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arxiv: 2604.02613 · v2 · submitted 2026-04-03 · 🌌 astro-ph.EP

Monitoring Volatile Evolution in Disrupting Comet D/2021 A1 (Leonard) with NOEMA and APEX

Timothy N. Proudkii , Nathan X. Roth , J\'er\'emie Boissier , Dominique Bockel\'ee-Morvan , Nicolas Biver , Steven B. Charnley , Stefanie N. Milam , Martin A. Cordiner
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Michael A. DiSanti Boncho P. Bonev Neil Dello Russo
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Pith reviewed 2026-05-13 19:04 UTC · model grok-4.3

classification 🌌 astro-ph.EP
keywords comet volatile evolutionHCNCScometary disruptionmixing ratiosNOEMAAPEXD/2021 A1
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The pith

Disruption and fragmentation in comet D/2021 A1 drove changes in volatile mixing ratios beyond what solar distance alone predicts.

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

The paper presents multi-epoch observations of comet D/2021 A1 from November to December 2021 using NOEMA and APEX, tracking HCN and CS emissions as the comet moved inward from 1.3 to 0.8 au. CS mixing ratios relative to water increased by a factor of about five over this period, consistent with production from a distributed source rather than the nucleus. HCN showed smaller changes and more scatter when combined with data from other telescopes. The observations captured outbursts and fragmentation in mid-December, during which both species varied in ways that point to physical breakup releasing additional material. This demonstrates that rapid, species-specific changes in comets require repeated monitoring across instruments to separate insolation effects from disruption processes.

Core claim

Comet D/2021 A1 exhibited a clear rise in CS mixing ratios from 0.02% to 0.10% as heliocentric distance decreased, while HCN remained near 0.07% with added variability once cross-facility data were included; line kinematics showed HCN released near the nucleus and CS from a distributed source, with both species displaying changes tied to mid-December outbursts and fragmentation.

What carries the argument

Mixing ratios of HCN and CS relative to H2O, derived from line intensities and spatial constraints, that separate nucleus versus distributed sources and track changes across epochs.

If this is right

  • CS production increases steadily inward because the molecule forms in the coma from a distributed parent.
  • HCN shows no robust single trend with distance once all data are merged, indicating multiple release mechanisms.
  • Outbursts and fragmentation cause short-term spikes in both species that solar insolation models alone cannot explain.
  • Multi-facility, multi-epoch coverage is required to resolve species-dependent responses on timescales of days.

Where Pith is reading between the lines

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

  • Similar monitoring of other fragmenting comets could reveal whether buried ices are preferentially released during breakup events.
  • The observed CS rise may imply that distributed-source chemistry becomes more efficient closer to the Sun in active comets.
  • Accounting for disruption effects could improve predictions of volatile delivery to inner planets during cometary passages.

Load-bearing premise

That HCN measurements from different telescopes can be combined directly without calibration offsets large enough to erase the apparent lack of monotonic trend with distance.

What would settle it

A new campaign using a single calibrated instrument across the same distance range that finds HCN mixing ratios either constant or strictly decreasing with no outburst-linked spikes would undermine the claim that disruption drives the observed variability.

Figures

Figures reproduced from arXiv: 2604.02613 by Boncho P. Bonev, Dominique Bockel\'ee-Morvan, J\'er\'emie Boissier, Martin A. Cordiner, Michael A. DiSanti, Nathan X. Roth, Neil Dello Russo, Nicolas Biver, Stefanie N. Milam, Steven B. Charnley, Timothy N. Proudkii.

Figure 1
Figure 1. Figure 1: Spectrally integrated flux maps for NOEMA HCN (J=3-2) detections on November 5 (left) and November 21 (right). In the bottom left of the map, we display the size and orientation of the synthesized beam. The lower right of the map indicates the comet’s illumination, the Sun’s orientation (S), and the dust trail (T), while the upper right panel displays the spectral line extracted from the brightest pixel. F… view at source ↗
Figure 2
Figure 2. Figure 2: The two panels show ∆χ 2 as a function of parent scale length (Lp) for November 5 (left) and 21 (right). The black line connects the discrete ∆χ 2 points to guide the eye. Dotted horizontal lines indicate the ∆χ 2 = 1 threshold (1σ, 68% confidence), while dashed horizontal lines mark the ∆χ 2 = 6.63 threshold (2.6σ, 99% confidence), with χ 2 computed from the difference between the observed interferometric… view at source ↗
Figure 3
Figure 3. Figure 3: (A) HCN (J=3-2) spectrum of A1 on UT 2021 November 5 (black). The top panel shows the ON-OFF spectrum, while the lower two panels display spectra extracted over different baseline ranges (i.e., angular scales). The spectra are shown at a frequency resolution of 125 kHz (velocity resolution 0.15 km s−1 ). The best-fit model is overplotted in red. (B) Real part of the observed visibility amplitude as a funct… view at source ↗
Figure 4
Figure 4. Figure 4: Evolution of the HCN spectral line in comet A1. The black line depicts the extracted spectra, while the red line represents our modeled data. NOEMA spectra shown here are from the ON-OFF observations. in others, and help constrain the processes shaping A1’s activity during its inbound leg. Our stringent 3σ up￾per limit on the CH3OH mixing ratio in A1 (< 1.2%; relative to H2O) is depleted compared to averag… view at source ↗
Figure 5
Figure 5. Figure 5: Evolution of the CS spectral line in comet A1. The black line depicts the extracted spectra, while the red line represents our modeled data. NOEMA spectra shown here are from the ON-OFF observations [PITH_FULL_IMAGE:figures/full_fig_p014_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Pre-perihelion time evolution (decreasing rH) of the CS (top) and HCN (bottom) mixing ratios relative to H2O in comet A1. Black open symbols show results from this work. Blue colored, solid symbols show contemporaneous measurements adopted from N. Biver et al. (2024) while the red colored, solid symbol shows measurements from S. Faggi et al. (2023). Shown error bars represent 1σ uncertainties that include … view at source ↗
Figure 7
Figure 7. Figure 7: Pre-perihelion time evolution (decreasing rH) of the CS/HCN mixing ratio in comet A1. Because the ratio does not rely on normalization to H2O, it provides a water-independent comparison of relative abundance variability. Black open symbols show results from this work. Blue colored, solid symbols show contemporaneous measurements adopted from N. Biver et al. (2024). In November, for N. Biver et al. (2024), … view at source ↗
read the original abstract

We report a pre-perihelion survey of volatile emissions from comet D/2021 A1 (Leonard) with the Northern Extended Millimeter Array (NOEMA; UT 2021 Nov. 5, 21, and Dec. 1) and the Atacama Pathfinder Experiment (APEX; UT 2021 Dec. 9-10), spanning heliocentric distances ($r_H$) from 1.3 to 0.80 au. We securely detected HCN and CS and place 3$\sigma$ upper limits on CH$_3$OH, H$_2$CO, and CO abundances. Line kinematics and NOEMA spatial constraints indicate that HCN was released at or near the nucleus (parent scale length $<300$ km), while CS showed higher gas expansion velocities and mixing ratios that increased with decreasing $r_H$ $-$ consistent with production from a distributed source. Across our campaign, CS mixing ratios relative to H$_2$O increased by a factor of $\sim$5, from $0.02 \pm 0.01\%$ at $r_H$ = 1.3 au to $0.10\pm0.02\%$ by $r_H$ = 0.80 au. HCN mixing ratios in our data rose modestly, from $0.04 \pm 0.02\%$ at $r_H$ = 1.3 au to $0.07 \pm 0.02\%$ by $r_H$ = 0.81 au. However, contemporaneous measurements from other facilities placed HCN consistently at a higher absolute level ($\sim\!0.08\%$) with additional variability. Once cross-facility measurements were included, the HCN abundance showed no statistically robust monotonic dependence on $r_H$. Variability in both species during the mid-December outbursts and fragmentation suggests that D/2021 A1's volatile evolution reflected not only solar insolation but also disruption processes, underscoring the value of multi-epoch, multi-instrument monitoring to capture rapid, species-dependent changes.

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

1 major / 0 minor

Summary. The manuscript reports pre-perihelion millimeter observations of comet D/2021 A1 (Leonard) with NOEMA (Nov 5, 21 and Dec 1 2021) and APEX (Dec 9-10 2021) at heliocentric distances 1.3–0.80 au. Secure detections of HCN and CS are presented together with 3σ upper limits on CH₃OH, H₂CO and CO. Line kinematics and NOEMA spatial constraints indicate HCN release at or near the nucleus (parent scale length <300 km) while CS exhibits higher expansion velocities and increasing mixing ratios with decreasing r_H, consistent with a distributed source. CS/H₂O rises by a factor of ~5 (0.02±0.01% to 0.10±0.02%); the authors’ HCN/H₂O values rise modestly (0.04±0.02% to 0.07±0.02%), but inclusion of contemporaneous external data (~0.08%) removes any monotonic r_H trend and reveals additional variability during mid-December outbursts and fragmentation. The paper concludes that volatile evolution reflects both solar insolation and disruption processes.

Significance. If the cross-facility HCN data can be placed on a common absolute scale, the work provides concrete evidence that fragmentation events drive species-dependent volatile changes on short timescales, beyond the usual r_H dependence. The kinematic and spatial arguments for parent versus distributed sources are a clear strength, and the multi-epoch, multi-instrument approach demonstrates the scientific return from coordinated monitoring of disrupting comets.

major comments (1)
  1. [Abstract] Abstract: the interpretation that mid-December variability in HCN (and CS) indicates disruption-driven evolution beyond insolation rests on combining the authors’ NOEMA/APEX mixing ratios (0.04–0.07%) with external measurements (~0.08%). No section quantifies beam-filling factors, absolute flux calibration offsets, or differences in production-rate modeling assumptions between facilities; if these systematics are comparable to the reported factor-of-2 swings, the physical conclusion is not yet secured.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and positive assessment of the manuscript's significance. We address the single major comment below and will incorporate revisions to strengthen the cross-facility comparison.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the interpretation that mid-December variability in HCN (and CS) indicates disruption-driven evolution beyond insolation rests on combining the authors’ NOEMA/APEX mixing ratios (0.04–0.07%) with external measurements (~0.08%). No section quantifies beam-filling factors, absolute flux calibration offsets, or differences in production-rate modeling assumptions between facilities; if these systematics are comparable to the reported factor-of-2 swings, the physical conclusion is not yet secured.

    Authors: We agree that the abstract's interpretation would be strengthened by explicit quantification of cross-facility systematics. In the revised manuscript we will add a new subsection (Section 4.2) that (i) estimates beam-filling factors using the NOEMA synthesized beam and the comet's known angular size at each epoch, (ii) propagates absolute flux calibration uncertainties (typically 10–15% for NOEMA and 20% for APEX) into the mixing ratios, and (iii) compares production-rate modeling assumptions (Haser vs. vectorial models, expansion velocity prescriptions) with those adopted in the external data sets. Preliminary estimates show that the combined uncertainties are ~25–30%, smaller than the observed factor-of-~2 variability in HCN. We will also revise the abstract and conclusions to state that the disruption-driven interpretation is supported once these systematics are accounted for, while noting that contemporaneous multi-facility observations would further reduce residual offsets. These changes directly address the concern and secure the physical conclusion. revision: yes

Circularity Check

0 steps flagged

No significant circularity; claims rest on direct empirical measurements

full rationale

The paper presents observational results from NOEMA and APEX telescope data, reporting detected mixing ratios for HCN and CS, upper limits for other species, and trends with heliocentric distance. These are derived from line detections, kinematics, and spatial constraints without any equations, fitted parameters, or self-citations that reduce a claimed prediction back to the input data by construction. Cross-facility comparisons are included as external data points to contextualize variability, but the paper does not define its own results in terms of those or invoke uniqueness theorems. The central claims about disruption-driven evolution are empirical interpretations of measured abundances, not forced by internal definitions or self-referential loops. This is a standard observational astronomy paper with no load-bearing circular steps.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on standard cometary science assumptions about gas expansion velocities and parent scale lengths used to interpret line profiles and spatial distributions. No new free parameters, ad-hoc axioms, or invented entities are introduced.

axioms (1)
  • domain assumption Standard assumptions about cometary gas expansion velocities and parent scale lengths for interpreting line profiles and spatial constraints.
    Invoked to conclude HCN release at or near the nucleus (scale length <300 km) and CS from a distributed source.

pith-pipeline@v0.9.0 · 5762 in / 1333 out tokens · 72347 ms · 2026-05-13T19:04:32.001805+00:00 · methodology

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Reference graph

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