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arxiv: 2604.27956 · v1 · submitted 2026-04-30 · 🌀 gr-qc · astro-ph.SR

Rotation-Induced Pressure Anisotropy in Newtonian White Dwarfs: Sequences and Applicability Criteria

Pith reviewed 2026-05-07 05:56 UTC · model grok-4.3

classification 🌀 gr-qc astro-ph.SR
keywords white dwarfsrotationpressure anisotropyChandrasekhar EOSNewtonian stellar structurelimiting masshydrostatic balancereduced models
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The pith

A one-dimensional Newtonian model encodes centrifugal support in rotating white dwarfs as pressure anisotropy, showing percent-level mass increases.

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

The authors present a reduced one-dimensional model for uniformly rotating Newtonian white dwarfs that incorporates centrifugal effects through an effective pressure anisotropy rather than solving the full two-dimensional structure equations. By averaging the centrifugal term from the Euler equation over angles with a fixed factor of 2/3, they derive a radial correction that enters the hydrostatic balance like an anisotropic pressure component. Applying this to sequences of white dwarfs using the Chandrasekhar degenerate electron equation of state across central densities from 10^6 to 10^11 g/cm³ and rotation parameters up to 0.35, they find that both maximum mass and radius grow steadily with increasing rotation. At the upper end of the rotation range the mass increase reaches a few percent. They also define diagnostics to check that the configurations remain sub-Keplerian and the anisotropy approximation stays small in the bulk interior, confirming validity within the explored domain and framing the model as a useful benchmark tool.

Core claim

We introduce a fast, one-dimensional Newtonian reduced model to capture uniform rotation in cold white dwarfs, encoding centrifugal support as an effective pressure anisotropy. Using Δ_rot(r)=(1/3)ρ(r)Ω² r² derived from the stationary Euler equation with <sin²θ>=2/3, the model incorporates rotation into hydrostatic balance without a two-dimensional solver. Applying the Chandrasekhar degenerate-electron equation of state, we compute interior structures and global sequences for ρ_c ∈ [10^6, 10^{11}] g cm^{-3} with rotation proxies f ≤ 0.35, finding monotonic increases in limiting mass and radius, with a percent-level mass gain at f = 0.35. We quantify applicability using sub-Keplerian diagonst

What carries the argument

The effective pressure anisotropy Δ_rot(r) = (1/3) ρ(r) Ω² r² with angular average <sin²θ> = 2/3 that reduces the centrifugal term to a one-dimensional correction in the hydrostatic equilibrium equation.

If this is right

  • Limiting mass and radius increase monotonically with rotation parameter f.
  • Mass gain reaches the percent level at f = 0.35.
  • Sub-Keplerian diagnostics max(Ω/Ω_K), max(ε) and A_{10^{-2}} stay below unity in the domain.
  • The model acts as a computationally efficient benchmark for slow-to-moderate rotation.

Where Pith is reading between the lines

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

  • This approach could support quick explorations of how rotation affects white dwarf cooling or merger rates in population models.
  • The anisotropy reduction might extend to models of other rotating compact objects like neutron stars in the Newtonian limit.
  • Validation against full multidimensional simulations at higher rotation rates would define the model's breakdown point more precisely.

Load-bearing premise

The centrifugal force can be reduced to an effective one-dimensional pressure anisotropy using a fixed angular average of two-thirds for sin squared theta, with the resulting models satisfying sub-Keplerian conditions.

What would settle it

A full two-dimensional axisymmetric computation of a white dwarf at central density 10^{11} g cm^{-3} and rotation proxy f = 0.35 yielding a mass or radius differing by more than a few percent from the one-dimensional result would show the approximation fails to capture the effects accurately.

Figures

Figures reproduced from arXiv: 2604.27956 by Aray Muratkhan, Hernando Quevedo, Saken Toktarbay.

Figure 1
Figure 1. Figure 1: FIG. 1: Normalized density profile view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: Normalized radial pressure profile view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Cumulative enclosed mass fraction view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: Rotation-induced effective anisotropy ∆ view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: Total mass view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Radius view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: Mass–radius relation view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8: Relative mass discrepancy between the one-dimensio view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: Relative radius discrepancy between the one-dimens view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10: Normalized applicability criteria for the rotatio view at source ↗
read the original abstract

We introduce a fast, one-dimensional Newtonian {reduced model} to capture uniform rotation in cold white dwarfs, encoding centrifugal support as an effective pressure anisotropy. Using $\Delta_{\rm rot}(r)=\frac{1}{3}\rho(r)\Omega^2 r^2$ derived from the stationary Euler equation with $\langle\sin^2\theta\rangle=2/3$, the model incorporates rotation into hydrostatic balance without a two-dimensional solver. Applying the Chandrasekhar degenerate-electron equation of state, we compute interior structures and global sequences for $ \rho_c \in [10^6, 10^{11}]~{\rm g\,cm^{-3}} $ with rotation proxies $f \le 0.35$, finding monotonic increases in limiting mass and radius, with a percent-level mass gain at $f = 0.35$. We quantify applicability using sub-Keplerian diagnostics evaluated on the rotating configurations, $\max(\Omega/\Omega_K)$ and $\max(\epsilon)$, together with a bulk-interior smallness measure $A_{10^{-2}}\equiv \max_{p_r/p_c\ge 10^{-2}}(\Delta_{\rm rot}/p_r)$. Within the scanned domain these diagnostics remain below unity. The model is therefore best viewed as a reduced Newtonian benchmark for slow-to-moderate rotation, not as a replacement for fully axisymmetric calculations of rotating stars.

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 / 2 minor

Summary. The manuscript introduces a one-dimensional Newtonian reduced model for uniformly rotating white dwarfs by representing centrifugal support through an effective pressure anisotropy Δ_rot(r) = (1/3) ρ(r) Ω² r², obtained by angular averaging the centrifugal acceleration in the stationary Euler equation assuming ⟨sin²θ⟩ = 2/3. Using the Chandrasekhar degenerate electron equation of state, the authors compute interior structures and global mass-radius sequences for central densities ρ_c from 10^6 to 10^11 g cm^{-3} and rotation parameters f ≤ 0.35. They report monotonic increases in limiting mass and radius, with a percent-level mass gain at the highest f. Applicability is quantified using sub-Keplerian diagnostics max(Ω/Ω_K), max(ε), and A_{10^{-2}} = max(Δ_rot / p_r for p_r/p_c ≥ 10^{-2}), all of which remain below unity in the explored domain. The model is positioned as a fast benchmark for slow-to-moderate rotation rather than a substitute for full axisymmetric calculations.

Significance. If the central approximation holds, this provides a computationally efficient 1D framework for incorporating uniform rotation into white dwarf models, enabling rapid parameter-space exploration without 2D solvers. The reported sequences quantify that rotation at f=0.35 induces only percent-level corrections to the Chandrasekhar mass and radius, serving as a useful Newtonian benchmark. Strengths include the direct derivation of Δ_rot from the Euler equation (no data fitting), consistent numerical sequences across the ρ_c range, and the inclusion of explicit applicability diagnostics that bound the anisotropy magnitude.

major comments (1)
  1. The fixed angular average ⟨sin²θ⟩ = 2/3 used to define Δ_rot(r) = (1/3) ρ Ω² r² (as described in the abstract and the derivation of the hydrostatic balance) assumes spherical isodensity surfaces and a purely monopolar gravitational potential. Insertion of the anisotropy term produces oblate deformations, which shift the density-weighted angular average and introduce quadrupole corrections to the potential absent from the 1D Poisson solver. The diagnostics max(Ω/Ω_K), max(ε), and A_{10^{-2}} only verify that |Δ_rot/p_r| and Ω/Ω_K remain O(1) or smaller; they do not recompute the average on the deformed geometry or bound the resulting error. Given that the reported mass increase is only a few percent at f = 0.35, an O(10 %) shift in the effective coefficient of ρ Ω² r² would alter the headline sequences at the same order. A quantitative error estimate (e.g., via comparison to a 2D axisym-m
minor comments (2)
  1. The rotation proxy f is used throughout but its precise definition (e.g., f = Ω/Ω_K at the equator or a normalized central value) is not stated in the abstract and should be given explicitly with an equation number in §2 or §3.
  2. The manuscript would benefit from additional references to prior 2D Newtonian and relativistic calculations of rotating white dwarfs (e.g., Ostriker & Bodenheimer 1968 and subsequent numerical work) to better situate the reduced model as a benchmark.

Simulated Author's Rebuttal

1 responses · 1 unresolved

We thank the referee for the careful reading and constructive comments on our manuscript. We appreciate the recognition of the model's utility as a computationally efficient 1D benchmark for incorporating uniform rotation. We address the major comment below, clarifying the scope of the approximations and indicating the revisions made to strengthen the discussion of limitations.

read point-by-point responses
  1. Referee: The fixed angular average ⟨sin²θ⟩ = 2/3 used to define Δ_rot(r) = (1/3) ρ Ω² r² (as described in the abstract and the derivation of the hydrostatic balance) assumes spherical isodensity surfaces and a purely monopolar gravitational potential. Insertion of the anisotropy term produces oblate deformations, which shift the density-weighted angular average and introduce quadrupole corrections to the potential absent from the 1D Poisson solver. The diagnostics max(Ω/Ω_K), max(ε), and A_{10^{-2}} only verify that |Δ_rot/p_r| and Ω/Ω_K remain O(1) or smaller; they do not recompute the average on the deformed geometry or bound the resulting error. Given that the reported mass increase is only a few percent at f = 0.35, an O(10 %) shift in the effective coefficient of ρ Ω² r² would alter the headline sequences at the same order. A quantitative error estimate (e.g., via comparison to a 2D axisym-m

    Authors: We agree that the fixed ⟨sin²θ⟩ = 2/3 average is derived under the assumption of spherical isodensity surfaces and a monopolar potential, and that the induced oblate deformation introduces higher-order corrections not captured by updating the average or including quadrupole terms in the 1D Poisson solver. However, the model is explicitly constructed as a reduced approximation whose validity is restricted to the regime where these corrections remain small. The diagnostics max(ε) and A_{10^{-2}} are designed precisely to bound the size of the anisotropy and the resulting deformation; within the scanned domain max(ε) ≪ 1, so the shift in the angular average is O(ε) while the leading centrifugal support enters at O(f). This ordering justifies retaining the spherical average to the accuracy needed for the reported percent-level mass and radius changes. We have added a dedicated paragraph in Section 4 of the revised manuscript that estimates the magnitude of the neglected quadrupole correction (scaling as ε²) and reiterates that the model is positioned only as a fast Newtonian benchmark, not a high-precision substitute for axisymmetric calculations. A direct numerical comparison against 2D solvers would furnish a sharper error bar but lies outside the intended scope of this work. revision: partial

standing simulated objections not resolved
  • A full quantitative error bound obtained by direct comparison of the 1D sequences against independent 2D axisymmetric integrations, as this would require developing or interfacing with a separate 2D solver and would undermine the purpose of providing a rapid 1D alternative.

Circularity Check

0 steps flagged

No significant circularity; derivation is self-contained

full rationale

The central reduction sets Δ_rot(r) = (1/3) ρ(r) Ω² r² by angular averaging of the centrifugal term in the stationary Euler equation under the explicit assumption of spherical isodensity surfaces (<sin²θ> = 2/3). This fixed functional form is inserted once into the 1D hydrostatic balance and Poisson equations; the resulting ODE system is then integrated numerically with the independent Chandrasekhar EOS for chosen ρ_c and f. The reported mass/radius sequences and the three applicability diagnostics (max(Ω/Ω_K), max(ε), A_{10^{-2}}) are direct numerical outputs of that integration and are not algebraically or statistically forced to equal the input assumptions. No parameters are fitted to the output data, no self-citation chain is invoked to justify the averaging step, and the paper explicitly frames the construction as an approximate benchmark rather than an exact or self-consistent solution. The spherical-averaging assumption is therefore an external modeling choice whose accuracy can be tested externally, not a tautology that renders the predictions equivalent to the inputs by construction.

Axiom & Free-Parameter Ledger

2 free parameters · 3 axioms · 1 invented entities

The central claim rests on the Newtonian hydrostatic balance, the fixed angular average <sin²θ> = 2/3, and the Chandrasekhar EOS; the only added construct is the effective anisotropy, which is derived rather than postulated independently.

free parameters (2)
  • f
    Dimensionless rotation proxy that scales Ω and is scanned up to 0.35; its value is chosen by the user rather than fitted to observations.
  • ρ_c
    Central density scanned over [10^6, 10^{11}] g cm^{-3} to generate sequences.
axioms (3)
  • domain assumption Newtonian hydrostatic equilibrium holds with the added anisotropic pressure term Δ_rot(r).
    Invoked when the centrifugal contribution is folded into the radial force balance.
  • domain assumption The angular average <sin²θ> = 2/3 is an adequate representation of uniform rotation for the purpose of obtaining a one-dimensional equation.
    Stated explicitly in the derivation of Δ_rot from the stationary Euler equation.
  • standard math The Chandrasekhar equation of state for completely degenerate, non-relativistic electrons is appropriate throughout the density range considered.
    Used without modification to close the hydrostatic equation.
invented entities (1)
  • effective pressure anisotropy Δ_rot(r) no independent evidence
    purpose: To encode the centrifugal support of uniform rotation inside a one-dimensional radial structure equation.
    Derived directly from the Euler equation via angular averaging; no independent observational or numerical evidence outside the model is supplied.

pith-pipeline@v0.9.0 · 5561 in / 2042 out tokens · 66749 ms · 2026-05-07T05:56:27.958269+00:00 · methodology

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Reference graph

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