Searches for Interstellar HCCSH and H$_2$CCS

Andrew M. Burkhardt; Anthony J. Remijan; Brett A. McGuire; Christopher N. Shingledecker; Ci Xue; Crystal L. Brogan; Eric R. Willis; Geoffrey A. Blake; Harold Linnartz; Kin Long Kelvin Lee

Searches for Interstellar HCCSH and H$_2$CCS

T0 review · 0 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read 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.

desk verdict 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. read the letter →

arxiv 1908.04247 v1 pith:EV7IOA53 submitted 2019-08-12 astro-ph.GA

Brett A. McGuire , Christopher N. Shingledecker , Eric R. Willis , Kin Long Kelvin Lee , Marie-Aline Martin-Drumel , Geoffrey A. Blake , Crystal L. Brogan , Andrew M. Burkhardt

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Paola Caselli Ko-Ju Chuang Samer El-Abd Todd R. Hunter Sergio Ioppolo Harold Linnartz Anthony J. Remijan Ci Xue Michael C. McCarthy

This is my paper · ORCID

classification astro-ph.GA

keywords astrochemistrymissinginterstellarsulfurHCCSH(ethynethiol)H2CCS(thioketene)moleculesearchesrotationalspectroscopyupperlimitsstar-formingregionsanddarkclouds

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 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.

What carries the argument

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.

What would settle it

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.

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Extended reading notes

Core claim

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.

Load-bearing premise

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.

Editorial extensions

If this is right

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.

Reading between the lines

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

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.

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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

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.

What'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.

Soft 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.

Citation 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.

Bottom 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.

Referee Report

0 major / 6 minor

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.

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.

minor comments (6)

[Section 3, Eq. (1)] The equation as typeset is garbled, so the optical-depth correction term cannot be verified; please ensure the final typeset version is correct.
[Section 3 and Tables 2/3] 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.
[Abstract and Section 5] 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 3, second paragraph] The molecule is once misspelled as 'HCSSH'; it should be 'HCCSH'.
[Figure B2, Orion-KL panel] 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.
[Table 2, NGC 6334I row] No N(H2) or X(H2) is given for this source; if this quantity cannot be reliably estimated, the omission should be noted.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity identified: upper limits are computed from external laboratory rest frequencies and observed spectra, not fitted to the data being predicted.

full rationale

The paper's derivation chain is self-contained against external benchmarks. The central claim, that neither HCCSH nor H2CCS is a major interstellar sulfur reservoir, rests on non-detections and column-density upper limits computed with Eq. 1 from measured or extrapolated rest frequencies. The HCCSH laboratory spectrum is from Lee et al. 2018 and the H2CCS spectrum from Winnewisser & Schäfer 1980; neither set of frequencies is derived from the astronomical data in this paper, and the present observations are not used to tune any spectroscopic parameter. The adopted excitation temperatures, line widths, and source sizes (Tables A1 and A2) are taken from chemically similar molecules and are plainly labeled as assumptions rather than fits to the target species, so the resulting upper limits are not forced predictions of the inputs. The conclusion is also explicitly scoped to 'the range of environments studied,' and the paper separately reports limits based on laboratory-covered lines when extrapolated frequencies are uncertain. No step in the argument reduces by construction to its own inputs, and the self-citations to co-authored laboratory and source-characterization work are used as external evidence with stated assumptions, not as a uniqueness theorem or ansatz that forbids alternatives.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

The central claim rests on assumed physical conditions for each source (Tex, source size, background temperature) and on the accuracy of laboratory rest frequencies. These are reasonable and standard assumptions, but they are not independently verified for the target molecules, so they are listed explicitly. No new entities are introduced.

free parameters (2)

Assumed excitation temperature Tex per source = 7 K (TMC1) to 280 K (Sgr B2(N))
Single-excitation temperature adopted from analogous molecules (Tables A1/A2). Upper limits depend on Q(Tex)/exp(-Eu/Tex), so these values are load-bearing for the derived column densities.
Assumed source size theta_s and background temperature Tbg = e.g., 2.3-20 arcsec; 2.7-28.2 K
Beam dilution and background subtraction affect line intensities; values are taken from prior source models, not measured for these molecules.

assumptions (4)

domain assumption The adopted single-excitation temperature and source physical parameters for each source are representative of the gas where HCCSH or H2CCS would reside.
Detailed in Section 4 and Tables A1/A2; parameters are borrowed from analogous molecules. If the true excitation is colder or the source size differs, the upper limits would weaken.
domain assumption Laboratory rest frequencies and their quoted accuracies, including extrapolation beyond 660 GHz for HCCSH and 230 GHz for H2CCS, are correct.
References Lee et al. (2018) and Winnewisser and Schäfer (1980). Some strong lines used for limits are extrapolated; the paper notes this caveat in Section 4.2.
standard math The partition function can be approximated by the product of rotational and harmonic vibrational sums with Qelec = 1.
Equations (2)-(4), standard for interstellar conditions; vibrational energies from CCSD(T)/cc-pVQZ calculations.
domain assumption Optically thin emission or absorption with a single excitation temperature (Eq. 1) adequately describes the lines.
Standard Hollis et al. (2004a) formalism; optical depth corrections applied as in Turner (1991).

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Cite this review

Pith. "Pith review of Searches for Interstellar HCCSH and H$_2$CCS." pith.science (2026). https://pith.science/paper/EV7IOA53

@misc{pith2026190804247,
  author       = {Pith},
  title        = {Pith review of: Searches for Interstellar HCCSH and H$_2$CCS},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/EV7IOA53}},
  note         = {Machine review of arXiv:1908.04247}
}

abstract

A long standing problem in astrochemistry is the inability of many current models to account for missing sulfur content. Many relatively simple species that may be good candidates to sequester sulfur have not been measured experimentally at the high spectral resolution necessary to enable radioastronomical identification. On the basis of new laboratory data, we report searches for the rotational lines in the microwave, millimeter, and sub-millimeter regions of the sulfur-containing hydrocarbon HCCSH. This simple species would appear to be a promising candidate for detection in space owing to the large dipole moment along its $b$-inertial axis, and because the bimolecular reaction between two highly abundant astronomical fragments (CCH and SH radicals) may be rapid. An inspection of multiple line surveys from the centimeter to the far-infrared toward a range of sources from dark clouds to high-mass star-forming regions, however, resulted in non-detections. An analogous search for the lowest-energy isomer, H$_2$CCS, is presented for comparison, and also resulted in non-detections. Typical upper limits on the abundance of both species relative to hydrogen are $10^{-9}$-$10^{-10}$. We thus conclude that neither isomer is a major reservoir of interstellar sulfur in the range of environments studied. Both species may still be viable candidates for detection in other environments or at higher frequencies, providing laboratory frequencies are available.

Figures

Figures reproduced from arXiv: 1908.04247 by the authors.

Discussion (0). Continue with ORCID to comment.

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Pith tools

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