A New Type of Stochastic Low Frequency Variable Star: the Extreme Helium Stars

The launch of TESS revealed a new type of variability known as stochastic low frequency (SLF) variations, characterized by a smoothly varying red noise in power density space, rather than the discrete peaks associated with coherent modes. SLF variability is common in massive stars such as O & B dwarfs and blue supergiants, which is similar to the granulation component of solar-like oscillators. Its physical origin remains uncertain, with proposed mechanisms including sub-surface convection zones, internal gravity waves, variable stellar winds, and surface granulation. Using TESS light curves, we report for the first time that Extreme Helium (EHe) stars also exhibit SLF variability.

The EHe stars are a class of low-mass (<1 Msun) B-type supergiants with atmospheres that are helium-rich (~98% by mass) and incredibly hydrogen-deficient (<1% by mass). The leading theory for their formation is that they are the remnant of a double-white-dwarf merger with total mass less than the Chandrasekhar limit, making them a low-mass counterpart to Type Ia supernovae. In this talk, I will present TESS observations of several EHe stars and our ongoing efforts to characterize their SLF variability using Gaussian Processes, which are well-suited to modeling correlated, stochastic signals. I will compare these results with recent studies of SLF behavior in massive and solar-like stars. Finally, I will discuss whether the variability mechanism in EHe stars aligns more closely with those of hot, massive stars or cool, low-mass stars. Our study will help to illuminate how surface and internal processes operate across different stellar masses and evolutionary histories, and shed light about these rare remnants of double white dwarf mergers.