pith. sign in

arxiv: 2510.02937 · v1 · submitted 2025-10-03 · 🌌 astro-ph.GA

A quest for sulfur-bearing refractory species. Identification of CaS in the interstellar medium

A. Tasa-Chaveli , \'A. S\'anchez-Monge , A. Fuente , A. Ginsburg , H. S. P. M\"uller , Th. M\"oller , P. Rivi\`ere-Marichalar , D. Navarro-Almaida
show 5 more authors
G. Esplugues P. Schilke M. Rodr\'iguez-Baras S. Thorwirth L. Beitia-Antero
This is my paper

Pith reviewed 2026-05-18 10:49 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords interstellar mediumrefractory moleculessulfur chemistryCaSmassive star formationG351.77-mm1gas-phase observationscolumn densities
0
0 comments X

The pith

Astronomers report the first detection of CaS in the interstellar medium, inside a massive star-forming disk.

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

The paper examines gas-phase observations toward the disk G351.77-mm1 to search for sulfur-containing refractory molecules. It presents evidence for a clear detection of CaS along with tentative signals from KS and KSH, marking the first identifications of these species outside the solar system. These findings matter because they let researchers trace how refractories behave in the hot, dense gas near massive young stars and test whether sulfur is locked into such compounds. The measured column densities for the new species fall three orders of magnitude below those of SO2, CH3SH, and SiS, showing they do not dominate the sulfur inventory at the probed scales. The work concludes that higher-resolution spectra at multiple wavelengths will be needed to confirm the lines and map refractory formation routes.

Core claim

Convincing evidence supports a reliable detection of CaS, with tentative detections of KS and KSH, in the disk G351.77-mm1. These are the first identifications of these refractory sulfur species in the interstellar medium. Their column densities lie roughly three orders of magnitude below those of the dominant sulfur carriers SO2, CH3SH, and SiS, demonstrating that the newly detected molecules are not the main sulfur reservoir at the spatial scales of the observations.

What carries the argument

Matching of observed spectral lines to laboratory transition frequencies of CaS, KS, and KSH under the physical conditions of the G351.77-mm1 disk.

If this is right

  • Refractory sulfur species can now be studied through their gas-phase rotational lines in the inner regions of massive-star disks.
  • CaS, KS, and KSH contribute only a minor fraction of the total sulfur at the observed scales.
  • Formation pathways for gas-phase refractory molecules can be constrained by comparing the new detections with known sulfur carriers.
  • Multi-wavelength, high-resolution follow-up is required to secure the identifications and map their spatial distribution.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If confirmed, the detections suggest that sulfur depletion onto grains or other undetected carriers may still dominate the sulfur budget in these environments.
  • Similar searches in additional massive disks could reveal whether CaS and related species are common or rare products of refractory chemistry.
  • The results raise the possibility of linking gas-phase refractory sulfur to the composition of planetesimals forming around massive stars.

Load-bearing premise

The observed spectral features arise from CaS, KS, and KSH with little blending from other molecules and the excitation conditions allow reliable conversion of line strengths into column densities.

What would settle it

Higher-angular-resolution spectra that show the candidate lines are absent, blended with unrelated species, or inconsistent with the expected excitation would falsify the identifications.

Figures

Figures reproduced from arXiv: 2510.02937 by A. Fuente, A. Ginsburg, \'A. S\'anchez-Monge, A. Tasa-Chaveli, D. Navarro-Almaida, G. Esplugues, H. S. P. M\"uller, L. Beitia-Antero, M. Rodr\'iguez-Baras, P. Rivi\`ere-Marichalar, P. Schilke, S. Thorwirth, Th. M\"oller.

Figure 1
Figure 1. Figure 1: ALMA 1.5 mm continuum emission towards the G351.77−0.54 star-forming region. Crosses mark the posi￾tions of the mm continuum sources identified by H. Beuther et al. (2019). The synthesized beam of the image (beam = 0. ′′21×0. ′′15, PA= −86◦ ; with an rms ≃ 0.32 mJy beam−1 ) is shown in the bottom-left corner. The spectra shown in Figs. 2 and A1 have been extracted towards the peak posi￾tion of G351.77-mm1,… view at source ↗
Figure 2
Figure 2. Figure 2: Zoom in panels around the CaS and KS transitions included in the spectra of G351.77-mm1. The observed spectra are shown with grey-filled histograms. The synthetic spectrum generated without including the S-bearing refractory species is shown with a blue line, while the best fits of CaS and KS are shown in red. The best fits of the KSH and CaSH lines are shown with a green line (see Fig. A2 for additional f… view at source ↗
read the original abstract

The recent detection of refractory molecules in massive star-forming regions provides a means of probing the innermost regions of disks around massive stars. These detections also make it possible to explore the chemical composition of refractories through gas-phase observations. In this regard, identifying refractory compounds containing sulfur could reveal potential connections between sulfur and refractories, as well as help determine the sulfur budget in these extreme environments. We find convincing evidence of a reliable detection of CaS, and tentative detections of KS and KSH in the disk G351.77-mm1. These are the first ever identifications of these species in the interstellar medium. The CaS, KS, and KSH column densities are about 3 orders of magnitude lower than those of the abundant sulfur compounds SO$_2$, CH$_3$SH and SiS, proving that these species are not the major reservoir of sulfur at the spatial scales probed by our observations. Higher angular resolution observations at different wavelengths are required to confirm these detections, which are of paramount importance to gain insights into the formation of gas-phase refractory molecules.

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

3 major / 1 minor

Summary. The manuscript reports the first claimed interstellar detections of the refractory sulfur-bearing species CaS (with convincing evidence) and tentative detections of KS and KSH toward the disk G351.77-mm1. Column densities for these species are stated to be ~3 orders of magnitude below those of SO2, CH3SH, and SiS, implying they are not the dominant sulfur reservoir at the observed spatial scales (~0.5–1 arcsec). The authors note that higher-angular-resolution observations at different wavelengths are required to confirm the assignments.

Significance. If the line identifications and column-density derivations are robust, the work would constitute the first gas-phase detections of these specific refractory sulfur compounds in the ISM. This would provide new observational constraints on refractory chemistry and the sulfur budget in the inner regions of massive protostellar disks, potentially linking sulfur to refractory species in extreme environments.

major comments (3)
  1. [Abstract] Abstract: The central claim of a 'reliable detection' of CaS (and tentative KS/KSH) rests on spectral-feature matching, yet no line lists, rest frequencies, intensities, or signal-to-noise values are supplied in the abstract or referenced sections. Without these, it is impossible to assess whether the features are unblended or whether the excitation conditions support the reported column densities.
  2. [Results] Results/Discussion: The assumption of negligible line blending at the current angular resolution is load-bearing for the detection claim but is not demonstrated. At ~0.5–1 arcsec scales, other sulfur-bearing or organic species can contribute within the same velocity channel and beam; the manuscript itself states that higher-resolution data are needed to confirm, indicating the present data cannot exclude misassignment.
  3. [Methods] Methods/Results: Column-density conversion assumes LTE excitation and accurate partition functions with no apparent verification for optical-depth effects or non-LTE deviations. Any such deviation would rescale the reported values (already stated to be three orders of magnitude below SO2 etc.) by large factors, directly affecting the conclusion that these species are not major sulfur reservoirs.
minor comments (1)
  1. [Title] The title emphasizes 'Identification of CaS' while the abstract and conclusions treat KS and KSH as tentative; a brief clarification of the differing confidence levels would improve precision.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their detailed and constructive comments on our manuscript. We address each of the major comments below and outline the revisions we plan to make to improve the clarity and robustness of our claims.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The central claim of a 'reliable detection' of CaS (and tentative KS/KSH) rests on spectral-feature matching, yet no line lists, rest frequencies, intensities, or signal-to-noise values are supplied in the abstract or referenced sections. Without these, it is impossible to assess whether the features are unblended or whether the excitation conditions support the reported column densities.

    Authors: The abstract is intended to provide a high-level summary of the key findings. Detailed information on the spectroscopic parameters, including line lists, rest frequencies from laboratory data, line intensities, and the measured signal-to-noise ratios of the detected features, is presented in the Results section of the manuscript, along with figures showing the spectra. We will revise the abstract to briefly reference these details and direct readers to the appropriate sections for a full assessment of the detection reliability. revision: yes

  2. Referee: [Results] Results/Discussion: The assumption of negligible line blending at the current angular resolution is load-bearing for the detection claim but is not demonstrated. At ~0.5–1 arcsec scales, other sulfur-bearing or organic species can contribute within the same velocity channel and beam; the manuscript itself states that higher-resolution data are needed to confirm, indicating the present data cannot exclude misassignment.

    Authors: We have performed checks using the available molecular line catalogs to ensure that the identified features for CaS, KS, and KSH do not coincide with known transitions of other species at the observed velocities. The current data support the assignments with multiple lines for CaS providing convincing evidence. The call for higher-angular-resolution observations is to further confirm and spatially resolve the emission, rather than to suggest that the current identifications are likely incorrect. We will expand the discussion to include a more explicit description of our blending checks and any potential alternative carriers considered. revision: partial

  3. Referee: [Methods] Methods/Results: Column-density conversion assumes LTE excitation and accurate partition functions with no apparent verification for optical-depth effects or non-LTE deviations. Any such deviation would rescale the reported values (already stated to be three orders of magnitude below SO2 etc.) by large factors, directly affecting the conclusion that these species are not major sulfur reservoirs.

    Authors: The derivations assume local thermodynamic equilibrium (LTE), which is appropriate given the high densities in the inner disk regions. Partition functions were taken from standard databases. The lines are weak, supporting the optically thin approximation. We will add a subsection or paragraph verifying the optical depth (e.g., via line intensity ratios where possible) and discussing the validity of LTE based on critical densities and collision rates. This will include a note on the potential impact of non-LTE effects on the column density estimates. revision: yes

Circularity Check

0 steps flagged

Observational detection report exhibits no circularity in line identification or column-density derivation

full rationale

The paper reports spectral-line detections of CaS (and tentative KS/KSH) by matching observed features in G351.77-mm1 to laboratory or computed rest frequencies, then converts integrated intensities to column densities under standard LTE assumptions. These steps rely on external laboratory data and measured intensities rather than any self-definition, fitted-parameter renaming, or self-citation chain that reduces the central claim to its own inputs by construction. The authors explicitly note the need for higher-resolution follow-up, confirming that the present result is not forced by internal normalization or prior author work. The derivation chain is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The detection claim rests on correct line identification and standard LTE excitation assumptions used to convert intensities to column densities; these are typical domain assumptions rather than new postulates.

free parameters (1)
  • CaS column density
    Fitted to match observed line intensities in the disk G351.77-mm1.
axioms (1)
  • domain assumption Local thermodynamic equilibrium applies to the excitation of CaS, KS, and KSH
    Standard assumption in radio astronomy for deriving molecular column densities from spectral lines.

pith-pipeline@v0.9.0 · 5798 in / 1209 out tokens · 35391 ms · 2026-05-18T10:49:14.143885+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

What do these tags mean?
matches
The paper's claim is directly supported by a theorem in the formal canon.
supports
The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends
The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses
The paper appears to rely on the theorem as machinery.
contradicts
The paper's claim conflicts with a theorem or certificate in the canon.
unclear
Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.

Reference graph

Works this paper leans on

37 extracted references · 37 canonical work pages

  1. [1]

    I., de Andres, P

    Agúndez, M., Martínez, J. I., de Andres, P. L., Cernicharo, J., & Martín-Gago, J. A. 2020, A&A, 637, A59, doi: 10.1051/0004-6361/202037496

    work page doi:10.1051/0004-6361/202037496 2020

  2. [2]

    , keywords =

    Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481, doi: 10.1146/annurev.astro.46.060407.145222

    work page doi:10.1146/annurev.astro.46.060407.145222 2009

  3. [3]

    C., et al

    Beuther, H., Ahmadi, A., Mottram, J. C., et al. 2019, A&A, 621, A122, doi: 10.1051/0004-6361/201834064

    work page doi:10.1051/0004-6361/201834064 2019

  4. [4]

    Boogert, A. C. A., Schutte, W. A., Helmich, F. P., Tielens, A. G. G. M., & Wooden, D. H. 1997, A&A, 317, 929

    work page 1997

  5. [5]

    P., Sheridan, P

    Bucchino, M. P., Sheridan, P. M., Young, J. P., et al. 2013, The Journal of Chemical Physics, 139, 214307, doi: 10.1063/1.4834656

    work page doi:10.1063/1.4834656 2013

  6. [6]

    2016, MNRAS, 462, S253, doi: 10.1093/mnras/stw2601

    Calmonte, U., Altwegg, K., Balsiger, H., et al. 2016, MNRAS, 462, S253, doi: 10.1093/mnras/stw2601

    work page doi:10.1093/mnras/stw2601 2016

  7. [7]

    Photoprocessing of H2S on dust grains

    Cazaux, S., Carrascosa, H., Muñoz Caro, G. M., et al. 2022, A&A, 657, A100, doi: 10.1051/0004-6361/202141861

    work page doi:10.1051/0004-6361/202141861 2022

  8. [8]

    2019, A&A, 627, L4, doi: 10.1051/0004-6361/201936040

    Cernicharo, J., Velilla-Prieto, L., Agúndez, M., et al. 2019, A&A, 627, L4, doi: 10.1051/0004-6361/201936040

    work page doi:10.1051/0004-6361/201936040 2019

  9. [9]

    R., Agúndez, M., et al

    Cernicharo, J., Pardo, J. R., Agúndez, M., et al. 2025, A&A, 700, L20, doi: 10.1051/0004-6361/202556370

    work page doi:10.1051/0004-6361/202556370 2025

  10. [10]

    2025, A&A, 700, A144, doi: 10.1051/0004-6361/202452044

    Das, A., Sil, M., & Caselli, P. 2025, A&A, 700, A144, doi: 10.1051/0004-6361/202452044

    work page doi:10.1051/0004-6361/202452044 2025

  11. [11]

    P., Schlemmer, S., Schilke, P., Stutzki, J., & Müller, H

    Endres, C. P., Schlemmer, S., Schilke, P., Stutzki, J., & Müller, H. S. P. 2016, Journal of Molecular Spectroscopy, 327, 95, doi: 10.1016/j.jms.2016.03.005

    work page doi:10.1016/j.jms.2016.03.005 2016

  12. [12]

    2025, A&A, 699, L2, doi: 10.1051/0004-6361/202555384

    Esplugues, G., Agúndez, M., Molpeceres, G., et al. 2025, A&A, 699, L2, doi: 10.1051/0004-6361/202555384

    work page doi:10.1051/0004-6361/202555384 2025

  13. [13]

    G., Caselli, P., et al

    Fuente, A., Navarro, D. G., Caselli, P., et al. 2019, A&A, 624, A105, doi: 10.1051/0004-6361/201834654

    work page doi:10.1051/0004-6361/201834654 2019

  14. [14]

    2023, A&A, 670, A114, doi: 10.1051/0004-6361/202244843

    Fuente, A., Rivière-Marichalar, P., Beitia-Antero, L., et al. 2023, A&A, 670, A114, doi: 10.1051/0004-6361/202244843

    work page doi:10.1051/0004-6361/202244843 2023

  15. [15]

    2024, A&A, 687, A87, doi: 10.1051/0004-6361/202449229

    Fuente, A., Roueff, E., Le Petit, F., et al. 2024, A&A, 687, A87, doi: 10.1051/0004-6361/202449229

    work page doi:10.1051/0004-6361/202449229 2024

  16. [16]

    2025, ApJL, 986, L17, doi: 10.3847/2041-8213/addbd3

    Fuente, A., Esplugues, G., Rivière-Marichalar, P., et al. 2025, ApJL, 986, L17, doi: 10.3847/2041-8213/addbd3

    work page doi:10.3847/2041-8213/addbd3 2025

  17. [17]

    2019, ApJ, 872, 54, doi: 10.3847/1538-4357/aafb71

    Ginsburg, A., McGuire, B., Plambeck, R., et al. 2019, ApJ, 872, 54, doi: 10.3847/1538-4357/aafb71

    work page doi:10.3847/1538-4357/aafb71 2019

  18. [18]

    A., Sanhueza, P., et al

    Ginsburg, A., McGuire, B. A., Sanhueza, P., et al. 2023, ApJ, 942, 66, doi: 10.3847/1538-4357/ac9f4a

    work page doi:10.3847/1538-4357/ac9f4a 2023

  19. [19]

    R., & Cuadrado, S

    Goicoechea, J. R., & Cuadrado, S. 2021, A&A, 647, L7, doi: 10.1051/0004-6361/202140517

    work page doi:10.1051/0004-6361/202140517 2021

  20. [20]

    2024, ApJ, 974, 95, doi: 10.3847/1538-4357/ad630f Jiménez-Escobar, A., & Muñoz Caro, G

    Ishihara, K., Sanhueza, P., Nakamura, F., et al. 2024, ApJ, 974, 95, doi: 10.3847/1538-4357/ad630f Jiménez-Escobar, A., & Muñoz Caro, G. M. 2011, A&A, 536, A91, doi: 10.1051/0004-6361/201014821

    work page doi:10.3847/1538-4357/ad630f 2024

  21. [21]

    Jongejan, S., Dominik, C., & Dullemond, C. P. 2023, A&A, 679, A45, doi: 10.1051/0004-6361/202245005

    work page doi:10.1051/0004-6361/202245005 2023

  22. [22]

    2019, arXiv e-prints, arXiv:1912.00844, doi: 10.48550/arXiv.1912.00844 Möller, T., Endres, C., & Schilke, P

    Lodders, K. 2019, arXiv e-prints, arXiv:1912.00844, doi: 10.48550/arXiv.1912.00844 Möller, T., Endres, C., & Schilke, P. 2017, A&A, 598, A7, doi: 10.1051/0004-6361/201527203 Müller, H. S. P., Schlöder, F., Stutzki, J., & Winnewisser, G. 2005, Journal of Molecular Structure, 742, 215, doi: 10.1016/j.molstruc.2005.01.027

    work page doi:10.48550/arxiv.1912.00844 2019

  23. [23]

    A., Godard, B., Gerin, M., et al

    Neufeld, D. A., Godard, B., Gerin, M., et al. 2015, A&A, 577, A49, doi: 10.1051/0004-6361/201425391

    work page doi:10.1051/0004-6361/201425391 2015

  24. [24]

    E., Tielens, A

    Palumbo, M. E., Tielens, A. G. G. M., & Tokunaga, A. T. 1995, ApJ, 449, 674, doi: 10.1086/176088

    work page doi:10.1086/176088 1995

  25. [25]

    M., Poynter, R

    Pickett, H. M., Poynter, R. L., Cohen, E. A., et al. 1998, JQSRT, 60, 883, doi: 10.1016/S0022-4073(98)00091-0

    work page doi:10.1016/s0022-4073(98)00091-0 1998

  26. [26]

    K., Todd, Z., et al

    Ranjan, S., Abd El-Rahman, M. K., Todd, Z., et al. 2022, in The Astrobiology Science Conference (AbSciCon) 2022, 330–07

    work page 2022

  27. [27]

    J., Menten, K

    Reid, M. J., Menten, K. M., Brunthaler, A., et al. 2014, ApJ, 783, 130, doi: 10.1088/0004-637X/783/2/130

    work page doi:10.1088/0004-637x/783/2/130 2014

  28. [28]

    2024, ApJ, 975, 174, doi: 10.3847/1538-4357/ad736e

    Rey-Montejo, M., Jiménez-Serra, I., Martín-Pintado, J., et al. 2024, ApJ, 975, 174, doi: 10.3847/1538-4357/ad736e

    work page doi:10.3847/1538-4357/ad736e 2024

  29. [29]

    K., & Tormen, G

    Ruffle, D. P., Hartquist, T. W., Caselli, P., & Williams, D. A. 1999, MNRAS, 306, 691, doi: 10.1046/j.1365-8711.1999.02562.x

    work page doi:10.1046/j.1365-8711.1999.02562.x 1999

  30. [30]

    N., Lamberts, T., Laas, J

    Shingledecker, C. N., Lamberts, T., Laas, J. C., et al. 2020, ApJ, 888, 52, doi: 10.3847/1538-4357/ab5360

    work page doi:10.3847/1538-4357/ab5360 2020

  31. [31]

    1989, Chemical Physics Letters, 159, 563, doi: 10.1016/0009-2614(89)87533-5

    Takano, S., Yamamoto, S., & Saito, S. 1989, Chemical Physics Letters, 159, 563, doi: 10.1016/0009-2614(89)87533-5

    work page doi:10.1016/0009-2614(89)87533-5 1989

  32. [32]

    Taleb-Bendiab, A., Scappini, F., Amano, T., & Watson, J. K. G. 1996, JChPh, 104, 7431, doi: 10.1063/1.471458

    work page doi:10.1063/1.471458 1996

  33. [33]

    Tanaka, K. E. I., Zhang, Y., Hirota, T., et al. 2020, ApJL, 900, L2, doi: 10.3847/2041-8213/abadfc

    work page doi:10.3847/2041-8213/abadfc 2020

  34. [34]

    2004, A&A, 422, 159, doi: 10.1051/0004-6361:20047186 Wallström, S

    Castets, A. 2004, A&A, 422, 159, doi: 10.1051/0004-6361:20047186 Wallström, S. H. J., Danilovich, T., Müller, H. S. P., et al. 2024, A&A, 681, A50, doi: 10.1051/0004-6361/202347632

    work page doi:10.1051/0004-6361:20047186 2004

  35. [35]

    2024, A&A, 687, A65, doi: 10.1051/0004-6361/202450289

    Marigo, P. 2024, A&A, 687, A65, doi: 10.1051/0004-6361/202450289

    work page doi:10.1051/0004-6361/202450289 2024

  36. [36]

    Xin, J., & Ziurys, L. M. 1998, ApJL, 495, L119, doi: 10.1086/311221

    work page doi:10.1086/311221 1998

  37. [37]

    M., et al

    Zeng, S., Jiménez-Serra, I., Rivilla, V. M., et al. 2018, MNRAS, 478, 2962, doi: 10.1093/mnras/sty1174

    work page doi:10.1093/mnras/sty1174 2018