lines and from lines within the lab-measured range, and are transparent about which frequencies are extrapolated. The tables and appendix figures make it easy to see exactly what was assumed. The paper also motivates the search with the SH + CCH formation route and compares to the [H2,C3,O] isomer family, which is sensible context.\n\nSoft spots: the excitation temperatures and source sizes are borrowed from structurally similar molecules (HN13CO, HNCO, H2CCO), not measured for HCCSH/H2CCS. That is the main modeling uncertainty. A much lower Tex or much smaller emitting region could shift the column-density limits by orders of magnitude. But the stress-test is right: even the least favorable limit (HCCSH toward Sgr B2(N), ~4e-7) is only about 1.5% of the solar sulfur abundance, and most limits are four to five orders of magnitude below the sulfur budget. To break the central claim you would need a contrived combination of small source and low Tex, and in cold gas the low-J lines would then become brighter and provide a better constraint. The Orion-KL H2CCS limit is the shakiest, since it relies on extrapolated rest frequencies and a small assumed source size, but it is not load-bearing.\n\nCitation pattern looks fine; they cite the lab spectroscopy, the surveys they reuse, and relevant astrochemical literature. No circularity: the column densities come from lab rest frequencies and observed noise levels, not from fitting any parameter to the astronomical data.\n\nBottom line: this paper is for astrochemists and observers working on sulfur chemistry. It deserves a serious referee; it will not change the world, but it is honest, reproducible, and useful for narrowing the search. I would accept it with minor revisions, mostly asking for a sentence or two more on how robust the limits are to the adopted excitation parameters.","headline":"A careful null result that uses new lab spectroscopy to place tight upper limits on two sulfur-bearing isomers across 14 sources; the conclusion that they are not major sulfur reservoirs holds up.","tokens_in":18695,"tokens_out":2204,"would_cite":true,"duration_ms":23110,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Neither sulfur-bearing isomer HCCSH nor H2CCS appears in any of 14 surveyed interstellar sources, ruling both out as major reservoirs of the missing interstellar sulfur.","keywords":["astrochemistry","missing interstellar sulfur","HCCSH (ethynethiol)","H2CCS (thioketene)","interstellar molecule searches","rotational spectroscopy","upper limits","star-forming regions and dark clouds"],"falsifier":"A confident spectral-line identification of either HCCSH or H2CCS, above the quoted upper limits, in any of the 14 surveyed sources would refute the central claim; the cleanest test is to observe HCCSH's strongest b-type transitions near 850 GHz and near 1.4 THz toward Orion-KL and Sgr B2(N) once laboratory frequencies cover those bands, since the paper's own strongest limits rest on extrapolated frequencies. Short of a detection, recomputing the limits with a substantially lower excitation temperature than the adopted 135–280 K for the warm sources would show whether the non-detection conclusion survives the largest modeling uncertainty.","tokens_in":17688,"feed_emoji":"🔭","tokens_out":13412,"duration_ms":118736,"temperature":0.7,"pith_summary":"This paper sets out to test whether two small sulfur-bearing molecules, HCCSH (ethynethiol) and its lowest-energy isomer H2CCS (thioketene), could account for the sulfur that astrochemical models cannot find in the interstellar medium. HCCSH looked especially promising because it can form through a barrierless, exothermic reaction between two abundant radicals, SH and CCH, and its large dipole moment should make it bright in the millimeter and submillimeter bands. Searching archival line surveys spanning centimeter to far-infrared wavelengths toward 14 sources — from dark clouds and Class 0/I protostars to the high-mass star-forming regions NGC 6334I, Orion-KL, and Sgr B2(N) — the authors find no trace of either molecule. The resulting upper limits on abundance relative to H$_2$ are typically $10^{-9}$–$10^{-10}$, so the paper concludes that neither isomer is a major interstellar sulfur reservoir in the environments studied. If right, this closes off two specific candidate hiding places for the missing sulfur and refocuses the search on other carriers.","feed_headline":"No trace of two promising sulfur molecules in 14 interstellar sources","feed_subtitle":"Upper limits down to one part in ten billion rule both candidates out as major sulfur reservoirs.","key_machinery":"The argument is carried by the combination of precise laboratory rest frequencies and a single-excitation-temperature column-density calculation. The new lab spectrum of HCCSH (a near-prolate asymmetric top with a strong $b$-type dipole of 0.80 D and a near-zero $a$-type dipole of 0.13 D) and the existing spectrum of H2CCS (a symmetric $C_{2v}$ molecule with a 1.01 D $a$-type dipole) fix the frequencies at which emission should appear. For each source, a simulated spectrum is generated using assumed values of excitation temperature, source size, line width, and background continuum temperature, and the rms noise at the strongest predicted line is converted into a $1\\sigma$ upper limit on column density through the Hollis et al. formalism with optical-depth corrections and a partition function that includes the lowest five vibrational states. The assumption that each molecule's excitation conditions match a structurally similar, already-detected species — HN$^{13}$CO for HCCSH in Orion-KL, HNCO in Sgr B2(N), H$_2$CCO for H2CCS — is what connects the noise level to a physical abundance.","core_discovery":"The central result is a systematic set of non-detections. Using newly measured laboratory rest frequencies for HCCSH up to 660 GHz and previously available ones for H2CCS up to 230 GHz, the authors generated predicted spectra for each source and compared them with existing observations, adopting excitation temperatures and source sizes from structurally similar molecules such as HN$^{13}$CO, HNCO, and H$_2$CCO. No line from either species could be confidently identified in any of the 14 sources, and in some cases the strongest predicted lines themselves lie beyond the measured laboratory range and had to be extrapolated, notably the strongest HCCSH transitions near 850 GHz. The upper limits on column density translate to abundances relative to H$_2$ that are typically $10^{-9}$–$10^{-10}$, bracketed by looser limits near $10^{-7}$ in warm compact regions and tighter limits near $10^{-12}$ in cold clouds. The authors conclude that neither HCCSH nor H2CCS is a major reservoir of interstellar sulfur in the range of environments studied, while leaving open the possibility of detection in other environments or at higher frequencies once better laboratory data exist.","pith_inferences":["Read together with the paper's own review of condensed-phase carriers, the non-detections point toward the solid phase: if these small gas-phase hydrocarbons are not present, carriers such as FeS grains, H2S ice, or OCS ice become the more plausible hiding places for the missing sulfur, which is a testable prediction for ice observations in the JWST era.","The paper treats destruction by atomic hydrogen as a plausible explanation for HCCSH's absence by analogy with propadienone; a natural next step would be quantum-chemical rate calculations or crossed-beam experiments for H + HCCSH and H + H2CCS, which would predict which sulfur isomers should be detectable.","The same observational machinery could be applied immediately to the third isomer, c-H2C2S (thiirene), once its laboratory spectrum is measured; the paper cannot search it now, so the [H2,C2,S] family is only two-thirds tested by this study.","Re-analyzing the cold sources with non-LTE radiative-transfer models rather than a single excitation temperature might tighten the limits below $10^{-12}$ relative to H$_2$, since the single-temperature assumption is the largest modeling uncertainty in the upper-limit calculation."],"forward_implications":["Neither HCCSH nor H2CCS can be invoked as the hidden gas-phase sulfur reservoir in dark clouds, protostellar cores, or high-mass star-forming regions; the missing-sulfur explanation must lie elsewhere, such as in ices, grains, or other molecules.","The barrierless SH + CCH formation route does not guarantee a detectable abundance of HCCSH: either gas-phase production is inefficient or the molecule is destroyed quickly, plausibly by atomic hydrogen in the same way the analogous [H2,C3,O] isomer propadienone is thought to be removed.","Both isomers remain reasonable future detection targets in sulfur-rich sources, but only after laboratory measurements extend to the strongest lines — HCCSH near 850 GHz and its 1.4 THz $b$-type branch — so that searches do not rely on uncertain extrapolations.","The fact that the most stable isomer (H2CCS) is absent while less stable isomers are known in similar families reinforces the message that kinetic formation and destruction, not thermodynamic stability, govern which isomers appear in space.","Because the upper limits are set by rms noise at assumed line positions, new laboratory spectroscopy directly sharpens the constraint: covering the strongest warm-environment transitions would improve the limits substantially in sources where line confusion does not dominate."],"supporting_citations":[{"why":"Supplies the laboratory rest frequencies of HCCSH up to 660 GHz, the b-type dipole moment, the energetics of the [H2,C2,S] isomers, and the barrierless SH + CCH formation reaction that motivated the search.","marker":"Lee et al. 2018"},{"why":"Provides the laboratory microwave spectrum and 1.01 D dipole moment of H2CCS on which the thioketene search frequencies and extrapolations are based.","marker":"Winnewisser & Schäfer 1980"},{"why":"Gives the single-excitation-temperature column-density equation used to convert predicted line intensities and rms noise into upper limits.","marker":"Hollis et al. 2004a"},{"why":"Supplies the optical-depth correction applied in the column-density calculation for the upper limits.","marker":"Turner 1991"},{"why":"Provides the Orion-KL Herschel/HIFI spectra and the HN13CO and H2CCO physical parameters adopted for HCCSH and H2CCS there.","marker":"Crockett et al. 2014"},{"why":"Provides the Sgr B2(N) Herschel HEXOS spectra and the HNCO-based parameters used for the sub-millimeter upper limits.","marker":"Neill et al. 2014"},{"why":"Provides the ALMA Band 7 and Band 10 observations of NGC 6334I and the physical model parameters adopted for both molecules.","marker":"McGuire et al. 2018b"},{"why":"Supplies the IRAM 30 m survey of Sgr B2(N) and the H2CCO-derived parameters used for the 3 mm upper limit.","marker":"Belloche et al. 2013"},{"why":"Describes the GBT PRIMOS survey and data reduction behind the cold-absorption centimeter-wave upper limit on H2CCS.","marker":"Neill et al. 2012"},{"why":"Describes the ASAI survey and its source sample, from which the dark-cloud and protostellar targets and their parameters are taken.","marker":"Lefloch et al. 2018"}],"fun_headline_variants":["Two sulfur molecules not found in 14 interstellar sources","HCCSH and H2CCS absent from 14 star-forming regions","Sulfur carriers HCCSH and H2CCS elude detection in space","No trace of HCCSH or H2CCS in interstellar gas"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the excitation temperature, line width, and source size adopted for each molecule in each source — borrowed from structurally similar species such as HN13CO, HNCO, and H2CCO — actually describe the gas where HCCSH and H2CCS would emit; if the true excitation temperature is much lower or the emitting region much smaller than assumed, the quoted upper limits could shift by orders of magnitude.","fun_headline_variants_meta":{"raw":{"variants":["Two sulfur molecules not found in 14 interstellar sources","HCCSH and H2CCS absent from 14 star-forming regions","Sulfur carriers HCCSH and H2CCS elude detection in space","No trace of HCCSH or H2CCS in interstellar gas"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000744,"raw_usage":{"total_tokens":3356,"prompt_tokens":1022,"completion_tokens":2334,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":638,"completion_tokens_details":{"reasoning_tokens":2258}},"tokens_in":638,"tokens_out":2334,"duration_ms":17898,"temperature":1.0,"reasoning_tokens":2258,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T13:46:12.782892+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A confident spectral-line identification of either HCCSH or H2CCS, above the quoted upper limits, in any of the 14 surveyed sources would refute the central claim; the cleanest test is to observe HCCSH's strongest b-type transitions near 850 GHz and near 1.4 THz toward Orion-KL and Sgr B2(N) once laboratory frequencies cover those bands, since the paper's own strongest limits rest on extrapolated frequencies. Short of a detection, recomputing the limits with a substantially lower excitation temperature than the adopted 135–280 K for the warm sources would show whether the non-detection conclusion survives the largest modeling uncertainty.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the laboratory rest frequencies of HCCSH up to 660 GHz, the b-type dipole moment, the energetics of the [H2,C2,S] isomers, and the barrierless SH + CCH formation reaction that motivated the search."},{"cited_title":"a fer, E. 1980, Zeitschrift f \\","cited_arxiv_id":null,"evidence_quote":"Provides the laboratory microwave spectrum and 1.01 D dipole moment of H2CCS on which the thioketene search frequencies and extrapolations are based."},{"cited_title":"R., Bergin, E","cited_arxiv_id":null,"evidence_quote":"Provides the Orion-KL Herschel/HIFI spectra and the HN13CO and H2CCO physical parameters adopted for HCCSH and H2CCS there."},{"cited_title":"L., Bergin, E","cited_arxiv_id":null,"evidence_quote":"Provides the Sgr B2(N) Herschel HEXOS spectra and the HNCO-based parameters used for the sub-millimeter upper limits."},{"cited_title":"L., Muckle, M","cited_arxiv_id":null,"evidence_quote":"Describes the GBT PRIMOS survey and data reduction behind the cold-absorption centimeter-wave upper limit on H2CCS."},{"cited_title":"2018, , 477, 4792","cited_arxiv_id":null,"evidence_quote":"Describes the ASAI survey and its source sample, from which the dark-cloud and protostellar targets and their parameters are taken."}],"review_version":1}