Discs surrounding both stars in a binary may help explain the range of orbital eccentricities—the measure of how far an orbit departs from a circle—seen in shell-burning stripped giant (SBSG) binaries. But the match appeared only when the systems entered the disc-interaction phase with eccentricities of at least 0.05, their circumbinary discs started with at least 0.1 solar masses, and accretion onto the stripped giant star was highly inefficient.
That is a conditional match, not a demonstration that the discs caused the observed orbits. The study was designed to test whether the interaction could explain the observed eccentricity distribution, so its conclusion depends on the starting conditions built into the model.
How the comparison was made
The authors used a circumbinary-disc/binary interaction formalism based on hydrodynamic simulations. They compared the final eccentricity distributions from alternative synthetic populations with the observed sample of 39 SBSG binaries whose orbits had been determined spectroscopically. Each synthetic population contained 1,000 sampled systems.
To assess consistency, the study used contest_dens and treated p-values above 0.05 as statistically significant; it also commonly checked the result with the standard Kolmogorov-Smirnov test.
The observed systems split into two patterns
The observed binaries did not show one uniform eccentricity pattern. Around 45% of post-RGB systems had e below 0.05, compared with approximately 15% of post-AGB systems. At the same time, about 60% of the post-AGB binaries clustered around e = 0.3 ± 0.1. Only post-AGB systems exceeded the study’s equilibrium eccentricity.
Model #4’s successful settings formed a broad solution valley rather than a single narrowly defined point. Its highest p-value occurred at δ = 0.35 and eb,max = 0.07, while the wider valley extended from δ = 0.15 with eb,max = 0.1 to δ = 0.5 with eb,max = 0.05.
The model’s price
The fit also imposed a very low ceiling on stellar accretion. At the highest-p-value setting for model #4, the estimated maximum stellar-accretion efficiency ranged from 5 × 10^-5 to 10^-2 for post-AGB stars. For post-RGB stars, the corresponding range was 7 × 10^-3 to 4 × 10^-2.
Within the model, however, changing the accretion efficiency had negligible effect on the final eccentricity distribution. The paper attributes that stability to the gravitational torques governing eccentricity pumping remaining unaffected by the accretion assumption. Low accretion was therefore important to the conditions for the fit, but it was not what set the final eccentricity pattern in this calculation.
Two unresolved mismatches
One gap concerns the disc itself. The model’s required mass proxy for circumbinary-disc mass was 0.1 to 1 solar masses, while current observed SBSG circumbinary discs typically have 10^-3 to 10^-2 solar masses. The paper treats the modeled quantity as a proxy, and the actual accreted mass could be lower.
A second gap appears at the beginning of the calculation. The model needed starting eccentricities much larger than the residual values of no more than about 10^-3 predicted after mass transfer. The authors identify phase-dependent Roche-lobe overflow, or RLOF, as the most promising additional mechanism to account for those starting values, but substantial uncertainty remains.
Orbital periods point elsewhere
The same setup struggled with orbital periods. For most observed post-AGB systems, the periods were up to 10 times shorter than the model predicted when stable mass transfer was followed by circumbinary-disc interaction. Only 11 of 26 systems were marginally consistent with the prediction, and using the metal-poor mass–period relation shortened the predicted periods by a factor of two.
The calculation therefore delivers a mixed result: it can match the eccentricity pattern under demanding conditions, but the stable-mass-transfer-plus-disc model does not account for most of the observed post-AGB periods.
A plausible explanation, still unconfirmed
The hydrodynamic calibration rests on a simplified picture. The simulations assumed that the stars were much smaller than their separation, that the disc’s self-gravity was negligible, that the disc was already viscously relaxed, and that it lost no material except through accretion. Those assumptions are part of the model conditions behind the population result.
The authors present circumbinary-disc interaction as a plausible explanation only if the progenitor systems began with massive discs and non-negligible post-mass-transfer eccentricities. But the study does not settle how those starting conditions arose; it points to phase-dependent RLOF as a leading candidate while acknowledging the gap between the modeled and theoretically expected eccentricities.
The work is an arXiv v1 preprint dated 26 August 2026. The authors say the underlying data will be shared upon reasonable request. Its central contribution is a modeling scenario that identifies the disc masses, initial eccentricities and accretion assumptions able to fit the observed distribution, while leaving the physical origin of those conditions open.
Paper data and sources
Original title: Can circumbinary discs produce the eccentricities of shell-burning stripped giant binaries?
Authors: C. A. S. Moltzer, O. R. Pols, H. Van Winckel
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
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