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

This study presents the first population-level analysis of TESS light curves for Extreme Helium stars, revealing that most exhibit stochastic low-frequency variability driven by subsurface convection, with timescales correlating to stellar parameters and metallicity.

Original authors: Courtney L. Crawford, C. Simon Jeffery, May G. Pedersen, Timothy R. Bedding, Benjamin T. Montet, Geoffrey C. Clayton, Patrick Tisserand

Published 2026-05-25

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CC BY 4.0

Original authors: Courtney L. Crawford, C. Simon Jeffery, May G. Pedersen, Timothy R. Bedding, Benjamin T. Montet, Geoffrey C. Clayton, Patrick Tisserand

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the night sky is filled with stars that are mostly made of helium, having lost almost all their hydrogen. Astronomers call these "Extreme Helium Stars" (EHes). For decades, we've watched them from the ground, trying to figure out why they flicker and change brightness. Some seemed to pulse like a heartbeat, while others just seemed to shimmer randomly. It was a confusing mess.

This paper is like turning on a high-definition, all-seeing camera (the TESS space telescope) to take a long, uninterrupted look at every single one of these stars we know about. The goal? To finally understand what makes them "wiggle."

Here is what the researchers found, explained simply:

1. The "Flicker" is Mostly Random, Not a Rhythm

For a long time, scientists hoped these stars were pulsing in a perfect, rhythmic beat (like a drum). They found two stars that do this perfectly, but for almost everyone else, the light doesn't pulse in a neat pattern.

Instead, most of these stars exhibit what the paper calls Stochastic Low-Frequency (SLF) variability.

The Analogy: Think of a calm pond. If you drop a single stone, you get a perfect, rhythmic ripple (that's a pulsating star). But if a gentle breeze blows across the water, you get a chaotic, shifting pattern of small waves that never repeat exactly. That is what these stars are doing. The "wind" is churning the star's surface, creating a random, low-frequency hum rather than a clear beat.

2. The "Churn" is Likely Subsurface Convection

So, what is causing this chaotic churning? The paper suggests it's convection happening just under the star's surface.

The Analogy: Imagine a pot of boiling water. You see bubbles rising and breaking at the surface (granulation). Usually, we think of this happening in cool stars. But these helium stars are incredibly hot. The researchers found that deep inside these hot stars, there is a thin layer where the star's own "iron fog" (opacity) traps heat, causing a tiny, churning convection zone right near the surface.
This churning creates waves that travel up to the surface, making the star's brightness jitter randomly. It's like the star has a tiny, invisible blender just under its skin.

3. The "Metal" Test

The researchers noticed something interesting about two specific stars that didn't show this random flickering.

The Analogy: These two stars were "metal-poor" (in astronomy, "metals" are elements heavier than hydrogen and helium). It's like trying to make a smoothie without any fruit; the blender just spins uselessly. Because they lack the necessary "ingredients" (heavy elements like iron), the churning mechanism doesn't start, and the star stays relatively calm. This confirms that the churning is driven by these heavy elements.

4. Size Matters

The paper also found a clear rule: Bigger stars flicker more slowly.

The Analogy: Think of a large, lazy ocean wave versus a small, choppy ripple. The bigger the star, the slower its "churn" moves. The researchers measured how long it takes for this random flickering to happen, and it perfectly matched predictions based on the star's size and how fast sound travels inside it.

5. The "Heartbeat" Stars (The Exceptions)

While most stars were doing the random "churn," two famous stars (V652 Her and BX Cir) were found to be doing something else entirely. They are the "perfect drummers."

They pulse with a very strong, stable rhythm. The paper updated our measurements of their "heartbeats," showing they are actually speeding up slightly as they shrink, much like a figure skater pulling in their arms to spin faster.

Summary

The paper concludes that for the vast majority of Extreme Helium stars, the light we see isn't a rhythmic pulse, but a random, low-frequency hum caused by a thin layer of churning gas just under the surface.

If the star has enough heavy elements (iron): The gas churns, creating a random flicker.
If the star is too small or lacks heavy elements: The churning stops, and the star appears steady.
If the star is one of the rare "heartbeat" types: It pulses in a perfect, rhythmic beat.

This study didn't just look at one star; it looked at the whole "population" of these strange stars for the first time, proving that this random "churning" is the standard behavior for them, driven by the physics of their unique, helium-rich atmospheres.

Problem Statement
Extreme Helium stars (EHes) are low-mass, hydrogen-deficient giants and supergiants thought to be the products of double white dwarf mergers. Despite decades of ground-based observation, there is no consensus regarding the nature of their photometric variability. Key unresolved questions include whether all EHes vary, whether their variations are periodic or stochastic, the existence of characteristic timescales, and the physical driving mechanisms behind any observed variability. Previous studies identified a few large-amplitude pulsators (e.g., V652 Her, BX Cir) and suggested periodicity in others, but extended campaigns failed to find stable pulsation periods for the majority of the population.

Methodology
This study presents the first population-level analysis of EHe photometric variability using data from the Transiting Exoplanet Survey Satellite (TESS). The authors analyzed TESS light curves for 23 of the 30 known EHe stars, utilizing data from the Quick Look Pipeline (QLP) and the Science Processing Operations Center (SPOC), as well as custom extractions using eleanor to address instrumental artifacts and barycentric corrections.

To characterize the variability, the authors employed Gaussian Process Regression (GPR) using a stochastically-driven/damped simple harmonic oscillator (SHO) kernel. This approach was chosen to model the covariance of the data in the time domain, avoiding artifacts associated with Fourier analysis of multi-sector data gaps. The analysis focused on fitting three key parameters: the power density at zero frequency ( $S_0$ ), the characteristic frequency ( $\omega_0$ ), and the quality factor ( $Q$ ). For stars exhibiting periodic signals (V652 Her, BX Cir, BD+37 442, BD+37 1977), iterative pre-whitening was used to extract frequencies before modeling the residual stochastic signal. Contamination from nearby stars was rigorously assessed and corrected using flux ratios derived from the TESS Input Catalogue.

Key Contributions and Results

Prevalence of Stochastic Low-Frequency (SLF) Variability: The study finds that the majority of EHe stars exhibit SLF variability, characterized by a power spectrum increasing smoothly toward low frequencies rather than distinct oscillation peaks. This signal is distinct from the periodic variations seen in the two confirmed large-amplitude pulsators (V652 Her and BX Cir) and the potential $r$ -mode pulsators (BD+37 442 and BD+37 1977).
Characterization of SLF Parameters: Using GPR, the authors measured the characteristic timescales ( $\tau_{SLF}$ ), low-frequency amplitudes, and quality factors for the sample. The variability timescales range from approximately 0.5 to 10 days.
Correlation with Stellar Parameters: The variability timescales correlate strongly with stellar parameters. Specifically, $\tau_{SLF}$ increases with stellar radius. The measured frequencies align with granulation scaling relations established for cool stars ( $\tau \propto L/M T_{eff}^{-3.5}$ ) and with convective turnover timescales predicted by one-dimensional EHe stellar models featuring a subsurface convection zone driven by the iron opacity bump (FeCZ).
Metallicity Dependence: Two metal-poor EHe stars (DN Leo and EC 19529-4430) showed no detectable SLF variability, consistent with a metallicity-dependent driving mechanism, likely related to the opacity-driven nature of the FeCZ.
Updated Pulsation Data: The paper provides updated TESS light curves and frequency analyses for V652 Her and BX Cir, confirming their harmonic structures and noting that the ephemeris for V652 Her requires revision due to period contraction. It also confirms the presence of low-frequency noise in the $r$ -mode candidates BD+37 442 and BD+37 1977 after removing their periodic signals.

Significance and Claims
The authors claim that this work transforms the understanding of EHe variability by demonstrating that stochastic low-frequency variability is the dominant mode for this population, rather than the periodic pulsations previously assumed for the bulk of the class. The strong agreement between observed timescales and predictions from both granulation scaling relations and FeCZ-driven convection models suggests that subsurface convection is the likely driver of SLF variability in EHes.

The paper modestly concludes that while the evidence points toward subsurface convection (specifically the FeCZ) as the driving mechanism, further theoretical work is required to distinguish this from other potential mechanisms, such as internal gravity waves or highly damped oscillation modes. The study establishes a baseline for future population-level studies of hydrogen-deficient stars and suggests that the GPR methodology can be applied to historical ground-based data to extend these findings.