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Two More Double-Faced White Dwarfs Discovered

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double-faced white dwarf
Illustration of a double-faced white dwarf. This shows an extreme example, in which the two halves of the white dwarf have different surface compositions. Other surface composition configurations are possible. [K. Miller, Caltech/IPAC]

A recent study brings the number of known double-faced white dwarfs to seven. These rare objects, which feature compositional changes across their surfaces, may arise under the influence of magnetic fields.

The Rise of Two-Faced Stars

Hubble image of the Ring Nebula

When a super-hot white dwarf illuminates the diffuse shells of gas that surround it, we see a glowing planetary nebula. The central white dwarf is visible in this image of the Ring Nebula. [NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]

At the end of their hydrogen-burning lifetimes, low- to intermediate-mass stars shed their outer layers and bare their cores, evolving into Earth-sized spheres of degenerate electron matter called white dwarfs.

In 2023, researchers reported the discovery of a bizarre white dwarf: one with a surface composed of hydrogen on one side and helium on the other. A handful of other white dwarfs with split or patchy surface compositions have since been discovered, giving rise to the growing class of double-faced white dwarfs.

Many aspects of these strange objects are still unknown, including exactly how they form, how common they are, and what kinds of surface composition distributions are possible. To answer these questions, researchers must find more double-faced white dwarfs — and the hunt is on.

A Closer Look at “Binary” White Dwarfs

Because the composition of a double-faced white dwarf varies across its surface, these objects can present different compositional “faces” as they spin, sometimes resembling one kind of white dwarf and sometimes another. To find white dwarfs with this behavior, Adam Moss (University of Oklahoma) and collaborators investigated six systems earmarked as being possible unresolved binary systems containing one hydrogen-rich white dwarf and one helium-rich white dwarf.

The team suspected that these “binary” systems might instead be double-faced white dwarfs exhibiting features of two types of white dwarfs. One last white dwarf, previously identified as having spectral features from both hydrogen and helium, rounded out the sample.

spectra of a double-faced white dwarf

Spectra of white dwarf J0847+4842 taken at the beginning, middle, and end of the night from Apache Point Observatory. The spectra have been offset vertically for clarity. Vertical blue lines mark the locations of hydrogen lines, and vertical red lines mark the locations of helium lines. [Adapted from Moss et al. 2025]

Analyzing spectra from the Gemini North telescope, Apache Point Observatory, and the MMT Observatory, Moss’s team found spectral variations in two of the seven targets. The five targets with no spectral variations are likely bona fide binary systems, while the two time-varying targets showed clear signs of the surface composition variations that characterize a double-faced white dwarf. The spectra of these two objects were well fit with a model featuring polar caps rich in hydrogen and an equatorial belt rich in helium.

Potential Magnetic Origins

These two newly discovered double-faced white dwarfs bring the number of known objects in this class to seven. What does the current sample tell us about the origins of double-faced white dwarfs?

Four of the seven sample members are known to have magnetic fields, which is a far larger proportion than the general population of white dwarfs. This suggests that magnetic fields play an important role in creating these objects’ patchwork surfaces.

Moss and collaborators favor a scenario in which magnetic fields affect the process of convection, which can alter the elemental makeup of a white dwarf’s surface as it cools. In this scenario, stronger magnetic fields at the magnetic poles suppress convection, while weaker fields near the magnetic equator allow convection to proceed. Convection brings helium-rich material to the star’s surface, creating a helium-rich belt sandwiched between hydrogen-rich polar caps. Future modeling will explore how complex magnetic field configurations affect convection and the composition of white-dwarf surfaces, advancing our understanding of this rare class of objects.

Citation

“The Emerging Class of Double-Faced White Dwarfs,” Adam Moss et al 2025 ApJ 983 14. doi:10.3847/1538-4357/adbd3a