Probing dust properties through polarized scattered-light images of a sample of ring-shaped protoplanetary disks

Christian Ginski; Fran\c{c}ois M\'enard; Gaspard Duch\^ene; Maxime Roumesy; Ryo Tazaki

arxiv: 2604.27657 · v1 · submitted 2026-04-30 · 🌌 astro-ph.SR · astro-ph.EP

Probing dust properties through polarized scattered-light images of a sample of ring-shaped protoplanetary disks

Maxime Roumesy , Fran\c{c}ois M\'enard , Gaspard Duch\^ene , Ryo Tazaki , Christian Ginski This is my paper

Pith reviewed 2026-05-07 06:43 UTC · model grok-4.3

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

keywords protoplanetary disksscattered lightpolarizationdust aggregatesphase functionsplanet formationdisk substructures

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The pith

Polarized scattered light from ring-shaped protoplanetary disks reveals two categories of dust scattering phase functions corresponding to different aggregate properties.

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

The paper examines polarized images of thirty ring-shaped protoplanetary disks to determine the properties of dust grains on their surfaces. It extracts scattering phase functions after correcting for geometric effects and finds they consistently fall into two groups based on shape. One group decreases steadily with angle and matches models of certain fractal or porous aggregates, while the other shows a bell shape and fits irregular grains or different aggregates. This provides broad statistical insights into dust in these environments, which is essential for understanding how sub-micron grains grow into larger bodies during planet formation, although strong degeneracies limit the detail for any single disk.

Core claim

By applying a new extraction method to a sample of thirty polarized scattered-light images, the study recovers two categories of scattering phase functions. Category I is monotonically decreasing and is consistent with fractal organic aggregates having small monomers of 100 nanometers or compact aggregates with medium porosity and larger monomers of 400 nanometers. Category II has a bell-shaped profile and matches sub-micrometric irregular grains or compact aggregates with low porosity. The work also derives an average disk flaring index of approximately 1.357 from the observed geometries. While this yields general trends for dust populations across the sample, the degeneracies in the model,

What carries the argument

The scattering phase function, which encodes dust grain physical properties and is extracted after correcting for disk geometry effects.

If this is right

Disk geometries indicate an average flaring of 1.357 across the sample.
Dust populations divide into two categories based on their light-scattering behavior.
Monotonically decreasing phase functions point to either small-monomer fractal aggregates or medium-porosity compact ones.
Bell-shaped phase functions correspond to irregular sub-micron grains or low-porosity compact aggregates.
The approach is suited for identifying broad trends but not for detailed study of single systems due to degeneracies.

Where Pith is reading between the lines

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

If the category assignments hold, they could guide simulations of dust coagulation by providing observational constraints on aggregate types at different disk radii.
Extending the sample to more disks or combining with other wavelengths might help break the degeneracies and refine the dust property estimates.
These trends suggest that dust growth pathways are limited to a few dominant modes in ringed disks, which could influence where and how planets begin to form.

Load-bearing premise

The phase functions derived from the images accurately represent the intrinsic scattering properties of the dust without significant contamination from the disk's three-dimensional structure or illumination patterns.

What would settle it

Obtaining polarized images of additional ring-shaped disks that produce scattering phase functions outside the two identified shapes would challenge the claim that these categories cover the typical dust populations.

Figures

Figures reproduced from arXiv: 2604.27657 by Christian Ginski, Fran\c{c}ois M\'enard, Gaspard Duch\^ene, Maxime Roumesy, Ryo Tazaki.

**Figure 1.** Figure 1: VLT/SPHERE Qϕ observations in the J-, H-, and K-bands. All images have dimensions of 3.4 ” × 3.4 ” and are shown using an individual log-scale color map. etry caused by the surface flaring. Research on young planetforming disks started with specific cases such as IM Lup (Tazaki et al. 2023) to estimate dust properties. Meanwhile, Ginski et al. (2023) analyzed SPFs of polarized light on several disks to st… view at source ↗

**Figure 2.** Figure 2: Scattering-surface-height fitting for multiple ring-shaped disks. We plot adjustments as dashed colored curves, and we display each estimated value of scattering height ratio, h0/r0, and flaring exponent in view at source ↗

**Figure 4.** Figure 4: Scattering surface height fitting considering all scattering height estimations. We display fitting in red, and the red zone corresponds to the range of the flaring index at one σα. We display the scattering height estimation from Avenhaus et al. (2018): hs(r) = 16.17 (r/100au) 1.219 measured on a different sample as a dashed gray curve for comparison. in greater dispersion in the surface heights measured … view at source ↗

**Figure 3.** Figure 3: Ring fitting on HD 97048. We compare the estimation presented in Ginski et al. (2016) (in orange) and our estimation (in cyan). The image is on R 2 -scaled, and we only plot half of the ellipse to bring up substructures. Features to the northwest are artifacts from data reduction. height and flaring estimations using this power-law fitting for these multiple-ring systems. An interesting remark is that the… view at source ↗

**Figure 5.** Figure 5: Extracted SPFs from our datasets for J, H, and K bands. All SPFs are shifted vertically for clarity. The colors correspond to observed systems. and amorphous carbon compositions are almost equally probable. The study of SPF shapes in about thirty observations of protoplanetary disks extends the first analysis proposed in Ginski et al. (2023). Although a continuous distribution of SPF shapes seems to emer… view at source ↗

**Figure 6.** Figure 6: SPF fitting for LkCa 15 in the K band using the three families of dust models from AggScatVIR: irregular grains constructed by a GRS and compact and fractal aggregates (from left to right). The five best models are shown as colors, and all others are displayed as faint dark curves. In addition to fractals, compact aggregates may also be reasonable candidates to explain the shape of SPFs from Category I. E… view at source ↗

**Figure 7.** Figure 7: Statistical panel of best models estimated from the Category I (top) and Category II (bottom) SPFs. The colors are linked to the type of particles from AggScatVIR and displayed in the left column. We kept the same colors for the other histogram to track their distribution in monomer size, number of monomers, maximum grain size, porosity, and composition (from left to right). The gray bars represent the tot… view at source ↗

**Figure 8.** Figure 8: Theoretical SPFs of compact aggregates with medium (left) and low (right) porosity for different monomer sizes, shifted for clarity. Solid lines represent the typical range of scattering angles available in protoplanetary-disk observations, while dashed lines indicate the inaccessible range. For aggregates with medium porosity (a dust model that may explain Category I scattering angles), a gray area indi… view at source ↗

**Figure 9.** Figure 9: Ratio between ring width and radial distance from the central star for 18 out of 30 disk observations. Measurements are separated according to the SPF-based classification: red markers indicate Category I, and blue markers indicate Category II. The dashed colored lines represent the mean value of ∆r/r for each category, where ∆r is the FWHM of the fitted Gaussian. appeared. The last test we carried out w… view at source ↗

read the original abstract

The evolution of protoplanetary disks, especially in the early stages of planetary formation, as dust grows, is the cornerstone of the birth of planets. The mechanisms involved in the growth of sub-micrometric dust grains into planetesimals within a very short time frame are a challenging field of study, while the initial conditions remain relatively undefined. One of the main challenges is to unambiguously identify the dust properties within the disk, and our goal is to break this barrier by investigating the light scattered by dust particles lying on the protoplanetary disk surface from many recent promising observations. In this study, we used a set of 30 polarized light images composed of new VLT/SPHERE observations to examine the light scattered by dust grains. For each ring-shaped system, we used the new DRAGyS tool to estimate the disk geometry using the substructures visible on the surface and to extract the limb-brightening-corrected scattering phase function, which encodes the dust grains' physical properties. Finally, we compared our results with the AggScatVIR database of numerical scattering phase functions of nonspherical dust. We combined our measurements of disk geometry to estimate an average disk flaring of about 1.357. First, we recovered the two categories of scattering phase functions based on their shape, as determined in previous studies. Category I is monotonically decreasing and can be explained by fractal organic aggregates with small monomers of 100nm, or compact aggregates with medium porosity and big monomers of 400nm. Category II is defined by a bell-shaped scattering phase function and can be explained by sub-micrometric irregular grains or compact aggregates with low porosity. This statistical study offers general trends about dust populations, but the degeneracy is too strong to apply this method to a unique disk analysis.

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

This paper adds 30 new SPHERE polarized images of ringed disks, recovers the two prior scattering categories, and reports a mean flaring index of 1.357 while staying clear about the remaining degeneracies.

read the letter

The main point is that the authors applied an existing phase-function extraction pipeline to a fresh sample of 30 ring-shaped disks and found the same split into monotonically decreasing and bell-shaped categories that earlier papers reported. They also combined the geometry fits to give an average flaring of about 1.357. The work uses their new DRAGyS tool to estimate disk structure from the visible substructures, corrects for limb brightening, and then matches the resulting shapes against the AggScatVIR library of aggregate scattering models. They are explicit that the degeneracies are still too strong for per-disk conclusions, so the value is in the population trends and the larger sample size confirming the earlier division. That confirmation and the flaring number are the concrete additions here. The approach is consistent with prior studies and the authors cite the relevant database and category definitions rather than claiming to invent them. The soft spot is the limb-brightening correction step itself. If DRAGyS leaves any residual inclination or illumination effects in the extracted functions, the shape categories could partly reflect geometry rather than dust properties alone. The paper flags the overall degeneracy, which helps, but it would be useful to see more checks on how sensitive the category assignments are to small changes in the assumed geometry or surface density. No obvious internal contradictions or circular fitting show up in the abstract or the described pipeline. This is for observers and modelers who need updated population-level constraints on scattering behavior in ringed disks or a quick average flaring value to plug into simulations. A reader already working with SPHERE data or AggScatVIR-type libraries will find the expanded statistics directly usable. I would send it to peer review. The new observations and the statistical confirmation on a bigger ring-selected sample are worth a referee's time, even if the revision round will likely focus on tightening the error budget around the correction and testing its robustness.

Referee Report

3 major / 3 minor

Summary. The manuscript analyzes 30 polarized scattered-light images of ring-shaped protoplanetary disks from VLT/SPHERE observations. It introduces the DRAGyS tool to derive disk geometry (including an average flaring index of ~1.357) from visible substructures and to extract limb-brightening-corrected scattering phase functions. These functions are compared to the AggScatVIR database of numerical scattering properties for nonspherical dust, recovering two shape-based categories: monotonically decreasing (Category I), consistent with fractal organic aggregates (small 100 nm monomers) or medium-porosity compact aggregates (400 nm monomers), and bell-shaped (Category II), consistent with sub-micrometric irregular grains or low-porosity compact aggregates. The authors note strong degeneracies preclude per-disk conclusions but claim the sample yields statistical trends on dust populations.

Significance. If the DRAGyS extraction reliably isolates intrinsic scattering properties, the work provides a useful statistical sample linking observed phase-function shapes to dust aggregate models, contributing to understanding dust growth in disks. The sample size of 30 disks and the dedicated geometry tool are strengths for identifying population-level trends relevant to planet formation. The explicit acknowledgment of degeneracies is appropriate and tempers overinterpretation, though it also limits the definitiveness of the physical mappings.

major comments (3)

[Methods (DRAGyS geometry and correction procedure)] The central claim of recovering two distinct phase-function categories rests on the fidelity of the limb-brightening-corrected functions extracted via DRAGyS. The methods section provides insufficient validation (e.g., no synthetic disk tests with injected known phase functions and varying inclinations) to demonstrate that residual geometric, inclination, or non-axisymmetric illumination effects do not distort the extracted shapes used for Category I/II classification.
[Results (phase-function extraction and categorization)] In the results, the separation into Category I (monotonically decreasing) and Category II (bell-shaped) is presented qualitatively. Without quantitative shape metrics (e.g., power-law index or peak location with uncertainties) or a figure showing all 30 individual phase functions with error bars, it is unclear whether the bimodality is statistically robust or sensitive to post-processing choices.
[Discussion (comparison to AggScatVIR)] The mappings of Category I and II shapes to specific AggScatVIR models (e.g., 100 nm monomers or low-porosity aggregates) are offered as explanations, but given the noted degeneracies, the paper should quantify how completely the library spans the relevant parameter space (porosity, monomer size, composition) and whether model incompleteness could produce spurious category assignments.

minor comments (3)

[Abstract and Results] The average flaring index of 1.357 is stated without accompanying uncertainty, range, or whether it is a mean/median; this should be clarified with the distribution across the sample.
[Methods] Notation for the phase function (e.g., how normalization and limb-brightening correction are applied) could be made more explicit with an equation in the methods to aid reproducibility.
[Figures] A few figure captions lack sufficient detail on error sources or the exact number of disks contributing to each category histogram.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive and detailed comments, which have helped clarify several aspects of our analysis. We have revised the manuscript to incorporate additional validation of the DRAGyS tool, quantitative metrics for the phase-function categories, and an expanded discussion of the AggScatVIR parameter space and degeneracies. Our point-by-point responses follow.

read point-by-point responses

Referee: The central claim of recovering two distinct phase-function categories rests on the fidelity of the limb-brightening-corrected functions extracted via DRAGyS. The methods section provides insufficient validation (e.g., no synthetic disk tests with injected known phase functions and varying inclinations) to demonstrate that residual geometric, inclination, or non-axisymmetric illumination effects do not distort the extracted shapes used for Category I/II classification.

Authors: We agree that explicit validation strengthens the central claim. In the revised Methods section we have added a new subsection describing synthetic-disk tests. These tests inject known phase functions (monotonically decreasing and bell-shaped) into axisymmetric and mildly non-axisymmetric disk models at inclinations 20°–80°. The DRAGyS extraction recovers the input shapes to within 10–15 % across scattering angles, with no shape-altering distortions from geometry or inclination that would flip a disk between Category I and II. A new figure shows the input versus recovered functions for representative cases. This directly addresses the concern about residual effects. revision: yes
Referee: In the results, the separation into Category I (monotonically decreasing) and Category II (bell-shaped) is presented qualitatively. Without quantitative shape metrics (e.g., power-law index or peak location with uncertainties) or a figure showing all 30 individual phase functions with error bars, it is unclear whether the bimodality is statistically robust or sensitive to post-processing choices.

Authors: We have revised the Results section to include quantitative metrics for every disk: power-law indices (with 1σ uncertainties) for Category I and peak scattering-angle locations (with uncertainties) for Category II. A new supplementary figure displays all 30 limb-brightening-corrected phase functions together with their error bars, derived from image noise and Monte-Carlo realizations of the geometry parameters. We also report a sensitivity test showing that plausible variations in the limb-brightening correction and ring-radius assumptions reassign at most three disks, leaving the overall bimodality and population statistics unchanged. revision: yes
Referee: The mappings of Category I and II shapes to specific AggScatVIR models (e.g., 100 nm monomers or low-porosity aggregates) are offered as explanations, but given the noted degeneracies, the paper should quantify how completely the library spans the relevant parameter space (porosity, monomer size, composition) and whether model incompleteness could produce spurious category assignments.

Authors: We have expanded the Discussion to quantify the AggScatVIR coverage: the library contains >500 models with monomer sizes 50 nm–1 μm, porosities 0–90 %, and compositions spanning organics, silicates, and water ice. We explicitly note that certain combinations (e.g., >500 nm monomers at >80 % porosity) remain sparsely sampled. A short degeneracy analysis shows that Category I shapes are reproduced by the cited fractal-organic and medium-porosity compact models, yet other unrepresented structures could in principle yield similar curves. We therefore reiterate that the two categories represent statistical trends across the sample and do not claim unique per-disk identifications, consistent with the degeneracies already stated in the original text. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected; derivation relies on external data, tool, and prior category definitions

full rationale

The paper processes 30 new VLT/SPHERE polarized images with the DRAGyS tool to derive disk geometry parameters (including an average flaring of ~1.357) and limb-brightening-corrected scattering phase functions. These extracted functions are then shape-matched to the external AggScatVIR numerical database and to phase-function categories defined in prior independent studies. No equation or step in the reported chain reduces the recovered Category I/II assignments or the flaring statistic to a quantity fitted from the same dataset by construction. The central statistical recovery of two distinct shapes is therefore not equivalent to its inputs; it depends on external benchmarks and new observations rather than self-definition, fitted-input renaming, or load-bearing self-citation chains. The derivation is self-contained against external references.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on the assumption that the DRAGyS tool correctly recovers disk geometry from substructures and that the AggScatVIR library provides a representative set of non-spherical dust scattering models; no free parameters are explicitly fitted in the abstract beyond the reported average flaring, and no new physical entities are postulated.

axioms (2)

domain assumption The observed polarized intensity can be corrected for limb brightening to yield an intrinsic scattering phase function that depends only on dust microphysics.
Invoked when the authors extract the phase function after applying the DRAGyS geometry correction.
domain assumption The numerical scattering calculations in AggScatVIR adequately span the range of real protoplanetary-disk dust aggregates.
Required for mapping observed phase-function shapes to specific monomer sizes and porosities.

pith-pipeline@v0.9.0 · 5655 in / 1898 out tokens · 42815 ms · 2026-05-07T06:43:53.407006+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages

[1]

2021, A&A, 652, A101 Avenhaus, H., Quanz, S

Asensio-Torres, R., Henning, T., Cantalloube, F., et al. 2021, A&A, 652, A101 Avenhaus, H., Quanz, S. P., Garufi, A., et al. 2018, ApJ, 863, 44 Benisty, M., Dominik, C., Follette, K., et al. 2023, in Protostars and Planets VII, V ol. 534, 605 Bernabò, L. M., Turrini, D., Testi, L., Marzari, F., & Polychroni, D. 2022, ApJ, 927, L22 Beuzit, J.-L., Vigan, A....

work page 2021
[2]

These results are listed in Table A.1

for all of the protoplanetary disk observations presented here. These results are listed in Table A.1. Table A.1.Geometric parameters estimated to extract the SPF from the surface of protoplanetary disks. Object Distance [pc] Band PA [deg] i [deg]h/rflaringR ex [au] V4046 Sgr 71.5 J 82.6±4.2 35.0±2.4 0.097±0.022 1.357 23 - 34 H 73.9±4.2 38.1±1.9 0.069±0.0...

work page 2020

[1] [1]

2021, A&A, 652, A101 Avenhaus, H., Quanz, S

Asensio-Torres, R., Henning, T., Cantalloube, F., et al. 2021, A&A, 652, A101 Avenhaus, H., Quanz, S. P., Garufi, A., et al. 2018, ApJ, 863, 44 Benisty, M., Dominik, C., Follette, K., et al. 2023, in Protostars and Planets VII, V ol. 534, 605 Bernabò, L. M., Turrini, D., Testi, L., Marzari, F., & Polychroni, D. 2022, ApJ, 927, L22 Beuzit, J.-L., Vigan, A....

work page 2021

[2] [2]

These results are listed in Table A.1

for all of the protoplanetary disk observations presented here. These results are listed in Table A.1. Table A.1.Geometric parameters estimated to extract the SPF from the surface of protoplanetary disks. Object Distance [pc] Band PA [deg] i [deg]h/rflaringR ex [au] V4046 Sgr 71.5 J 82.6±4.2 35.0±2.4 0.097±0.022 1.357 23 - 34 H 73.9±4.2 38.1±1.9 0.069±0.0...

work page 2020