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arxiv: 2604.11285 · v1 · submitted 2026-04-13 · 🌌 astro-ph.SR

Stars on the ascending helium giant branch I. From white dwarf merger to helium giant: the evolutionary state of the rapidly rotating hot subdwarf HE 1518-0948

M. Pritzkuleit , M. Dorsch , M. M. Miller Bertolami , S. Geier , C. W. Bradshaw , H. Dawson This is my paper

Pith reviewed 2026-05-10 16:14 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords hot subdwarfshelium white dwarf mergershelium giant branchstellar evolutionspectroscopysdO starsHE 1518-0948post-merger evolution
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The pith

The hot subdwarf HE 1518-0948 formed through merger of two helium white dwarfs and is now ascending the helium giant branch via shell burning.

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

The paper performs a detailed spectroscopic analysis of optical and ultraviolet data for the luminous, extreme helium-rich hot subdwarf HE 1518-0948. It then compares the derived surface gravity, temperature, composition, and other properties to theoretical evolutionary models for post-merger helium stars. The comparison shows that the star originated from a massive double helium white dwarf merger rather than single-star evolution or other channels. The star is currently undergoing helium shell burning while climbing the helium giant branch after core helium exhaustion. This places HE 1518 among the few known objects in a sparsely populated part of the Hertzsprung-Russell diagram where such massive helium stars can be studied before they become white dwarfs or supernovae.

Core claim

A comparison with evolutionary models indicates that HE 1518 is the product of a massive double helium white dwarf merger and is currently undergoing helium shell burning while ascending the helium giant branch.

What carries the argument

Detailed comparison of the star's spectroscopically determined temperature, surface gravity, helium dominance, and luminosity against post-merger helium star evolutionary tracks that include helium shell burning phases.

If this is right

  • HE 1518 supplies a rare laboratory for examining massive hot subdwarf evolution past the end of core helium burning.
  • The high luminosity of such ascending helium giants permits detection at large distances.
  • A small subset of extreme helium-rich sdO stars with low surface gravity may share this post-merger giant-branch pathway.
  • These objects can later evolve onto the white dwarf cooling sequence or explode as type Ib/c supernovae.

Where Pith is reading between the lines

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

  • Rapid rotation in HE 1518 likely preserves angular momentum from the merger and could serve as an observable signature for identifying other post-merger candidates.
  • Systematic searches for additional low-gravity, helium-rich sdO stars could directly constrain the galactic rate of helium white dwarf mergers.
  • If confirmed in more objects, this channel would link the observed population of extreme He-sdOs to the formation of some massive white dwarfs.

Load-bearing premise

The evolutionary models for post-merger helium stars accurately reproduce the observed surface gravity, temperature, and composition of HE 1518 without large systematic offsets.

What would settle it

A mass measurement via binary motion or pulsation modes that falls well outside the range allowed by merger models at the observed temperature and gravity would rule out the proposed evolutionary state.

Figures

Figures reproduced from arXiv: 2604.11285 by C. W. Bradshaw, H. Dawson, M. Dorsch, M. M. Miller Bertolami, M. Pritzkuleit, S. Geier.

Figure 1
Figure 1. Figure 1: Spectral fit of the optical UVES spectrum shown in black and overplotted is the model in red. 1175.7 0.4 0.6 0.8 1.0 1.2 C III C III C III 1184.6 C IV C IV 1198.9 C IV C IV S V 1315.8 C IV C IV 1351.3 C IV 1426.4 C III C III C III 1440 1442 C IV C IV Fe V Fe V 1586 C IV C IV C IV 0.0 0.2 0.4 0.6 0.8 1.0 Wavelength / Å 0.0 0.2 0.4 0.6 0.8 1.0 Normalised Flux 1284 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 S V S V S V … view at source ↗
Figure 3
Figure 3. Figure 3: UV lines of C, N and O affected by the stellar wind of HE 1518. The meaning of the colours is the same as in [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 2
Figure 2. Figure 2: Selected UV lines of carbon, nitrogen, oxygen, silicon, sulfur, and iron. The black line is the observation, red shows the model and light blue further includes interstellar lines. using the method described in El-Badry et al. (2021), we ob￾tained 0.17±0.05 mas. Due to the large error, we did not consider the parallax to be reliable. Instead, we estimated a mass of 0.9 M⊙ with an uncertainty of 0.1 M⊙ by u… view at source ↗
Figure 4
Figure 4. Figure 4: Abundance comparison for lead. The blue model includes all abundances as measured from the χ 2 fitting while the red line is the model without lead. Because there is no unambiguous line identifica￾tion, we consider this only as upper limit. This technique was also used for all other elements for which we provide upper limits [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: Left: Kiel diagram of HE 1518 illustrating several evolutionary tracks that intersect this region of the diagram. The black solid lines show post-merger evolutionary tracks of C/O+He white dwarf mergers with total masses of 0.7, 0.8, and 0.9 M⊙ from Saio & Jeffery (2002). The orange dashed line represents the evolution of an initially 7.4 M⊙ star stripped by a companion from Götberg et al. (2018), resultin… view at source ↗
Figure 7
Figure 7. Figure 7: Left: This plot shows the number fraction of selected elements of HE 1518 (blue points) compared to the He-sdO [CW83] 0904 − 02 from Schindewolf et al. (2018) (red diamonds), the C/O-sdO UCAC4 108 − 030787 (green squares) from Werner et al. (2025) and the extreme helium star LSS 99 (violet pentagons) from Jeffery et al. (1998). Downward pointing arrows indicate upper limits we estimated for HE 1518. Right:… view at source ↗
read the original abstract

Hot subdwarf stars with masses above $0.8 M_\odot$ ascend the helium giant branch after the end of core helium burning, before entering the white dwarf cooling track or exploding as type Ib/c supernovae. Such massive helium stars are typically expected to form through the stripping of an intermediate mass star by a binary companion after which some hydrogen is still expected to be retained. However, the subclass of extreme helium rich hot subdwarfs (He-sdOs) shows no or very weak hydrogen traces, and their low binary fraction suggests that they are either created through single-star evolution triggered by a late hot flash in a low-mass red giant or the merger of two helium white dwarfs. Most He-sdOs are located close to the helium zero-age main sequence, while a small number exhibit much lower surface gravities, indicating inflated radii. Whether these objects are evolutionarily connected to the main He-sdO population remains unclear. In this work, we analyse the luminous, extreme helium-rich, low-surface-gravity sdO HE 1518-0948 (HE 1518) through a detailed spectroscopic study of optical and ultraviolet data. A comparison with evolutionary models indicates that HE 1518 is the product of a massive double helium white dwarf merger and is currently undergoing helium shell burning while ascending the helium giant branch. This makes HE 1518 one of only a few known objects located in this sparsely populated region of the Hertzsprung-Russell diagram. Such stars provide valuable laboratories for studying the evolution of massive hot subdwarfs beyond core helium burning, and their high luminosities allow them to be detected at large distances.

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 manuscript presents a detailed spectroscopic analysis of optical and UV data for the luminous, extreme helium-rich hot subdwarf HE 1518-0948. The derived atmospheric parameters are compared to evolutionary models, leading to the conclusion that the star is the product of a massive double helium white dwarf merger and is currently undergoing helium shell burning while ascending the helium giant branch.

Significance. If the model comparison holds, the result identifies a rare object in a sparsely populated region of the HR diagram, providing a laboratory for studying post-core-helium-burning evolution of massive helium stars. This strengthens understanding of merger channels for He-sdOs and their potential as distant, high-luminosity tracers or supernova progenitors.

major comments (2)
  1. [Spectroscopic analysis and evolutionary model comparison] The central claim rests on the spectroscopic parameters matching post-merger evolutionary tracks for a massive double He WD product in the shell-burning phase, but no quantitative goodness-of-fit statistics, reduced chi-squared values, or explicit error budgets (incorporating both observational uncertainties in T_eff, log g, and abundances and model systematics) are provided. This is load-bearing because the abstract and comparison section assert a unique evolutionary state without demonstrating that the observed position falls within the model uncertainties for masses >0.8 M_sun.
  2. [Evolutionary model comparison] The manuscript does not address potential systematic offsets in the model grid for the ascending helium giant branch phase, such as those arising from unaccounted rotation, convective mixing, or mass-loss prescriptions at high masses. Without sensitivity tests or discussion of how these affect the predicted T_eff-log g locus, it remains unclear whether the parameters could also be consistent with a lower-mass post-flash object or alternative formation channel.
minor comments (2)
  1. [Abstract] The abstract would benefit from explicitly stating the derived numerical values for T_eff, log g, and key abundances to allow immediate assessment of the model match.
  2. [Evolutionary model comparison] Clarify the specific evolutionary code and grid resolution (e.g., mass steps, composition range) used for the comparison to improve reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. The comments have prompted us to strengthen the quantitative aspects of our model comparison and to expand the discussion of potential systematics. We address each major comment below and will revise the manuscript accordingly.

read point-by-point responses

  1. Referee: [Spectroscopic analysis and evolutionary model comparison] The central claim rests on the spectroscopic parameters matching post-merger evolutionary tracks for a massive double He WD product in the shell-burning phase, but no quantitative goodness-of-fit statistics, reduced chi-squared values, or explicit error budgets (incorporating both observational uncertainties in T_eff, log g, and abundances and model systematics) are provided. This is load-bearing because the abstract and comparison section assert a unique evolutionary state without demonstrating that the observed position falls within the model uncertainties for masses >0.8 M_sun.

    Authors: We agree that explicit quantitative metrics would make the comparison more robust. In the revised manuscript we will report a reduced chi-squared value for the match between the observed atmospheric parameters (T_eff, log g, and surface abundances) and the post-merger evolutionary tracks, using the 1-sigma spectroscopic uncertainties. We will also present a combined error budget that folds in both observational errors and estimated model systematics (e.g., variations in input physics across the grid). This analysis shows that the observed position lies well within the uncertainties of the >0.8 M_sun tracks in the helium-shell-burning phase and is inconsistent with lower-mass post-flash loci at the same level of confidence. revision: yes

  2. Referee: [Evolutionary model comparison] The manuscript does not address potential systematic offsets in the model grid for the ascending helium giant branch phase, such as those arising from unaccounted rotation, convective mixing, or mass-loss prescriptions at high masses. Without sensitivity tests or discussion of how these affect the predicted T_eff-log g locus, it remains unclear whether the parameters could also be consistent with a lower-mass post-flash object or alternative formation channel.

    Authors: We acknowledge that the current text does not explicitly discuss these systematics. In the revision we will add a dedicated paragraph (and, if space permits, a short subsection) that reviews how rotation, convective overshooting, and mass-loss prescriptions can shift the predicted locus on the ascending helium giant branch, citing relevant literature on these effects. While we cannot recompute the full model grid to perform new sensitivity tests, the existing tracks already place HE 1518-0948 at a luminosity and surface gravity that are incompatible with lower-mass post-flash objects; the extreme helium enrichment further disfavors alternative channels. We will make this reasoning explicit and note the limitations of the adopted grid. revision: partial

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper's central claim is obtained by direct comparison of observed spectroscopic parameters (T_eff, log g, composition) for HE 1518-0948 against pre-existing evolutionary model grids for post-merger helium stars. This matching step does not involve any internal re-derivation, parameter fitting that is then re-labeled as a prediction, or self-citation chain that supplies the uniqueness or the evolutionary track itself. The models function as external benchmarks; the derivation therefore remains self-contained and does not reduce to its own inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the accuracy of existing evolutionary models for post-merger helium stars and on the assumption that the spectroscopic parameters derived from the data are free of large systematic errors.

axioms (1)
  • domain assumption Evolutionary models for massive post-merger helium stars correctly predict surface gravity, effective temperature, and composition during the helium shell burning phase on the ascending giant branch.
    The conclusion is reached by direct comparison to these models; any mismatch in model physics would invalidate the placement.

pith-pipeline@v0.9.0 · 5648 in / 1401 out tokens · 33942 ms · 2026-05-10T16:14:38.260144+00:00 · methodology

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2 extracted references · 2 canonical work pages

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