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First detection of C2H+ in the interstellar medium

T0 review · 2 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read This paper reports the first interstellar detection of the ethynylium ion C2H+ toward the Orion Bar, identified through the resolved Lambda-doubling and hyperfine structure of its lowest J=3-2 rotational transition near 211 GHz.

desk verdict A plausible, well-presented first detection of C2H+ in the ISM, but the evidence is a single rotational transition with a partly contaminated pattern, so treat it as a strong candidate rather than a closed case. read the letter →

arxiv 2608.08014 v1 pith:YXEAJWLT submitted 2026-08-08 astro-ph.GA

Arshia M. Jacob , Karl M. Menten , Sandra Brünken , Arnaud Belloche , Friedrich Wyrowski , Weslley G. D. P. Silva , Oskar Asvany , Sarwar Khan
show 5 more authors
Slawa Kabanovic Kim Steenbakkers Gerrit C. Groenenboom Britta Redlich Stephan Schlemmer
This is my paper · ORCID
classification astro-ph.GA
keywords C2H+ethynyliuminterstellarionsphotodissociationregionsOrionBarion-moleculechemistrycarbon-chainmoleculessubmillimetrespectroscopy
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 interstellar detection of the ethynylium cation C$_2$H$^+$, the simplest carbon-chain ion carrying two carbon atoms, toward the Orion Bar photodissociation region. The identification rests on the resolved $\Lambda$-doubling and hyperfine structure of the lowest $J=3$–$2$ ($\Omega=2$) rotational transition near 211 GHz, with laboratory rest frequencies and line intensities measured beforehand. The inferred column density is $0.3\times10^{11}$ to $2.2\times10^{11}$ cm$^{-2}$, corresponding to an abundance of at most a few $\times10^{-12}$ relative to total hydrogen. Gas-phase photodissociation-region models reproduce this abundance and place C$_2$H$^+$ formation at the outer edge of the PDR, where vibrationally excited H$_2$ drives CH$^+$ and CH$_3^+$ formation. If correct, the detection shows that small hydrocarbon ions can seed larger molecules at PDR surfaces, supporting a bottom-up route to molecular complexity.

What carries the argument

The load-bearing object is the rotational spectrum of the ethynylium cation itself. C$_2$H$^+$ is a linear molecule in a $X{}^3\Pi$ ground state, and its lowest $J=3$–$2$ ($\Omega=2$) transition near 211 GHz is split by $\Lambda$-doubling into two groups separated by roughly 200 MHz, each further split into three hyperfine components spread over about 30 MHz. That pattern—with rest frequencies, relative intensities, and Einstein $A$ coefficients measured in the laboratory by leak-out spectroscopy on a cryogenic ion trap—acts as a fingerprint that identifies the molecule from a single rotational transition. The same measured line data, combined with an LTE fitting tool and the rotational temperature range derived from companion hydrocarbons, converts the observed line intensities into column densities, while the photodissociation-region model supplies the chemical network that ties C$_2$H$^+$ to C$_2^+$ + H$_2$ and to the vibrationally excited H$_2$-driven CH$^+$/CH$_3^+$ chain.

What would settle it

Observe a second rotational transition of C$_2$H$^+$, such as the $J=4$–$3$ line whose frequency the laboratory fit predicts, toward the same Orion Bar position: if no feature appears there with the expected velocity (about 10.3 km s$^{-1}$) and linewidth (about 3.8 km s$^{-1}$) at comparable sensitivity, the reported lines are likely blends or noise. An interferometric map at arcsecond resolution could provide the same test spatially by checking whether the contaminated 211435.49 MHz component shares the distribution of the cleaner 211658.09 MHz line.

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Extended reading notes

Core claim

Toward the CO$^+$ peak of the Orion Bar, four of the six hyperfine components of the C$_2$H$^+$ $J=3$–$2$ ($\Omega=2$) transition are detected at roughly 3–4 mK in a 12 m submillimetre spectrum with 0.77 mK root-mean-square noise. The two strongest components of each $\Lambda$-doublet appear at the laboratory rest frequencies with a centroid velocity of about 10.3 km s$^{-1}$ and a linewidth of about 3.8 km s$^{-1}$, while the weakest component of each doublet is not detected. Using the rotational temperature range of 14–138 K obtained from co-detected hydrocarbons and an assumed 20 arcsec emitting region, the authors derive a C$_2$H$^+$ column density between $0.3\times10^{11}$ and $2.2\times10^{11}$ cm$^{-2}$, i.e. a fractional abundance of at most a few $\times10^{-12}$. A photodissociation-region model without PAH fragmentation or grain-surface chemistry reproduces this range, peaking at $A_V \simeq 1.3$ mag, and matches the observed [C$_2$H]/[C$_2$H$^+$] and [C$_3$H]/[C$_3$H$^+$] ratios at $A_V \simeq 2.7$–$3.2$ mag. The paper identifies C$_2^+$ + H$_2 \rightarrow$ C$_2$H$^+$ + H as the dominant formation path, nested in the network initiated by FUV-pumped vibrationally excited H$_2$.

Load-bearing premise

The claim stands on two premises: that the laboratory-measured line positions and relative intensities of C2H+ are accurate, and that the faint detected features are not blends with unrelated molecules; the paper itself notes that one of the four detected lines is partly contaminated, a neighbouring unidentified line remains unmatched, and the weakest hyperfine component of each doublet is not detected.

Editorial extensions

If this is right

  • C$_2$H$^+$ joins the small family of hydrocarbon ions detected in PDRs, giving observers a direct probe of the gas layer where ion-mediated growth begins.
  • The abundance is reproduced without invoking PAH or grain-surface chemistry, so bottom-up ion–molecule reactions can account for at least this part of the hydrocarbon budget in the Orion Bar.
  • The formation chain C$_2^+$ + H$_2 \rightarrow$ C$_2$H$^+$ + H, followed by C$_2$H$^+$ + H$_2 \rightarrow$ C$_2$H$_2^+$ + H, connects the observed CH$^+$/CH$_3^+$ layer to neutral acetylene and larger carbon chains.
  • In cold cloud conditions like TMC-1 the model predicts a C$_2$H$^+$ column density of about $6.4\times10^{10}$ cm$^{-2}$, so a targeted search there—even an upper limit—would test whether cosmic-ray-driven ion chemistry sustains hydrocarbon growth at 10 K.

Reading between the lines

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

  • If the detection holds, C$_2$H$^+$ could act as an observational tracer of the dissociation front's warm boundary: its modelled peak sits deeper than the H$_2$ (1–0) S(1) layer but just above the C$_2$H ridge, so arcsecond-scale imaging would separate the vibrationally excited H$_2$ zone from the dense molecular gas that the 29 arcsec beam currently blends together.
  • A confirmation that would not require new laboratory work is to detect a second rotational transition of C$_2$H$^+$, for instance $J=4$–$3$, at the same position; if no line appears at the predicted frequency with the same velocity and width, the reported features would most likely be blends or noise.
  • Because the [C$_2$H]/[C$_2$H$^+$] ratio varies steeply with depth in the models, measuring both species with a common beam across several PDRs could yield a depth gauge for the dissociation front that is insensitive to absolute calibration.
  • The paper's own caveats about one contaminated component and an unmatched neighbouring line imply that the cleanest abundance test is to redo the fit using only the two strongest, uncontaminated components, treating the remaining range as bracketed by source-size and excitation assumptions rather than by noise alone.
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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

2 major / 5 minor

Summary. The paper reports the first interstellar detection of the ethynyl cation C2H+ toward the Orion Bar, based on APEX 12 m observations of the J=3–2 (Ω=2) rotational transition near 211 GHz. The identification rests on the resolved Λ-doubling and hyperfine structure, with four of the six predicted components detected at 3–5 mK; the two weakest components are not detected, and one detected component is explicitly acknowledged to be contaminated. Assuming LTE and a 20″ source size, the authors derive C2H+ column densities of (0.3–2.2)×10^11 cm^-2 for excitation temperatures of 14–138 K, corresponding to abundances of at most a few×10^-12 relative to hydrogen nuclei. Meudon PDR models reproduce this abundance range near A_V ≈ 1–3 mag and link C2H+ to CH+/CH3+ chemistry driven by vibrationally excited H2. The paper also argues that C2H+ is a key intermediate in bottom-up hydrocarbon growth and suggests that a targeted search in TMC-1 may be feasible.

Significance. If the identification is secure, this is the first interstellar detection of C2H+ and a valuable new constraint on ion–molecule chemistry in photodissociation regions. The paper's strengths include a very deep integration (118.6 hours, 0.77 mK rms), the use of laboratory-measured rest frequencies and relative intensities for all six hyperfine components, and a transparent account of the contaminated component and the nearby unidentified line. The velocity agreement of the clean components with the secondary hydrocarbon component and the LTE-consistent relative intensities provide a genuine spectroscopic fingerprint. The Meudon PDR modeling gives a plausible chemical context. However, the central claim rests on a single rotational transition with only three cleanly detected components, so the identification is strong but not fully independent; the abundance range is also broad because the excitation temperature and source size are not directly measured. The paper is careful to state these limitations, which makes the residual risk one of correctness rather than internal inconsistency.

major comments (2)
  1. [§4.1, Fig. 3, Table 1] The first-detection claim relies on four of the six hyperfine components, one of which (211435.49 MHz) is acknowledged to be contaminated, and the two weakest components (211417.82 and 211640.65 MHz) are not detected. I request that the authors refit the data excluding the contaminated component and show that the remaining three clean components (211447.14, 211658.09, and 211670.03 MHz) independently reproduce the laboratory frequency pattern and the LTE intensity ratios. In addition, provide a quantitative estimate of the probability that the observed 3–5 mK features arise by chance from unknown lines in this crowded band, based on the density of unidentified features in Fig. A.1 and the number of independent velocity channels searched. This would directly address the residual risk that coincidental blending, not C2H+, produces part of the observed pattern.
  2. [§4.1, Table 2] The quoted column densities depend on the calculated permanent dipole moment (1.06 D) and the partition function from Steenbakkers et al. (2026b), both of which are explicitly flagged as uncertain in §2. Since the Einstein A coefficients scale as the square of the dipole moment and the partition function neglects low-lying vibrational states that can contribute at the upper end of the adopted temperature range, the authors should propagate these uncertainties into the reported column-density range. The comparison with the Meudon PDR models in §4.2 and §5 is a central quantitative result, so the robustness of N(C2H+) to the dipole-moment and partition-function uncertainties should be quantified rather than stated qualitatively.
minor comments (5)
  1. [§2] The phrase "six observed fine-structure lines belonging to the one rotational transition" is imprecise; the six lines are hyperfine components of two Λ-doublet transitions, not separate fine-structure lines. Please rephrase to "six hyperfine components" or similar.
  2. [Table 1] The column header "ν [MHz]×10^-6" is ambiguous. It appears that the Aul column is expressed in units of 10^-6 s^-1, but the table header should state the units of each column explicitly to avoid confusion between the frequency uncertainty and the Einstein-A scaling.
  3. [Fig. 3 caption] The velocity axis is defined relative to the strongest hyperfine component of each Λ-doublet, so the two panels have different zero-points. The caption should state this convention explicitly so that the negative velocities of the 2.5→1.5 components are not misread as blueshifted emission with respect to a common barycentric frame.
  4. [§4.1] Since the contamination of the 211435.49 MHz line is not identified, please show the residual spectrum around this component after subtracting the LTE model, so that the reader can judge whether the excess is consistent with a weak blend or with a more serious perturbation of the line profile.
  5. [Abstract and §6] The abstract and conclusion describe C2H+ as "ethynylium"; for readers not familiar with this nomenclature, consider adding the chemical formula "HCCH+" or "C2H+" in parentheses at first occurrence in the abstract.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the identification rests on externally measured laboratory spectroscopy and the PDR comparison uses independent model inputs.

full rationale

The paper's central chain is: (1) detect four weak HFS features near the laboratory rest frequencies of the C2H+ J=3-2 (Omega=2) transition; (2) convert observed intensities to column densities using laboratory line strengths, Einstein coefficients, and partition functions; (3) compare with Meudon PDR models computed from literature parameters (G0 from FUV-pumped IR fluorescent lines, thermal pressure from prior Orion Bar studies, elemental abundances and cosmic-ray ionization rate from the literature). None of these steps fits the astronomical C2H+ data back into the model or defines the detection in terms of the derived abundance. The rest frequencies and relative intensities of Steenbakkers et al. (2026b) are external laboratory measurements, not derived from the Orion Bar spectra, and the cited authors' overlap with the present team does not make the measurement circular: it is independently falsifiable laboratory data. The rotational temperature bracket (14-138 K) is obtained from other hydrocarbons and applied to C2H+, not fitted to the C2H+ lines themselves, so the resulting column-density range is an interpretation rather than a hidden input. The manuscript is also transparent about genuine weaknesses (the contaminated 211435.49 MHz component, the unidentified 211635 MHz line, and the undetected weakest HFS components), which are correctness risks in the identification claim, not evidence of circularity. Overall, the derivation is self-contained against external benchmarks and no load-bearing step reduces to its own inputs.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

The detection rests on the laboratory spectrum from the same group, on a series of observational assumptions (LTE, source size, excitation temperature bracketing), and on literature-adopted PDR model parameters. No new physical entities are introduced.

free parameters (4)
  • Rotational temperature T_rot for C2H+ = 14.3 K and 138.5 K (bracketed range)
    Only one rotational transition was detected, so T_rot cannot be constrained; the paper adopts the extremes of rotational temperatures measured for other hydrocarbons in the same beam (14-138 K) to bracket the column density.
  • Assumed emitting source size theta_source = 20 arcsec
    Adopted as a lower limit on the emitting region based on C2H HFS optical depth modeling; the APEX beam is 29 arcsec. Smaller source sizes produce appreciable optical depths, and the column densities are corrected for beam dilution.
  • C2H+ permanent dipole moment used for Einstein A coefficients = 1.06 D
    Calculated at MRCI/ANO1 and HF/cc-pVQZ level of theory; the paper cautions that it needs to be treated with caution. It directly scales the line intensities and hence the derived column densities.
  • Partition function Q for C2H+ = Computed with PGOPHER up to J<30; vibrational ground state only
    The partition function neglects low-lying vibrational states that may be populated above 100 K; the paper notes this limitation for intensity and abundance estimates.
assumptions (5)
  • domain assumption Laboratory rest frequencies and HFS patterns from Steenbakkers et al. (2026b) are accurate to about 80 kHz.
    The astronomical identification in Sect. 4 relies entirely on these frequencies; if they were in error by more than the line width, the line stacking and velocity alignment would fail.
  • domain assumption C2H+ emission is optically thin and in LTE, characterized by a single excitation temperature.
    The Weeds LTE fitting and column density derivation in Sect. 4.1 assume LTE. The observed HFS ratios are consistent with optically thin LTE, but this remains an assumption given only one rotational transition.
  • domain assumption The Meudon PDR code with adopted parameters (G0 = 2e4, P_th = 1e8 K cm^-3, zeta_H = 1e-16 s^-1, R_V = 5.5) describes the beam-averaged gas at the Orion Bar pointing position.
    Model parameters are taken from recent literature, not measured in this work. The region is modeled as a 1D isobaric slab, while the 29 arcsec beam encompasses multiple dissociation fronts.
  • domain assumption The gas-phase chemical network is sufficient to reproduce the C2H+ abundance; top-down processes and grain-surface chemistry are not needed.
    The model excludes PAH fragmentation and gas-grain exchange except H2 formation. The paper uses the agreement between model and observation to argue for bottom-up chemistry, so this modeling choice is load-bearing for the interpretation.
  • domain assumption C2H+ traces the same gas component as C2H, justifying the 20 arcsec source size lower limit.
    Used in Sect. 4.1 to set the emitting size. If C2H+ arises in a more compact region, the beam-dilution correction would be larger and the column density higher.

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Cite this review

Pith. "Pith review of First detection of C2H+ in the interstellar medium." pith.science (2026). https://pith.science/paper/YXEAJWLT

@misc{pith2026260808014,
  author       = {Pith},
  title        = {Pith review of: First detection of C2H+ in the interstellar medium},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/YXEAJWLT}},
  note         = {Machine review of arXiv:2608.08014}
}
read the original abstract

Despite the detection of nearly 350 molecules in the interstellar medium, almost half of which are carbon chains, the pathways that build molecular complexity remain poorly understood. Observed abundances of carbon-chain and aromatic species are difficult to reconcile with existing top-down or bottom-up formation scenarios, due in part to limited observational constraints and incomplete theoretical understanding. In particular, small intermediary ions, key drivers of ion-molecule reactions capable of seeding larger hydrocarbons and aromatic rings, could provide critical support for the bottom-up formation scenario. Constraining the abundance and chemistry of these ions is therefore essential to test whether bottom-up growth can operate efficiently under interstellar conditions. Here, we report the first detection of the small hydrocarbon cation ethynylium, C2H+, toward the Orion Bar, based on observations with the APEX 12m sub-mm telescope of its lowest-lying J=3-2 rotational transition near 211GHz, which exhibits a unique spectroscopic fingerprint through resolved Lambda-doubling and hyperfine splitting components, as recently measured in the laboratory. Meudon PDR models successfully reproduce these values, placing C2H+ formation at the outer edges of PDR fronts. Our results link C2H+ production to CH+ and CH3+ within a network of ion-molecule reactions driven by vibrationally excited H2, a scenario now further supported by recent detections of these species in PDRs like the Orion Bar with JWST observations. The importance of C2H+ lies in its role as a key intermediate: it produces C2H2+ and subsequently C2H3+, effectively channelling small C2 building blocks toward larger hydrocarbons and facilitating bottom-up growth at the PDR surface. Targeted searches for C2H+ in other regions promise to provide a potentially decisive probe of ion-driven bottom-up chemistry in the ISM.

Figures

Figures reproduced from arXiv: 2608.08014 by the authors.

Figure 1
Figure 1. Chemical network displaying the relevant gas-phase ion-neutral and neutral-neutral reactions in the early stages of interstellar carbon chemistry. The dominant formation and destruction pathways of C2H + are marked by broader arrows. Unless otherwise labelled, the arrows indicate reactions with electrons, photons (hν), H2, C, and C+ in red, black, blue, pink, and green, respectively. Adapted from [PITH_FULL_IMAGE:f… view at source ↗
Figure 2
Figure 2. Left: Contours of the l-C3H + J = 10–9 integrated line intensity (in white) at levels of 0.97, 1.07, and 1.17 K km s−1 , together with Spitzer 8 µm PAH emission (in blue) shown at 3000, 4000, 4900, and 5 700 MJy sr−1 (Walmsley et al. 2000), are overlaid on the C2H J = 3–2 integrated intensity map (5–15 K km s−1 , colour scale; Brinkmann et al. 2020) toward the Orion Bar. Beam sizes of the different observations are … view at source ↗
Figure 3
Figure 3. Spectra of the HFS-resolved Λ-doubling components (upper and lower panel) of the J = 3–2 (Ω = 2) rotational transition of C2H + to￾ward the Orion Bar single dish line survey position. The observation is given in black, with the GILDAS-Weeds model fit assuming Trot = 14 K and a source size of 20′′ overlaid in magenta. The relative intensities of the HFS components are marked in teal as determined by Steenbakkers et a… view at source ↗
Figures from the paper (6 more)
Figure 4
Figure 4. Figure 4: Variation of N(C2H) (blue) and N(C2H + ) (red) as a function of excitation temperature, assuming an emitting region of 20′′. The er￾ror bars denote the formal 3σ uncertainties derived from the non-linear least-squares fitting procedure. The "*" markers indicate the rot…
Figure 6
Figure 6. Figure 6: Meudon PDR model-predicted abundance ratios for [C2H]/[C2H + ] (red) and [C3H]/[C3H + ] (blue), with the corre￾sponding ranges of observed abundance ratios in the Orion Bar shown in the same colours. these energetic conditions are not met, CH+ and related species are f…
Figure 5
Figure 5. Figure 5: Isobaric PDR model with thermal pressure Pth = 108 K cm−3 , computed for fixed values of G0 = 2 × 104 and ζH = 10−16 s −1 . Top panel: Gas density, gas temperature, dust temperature (shown for the minimum and maximum grain-size distributions), and the abundance of FUV-…
Figure 7
Figure 7. Figure 7: Normalised contributions of the formation (left) and destruction (right) pathways for C2H + for the physical conditions in the Orion Bar modelled with the Meudon PDR code. In both panels the dominant reaction pathway for AV ranges of interest (>1 mag) are displayed in …
Figure 8
Figure 8. Figure 8: Normalised line intensities of H2 (0–0) S(1) (dashed grey) and H2 (1–0) S(1) (solid grey) alongside that of CH+ (blue) and CH+ 3 (brown) on the left-hand panel and that of C2H + (red), C2H (magenta) and C2H + 2 (dark orange) for the physical conditions in the Orion Bar…
Figure 9
Figure 9. Figure 9: Model-predicted column densities from the Meudon PDR code (open diamonds) for C2H + and related small hydrocarbons, including C + 2 , toward the Orion Bar and TMC-1 compared with column densi￾ties determined from observations (filled squares). Error bars indicate the r…

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