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REVIEW 3 major objections 5 minor 166 references

SN 1987A's HCO+ emission shows hydrogen was mixed into the remnant's carbon–oxygen core.

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

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2026-08-01 00:33 UTC pith:5AEYO4YH

load-bearing objection First resolved HCO+ map in SN 1987A is a real advance, but the mass and mixing conclusions lean on a J=4-3 detection that the paper's own error budget makes marginal. the 3 major comments →

arxiv 2607.26189 v1 pith:5AEYO4YH submitted 2026-07-28 astro-ph.SR

HCO^+ and the Effect of Mixing in SN 1987A

H. M. Davies , M. Matsuura , H. L. Gomez (Cardiff) , A. M. S. Richards (Manchester) , P. Cigan (George Mason University , United States Naval Observatory) , R. Indebetouw (University of Virginia , National Radio Astronomy Observatory)
show 7 more authors
F. D. Priestley (Cardiff) R. Wesson (UCL Cardiff Maynooth University) M. J. Barlow (UCL) J. Larsson (KTH Royal Institute of Technology) C. Fransson (Stockholm University)
This is my paper
classification astro-ph.SR
keywords HCO+SN 1987Asupernova remnantsmolecular ejectachemical mixingastrochemistryALMAradiative transfer
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

SN 1987A is a rare supernova remnant where HCO+ is detected in the ejecta itself, and this paper uses that fact to read the explosion's mixing history. The authors establish that the HCO+ J=3–2 emission is co-spatial with CO, quantify the HCO+ mass at 3–9 × 10^-6 solar masses from two rotational transitions, and argue that the abundance ratio to CO can only be reached if hydrogen from the star's envelope was transported inward to the carbon- and oxygen-rich nuclear zones. This makes HCO+ a tracer of mixing that other remnant observations do not provide. A simple formation-rate estimate falls short of the observed mass unless the ionisation rate is higher than the canonical interstellar value or additional hydrogen sits with the CO — either way, hydrogen must be co-located with CO.

Core claim

The central claim is that HCO+ in SN 1987A exists because the ejecta are not radially stratified: hydrogen reached the CO-rich nuclear zones before and during the explosion. High-resolution ALMA maps show the J=3–2 HCO+ emitting region overlaps the CO J=2–1 emission (Spearman correlation 0.72), while the brighter HCO+ peaks sit apart from Hα and H2, placing HCO+ in compact, mildly ionised gas inside the ejecta. Using the J=3–2 and J=4–3 line intensities with a non-LTE radiative-transfer calculation, the paper derives column density and kinetic temperature for two assumed H2 collision densities, giving an HCO+ mass of 3–9 × 10^-6 solar masses and a fractional abundance relative to CO of 3×10^

What carries the argument

The load-bearing object is HCO+ itself, a molecular ion that requires carbon, oxygen, and hydrogen to meet: the proposed formation route CO + H3+ → HCO+ + H2. Two ALMA lines (J=3–2 and J=4–3) feed a non-local-thermodynamic-equilibrium radiative-transfer calculation that turns observed intensities into column density and kinetic temperature, with the H2 collision-partner density fixed at 10^6 or 10^5 cm^-3. Spatial comparison with CO, SiO, Hα, and H2 isolates where the ion forms, and a timescale-integrated formation-rate estimate tests whether the reaction can account for the observed mass. The electron destruction channel (dissociative recombination) sets the condition that HCO+ survives onl

Load-bearing premise

The HCO+ mass rests on an assumed H2 collisional-partner density that is obtained by spreading the star's entire 6-solar-mass hydrogen envelope uniformly through the ejecta volume — effectively assuming hydrogen is already everywhere, which is exactly the mixing the paper sets out to infer.

What would settle it

High-spatial-resolution maps of H2 emission in the same velocity channels as the HCO+ clumps: if the H2 there is far below 10^5 cm^-3, or absent, the adopted collision densities collapse and the inferred HCO+ mass and mixing conclusion no longer hold. A cleaner test would be detection of the HCO+ J=1–0 line: its intensity, combined with J=3–2 and J=4–3, would break the column-density/temperature degeneracy and settle the mass without fixing the collision density.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • HCO+ becomes a quantitative tracer of hydrogen transport into metal-rich ejecta, giving models of Rayleigh–Taylor and smaller-scale mixing a new observable to match.
  • The HCO+/CO ratio of 3×10^-6 to 3×10^-4 sets a floor on how much hydrogen must be mixed into the C/O zones; reproducing it requires either an elevated H2 ionisation rate (about 3×10^-16 s^-1) or additional co-located H2.
  • Because HCO+ tracks CO spatially and forms from CO, CO maps can be used to predict where HCO+ emission should appear in other remnants.
  • The non-coincidence of HCO+ with Hα and H2 implies HCO+ marks shielded, low-ionisation gas and supports UV irradiation, not electron collisions, as the H2 excitation mechanism in SN 1987A's ejecta.

Where Pith is reading between the lines

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

  • If the true H2 distribution is clumpy rather than uniform, the density used in the radiative-transfer calculation could be locally higher or lower; high-resolution H2 maps would test whether the assumed 10^5–10^6 cm^-3 densities coincide with the HCO+ clumps.
  • The same technique applied to other young core-collapse remnants with detectable molecular ejecta could show whether deep hydrogen mixing is a generic feature of the explosion mechanism or peculiar to SN 1987A's blue supergiant progenitor.
  • A full chemical network including CH+, OH+, H2O, and CO+ would likely change the inferred required hydrogen fraction; the paper's single-reaction estimate is deliberately minimal, so the mixing conclusion is more robust than the specific mass of hydrogen inferred.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The paper reports ALMA observations of HCO+ J=3-2 and J=4-3 emission from the ejecta of SN 1987A. It finds the J=3-2 emission to be clumpy and co-spatial with CO, with a Spearman correlation of 0.72, and less similar to SiO and Hα. Using RADEX non-LTE radiative transfer on the two line intensities, the authors derive an HCO+ column density and mass of 3–9×10^-6 M_sun (depending on the adopted H2 collisional density, 1e5 or 1e6 cm^-3). They compare this to the CO mass and conclude that the fractional abundance M_HCO+/M_CO = 3×10^-6–3×10^-4 implies that a moderate amount of hydrogen was mixed into the carbon- and oxygen-rich zones. A chemical feasibility estimate for formation via CO + H3+ yields ~1e-7 M_sun, an order of magnitude below the claimed mass unless the H2 ionization rate is raised by an order of magnitude.

Significance. If the quantitative result were robust, this would be an important observational constraint: HCO+ is a direct tracer of hydrogen transport into the metal-rich core of an SN remnant, and no other remnant currently offers a comparable measurement. The paper's qualitative contributions are solid and should be credited: the spatially resolved ALMA detection of HCO+, the morphological comparison with CO/SiO/Hα/H2, and the explicit attempt to connect the observed molecular gas to hydrodynamic mixing scenarios. However, the quantitative anchor for the central claim—the HCO+ mass and the inferred 'moderate' hydrogen mixing—is currently too fragile. The J=4-3 detection is marginal under the paper's own error budget, and the formation-rate estimate does not independently validate the mass. The paper is worth pursuing, but the load-bearing quantitative steps need substantial revision.

major comments (3)
  1. [§3.4 / Table 2] The J=4-3 line, which anchors the RADEX mass, is not a secure detection under the paper's own error budget. Table 2 lists σ_cal=0.40, σ_RMS=0.24, and σ_cont=2.54 (10^-20 W/m^2). Propagating in quadrature gives σ≈2.58, so the continuum-subtracted line (5.69×10^-20) is only ≈2.2σ, not the ±1.0 quoted in the text. Moreover, §3.1 gives the MC SED continuum uncertainty at 356 GHz as ±0.92×10^-19, roughly 90% of the continuum; if that uncertainty is used, the line is sub-significant. Since the J=3-2 line alone cannot break the T_kin–N_HCO+ degeneracy, the derived N_HCO+ and mass (3–9×10^-6 M_sun) are not robust. The error budget and the quoted line intensities must be reconciled before the central quantitative claim can be accepted.
  2. [§5.1] The formation-rate 'feasibility test' yields ~1×10^-7 M_sun, an order of magnitude below the adopted 3–9×10^-6 M_sun, and the gap is closed by assuming ζ=3×10^-16 s^-1. Because ζ is not independently constrained for SN ejecta, this does not demonstrate that the observed HCO+ can be formed; it simply adjusts a free parameter. The calculation also assumes spatially uniform CO and H3+, which §5.2 later concedes is unrealistic. Consequently, the abstract's statement that the fractional abundance 'suggests a moderate amount of hydrogen was mixed' is not quantitatively supported by this section. Either an independent constraint on ζ must be provided, or the chemistry should be presented as a qualitative plausibility argument only.
  3. [§4.1] The H2 collisional density input to RADEX is derived by spreading the full 6 M_sun hydrogen envelope uniformly through the ejecta volume. This presumes the very H–C/O co-location the paper aims to infer. The two tested densities (1e5 and 1e6 cm^-3) change the derived mass by only a factor ~3, but they do not sample the possibility that H2 is absent or clumpy in the HCO+ region; if the local n_H2 differs, the excitation solution and N_HCO+ shift. The paper should state explicitly that the mass is conditional on H2 being co-located with HCO+ and should quantify how an H2 filling factor changes the allowed mass. As written, the RADEX mass cannot independently support the 'moderate mixing' conclusion.
minor comments (5)
  1. [Table 2] Errorc for J=4-3 is listed as 2.54×10^-20, but 30% of Cdust=10.17×10^-20 is 3.05×10^-20. Reconcile the table, the text, and the quoted total uncertainty ±1.0×10^-20.
  2. [Fig. 1 / §3.1] The continuum contour levels are inconsistent: the figure caption lists 4.5e-5, 6e-5, 8e-5 Jy/beam, while the text quotes 7e-5 and 9e-4 Jy/beam. Please check and unify.
  3. [§3.4] The J=4-3 Gaussian fit fixes the FWHM to the J=3-2 value (1906 km/s). Given the limited spectral coverage of the J=4-3 data, state explicitly how this assumption affects the integrated intensity and the subsequent RADEX fit.
  4. [§5.1 / §5.2] The paper first assumes a uniform distribution of CO and H3+ for the formation estimate and then argues that uniform mixing is unrealistic. This tension should be acknowledged earlier and the quantitative conclusions adjusted accordingly.
  5. [Abstract / §4.2] The mass range 3–9×10^-6 M_sun and the fractional abundance range spanning two orders of magnitude are conditional on two fixed H2 densities and f=1. The abstract should present these as conditional estimates, not as a measured range.

Circularity Check

0 steps flagged

No significant circularity: the HCO+ mass is a RADEX fit to observed line intensities, and the mixing inference is a plausibility argument with acknowledged assumptions.

full rationale

The derivation chain for the central mass estimate is a standard RADEX fit: two observed continuum-subtracted line intensities (Table 2) are compared to RADEX models varying T_kin and N_HCO+, and the mass follows from the fitted column density via M = f Ω N d^2 m (Sects 4.1-4.2). The J=4-3 line is an independent second observable; no equation defines the output mass from the assumed inputs by construction. The H2 collisional density is assumed (Sect. 4.1: 'We estimate an approximate H2 density by calculating it from the mass of the hydrogen envelope... 6 M_sun... and the volume of the ejecta'), but the paper explicitly brackets the assumption with two densities (1e6 and 1e5 cm^-3) and the resulting mass changes by only a factor ~3. The conclusion that hydrogen was mixed is anchored by the detection of HCO+ itself and its co-spatiality with CO, not solely by the assumed density. Section 5.1's formation-rate estimate is presented as a feasibility test, not a prediction; the ionization rate is adjusted to match the observed mass, and the paper flags this ('we suspect this is predominantly due to the assumption that the rate of ionisation of H2 is ζ = 3e-17 s^-1'). The paper also states the uniform-mixing assumption is 'unrealistic' (Sect. 5.2) and that a filling factor of 1 may overestimate the mass (Sect. 5.1), so the limitations are on the table. Self-citations (Matsuura et al. 2017, Larsson et al. 2023, Matsuura et al. 2024) provide prior observational/context data, but are not invoked as an unverified uniqueness theorem and are not load-bearing for the mass fit. The apparent J=4-3 continuum-uncertainty inconsistency (Table 2 vs Sect. 3.1) is a robustness concern, not circularity.

Axiom & Free-Parameter Ledger

5 free parameters · 7 axioms · 0 invented entities

The central mass/mixing claim rests on several hand-chosen inputs. The H2 collisional density is derived by diluting the whole 6 M_sun hydrogen envelope uniformly over the ejecta volume — an assumption of the very mixing under test. The chemical feasibility estimate agrees with the observed mass only after a post-hoc tenfold boost of the ionisation rate ζ. The dust-continuum subtraction, which removes 64% of the raw J=4-3 flux, depends on a fitted modified-blackbody SED with β, T, M_d free. Filling factor f=1 and a Gaussian line profile with fixed FWHM=1906 km/s close the chain. No invented entities are introduced.

free parameters (5)
  • H2 collisional partner density n_H2 = 1e6 and 1e5 cm^-3 (two fixed cases)
    Input to RADEX (Sect. 4.1); drives the T_kin/N_HCO+ fit and hence the mass. Derived from diluting the 6 M_sun hydrogen envelope uniformly over the ejecta volume, i.e., it presumes the mixing conclusion.
  • Ionisation rate ζ of H2 = 3e-17 s^-1 (canonical ISM); 3e-16 s^-1 to match observed mass
    The formation estimate (Sect. 5.1) gives ~1e-7 M_sun; agreement with 3-9e-6 M_sun requires the order-of-magnitude boost. Post-hoc tuning of the chemistry.
  • Filling factor f for HCO+ mass = 1
    Assumed uniform (Sect. 4.2). Directly multiplies the mass; with clumpy emission (Fig. 3) the mass could be overestimated, as the paper concedes.
  • Dust SED parameters β, T, M_d = β=2.1, T=17.8 K, M_d=1.4 M_sun
    Fitted modified blackbody (MC, 5000 draws) setting the continuum subtracted from both HCO+ lines. The J=4-3 line is only 5.69e-20 vs 10.17e-20 W/m2 continuum, so the mass depends on this fit.
  • Gaussian FWHM of HCO+ lines = 1906 km/s (fit to J=3-2; fixed for J=4-3)
    Used to extrapolate flux beyond the observed ±1650 km/s coverage and as the RADEX line width; a non-Gaussian profile would shift the integrated intensity and mass.
axioms (7)
  • domain assumption HCO+ forms predominantly via CO + H3+ → HCO+ + H2 with UMIST rate coefficients; destruction is dominated by dissociative recombination with electrons.
    Central chemistry in Sect. 5.1 and 5.3; alternative routes (CH+ + H2O, CO+ + HCO) are set aside.
  • domain assumption Steady-state H3+ in the SN ejecta follows the ISM relation n_H3+ = ζ n_H2 / k_H3+, with k_H3+ set equal to the HCO+ formation rate k(T).
    Sect. 5.1, citing Oka (2006) for dense clouds; applied to SN ejecta without in-situ validation.
  • ad hoc to paper Ejecta is chemically well-mixed and uniform (uniform CO and H3+ distributions) for the formation estimate; density evolves as t^-3 (homologous expansion).
    Stated in Sect. 5.1 and later conceded as 'unrealistic given the partial retention of the progenitor structure' (Sect. 5.2). Load-bearing for the 1e-7 M_sun estimate and the mixing inference.
  • domain assumption H2 is the dominant collisional partner, and the LAMDA HCO+-H2 collisional rate data apply.
    Sect. 4.1; in partially ionised ejecta, electrons could be a significant collider, changing the RADEX excitation solution and derived column/mass.
  • domain assumption Dust continuum is a single-temperature modified blackbody with κ ∝ λ^-β, and the FIR/sub-mm SED from ~7 years earlier (Herschel) is representative of the HCO+ epoch to within ~3%.
    Sect. 3.1; the J=4-3 continuum (10.17e-20 W/m2) is 64% of the raw flux, so the mass depends on this SED assumption. A 44Ti-decay heating correction is applied.
  • standard math Free expansion at 1300 km/s (from FWHM 1900 km/s via the McCray 1993 relation) gives the ejecta volume; SN distance is 51.2 kpc.
    Sect. 4.1 mass/volume/density conversions.
  • domain assumption Macroscopic mixing from Rayleigh-Taylor instabilities occurs as in 3D CCSN models (Wongwathanarat et al. 2015; Utrobin et al. 2019).
    Background for the interpretation (Sects. 1 and 5.2); not derived here.

pith-pipeline@v1.3.0-alltime-deepseek · 5927 in / 7533 out tokens · 267855 ms · 2026-08-01T00:33:49.305619+00:00 · methodology

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read the original abstract

We present high angular resolution observations of the HCO$^+$ emission in the central ejecta of the supernova remnant SN 1987A using the Atacama Large Millimeter Array (ALMA). We use this to infer the degree and type of mixing required within the ejecta in order to form HCO$^+$. The distribution of the $J=3-2$ HCO$^+$ emission is co-spatial with that of the $J=2-1$ CO emission, with an overlap between their brightest peaks. The correlation between the two molecules is strong and suggests that HCO$^+$ could form from reactions involving CO. We obtain additional observations of the $J=4-3$ HCO$^+$ emission to calculate the mass of HCO$^+$. The estimated HCO$^+$ mass is $3\text{--}9 \times 10^{-6}\,M_{\odot}$. The relatively large fractional abundance of HCO$^+$ with respect to CO ($M_{\mathrm{HCO}^+}/M_{\mathrm{CO}} = 3 \times 10^{-6}\text{--}3 \times 10^{-4}$) suggests that a moderate amount of hydrogen was mixed into the carbon- and oxygen-rich nuclear zones of the ejecta prior to and during the supernova explosion, in addition to large-scale macroscopic mixing.

Figures

Figures reproduced from arXiv: 2607.26189 by A. M. S. Richards (Manchester), Cardiff, C. Fransson (Stockholm University), F. D. Priestley (Cardiff), H. L. Gomez (Cardiff), H. M. Davies, J. Larsson (KTH Royal Institute of Technology), Maynooth University), M. J. Barlow (UCL), M. Matsuura, National Radio Astronomy Observatory), P. Cigan (George Mason University, R. Indebetouw (University of Virginia, R. Wesson (UCL, United States Naval Observatory).

Figure 1
Figure 1. Figure 1: (a) An image of the brightness distribution of the continuum-included J = 3 − 2 HCO+ emission, integrated across its velocity space. Overlain are contours of the 315 GHz con￾tinuum emission from Matsuura et al. (2024), which is dis￾played in panel (b). The continuum consists of dust emission from the ejecta and synchrotron emission from the equatorial ring. The continuum contour levels correspond to 4.5×10… view at source ↗
Figure 2
Figure 2. Figure 2: A modified blackbody fit to sub-millimeter and far￾infrared observations of thermal dust emission using a Monte-Carlo simulation. Circles show ALMA dust observations in the sub-mm where the temporal span of observations is averaged to SN day 10402 (Cigan et al. 2019). Diamonds show Herschel dust observa￾tions in the far-infrared taken on SN days 9090 and 9122 (Matsuura et al. 2015). The blue error bar on t… view at source ↗
Figure 3
Figure 3. Figure 3: Velocity channel map of the J = 3 − 2 transition of HCO+ emission from velocities beginning at −1950 km s−1 to 1650 km s−1 , binned in 300 km s−1 increments. Each image channel is labelled with the central velocity of the 300 km s−1 bin. Velocities are in LSRK. Continuum is included in this figure which, if uniformly spread out over the region, is ∼ 0.02 mJy beam−1 , i.e. very low level. The typical RMS no… view at source ↗
Figure 4
Figure 4. Figure 4: Comparison of (a) ALMA J = 3–2 HCO+, (b) HST Hα, and (c) JWST H2 emission. (a) Continuum-included J = 3– 2 HCO+ image, obtained by integrating over velocity space. The white contours indicate HCO+ emission levels of 0.3, 0.5 and 0.85 Jy beam−1 km s−1 in all three panels. The emission from the ring is due to synchrotron radiation, not HCO+ emission. (b) HST WFC3 F625W image, which predominantly traces Hα em… view at source ↗
Figure 5
Figure 5. Figure 5: Upper: Flux density profile for the J = 3 − 2 HCO+ with continuum included in the flux density. This profile was fit￾ted with a gaussian curve to retrieve the FWHM of the line which was calculated to be +1906 km s−1 shown as a black line. Lower: Flux density profile for the J = 4 − 3 HCO+ transition with con￾tinuum included and fitted with a Gaussian curve with FWHM fixed at 1906 km s−1 (black line). Both … view at source ↗
Figure 6
Figure 6. Figure 6: χ 2 distribution for comparisons of RADEX line intensities with observed line fluxes with respect to the RADEX input parameters of kinetic temperature, Tkin, and column density, NHCO+ , of HCO+. (a) and (b) show the results using the continuum￾subtracted HCO+ line intensity at a H2 collisional partner density of at 1×106 and 1×105 cm−3 respectively. The white space in the figures indicate the negative τ en… view at source ↗

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Reference graph

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