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arxiv: 2603.08470 · v1 · submitted 2026-03-09 · 🌌 astro-ph.SR

Extreme mass loss during common envelope evolution: the origin of the double low-mass white dwarf system J2102--4145

Leandro G. Althaus , Alejandro H. Corsico , Monica Zorotovic , Maja Vuckovic , Alberto Rebassa-Mansergas , Santiago Torres This is my paper

Pith reviewed 2026-05-15 13:46 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords white dwarfscommon envelope evolutionbinary starsstellar radiihydrogen envelopeeclipsing binarieslow-mass stars
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The pith

The secondary white dwarf in J2102-4145 lost nearly all its hydrogen envelope during common-envelope evolution.

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

This paper examines the eclipsing double white dwarf system J2102-4145 to probe how much hydrogen envelope remains after common-envelope phases in low-mass stars. By matching the stars' measured masses, radii, and temperatures to evolutionary tracks, it finds the 0.314 solar mass secondary must have an extremely thin envelope of less than 10 to the minus 7 solar masses, while the primary retains a thicker one consistent with stable mass transfer. The cooling ages suggest the primary formed first via Roche-lobe overflow, followed by the common-envelope phase that created the secondary. This system thus supplies a tight observational limit on envelope retention in common-envelope events.

Core claim

Both components are helium-core white dwarfs. The primary's properties match models with thick hydrogen envelopes from stable Roche-lobe overflow and ongoing nuclear burning, while the secondary's small radius requires an almost completely stripped envelope produced by common-envelope evolution. The inferred envelope mass for the secondary lies below 10^{-7} solar masses, far thinner than standard models predict.

What carries the argument

Matching observed radii and effective temperatures to updated evolutionary models for common-envelope evolution and stable Roche-lobe overflow, combined with cooling age estimates and energy budget reconstruction for the common-envelope phase.

If this is right

  • The primary formed first through stable Roche-lobe overflow.
  • The secondary formed later via a common-envelope phase.
  • The common-envelope energy budget reconstruction is consistent with the observed orbital parameters.
  • Current prescriptions for envelope ejection during common-envelope evolution must account for cases of extreme mass loss.
  • The system sets a benchmark for the minimum hydrogen-envelope mass on post-common-envelope low-mass white dwarfs.

Where Pith is reading between the lines

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

  • Similar eclipsing systems could be used to map the range of envelope retention across different progenitor masses.
  • Models of common-envelope ejection efficiency may need revision if they cannot produce envelopes this thin.
  • Future observations of the system's cooling rates could test the residual burning contributions.
  • Other double white dwarf binaries might show a similar formation sequence if their parameters are measured precisely.

Load-bearing premise

That the observed radii and temperatures map directly to hydrogen-envelope masses using the evolutionary models without significant influences from rotation, magnetic fields, or unmodeled composition variations.

What would settle it

Measuring a radius for the secondary that is larger than predicted for an envelope thinner than 10^{-7} solar masses, or finding that the cooling ages do not support the primary forming first.

Figures

Figures reproduced from arXiv: 2603.08470 by Alberto Rebassa-Mansergas, Alejandro H. Corsico, Leandro G. Althaus, Maja Vuckovic, Monica Zorotovic, Santiago Torres.

Figure 1
Figure 1. Figure 1: Internal structure of a 1.5 M⊙ pre-CE RGB star at the epoch when the H-free core is 0.3202 M⊙ (plotted versus the Lagrangian mass coordinate mr). Shown are the H abundance by mass, XH(mr), and the cumulative H mass interior to mr , MH(< mr), which increases outward. Vertical dashed lines mark the BP criteria (XH = 0.1, peak nuclear en￾ergy generation, and maximum compression). The dashed green line marks M… view at source ↗
Figure 2
Figure 2. Figure 2: Stellar radius (R⊙) versus Teff for He-core WD sequences with different MH. Shown are CE models at M = 0.3208 M⊙ (MH = 6.6 × 10−6 , 10−6 , and 10−7 M⊙) and at M = 0.363 M⊙ (MH = 5 × 10−6 M⊙) (Althaus et al. 2025), together with SRLOF tracks from Althaus et al. (2013) (blue dashed). The J2102–4145 components are shown with 1σ errors in R and Teff, including the observed mass ranges of the primary (M1) and s… view at source ↗
Figure 3
Figure 3. Figure 3: Evolution of Teff since the end of mass loss for selected He￾core WD models. The red curve shows the CE track (M = 0.3208 M⊙, MH = 10−7 M⊙) used for the secondary. Solid blue is the post-SRLOF model for the primary from Althaus et al. (2013) with residual burn￾ing Lnuc/L ≃ 0.25, and dashed blue is a variant computed here with increased residual burning (Lnuc/L ≃ 0.70) to illustrate its impact on the coolin… view at source ↗
Figure 4
Figure 4. Figure 4: Schematic diagram illustrating the proposed formation sequence of J2102–4145, including the two mass-transfer episodes. Stellar sepa￾rations are not to scale. tent with the twin peak in the mass-ratio distribution of main￾sequence binaries, particularly in short-period systems (e.g., Halbwachs et al. 2003). With nearly equal masses, the first episode of stable mass transfer (SRLOF) is expected to widen the… view at source ↗
Figure 5
Figure 5. Figure 5: illustrates the dependence of the CE efficiency on the residual H mass in the secondary WD. For all three pro￾genitor models, αCE flattens into a nearly constant plateau for log MH ≲ −6.5, corresponding to the regime constrained by the observed radius of the secondary (grey band). In this range, the values of αCE coincide with those listed in [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
read the original abstract

Eclipsing close double white dwarf (WD) systems provide a unique opportunity to directly constrain hydrogen-envelope retention and test common-envelope (CE) evolution in low-mass stars, since they allow precise determinations of stellar masses and radii. We analyze J2102-4145, an eclipsing binary composed of two low-mass helium-core white dwarfs in a 2.4-hour orbit. By comparing the observed radii and effective temperatures with updated evolutionary models for CE evolution and stable Roche-lobe overflow (SRLOF), we confirm that both stars are helium-core white dwarfs. The primary, with a mass of 0.375 solar masses, is consistent with SRLOF models that retain thick hydrogen envelopes and sustain residual nuclear burning, whereas the secondary, with a mass of 0.314 solar masses, can only be reproduced by CE models in which the hydrogen envelope is almost completely removed. The inferred cooling ages (approximately 220 Myr for the secondary and between about 260 and 510 Myr for the primary, depending on the contribution of residual nuclear burning) support a formation sequence in which the primary formed first through SRLOF, followed by a CE phase that produced the compact secondary. Reconstruction of the CE energy budget yields progenitor and orbital parameters consistent with this evolutionary picture. The unusually small radius of the secondary requires an extremely thin hydrogen envelope, with a mass below about 10e-7 solar masses, well below the values predicted by standard bifurcation criteria. J2102-4145 therefore provides one of the strongest observational constraints on the hydrogen-envelope mass of post-CE low-mass white dwarfs and represents a benchmark challenge for current prescriptions of envelope ejection.

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 analyzes the eclipsing double white dwarf binary J2102-4145 with a 2.4-hour orbit. Precise masses (0.375 and 0.314 solar masses) and radii are derived from eclipse photometry and spectroscopy. Comparison to updated CE and SRLOF evolutionary tracks shows the primary is consistent with SRLOF retaining a thick hydrogen envelope and residual burning, while the secondary requires CE models with an extremely thin envelope (<10^{-7} solar masses). Cooling ages support a formation sequence with the primary forming first, followed by CE for the secondary. The system is presented as providing one of the strongest observational constraints on post-CE hydrogen-envelope masses and a benchmark for envelope-ejection prescriptions.

Significance. If the model-to-observation mapping holds, the work supplies a rare, precisely measured post-CE white-dwarf system that directly challenges standard bifurcation criteria for envelope retention. The eclipsing geometry yields model-independent masses and radii, strengthening the constraint relative to single-star or non-eclipsing systems. The CE energy-budget reconstruction further ties the parameters to a coherent evolutionary channel, offering a falsifiable test case for CE prescriptions that is currently scarce in the literature.

major comments (2)
  1. [Model comparison section] Model comparison section (abstract and associated figures): The central inference that the secondary's radius and Teff require an envelope mass below ~10^{-7} Msun rests on the assumption that the cited CE/SRLOF tracks provide a unique, monotonic mapping from envelope thickness to radius at fixed mass. The manuscript does not present sensitivity tests to plausible variations in unmodeled physics (e.g., residual rotation, magnetic fields, or small metallicity offsets), which the skeptic note correctly flags as a potential degeneracy. Without such tests the lower bound on envelope mass is not yet demonstrated to be robust.
  2. [Cooling ages discussion] Abstract and § on cooling ages: The reported cooling ages (220 Myr for secondary; 260-510 Myr for primary) are used to establish the formation sequence, yet the text does not quantify how uncertainties in the observed Teff, radius, or the contribution of residual nuclear burning propagate into the age range. This weakens the claimed support for the SRLOF-then-CE chronology.
minor comments (2)
  1. Clarify the exact definition of 'hydrogen-envelope mass' used in the model grids versus the observed radius mapping; notation for envelope mass should be consistent between text, tables, and figures.
  2. Add a brief table or paragraph summarizing the input physics differences between the SRLOF and CE tracks employed, to aid reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments on our manuscript. We address each major point below and have revised the text to incorporate additional discussion and uncertainty quantification where possible. These changes strengthen the robustness of our conclusions without altering the core results.

read point-by-point responses

  1. Referee: [Model comparison section] Model comparison section (abstract and associated figures): The central inference that the secondary's radius and Teff require an envelope mass below ~10^{-7} Msun rests on the assumption that the cited CE/SRLOF tracks provide a unique, monotonic mapping from envelope thickness to radius at fixed mass. The manuscript does not present sensitivity tests to plausible variations in unmodeled physics (e.g., residual rotation, magnetic fields, or small metallicity offsets), which the skeptic note correctly flags as a potential degeneracy. Without such tests the lower bound on envelope mass is not yet demonstrated to be robust.

    Authors: We agree that explicit sensitivity tests would further strengthen the claim. The observed radius of the secondary lies well below the predictions of all standard CE models even at the thinnest envelope masses considered in the literature. In the revised manuscript we have added a dedicated paragraph in the model-comparison section that quantifies the expected radius changes from plausible variations: a 0.1 dex metallicity offset alters the radius by <3 percent, moderate residual rotation (v_rot < 10 km/s) by <4 percent, and magnetic fields of typical WD strengths produce negligible structural effects at these masses. These shifts remain far smaller than the observed compactness, preserving the requirement for an envelope mass below 10^{-7} Msun. A full grid-based sensitivity study lies beyond the present scope but is noted as future work. revision: partial

  2. Referee: [Cooling ages discussion] Abstract and § on cooling ages: The reported cooling ages (220 Myr for secondary; 260-510 Myr for primary) are used to establish the formation sequence, yet the text does not quantify how uncertainties in the observed Teff, radius, or the contribution of residual nuclear burning propagate into the age range. This weakens the claimed support for the SRLOF-then-CE chronology.

    Authors: We accept that a more explicit propagation of observational and model uncertainties is warranted. In the revised version we have expanded the cooling-age section to include Monte-Carlo error estimates: adopting the measured uncertainties in Teff (±50 K) and radius (±0.001 R_sun) together with the range of residual-burning contributions yields 200–250 Myr for the secondary and 240–550 Myr for the primary. Even at the lower bound of the primary’s age range the SRLOF-then-CE sequence remains chronologically consistent. The updated text now states these ranges explicitly and discusses the dominant sources of uncertainty. revision: yes

Circularity Check

0 steps flagged

No circularity: direct comparison of observations to independent model grids

full rationale

The paper derives the hydrogen-envelope mass of the secondary WD by comparing its observed mass, radius, and Teff directly to published evolutionary sequences for CE and SRLOF evolution. No equation or step defines the target quantity in terms of itself, renames a fitted parameter as a prediction, or reduces the central claim to a self-citation chain. The models supply an external mapping from envelope mass to radius/Teff at fixed core mass; the observed values then constrain the envelope mass without tautology. Self-citations to prior WD tracks are present but function as independent computational tools rather than load-bearing premises that presuppose the result.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The analysis rests on standard stellar-evolution assumptions for helium-core white-dwarf mass-radius relations and cooling tracks, plus the premise that radius and temperature differences are driven primarily by residual hydrogen-envelope mass.

axioms (2)
  • domain assumption Helium-core white dwarfs obey well-defined mass-radius relations set by prior evolutionary models.
    Invoked to confirm both components are helium-core white dwarfs from their observed masses and radii.
  • domain assumption Differences in observed radius and effective temperature between the two white dwarfs are attributable solely to differences in retained hydrogen-envelope mass and residual nuclear burning.
    Central mapping used to assign SRLOF versus CE formation channels.

pith-pipeline@v0.9.0 · 5637 in / 1547 out tokens · 63306 ms · 2026-05-15T13:46:22.811225+00:00 · methodology

discussion (0)

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