TESS Observations of Stochastic Low-frequency Variability in Extreme Helium Stars
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 such as density and radius. Further theoretical work is needed to determine the physical driving mechanism for the variability, but 1-D models of EHe stars suggest that thin subsurface convection zones may play a role.
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BibTeX
@misc{crawford2026tess,
title = {{TESS Observations of Stochastic Low-frequency Variability in Extreme Helium Stars}},
author = {Courtney L. Crawford and C. Simon Jeffery and May G. Pedersen and Timothy R. Bedding and Benjamin T. Montet and Geoffrey C. Clayton and Patrick Tisserand},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2605.23209},
eprint = {2605.23209},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.23209},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - TESS Observations of Stochastic Low-frequency Variability in Extreme Helium Stars
AU - Courtney L. Crawford
AU - C. Simon Jeffery
AU - May G. Pedersen
AU - Timothy R. Bedding
AU - Benjamin T. Montet
AU - Geoffrey C. Clayton
AU - Patrick Tisserand
PY - 2026
DA - 2026/08/26
PB - arXiv
DO - 10.48550/arxiv.2605.23209
UR - https://arxiv.org/abs/2605.23209
AB - 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 such as density and radius. Further theoretical work is needed to determine the physical driving mechanism for the variability, but 1-D models of EHe stars suggest that thin subsurface convection zones may play a role.
ER -