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arxiv: 2605.23209 · v1 · pith:QP6Z5YSXnew · submitted 2026-05-22 · 🌌 astro-ph.SR

TESS Observations of Stochastic Low-frequency Variability in Extreme Helium Stars

Courtney L. Crawford , C. Simon Jeffery , May G. Pedersen , Timothy R. Bedding , Benjamin T. Montet , Geoffrey C. Clayton , Patrick Tisserand This is my paper

Pith reviewed 2026-05-25 03:22 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords extreme helium starsstochastic low-frequency variabilityTESS photometrysubsurface convectiongranulation scalinghydrogen-deficient starsstellar variability
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The pith

Most extreme helium stars show stochastic low-frequency variability whose timescales match predictions from subsurface convection zones.

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

The paper examines TESS light curves for every known extreme helium star and reports that the majority display stochastic low-frequency variability instead of discrete pulsation modes. This signal is characterized by characteristic timescales between 0.5 and 10 days that scale with stellar parameters according to both established granulation relations and the convective turnover times calculated from one-dimensional models of the iron-opacity subsurface convection zone. Two metal-poor stars lack detectable variability, which the authors link to a metallicity-dependent driving process. The results indicate that photometric monitoring can probe the internal structure of these hydrogen-deficient merger products through their surface variability.

Core claim

The majority of EHe stars exhibit stochastic low-frequency variability with power increasing toward low frequencies; this variability is characterized using Gaussian process regression with a stochastically driven and damped simple harmonic oscillator kernel, yielding timescales, amplitudes, and quality factors that correlate strongly with stellar parameters and match both granulation scaling relations for cool stars and the convective turnover timescales predicted by the Fe-opacity subsurface convection zone in one-dimensional EHe models, with the signal absent in two metal-poor stars.

What carries the argument

Gaussian process regression with a stochastically-driven/damped simple harmonic oscillator kernel applied to TESS light curves to extract the characteristic timescale, low-frequency amplitude, and quality factor of the stochastic low-frequency variability.

If this is right

  • SLF variability provides a photometric probe of subsurface convection in EHe stars without requiring spectroscopy.
  • The correlation with stellar parameters allows population-level inference of convection zone properties across the known EHe sample.
  • Absence of the signal in metal-poor stars implies that driving efficiency depends on iron abundance.
  • Updated one-dimensional EHe models can be directly tested against the observed range of variability timescales.

Where Pith is reading between the lines

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

  • Similar SLF signals may appear in other classes of hot, hydrogen-deficient stars if they also host iron-opacity convection zones.
  • Longer-baseline photometry could distinguish whether the variability persists or evolves on timescales longer than the TESS sectors.
  • If the convection interpretation holds, it supplies an independent constraint on the depth of the subsurface zone that can be compared with pulsation models for the two known large-amplitude pulsators.

Load-bearing premise

That the observed stochastic low-frequency signal is generated by the iron-opacity subsurface convection zone rather than by other possible mechanisms.

What would settle it

Finding stochastic low-frequency variability with matching timescales in additional metal-poor extreme helium stars, or measuring timescales that systematically deviate from the convective turnover times in updated stellar models, would falsify the proposed driving mechanism.

Figures

Figures reproduced from arXiv: 2605.23209 by Benjamin T. Montet, Courtney L. Crawford, C. Simon Jeffery, Geoffrey C. Clayton, May G. Pedersen, Patrick Tisserand, Timothy R. Bedding.

Figure 2
Figure 2. Figure 2: Upper: The TESS light curve of V652 Her folded using a linear ephemeris with period calculated via iterative prewhitening (See [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 1
Figure 1. Figure 1: HRD of the EHe stars, with three evolutionary tracks of WD+WD mergers of varying total masses denoted with grey lines (Zhang et al. 2014; Crawford et al. 2024). Upper: The EHe stars are denoted in varying colours according to their brightness in the TESS filter. Lower: the symbols and markers denote the different classes of variables discussed in the text. The instability strip for the large amplitude puls… view at source ↗
Figure 3
Figure 3. Figure 3: Upper: The TESS light curve of BX Cir folded using the cubic ephemeris from Kilkenny et al. (2024). Different coloured points denote different TESS sectors as indicated in the colorbar. Vertical dashed grey lines indicate the beginning and end of the cycle. Middle: Same as the upper plot except the flux data has been binned to a width of 0.02 in phase to showcase the slight differences between sectors. Not… view at source ↗
Figure 4
Figure 4. Figure 4: Amplitude spectrum of BD+37 442 before and after iterative prewhitening (red and black, respectively). Upper: Frequency and ampli￾tude are in linear space and showcase the lowest frequencies to highlight the modes. Lower: Frequency and amplitude are in logarithmic space and showcase the full spectral range, highlighting the SLF variability signal [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Amplitude spectrum of BD+37 1977 before and after iterative prewhitening (red and black, respectively). Upper: Frequency and ampli￾tude are in linear space and showcase the lowest frequencies to highlight the modes. Lower: Frequency and amplitude are in logarithmic space and showcase the full spectral range, highlighting the SLF variability signal. 3.2 Stochastic Variables The majority of EHe light curves … view at source ↗
Figure 6
Figure 6. Figure 6: Example GPR fit to the TESS light curve for a representative target (V821 Cen) and its corresponding PSD. Top panels: Individual sector light curves (black points) shown in ppm, with the GPR fit overplotted (blue line). The blue shaded region indicates the 1𝜎 confidence interval. Bottom panel: PSD of the observed light curve (grey), with a smoothed version (dark grey). The dashed red line indicates the whi… view at source ↗
Figure 7
Figure 7. Figure 7: Three figures showing the HRD of known EHe stars with coloured points representing the three parameters fit using GPR as described in the text. Lines of constant radius for 1 𝑅⊙ and 20 𝑅⊙ are plotted as grey dashed lines. Upper: the logarithm of power density at zero frequency in ppm2 /d−1 , 𝑆0. Middle: the logarithm of the central frequency of the variability signal 𝜈SLF. Lower: the quality factor of the … view at source ↗
Figure 9
Figure 9. Figure 9: A zoomed in HRD of the EHe stars with measured SLF variability. Filled circles show the observed stars, colour-coded by log10 (𝜈SLF ) (in c/d), with error bars indicating uncertainties in L and 𝑇eff. Squares and triangles denote predicted 𝜈SLF = 1/𝜏conv from EHe evolutionary models at 0.8 and 0.7 𝑀⊙, respectively. The corresponding evolutionary tracks are shown as solid lines. The model predicted timescale… view at source ↗
Figure 10
Figure 10. Figure 10: SLF variability timescale, 𝜏SLF, as a function of √︁ 𝜌/𝜌⊙, com￾pared to a timescale predicted for a constant pulsation constant 𝑄puls = 0.075 days (dotted line). Error bars indicate uncertainties on the measured quanti￾ties. radiatively-driven winds causing modulated wind blanketing ef￾fects that stochastically change the brightness of the star over time. The example simulation used by Krtička & Feldmeier… view at source ↗
read the original abstract

Extreme helium stars (EHes) are low-mass hydrogen-deficient stars thought to be the products of double white dwarf mergers. Despite prolonged ground-based observations, there is no consensus on the properties of their photometric variability. In this article, we present an analysis of TESS light curves for all known EHe stars, constituting the first population-level study of EHe photometric variability. We present updated TESS light curves for the two confirmed large-amplitude pulsators, V652 Her and BX Cir, and discuss the potential r-mode pulsators BD+37 442 and BD+37 1977. Notably, we found that the majority of EHe stars exhibit stochastic low-frequency (SLF) variability, or a signal with power increasing smoothly towards low frequencies, rather than peaks in the power spectrum corresponding to oscillation modes. We characterised the SLF variability of EHe stars using Gaussian process regression with a stochastically-driven/damped simple harmonic oscillator kernel and measured the characteristic timescale, low-frequency amplitude, and quality factor for each star. The variability timescales range from approximately 0.5 to 10 d and correlate strongly with stellar parameters, following both the granulation scaling relations established for cool stars and the convective turnover timescales predicted by the Fe opacity subsurface convection zone in one-dimensional EHe stellar models. Two metal-poor EHe stars show no detectable SLF variability, consistent with a metallicity-dependent driving mechanism. Our results suggest that SLF variability in EHe stars may be driven by subsurface convection, though further theoretical work is needed to distinguish between other potential driving mechanisms.

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 the first population-level study of TESS light curves for all known extreme helium stars (EHes). It reports that the majority exhibit stochastic low-frequency (SLF) variability rather than coherent pulsation modes, characterized via Gaussian process regression with a stochastically-driven/damped simple harmonic oscillator kernel. Measured variability timescales (0.5–10 d) are found to correlate strongly with stellar parameters, matching both established granulation scaling relations and convective turnover timescales from 1D EHe models of the Fe-opacity subsurface convection zone. Two metal-poor EHes show no detectable SLF, and the authors suggest subsurface convection as a possible driver while noting that further theoretical work is required.

Significance. If the results hold, this constitutes the first systematic characterization of photometric variability across the known EHe population. The reported empirical correlations between observed timescales and both granulation relations and model convective times provide a new observational constraint on variability mechanisms in hydrogen-deficient merger products, with potential implications for convection theory and asteroseismology in extreme stellar conditions. The work is primarily observational and empirical, with appropriately cautious framing of the physical interpretation.

major comments (2)
  1. [Abstract] Abstract: the central claim that 'the majority of EHe stars exhibit stochastic low-frequency (SLF) variability' is presented without any accompanying sample statistics (total number of stars analyzed, number with detections, or detection thresholds), which prevents assessment of the robustness of the population-level result and the handling of non-detections or exclusion criteria.
  2. [Results] The reported strong correlations between GP-derived timescales and stellar parameters (and model convective times) are load-bearing for the main result, yet the manuscript provides no details on error propagation from the GP fits, the statistical method used to quantify the correlations, or uncertainties on the model timescales; this information is required to evaluate whether the agreement is significant or could arise from the fitting procedure itself.
minor comments (2)
  1. [Abstract] Abstract: the discussion of potential r-mode pulsators (BD+37 442 and BD+37 1977) would benefit from a brief statement of the evidence or prior references supporting this classification.
  2. [Table 1] The manuscript should include a table or explicit list of all analyzed EHe stars with their GP parameters, stellar parameters, and detection status to allow reproducibility and independent verification of the 'majority' claim.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their positive assessment and constructive comments, which will improve the clarity of the manuscript. We address each major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the central claim that 'the majority of EHe stars exhibit stochastic low-frequency (SLF) variability' is presented without any accompanying sample statistics (total number of stars analyzed, number with detections, or detection thresholds), which prevents assessment of the robustness of the population-level result and the handling of non-detections or exclusion criteria.

    Authors: We agree that the abstract would benefit from these statistics for better context. The full manuscript (Section 2) details the sample of all known EHe stars with TESS coverage, the number analyzed, detection criteria, and handling of non-detections. We will revise the abstract to incorporate the relevant numbers and thresholds. revision: yes

  2. Referee: [Results] The reported strong correlations between GP-derived timescales and stellar parameters (and model convective times) are load-bearing for the main result, yet the manuscript provides no details on error propagation from the GP fits, the statistical method used to quantify the correlations, or uncertainties on the model timescales; this information is required to evaluate whether the agreement is significant or could arise from the fitting procedure itself.

    Authors: We acknowledge that these methodological details are not explicitly stated. In the revised manuscript we will add a description of uncertainty propagation from the GP kernel parameters, specify the correlation statistic and significance test employed, and report uncertainties on the model convective timescales derived from the 1D EHe models. revision: yes

Circularity Check

0 steps flagged

Empirical GP analysis with external model comparisons; no internal reduction

full rationale

The paper's core pipeline fits a stochastically-driven damped SHO Gaussian process kernel to TESS light curves, extracts characteristic timescales/amplitudes/quality factors, and reports empirical correlations against pre-existing granulation scaling relations (for cool stars) plus convective turnover times taken from independent 1D EHe models. These comparisons are post-hoc and do not reduce any claimed prediction to quantities defined by the paper's own fitted parameters. The subsurface-convection interpretation is explicitly labeled a suggestion requiring further theoretical work, with no self-citation invoked as a uniqueness theorem or load-bearing premise. The analysis is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard assumptions about TESS data quality and the applicability of one-dimensional stellar models; GP kernel parameters are fitted per star but do not constitute free parameters invented for the claim. No new entities postulated.

free parameters (1)
  • GP kernel parameters (timescale, amplitude, quality factor)
    Fitted individually to each star's light curve using the stochastically-driven/damped SHO kernel; these are data-driven rather than ad hoc for the population claim.
axioms (2)
  • domain assumption TESS light curves accurately capture intrinsic stellar variability after standard processing
    Invoked implicitly when attributing SLF signals to stellar physics rather than instrumental effects.
  • domain assumption One-dimensional EHe stellar models correctly predict Fe-opacity subsurface convection zone properties
    Used when comparing observed timescales to model convective turnover times.

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