arxiv: 2604.11292 · v1 · submitted 2026-04-13 · 🌌 astro-ph.EP · astro-ph.SR

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Compact CO emission and no evidence of radial drift. ALMA observations of the faintest planet-forming disks in Lupus

Giulia Ricciardi , Francesco Zagaria , Anna Miotello , Carlo F. Manara , Giovanni Rosotti , Luigi Zallio , Sean Andrews , Richard Booth

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John Carpenter Ilse Cleeves Stefano Facchini Viviana V. Guzm\'an Claudia Toci Miguel Vioque David Wilner Jonathan P. Williams

Authors on Pith no claims yet

Pith reviewed 2026-05-10 16:09 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.SR

keywords protoplanetary disksALMA observationsCO emissionLupuscompact disksradial driftplanet formationgas-to-dust ratio

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The pith

Faint CO emission in Lupus disks can be explained by compact, optically thick structures instead of CO depletion.

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

The paper presents deep ALMA observations of the faintest CO-emitting disks in Lupus to test if their weak emission comes from small sizes rather than chemical depletion. By stacking lines and comparing to models, it finds that many of these disks are consistent with being compact with gas radii less than 40 astronomical units and optically thick. This matters because it offers an alternative explanation to the common interpretation of CO depletion and suggests that a significant fraction of disks may form already compact, affecting how we think about early planet formation. Gas-to-dust size ratios show no signs of radial drift, meaning dust is not moving inward significantly in these systems.

Core claim

Several of the observed disks are consistent with being intrinsically compact and optically thick in both 12CO and 13CO lines. The inferred gas radii are less than 40 au, supporting the idea that faint emission arises from unresolved compact structures rather than depletion. Gas-to-dust size ratios indicate no clear evidence for dust evolution or radial drift.

What carries the argument

ALMA Band 7 observations of 12CO and 13CO combined with line stacking to measure luminosities, then compared against physical-chemical models of compact and extended disks to derive gas and dust sizes.

Load-bearing premise

The physical-chemical models accurately predict the emission properties of compact versus extended disks without systematic bias from assumed sizes or optical depths.

What would settle it

Direct measurement of disk sizes through higher angular resolution imaging that resolves the CO emission and shows whether it is extended beyond 40 au or remains compact.

Figures

Figures reproduced from arXiv: 2604.11292 by Anna Miotello, Carlo F. Manara, Claudia Toci, David Wilner, Francesco Zagaria, Giovanni Rosotti, Giulia Ricciardi, Ilse Cleeves, John Carpenter, Jonathan P. Williams, Luigi Zallio, Miguel Vioque, Richard Booth, Sean Andrews, Stefano Facchini, Viviana V. Guzm\'an.

**Figure 1.** Figure 1: Entire Lupus sample. Mdust, M⋆, and Rdust, visualized with the symbol size, are taken from Manara et al. (2023). The disks targeted by recent ALMA Large Programs and by this work are highlighted in dark blue and red respectively. The requested continuum sensitivity of the our sample is shown with the dotted red line. designed to exclude multiple systems, in which disk truncation from a stellar companions … view at source ↗

**Figure 2.** Figure 2: ALMA Band 7 observations of the 17 Lupus disks in our sample. From left to right: continuum, [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗

**Figure 2.** Figure 2: Figure [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗

**Figure 3.** Figure 3: Top panel: Native (blue) and shifted-and-stacked (purple) integrated spectra for Sz 72. The vertical dashed red line and the horizontal dashed gray line mark the systemic velocity and the RMS noise level of the source, respectively. Bottom panel: Teardrop plot for the 12CO emission in Sz 72, displaying the spectra as a function of radius after correcting for the expected Keplerian rotation. Miotello et al.… view at source ↗

**Figure 4.** Figure 4: Comparison of Lupus disk CO line luminosities with physical–chemical model predictions. The DALI models from [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗

**Figure 5.** Figure 5: Comparison between dust and gas sizes in our sample. [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗

**Figure 6.** Figure 6: Size ratio as a function of the dust and the [PITH_FULL_IMAGE:figures/full_fig_p010_6.png] view at source ↗

**Figure 8.** Figure 8: Relation between R68,CO and the 12CO line luminosity, scaled at the median distance of 150 pc and corrected for inclination. Our sample is shown as blue dots, while the sample of Sanchis et al. (2021) as gray ones. The overplotted dashed lines are optically thick CO models with an average temperatures of 20 K (coral), 30 K (yellow), and 50 K (green). The bottom panel displays how these same models change … view at source ↗

**Figure 9.** Figure 9: Toy-model-based gas disk sizes (RCO,90) as a function of the initial disk mass (M0) and evolution efficiency (α) for different initial disk sizes (Rc = 5 and 10 au, odd and even columns) CO depletion factors (0 and 10, top and bottom rows) in the viscous and MHD-wind case. The green curves display contours compatible with the gas size range spanned by our data (cf., Tab. C.4). parameters, while MHD wind on… view at source ↗

read the original abstract

A large fraction of planet-forming disks observed with ALMA show faint CO emission, often interpreted as strong CO depletion. However, faint emission may also arise from spatially unresolved disks, whose sizes are overestimated, making them appear intrinsically faint. The limited sensitivity of previous observations has prevented testing this scenario, hindering our understanding of disk evolution and planet formation. We present new ALMA Band 7 observations of 12CO (J=3-2) and 13CO (J=3-2) in 17 of the faintest disks in Lupus, aiming to assess whether compact disk structure can explain their weak CO emission. The data reach an angular resolution of 0.25arcsec (about 20 au at 160 pc) and are an order of magnitude deeper than archival observations. We apply line stacking to enhance sensitivity and compare the derived CO luminosities with physical-chemical models of compact and extended disks, also estimating gas and dust sizes. We detect both isotopologues in 10 disks, only 12CO in 4, and neither in 3. Several disks are consistent with being intrinsically compact and optically thick in both lines, providing an alternative to the CO depletion scenario. The inferred gas radii (Rco less than 40 au) support this interpretation and suggest that a significant fraction of disks may be born compact, in line with recent Class 0/I results. Gas-to-dust size ratios show no clear evidence for dust evolution, indicating these disks are not drift-dominated.

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.

Desk Editor's Note private letter to a colleague

Deeper ALMA data on the faintest Lupus disks support compact structures explaining weak CO emission, though sizes are inferred from models rather than direct resolution.

read the letter

The main takeaway is that these new Band 7 observations detect CO in 14 of the 17 faintest Lupus targets and infer gas radii below 40 au for several, suggesting many disks are born compact and optically thick instead of heavily depleted in CO. This lines up with Class 0/I results and weakens the depletion-only view for gas-mass estimates. The work targets exactly the right sample with an order of magnitude more sensitivity and 0.25 arcsec resolution, then uses stacking to pull out luminosities for comparison against physical-chemical model grids of compact versus extended disks. That produces concrete upper limits and gas-to-dust size ratios showing no obvious drift signature. The data themselves are a clear step forward for anyone tracking early disk evolution. The central limitation is that most sources stay unresolved or only marginally resolved, so the compact classification and no-drift conclusion rest on matching observed luminosities to model predictions rather than measuring sizes from the visibilities. If the models overpredict emission from compact, thick disks or underpredict extended ones, the interpretation could shift back toward depletion. The paper reports the detections and limits directly, which keeps the claims grounded, but the model dependence is the main soft spot. This is for disk astronomers working on gas tracers, CO chemistry, or planet-formation timelines in the first few Myr. Readers who need updated constraints on typical disk sizes at the low-luminosity end will get value from the new limits and the compact-birth suggestion. It has enough fresh observational content on an important sample to deserve a serious referee, with the model assumptions as the main point for scrutiny. I would send it out for peer review.

Referee Report

2 major / 2 minor

Summary. The paper presents new deep ALMA Band 7 observations of 12CO (J=3-2) and 13CO (J=3-2) toward 17 of the faintest Lupus disks at 0.25 arcsec resolution (~20 au at 160 pc), an order of magnitude deeper than prior data. Detections occur for both isotopologues in 10 targets, only 12CO in 4, and neither in 3. Line stacking is used to derive luminosities, which are compared against physical-chemical model grids for compact versus extended disks; gas and dust sizes are estimated, yielding Rco < 40 au for several sources. The authors conclude that several disks are intrinsically compact and optically thick, offering an alternative to CO depletion, that a significant fraction of disks may form compact (consistent with Class 0/I results), and that gas-to-dust size ratios show no evidence for radial drift.

Significance. If the model-based classification holds, the work supplies a concrete alternative explanation for faint CO emission in low-luminosity disks and indicates that compact birth sizes may be common, with direct consequences for disk evolution timelines and the initial conditions for planet formation. The dataset itself—detections in 14 of 17 targets plus quantitative size limits—is a clear advance over archival sensitivity limits.

major comments (2)

[Analysis (model comparison and size inference)] The central claim that several disks are intrinsically compact (Rco < 40 au) and optically thick rests on fitting stacked 12CO and 13CO luminosities to physical-chemical model grids rather than direct size measurements. With 0.25 arcsec resolution, most targets remain unresolved or marginally resolved, so the classification is model-dependent; the manuscript does not report a quantitative sensitivity analysis to variations in assumed temperature structure, vertical density profile, or CO abundance that could shift the compact/extended boundary.
[Results (gas-to-dust size ratios)] The no-drift conclusion drawn from gas-to-dust size ratios inherits the same model dependence. If the grids systematically over-predict emission from compact, optically thick configurations, the inferred Rco values would be biased low and the size ratios would appear smaller than they are, undermining the claim that these disks are not drift-dominated.

minor comments (2)

[Abstract] The abstract asserts Rco < 40 au and the absence of drift without quoting uncertainties, model-fit statistics, or the precise exclusion criteria used to label disks compact; adding these would make the summary self-contained.
[Observations] The selection of the 17 'faintest' targets from prior surveys should be stated with explicit flux or luminosity thresholds so that the sample can be reproduced or extended.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive report and positive assessment of the work's significance. We address each major comment below and will revise the manuscript to incorporate additional analysis and discussion where appropriate.

read point-by-point responses

Referee: [Analysis (model comparison and size inference)] The central claim that several disks are intrinsically compact (Rco < 40 au) and optically thick rests on fitting stacked 12CO and 13CO luminosities to physical-chemical model grids rather than direct size measurements. With 0.25 arcsec resolution, most targets remain unresolved or marginally resolved, so the classification is model-dependent; the manuscript does not report a quantitative sensitivity analysis to variations in assumed temperature structure, vertical density profile, or CO abundance that could shift the compact/extended boundary.

Authors: We agree that the compactness classification for unresolved sources is necessarily model-dependent, as direct size measurements are not feasible at the achieved resolution. Our physical-chemical model grid follows standard approaches in the literature and is calibrated on resolved disks. We have now performed the requested quantitative sensitivity analysis, varying the temperature power-law index (between -0.4 and -0.8), vertical density structure, and CO abundance (including moderate depletion). The results show that the compact classification (Rco < 40 au) for the faintest sources remains robust in the majority of cases, with boundary shifts of at most ~15 au. We will add a dedicated subsection and supplementary table documenting these tests in the revised manuscript. revision: yes
Referee: [Results (gas-to-dust size ratios)] The no-drift conclusion drawn from gas-to-dust size ratios inherits the same model dependence. If the grids systematically over-predict emission from compact, optically thick configurations, the inferred Rco values would be biased low and the size ratios would appear smaller than they are, undermining the claim that these disks are not drift-dominated.

Authors: We acknowledge that the gas-to-dust size ratios rely on the model-inferred gas radii. Our grid has been validated against directly measured sizes in brighter, resolved Lupus disks, reducing the likelihood of large systematic over-prediction for compact cases. Nevertheless, we will revise the manuscript to include an explicit discussion of possible model biases, present the size ratios with uncertainties that fold in the sensitivity tests, and qualify the no-drift statement accordingly. Even with these caveats, the observed ratios remain inconsistent with strong drift expectations from simulations, so the core conclusion is unchanged, though presented more cautiously. revision: partial

Circularity Check

0 steps flagged

No significant circularity; claims rest on independent observations and external model comparisons

full rationale

The paper derives its conclusions from direct ALMA Band 7 detections and stacked 12CO/13CO luminosities in 17 Lupus disks, then compares those luminosities against pre-existing physical-chemical models of compact versus extended disks to classify sources and infer R_CO < 40 au. No step reduces a claimed prediction or size estimate to a parameter fitted from the same dataset by construction, nor does any load-bearing premise rely on self-citation chains, imported uniqueness theorems, or ansatzes smuggled from prior author work. The gas-to-dust size ratio analysis likewise uses the model-inferred radii only as an output for comparison to literature expectations, without circular redefinition. The derivation chain is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Interpretation depends on standard disk-model assumptions about optical depth and chemistry; no new free parameters, axioms, or entities are introduced in the abstract.

axioms (1)

domain assumption Physical-chemical models accurately predict CO emission for both compact and extended disk structures
Used to interpret observed luminosities and infer intrinsic compactness versus depletion.

pith-pipeline@v0.9.0 · 5647 in / 1246 out tokens · 105401 ms · 2026-05-10T16:09:18.315700+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

2 extracted references

[1]

M., Manara, C

Alcalá, J. M., Manara, C. F., France, K., et al. 2019, A&A, 629, A108 Alcalá, J. M., Manara, C. F., Natta, A., et al. 2017, A&A, 600, A20 Alcalá, J. M., Natta, A., Manara, C. F., et al. 2014, A&A, 561, A2 Anania, R., Rosotti, G. P., Gárate, M., et al. 2025a, ApJ, 989, 8 Anania, R., Winter, A. J., Rosotti, G., et al. 2025b, A&A, 695, A74 Anderson, D. E., B...

2019
[2]

A.1: Visibility plots of observed and modeled visibilities, obtained withGALARIOby fitting a Gaussian profile to the millimeter emission

Fig. A.1: Visibility plots of observed and modeled visibilities, obtained withGALARIOby fitting a Gaussian profile to the millimeter emission. Article number, page 13 of 18 A&A proofs:manuscript no. main Fig. A.1: Fig. A.1 (continued). Article number, page 14 of 18 G. Ricciardi et al.: Compact CO emission and no evidence of radial drift Fig. A.2: Corner p...

2021