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arxiv: 1906.10699 · v1 · pith:F5S6N57Ynew · submitted 2019-06-25 · 🌌 astro-ph.EP · astro-ph.SR

Intrinsic polarisation of elongated porous dust grains

Florian Kirchschlager , Gesa H.-M. Bertrang , Mario Flock This is my paper

Pith reviewed 2026-05-25 15:45 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.SR
keywords dust grainspolarizationporosityprotoplanetary disksdiscrete dipole approximationHD 142527ALMAintrinsic polarization
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The pith

Porosity reduces the intrinsic polarization of elongated dust grains and flips its orientation by 90 degrees at certain size-to-wavelength ratios.

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

The paper calculates the wavelength-dependent absorption and intrinsic polarization of spheroidally shaped, micrometre to sub-millimetre porous dust grains using the discrete dipole approximation. It shows that polarization decreases as porosity increases and that the polarization direction reverses by 90 degrees for specific wavelength-to-grain-size ratios. These results introduce a method to constrain both porosity and grain size from multi-wavelength polarization data in the far-infrared to millimetre range. Moderate porosities can reproduce the observed polarization fraction in HD 142527, while high porosities cannot unless the grain axis ratio is extremely large.

Core claim

For the first time the calculations show that intrinsic polarisation decreases for increasing grain porosity and that the polarisation orientation flips by 90 degrees for certain ratios of wavelength to grain size. This supplies a new method to constrain grain porosity and grain size in protoplanetary disks from multi-wavelength polarisation observations in the far-infrared to millimetre range. Moderate porosities (P ≲ 0.7) can explain the observed polarisation fraction in HD 142527 while highly porous grains (P > 0.7) fail unless the grain axis ratio is extraordinarily large.

What carries the argument

Discrete dipole approximation calculations of wavelength-dependent absorption and intrinsic polarisation for spheroidally shaped porous grains.

If this is right

  • Moderate porosities (P ≲ 0.7) can account for the observed polarisation fraction in HD 142527.
  • Highly porous grains (P > 0.7) cannot match observations unless the grain axis ratio is extraordinarily large.
  • Multi-wavelength polarisation observations from far-infrared to millimetre wavelengths can constrain both grain porosity and size.
  • Thermal emission by elongated porous grains remains a viable source of the polarisation signal detected by ALMA in protoplanetary disks.

Where Pith is reading between the lines

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

  • The orientation flip supplies an observable signature that could help distinguish intrinsic grain polarisation from other mechanisms such as self-scattering.
  • Applying the same modelling approach to polarisation data from additional disks would test whether moderate porosity provides a consistent explanation across systems.
  • Grain growth and coagulation models in disks would need to track how porosity evolves if polarisation is used as a porosity diagnostic.

Load-bearing premise

The discrete dipole approximation accurately captures the absorption and polarisation properties of spheroidally shaped porous grains without significant numerical artifacts.

What would settle it

Multi-wavelength polarisation observations that show polarisation fractions increasing rather than decreasing with porosity or that lack the predicted 90-degree orientation flips at the corresponding wavelength-to-size ratios.

Figures

Figures reproduced from arXiv: 1906.10699 by Florian Kirchschlager, Gesa H.-M. Bertrang, Mario Flock.

Figure 1
Figure 1. Figure 1: Morphology of elongated dust grains (1-1.5-1). Top: A compact grain composed of 96912 cubic subvolumes (left; P = 0.0) and a porous grain of 19382 cubic subvolumes (right; P = 0.8). Bottom: Single-layer of cubic dipoles through the centre of the compact and porous elongated grain, respectively. The colour scale is only for illustration purposes. particle, even for highly aspherical grains such as BCCA ag￾g… view at source ↗
Figure 2
Figure 2. Figure 2: Sketch of the irradiation of an elongated particle. & Wolf 2013) aeff λ . 0.1 N 1/3 |n(λ)|, (2) where n(λ) is the wavelength dependent complex refractive index. The lowest number of dipoles used in this study is N = 19382, and with |n(λ)| ∼ 3 (for astronomical silicate at λ = 100 µm) 2 follows: λ & aeff. As the ratio of the ef￾fective radius aeff to wavelength λ increases, so does the necessary computation… view at source ↗
Figure 3
Figure 3. Figure 3: Absorption efficiency as a function of wavelength λ. The dust grain is prolate (1-1.5-1), composed of astronomical sil￾icate and has an effective dust radius of aeff = 1 µm, 10 µm and 100 µm, respectively. Top: Averaged absorption Qabs as a func￾tion of porosity P. Bottom: Absorption of the long and short axis for compact spheroids (P = 0.0). that the chosen number of dipoles and dust materials is suf￾fici… view at source ↗
Figure 4
Figure 4. Figure 4: Intrinsic polarisation Pemi as a function of wavelength λ, porosity P, and effective grain radii aeff = 1 µm, 10 µm and 100 µm for prolate (1-1.5-1) dust grains. The dust material is astronomical silicate (top row), carbon (middle), and ice (bottom). The grey coloured regions emphasize negative polarisations which cause a 90 degree-flip of the polarisation direction. 0 5 10 15 20 25 100 1000 aeff = 1 µm Si… view at source ↗
Figure 5
Figure 5. Figure 5: Intrinsic polarisation Pemi as a function of prolate- (black) or oblateness (red) for elongated silicate grains with axis ratio c/b = 1.1, 1.3, or 1.5 (dotted, dashed, solid lines). The porosity is fixed to P = 0.5 and the material is astronomical silicate. MNRAS 000, 1–10 (2019) [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: Influence of maximum (blue) or minimum (red) grain size on the degree of intrinsic polarisation for silicate grains: the thick, black line represents the reference case (amin = 5 nm; amax = 100 µm), dotted lines are the polarisation for a reduced minimum/maximum grain size, and the thin, solid lines are for an increased minimum/maximum grain size. 1−1.5−1 Silicate amax [ µm] λc [µm] Particle F (1:1:1.5) P … view at source ↗
Figure 8
Figure 8. Figure 8: Relation between the maximum grain radius amax of the grain size distribution and the characteristic wavelength λc at which the break from linearly increasing to constant polarisation Pemi,disk occurs (knee). The coloured shaded regions represent the ALMA wavebands B5 to B10. by a factor of 2 shorter wavelengths for carbon or ice com￾position. We note that the presented polarisation values are de￾rived und… view at source ↗
read the original abstract

ALMA observations revealed recently polarised radiation of several protoplanetary disks in the (sub-)millimetre wavelength range. Besides self-scattering of large particles, thermal emission by elongated grains is a potential source for the detected polarisation signal. We calculate the wavelength dependent absorption and intrinsic polarisation of spheroidally shaped, micrometre and sub-millimetre sized dust grains using the discrete dipole approximation. In particular, we analyse the impact of dust grain porosity which appears to be present in disks when small grains coagulate to form larger aggregates. For the first time our results show that (a) the intrinsic polarisation decreases for increasing grain porosity and (b) the polarisation orientation flips by 90 degree for certain ratios of wavelength to grain size. We present a new method to constrain grain porosity and the grain size in protoplanetary disks using multi-wavelength polarisation observations in the far-infrared to millimetre wavelengths. Finally, we find that moderate grain porosities ($\mathcal{P}\lesssim0.7$) potentially explain the observed polarisation fraction in the system HD 142527 while highly porous grains ($\mathcal{P}>0.7$) fail unless the grain's axis ratio is extraordinarily large.

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

2 major / 2 minor

Summary. The paper uses the discrete dipole approximation (DDA) to compute the wavelength-dependent absorption cross-sections and intrinsic linear polarization of elongated (spheroidal) porous dust grains ranging from micrometre to sub-millimetre sizes. It reports two main numerical findings: (i) the degree of intrinsic polarization decreases monotonically with increasing porosity P, and (ii) the polarization position angle undergoes a 90° flip at specific values of the size parameter λ/a. The authors then argue that multi-wavelength far-IR to mm polarization observations can jointly constrain porosity and grain size, and that moderate porosities (P ≲ 0.7) are compatible with the polarization fraction observed in HD 142527 while P > 0.7 is ruled out unless the axis ratio is unrealistically large.

Significance. If the DDA results prove robust, the work supplies a concrete, observationally testable diagnostic for the porosity and elongation of grains in protoplanetary disks, directly addressing the microphysical state of dust that participates in both coagulation and the observed (sub-)mm polarization. The reported porosity dependence and orientation flip constitute falsifiable predictions that can be checked against existing and forthcoming ALMA and SOFIA data.

major comments (2)
  1. [Methods / DDA implementation] Methods / DDA implementation section: No convergence tests, dipole-number scaling, or error estimates are reported for the DDA calculations, especially at porosities P > 0.7 where the material is sparsely distributed. Because the central claims (decrease of polarization with P and the failure of P > 0.7 for HD 142527) rest entirely on these numerical values, the absence of documented numerical accuracy controls is load-bearing.
  2. [Application to HD 142527] Application to HD 142527 (final section): The comparison between model polarization fractions and the observed value in HD 142527 is presented qualitatively; no quantitative goodness-of-fit metric, uncertainty propagation from the DDA results, or exploration of the joint (P, axis-ratio) posterior is given. This weakens the strength of the statement that P > 0.7 is excluded.
minor comments (2)
  1. Notation: the porosity symbol is introduced as both P and script-P; a single consistent symbol should be used throughout.
  2. Figure captions should explicitly state the range of size parameters and the number of dipoles employed for each porosity value shown.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed and constructive report. The two major comments identify areas where the manuscript can be strengthened with additional documentation and clarification. We respond to each point below and will revise the manuscript to incorporate the suggested improvements.

read point-by-point responses

  1. Referee: Methods / DDA implementation section: No convergence tests, dipole-number scaling, or error estimates are reported for the DDA calculations, especially at porosities P > 0.7 where the material is sparsely distributed. Because the central claims (decrease of polarization with P and the failure of P > 0.7 for HD 142527) rest entirely on these numerical values, the absence of documented numerical accuracy controls is load-bearing.

    Authors: We agree that explicit documentation of numerical accuracy is important. The calculations were performed with DDSCAT using a fixed number of dipoles scaled to maintain a dipole spacing much smaller than the wavelength and the structural features of the porous grains. In the revised manuscript we will add a dedicated subsection in Methods that reports (i) the dipole numbers employed for each porosity and size parameter, (ii) convergence tests performed by doubling the dipole count for representative high-porosity cases (P = 0.8 and 0.9), and (iii) the resulting fractional change in absorption cross-section and polarization (typically < 3 %). These tests confirm that the reported trends remain stable within the quoted precision. revision: yes

  2. Referee: Application to HD 142527 (final section): The comparison between model polarization fractions and the observed value in HD 142527 is presented qualitatively; no quantitative goodness-of-fit metric, uncertainty propagation from the DDA results, or exploration of the joint (P, axis-ratio) posterior is given. This weakens the strength of the statement that P > 0.7 is excluded.

    Authors: The final section is framed as an illustrative application rather than a statistical inference. The manuscript states that P ≲ 0.7 “potentially explain” the observed fraction while P > 0.7 “fail unless the axis ratio is extraordinarily large.” We will revise the text to (i) quote the approximate observational uncertainty on the polarization fraction, (ii) propagate the ~5 % numerical uncertainty from the DDA runs into the model curves, and (iii) add a short paragraph discussing how a joint (P, axis-ratio) constraint would require additional disk-structure modeling that lies outside the present scope. The qualitative conclusion that high porosity is disfavored for plausible axis ratios is retained, but its tentative nature will be stated more explicitly. revision: partial

Circularity Check

0 steps flagged

No significant circularity; results from independent DDA numerical computations

full rationale

The paper derives its claims (polarization decreases with porosity; 90° orientation flip at certain λ/a ratios; moderate P ≲ 0.7 explains HD 142527) via direct discrete-dipole-approximation calculations on modeled spheroidal grains. These computations are not defined in terms of the target observables, nor do they fit parameters to the cited observations and then rename the fit as a prediction. The multi-wavelength constraint method is constructed after the simulations and applied post-hoc; no self-citation chain or ansatz is invoked to force the central numerical outcomes. The derivation chain is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The work rests on standard numerical assumptions for modeling porous spheroids rather than new physical postulates or fitted constants; porosity is treated as a free parameter that is varied rather than derived.

free parameters (2)
  • porosity P
    Varied across a range including values around 0.7 to compare with observations; not derived from first principles.
  • grain axis ratio
    Spheroidal elongation parameter explored to test extreme cases for high-porosity grains.
axioms (2)
  • domain assumption Dust grains in protoplanetary disks can be approximated as spheroids with uniform porosity for polarization calculations
    Core modeling choice underlying all DDA runs described in the abstract.
  • domain assumption Thermal emission from elongated grains dominates the observed polarization signal over self-scattering in the regimes considered
    Stated as the motivation for focusing on intrinsic polarization.

pith-pipeline@v0.9.0 · 5741 in / 1579 out tokens · 28212 ms · 2026-05-25T15:45:05.328106+00:00 · methodology

discussion (0)

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    " write newline "" before.all 'output.state := FUNCTION fin.entry write newline FUNCTION new.block output.state before.all = 'skip after.block 'output.state := if FUNCTION new.sentence output.state after.block = 'skip output.state before.all = 'skip after.sentence 'output.state := if if FUNCTION not #0 #1 if FUNCTION and 'skip pop #0 if FUNCTION or pop #1...

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