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The sulfur plume in the Horsehead nebula: New detections of S$_2$H, SH$^+$, and CO$^+$

T0 review · 3 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read The paper reports the first detections of SH+ and CO+ in the Horsehead nebula, plus a new S2H transition and the first resolved image of S2H emission, and argues from the spatial offset between S2H and the reactive ions that S2H forms on…

desk verdict Solid new detections and a clean spatial story, but the sulfur abundance claim leans on a model with unquantified reaction-rate errors. read the letter →

arxiv 2505.22117 v1 pith:3WCLGY2V submitted 2025-05-28 astro-ph.GA

Asunción Fuente , Gisela Esplugues , Pablo Rivière-Marichalar , David Navarro-Almaida , Rafael Martín-Doménech , Guillermo M. Muñoz-Caro , Álvaro Sánchez-Monge , Angèle Taillard
show 8 more authors
Héctor Carrascosa Julián J. Miranzo-Pastor Aitana Tasa-Chaveli Patricia Fernández-Ruiz Viviana V. Guzmán Javier R. Goicoechea Maryvonne Gerin Jerome Pety
This is my paper
classification astro-ph.GA
keywords AstrochemistryChemicalabundancesMolecularcloudsPhotodissociationregionssulfurdepletionS2HHorseheadnebulainterstellarices
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 detections of the reactive ions SH+ and CO+ in the Horsehead nebula, plus a new high-excitation S2H line and a spatially resolved image of S2H emission. The S2H emission peaks in a warm 'sulfur plume' that is offset from the ionization front where SH+, CO+, and SO+ peak, which the authors take as evidence that gas-phase ion-neutral reactions are not the dominant formation route for S2H; they instead propose that S2H forms on UV-irradiated grain surfaces and is released by non-thermal desorption, likely cosmic-ray sputtering. Using the observed SH+/CO+ column density ratio together with the Meudon PDR model, they derive a lower limit of $S/H > 1.5\times10^{-6}$, close to the solar sulfur abundance, indicating that sulfur is not significantly depleted at this cloud edge. If correct, the result supports the idea that the 'missing' sulfur in cold molecular clouds is stored in volatile or semi-refractory carriers such as sulfur chains and allotropes, rather than in refractory dust.

What carries the argument

The argument is carried by two coupled ingredients: the spatially resolved spectroscopic maps that locate S2H and the reactive ions in different layers, and the Meudon PDR model's prediction of the $N(\mathrm{SH^+})/N(\mathrm{CO^+})$ ratio as a function of the elemental sulfur abundance $S/H$. The model includes H2 vibrational-state-dependent reactions, in particular the $S^+ + H_2(v) \rightleftharpoons SH^+ + H$ rate coefficients, which are essential for forming SH+ in a low-UV PDR; comparing the predicted ratio with the observed value toward the ionization front sets the sulfur abundance lower limit. The spatial offset between S2H and the ions is the mechanism that points to grain-surface synthesis followed by non-thermal desorption rather than gas-phase ion-neutral chemistry.

What would settle it

Measure the neutral atomic sulfur abundance in the Horsehead PDR directly, for example through the [S I] 25.25 µm fine-structure line as done toward the Orion Bar; if the observed atomic sulfur column is far below the value required by the model to match the SH+/CO+ ratio, then the inferred near-solar sulfur abundance would be falsified.

Watch

Extended reading notes

Core claim

The central discovery is that S2H emission in the Horsehead PDR is spatially decoupled from the reactive ions SH+, CO+, and SO+. S2H peaks in the 'Sulfur-plume' region, which also shows enhanced SO, SO2, and H2CO emission, while the ions peak at the ionization front. The authors interpret this offset as ruling out gas-phase formation of S2H through reactions such as SH+ + H2S or S+ + H2S followed by dissociative recombination, and instead conclude that S2H is formed on the surfaces of UV-irradiated icy grains and then released non-thermally; photodesorption is disfavored because the measured upper limit to its yield is low ($<10^{-5}$ molecules per incident photon), leaving cosmic-ray sputtering or chemical desorption as likely mechanisms. In parallel, the first detections of SH+ and CO+ in the Horsehead allow the authors to use the model-predicted ratio $N(\mathrm{SH^+})/N(\mathrm{CO^+})$ as a sulfur abundance diagnostic; the observed ratio, with a factor-of-2 uncertainty, is consistent only with $S/H > 1.5\times10^{-6}$, i.e., near-solar sulfur. This argues against a major refractory sulfur reservoir in the cloud interior, at least at the UV-illuminated edge.

Load-bearing premise

The conclusion that sulfur is near solar at the Horsehead edge rests on the chemical model's predicted ratio of SH+ and CO+ column densities as a function of the sulfur elemental abundance; if that model's sulfur network is incomplete, for example in the $S^+ + H_2(v)$ formation rate or the destruction channels of SH+, the derived lower limit $S/H > 1.5\times10^{-6}$ could be wrong.

Editorial extensions

If this is right

  • If S2H is formed on grains and released non-thermally in the Horsehead, similar sulfur-chain molecules (e.g., S3, H2S2) should be detectable in other UV-illuminated PDRs and in regions with cosmic-ray or shock sputtering, making them observable tracers of ice chemistry.
  • The inferred $S/H > 1.5\times10^{-6}$ at the PDR edge means the bulk of sulfur in cold clouds is likely in volatile or semi-refractory carriers that are destroyed or desorbed once the cloud edge is illuminated; this reframes the sulfur depletion problem from 'missing sulfur' to 'hidden sulfur reservoirs'.
  • The successful detection of CO+ and SH+ in a moderate-UV PDR establishes these reactive ions as practical tracers of the HI/H2 transition layer in a variety of interstellar environments, not just high-UV PDRs.
  • The N(SH+)/N(CO+) ratio can be applied to other PDRs to measure sulfur abundance without requiring detections of many sulfur species, provided the model's sulfur network holds.

Reading between the lines

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

  • If the grain-surface formation picture is correct, S2H might also be present in prestellar cores whenever cosmic rays or UV photons hit the icy mantles; mapping S2H in a core would test whether non-thermal desorption alone can release it without a PDR.
  • A testable extension would be to search for S3 and H2S2 in the same 'Sulfur-plume' position; their detection would strengthen the sulfur-chain reservoir hypothesis, while non-detection would suggest that the near-solar S/H is carried mainly by atomic sulfur rather than chains.
  • The factor-of-2 uncertainty adopted for the observed SH+/CO+ ratio is ad hoc; a more rigorous treatment of excitation, opacity, and beam dilution could either tighten or relax the S/H lower limit, so the quantitative claim should be read with that uncertainty in mind.
  • If sulfur is close to solar at this PDR edge but dramatically depleted in dense cores, then the depletion is a local environmental effect rather than a global property of molecular clouds; mapping the same ions across a range of cloud depths in a single region could directly test this.
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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 / 4 minor

Summary. Fuente et al. present ALMA total-power observations of sulfur-bearing species toward the Horsehead nebula, reporting the first detections of SH+ and CO+ in this source and the first S2H image. The maps show S2H emission peaking in a warm 'sulfur-plume' layer offset from the reactive ions SH+, CO+, and SO+, which peak at the ionization front. From this spatial segregation the authors argue that gas-phase routes via SH+ and H2S are not the dominant S2H formation channel and propose formation on UV-irradiated ices followed by non-thermal desorption. Using the N(SH+)/N(CO+) column-density ratio at the ionization front together with Meudon PDR models run at fixed physical parameters, they infer S/H > 1.5e-6, concluding that sulfur is not strongly depleted at the PDR edge.

Significance. The new detections are observationally valuable: the line identification is careful, with CDMS/JPL catalog checks, mirror-frequency rejection, a methanol non-detection test, and detection of two CO+ lines. The forward model is not fitted to the target ratio and uses physical parameters fixed by earlier CO and HCO+ observations, so the argument is not circular. If confirmed, the S2H spatial offset provides a new constraint on sulfur formation pathways in low-UV PDRs. However, the quantitative S/H lower limit rests on two unaddressed modeling issues (beam averaging and reaction-rate uncertainties), so the paper's strongest astrochemical conclusion is not yet established.

major comments (3)
  1. [5.2, Figure 3] The observed N(SH+)/N(CO+) ratio is compared directly with model curves plotted as a function of A_V, but the observed value is a column-density ratio averaged over the 29.8" beam and integrated along the line of sight through a PDR that spans a range of visual extinctions. The manuscript does not describe integrating the model abundances along the line of sight or convolving the predicted emission with the total-power beam before computing the ratio. Without this step, the statement that only S/H > 1.5e-6 is compatible with the data is not demonstrated, because the A_V profiles of the model ratio differ strongly between the S/H curves in the outer layers that the beam would sample.
  2. [5.2] The inferred lower limit on the sulfur elemental abundance is not robust to uncertainties in the sulfur chemical network. The model includes the Zanchet et al. (2019) S+ + H2(v) ⇌ SH+ + H rate, but no uncertainty is assigned to this rate or to SH+ destruction routes such as reactions with O, C, and electrons. The observed constraint is quoted as a factor-of-2 band, while the model curves in Figure 3 differ by factors of 10 in S/H; a factor-of-2 change in a key rate coefficient can shift the predicted ratio by an amount comparable to the band width and move the inferred lower limit by roughly an order of magnitude in S/H. The paper should include a sensitivity analysis that varies these rate coefficients within plausible uncertainties and recomputes the allowed S/H range.
  3. [4.1, Table 2] The S2H column density and the derived S2H/H2S > 10 in the warm layer assume LTE with Trot = 17 K for S2H and Trot = 20 K for the H2S upper limit, yet the rotational diagram in Figure 4 does not reproduce the upper limit to the S2H 4(1,4)->5(0,5) b-type transition, which the authors interpret as possible non-LTE excitation. A non-LTE excitation calculation for the local density and temperature conditions should be performed to check the size of any bias in the LTE column density. The spatial offset of S2H relative to the ions is robust, but the quantitative abundance ratio used to support the formation scenario is not.
minor comments (4)
  1. [Figure 1 caption] The S2H transition is written as 14(1,14)->13(0,13) in the caption but as 14(0,14)->13(0,13) in Table A.1 and in Section 3; the notation should be made consistent throughout.
  2. [Table A.2] In the IF2 row for 13CS, the quoted noise is "rms = 1.83 K"; this should presumably be 1.83 mK, consistent with the other entries.
  3. [Section 3] The sentence stating that CO+ is destroyed by reactions with H2 to form HCO+ and HOC+ would benefit from a citation to the relevant rate coefficients or to earlier PDR studies, since this chemical assumption underlies the interpretation of the ion maps.
  4. [Figure 1] The mixed units (K km/s for H2S and Jy/beam MHz for the ALMA maps) make direct visual comparison difficult; overlaying contours with a common intensity scale or converting all panels to a single brightness-temperature unit would improve readability.

Circularity Check

0 steps flagged · score 2.0 of 10

No load-bearing circularity: the new detections are independent data and the S/H inference is a forward model comparison, not a fit to the target ratio.

full rationale

The paper's central claims rest on new ALMA total-power observations and a forward comparison with the Meudon PDR code, not on a parameter fitted to the target quantity. The S/H lower limit is obtained by comparing the observed N(SH+)/N(CO+) ratio with model predictions computed for three fixed sulfur abundances (S/H = 1.5e-5, 1.5e-6, 1.5e-7); no model parameter is tuned to reproduce the observed ratio, and the ratio is not defined in terms of sulfur abundance. The model itself uses physical parameters from Hernández-Vera et al. (2023) and the Zanchet et al. (2019) state-dependent S+ + H2(v) rate, both of which are independent, externally documented inputs rather than conclusions of this paper. The proposed grain-surface formation of S2H is supported by the spatial offset between S2H and the reactive ions, by binding-energy arguments, and by prior laboratory measurements, so it does not reduce to the model output. Although the paper cites several works by the same group (Fuente et al. 2017, 2024; Goicoechea & Cuadrado 2021; Zanchet et al. 2019), these citations supply observational anchors, code descriptions, and reaction-rate calculations that are not themselves the result being claimed. There is therefore no step in the derivation that is equivalent to its input by construction. The modest score of 2 reflects the presence of self-citation in the code and sulfur-chemistry background, but this self-citation is not load-bearing.

Assumptions & free parameters 6 free parameters · 5 assumptions · 0 invented entities

The central claims rest on standard radio-astronomy line identification, LTE excitation assumptions, and a 1D PDR model with fixed physical parameters. No entirely new physical entities are introduced. The main model-dependence is the Meudon chemical network for sulfur.

free parameters (6)
  • S2H rotational temperature (Trot) = 17 ± 1 K
    Fitted from rotational diagram combining Fuente et al. (2017) data and the new ALMA line (Fig. 4); used to derive N(S2H).
  • Assumed rotation temperatures for column densities = 10 K and 20 K
    Chosen as lower and upper limits for LTE column density estimates of all species except S2H (Section 4.2); affects H2S, S2H, OCS, SO+ by factors of 5 to 10.
  • Ortho-to-para ratio for H2S = 3
    Assumed standard value to derive the H2S rotation temperature upper limit (Section 4.1).
  • Beam filling factor = 1
    Assumed for the S2H rotational diagram; the authors note it does not affect Trot because the beam sizes are similar.
  • Sulfur elemental abundance S/H in PDR model = sampled 1.5e-5, 1.5e-6, 1.5e-7
    Varied in Meudon PDR model runs (Fig. 3) to constrain depletion; the paper concludes S/H > 1.5e-6.
  • PDR model physical parameters (G0, P_th, zeta) = G0=60/186 Mathis field, P_th=3e6 K cm-3, zeta=5e-17 s-1
    Taken from Hernandez-Vera et al. (2023), not fitted here; these set the physical structure and radiation field for the N(SH+)/N(CO+) predictions.
assumptions (5)
  • domain assumption LTE holds for excitation of the observed transitions
    Used in Sections 4.1 and 4.2 for column densities; the paper notes possible non-LTE for S2H from the rotational diagram tension.
  • domain assumption Steady-state chemistry in the outer PDR layers (AV < 2 mag)
    Assumed in Section 5.2 to justify using the Meudon PDR code for the S/H inference.
  • domain assumption The Meudon PDR sulfur chemical network is sufficiently accurate
    The S/H lower limit depends on the network's predicted N(SH+)/N(CO+) ratio; the paper's own text notes the network is insufficient to explain S2H and complex sulfides.
  • domain assumption Physical structure parameters from Hernandez-Vera et al. (2023) apply to the Horsehead PDR
    Used in Section 5.2 and Table A.3; transferability is assumed because those parameters reproduce CO and HCO+ observations.
  • domain assumption N(SH+)/N(CO+) is a reliable tracer of elemental sulfur abundance
    Central to the S/H inference in Section 5.2; the ratio is compared with model predictions at three S/H values.

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Cite this review

Pith. "Pith review of The sulfur plume in the Horsehead nebula: New detections of S$_2$H, SH$^+$, and CO$^+$." pith.science (2026). https://pith.science/paper/3WCLGY2V

@misc{pith2026250522117,
  author       = {Pith},
  title        = {Pith review of: The sulfur plume in the Horsehead nebula: New detections of S$_2$H, SH$^+$, and CO$^+$},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/3WCLGY2V}},
  note         = {Machine review of arXiv:2505.22117}
}
abstract

Sulfur is essential for life, but its abundance and distribution in the interstellar medium remain uncertain, with over 90% of sulfur undetected in cold molecular clouds. Sulfur allotropes (S$_{\rm n}$) have been proposed as possible reservoirs, but the only detected interstellar molecule with a disulfide bond is S$_2$H in the Horsehead Nebula, making the estimation of sulfur chains abundances difficult. Here we present total-power ALMA images of H$_2$S, S$_2$H, SO$_2$, CO$^+$, and SH$^+$ towards the Horsehead nebula. These observations, with unprecedented sensitivity (rms $\sim$ 1.5 mK), provide the first detections of SH$^+$ and CO$^+$ in this region, together with the identification of a new S$_2$H line. The comparison of the spectroscopic images of H$_2$S, S$_2$H, SO$_2$, CO$^+$ and SH$^+$ shows that the S$_2$H emission originates from a warm gas layer adjacent to the photodissociation front. The emission peak of S$_2$H is offset from those of reactive ions such as SH$^+$, CO$^+$, and SO$^+$, suggesting that gas-phase reactions involving SH$^+$ and H$_2$S are not the dominant formation pathway of S$_2$H. Instead, we propose that S$_2$H is desorbed from irradiated grain surfaces by non-thermal processes. The SH$^+$ detection indicates that sulfur is not significantly depleted at the UV-irradiated edge of the molecular cloud, arguing against a major refractory sulfur reservoir in the interior of molecular clouds.

Figures

Figures reproduced from arXiv: 2505.22117 by the authors.

Figure 1
Figure 1. Zero-moment images of the H2S 1(1,0)→1(0,1), S2H 14(1,14)→13(0,13), SO2 4(2,2)→3(1,3), CO+ N=2→1, J=5/2→3/2 , SO+ J=15/2→13/2, Ω=1/2, l=e, and SH+ N=0→1, J= 1→0, F=3/2→1/2 lines. The H2S line is taken from Fuente et al. (2017). Units are K km s−1 for H2S and Jy/beam × MHz for the ALMA maps presented in this work. In colors, we indicate the regions selected to extract the spectra shown in [PITH_FULL_IMAGE:figures/fu… view at source ↗
Figure 2
Figure 2. Averaged spectra of the H2S 2(2,0)→2(1,1), S2H 14(1,14)→13(0,13), SO2 4(2,2)→3(1,3), CO+ N=2→1, J=5/2→3/2 , SO+ J=15/2→13/2, Ω=1/2, l=e, and SH+ N=0→1, J= 1→0, F=3/2→1/2 lines in the areas defined in [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. The N(CO+)/N(SH+) ratio as a function of vi￾sual extinction as predicted by the Meudon PDR code using the parameters shown in Table A.3. Different colors cor￾respond to different values of the sulfur elemental abun￾dance: S/H=1.5×10−5 (red), S/H=1.5×10−6 (blue), and S/H=1.5×10−7 (black). Solid and dashed lines correspond to G0 = 60 and 186 Mathis field, respectively. The blue band indicates values of N(CO+)/N(SH+)=0… view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Rotational diagram of S2H based on 30m data from Fuente et al. (2017) (black squares) and the ALMA to￾tal power data presented in this work (red squares) assuming a beam-filling factor of 1. It should be noticed that the as￾sumed beam filling factor is not relevant for…

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Forward citations

Cited by 2 Pith papers

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    HS2 has been detected for the first time in a cold dark cloud, TMC-1, with an abundance of about 6e-11 relative to H2, inferred from five hyperfine lines.

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Reference graph

Works this paper leans on

3 extracted references · 1 canonical work pages · cited by 2 Pith papers

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    It should be noticed that the as- sumed beam filling factor is not relevant for the calculation of T rot since the observations were performed with similar angular resolution (HPBW ∼ 24”-29”). Fuente, A., Goicoechea, J. R., Pety, J., et al. 2017, ApJL, 851, L49, doi: 10.3847/2041-8213/aaa01b Fuente, A., Navarro, D. G., Caselli, P., et al. 2019, A&A, 624, ...

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    (2017) (black squares) and the ALMA to- tal power data presented in this work (red squares) assuming a beam-filling factor of

    Rotational diagram of S 2H based on 30m data from Fuente et al. (2017) (black squares) and the ALMA to- tal power data presented in this work (red squares) assuming a beam-filling factor of

    work page 2017

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