Title: The prototype double-faced white dwarf has a thin hydrogen layer across its entire surface
Authors: Antoine Bédard, Pier-Emmanuel Tremblay
First Author’s Institution: Department of Physics, University of Warwick, Coventry, UK
Status: Published in Monthly Notices of the Royal Astronomical Society Letters [open access]
Most of the stars in our universe end their lives as white dwarfs. These stellar remnants are incredibly dense, packing 60% of the Sun’s mass into an object the size of Earth. As they cool, their immense gravity pulls heavy elements inwards, creating a purified atmosphere consisting of lighter elements: hydrogen most of the time, or helium if previous stages of evolution depleted the star’s hydrogen. This process, called gravitational settling, explains the properties of most known white dwarfs.
Some white dwarfs defy the norm and undergo spectral evolution, where the composition of their atmosphere changes over time. These changes are attributed to two processes:
- Diffusion causes lighter elements to float to the surface. This process is thought to occur in hotter white dwarfs (35,000 to 55,000 K). For example, residual hydrogen may diffuse to the surface of a white dwarf with a helium-dominated atmosphere, causing hydrogen lines to appear in the spectrum. The result is a stratified atmosphere, with hydrogen forming a thin layer over the helium beneath.
- Convection mixes elements from lower layers up into the atmosphere, like bubbles rising in a pot of boiling water. This process is thought to occur in cooler white dwarfs (< 20,000 K). For example, helium may be mixed up into a hydrogen-rich layer, causing helium lines to appear in the spectrum. The result is a mixed atmosphere, where hydrogen and helium coexist.
The white dwarf’s surface composition changes uniformly in both cases, meaning that all sides should show the same spectrum as the change occurs. However, astronomers have recently identified a small subset of white dwarfs whose surface compositions vary periodically on timescales of minutes to days. These variations usually follow the star’s rotation period, suggesting that zones with different surface compositions are rotating in and out of view.
The most extreme example of this phenomenon, ZTF J2033, was discovered in 2023. Over the course of its ~15 minutes rotation period, this wacky white dwarf alternates between a spectrum showing only hydrogen lines and a spectrum showing only helium lines, with a smooth transition in between. Astronomers initially interpreted this to mean that ZTF J2033 was double-faced, with a hydrogen atmosphere on one side and a helium atmosphere on the other. But today’s paper proposes a simpler explanation for the star’s unusual behavior.
In the original discovery paper for ZTF J2033, the authors attempted to characterize the star’s two faces by fitting standard white dwarf atmosphere models – which are either pure hydrogen, pure helium, or a mix of both – to the observed spectra. However, they found that their models overestimated either the star’s temperature or the strength of the spectral lines. Today’s authors reproduce these results and argue that the failure of standard models to explain ZTF J2033’s behavior suggests that the star isn’t double-faced at all. Instead, they fit stratified atmosphere models to the data to see if it would provide a better match (Figure 1).
Excitingly, the stratified atmosphere models matched the data much more closely than the standard models! This suggests that instead of having two sides with completely different compositions, ZTF J2033 is covered with a hydrogen layer that varies in thickness (Figure 2). On one side, the layer is very thin, so we see the absorption lines from the helium underneath. On the other side, the layer is much thicker, so we only see absorption lines from hydrogen. In between, the hydrogen layer must smoothly transition from thin to thick, which explains why the spectral lines show the same behavior.
But the fits still aren’t perfect, which the authors attribute to two simplifying assumptions. First, their models assume that the hydrogen layer is the same thickness across the stellar disk at each point in time, which might not be true when the thickness is changing rapidly from one side of the star to the other (like at phases 0.36 and 0.64). Second, their models only include diffusion, not convection, which could create a mixed hydrogen-and-helium zone just below the outer hydrogen layer that affects the appearance of the spectral lines. Since the effect from the first assumption is probably quite small, the authors focus on addressing the second.
Using a code called STELUM, the authors create one-dimensional white dwarf models incorporating both diffusion and convection. They find that both the pure hydrogen and pure helium spectra can be reproduced by models with an outer layer in diffusive equilibrium and a small convective zone underneath. For the pure hydrogen spectrum, the convective layer is closer to the surface. The authors theorize that this structure could suppress convection, which would increase the longevity of the outer layer of hydrogen. However, the origins of ZTF J2033 remain mysterious. We generally expect stars to be symmetric, since gravity should even out any bumps relatively quickly. So what could account for the varying thickness of ZTF J2033’s hydrogen layer?
The authors propose three possible mechanisms. The first two involve an asymmetric magnetic field, which could either inhibit convection over just part of the white dwarf’s surface, or cause ionized hydrogen atoms to accumulate at its magnetic poles. The third doesn’t require a magnetic field at all, but instead assumes that ZTF J2033 is the product of a stellar merger, which might have created an uneven internal distribution of hydrogen. All three of these mechanisms are both complicated and poorly understood, so the authors defer further investigation to more advanced, three-dimensional models. In the meantime, they note that ZTF J2033 could also have four “faces” (a dipolar configuration) with a rotation period twice as long – technically, none of their analysis rules out this possibility!
Even though ZTF J2033 is the most extreme inhomogeneous white dwarf known, the findings of today’s paper have implications for the other members of this small but growing class. The idea of a non-uniform hydrogen layer is new, but provides a much simpler explanation than a star with two entirely different zones. Could this idea explain the behavior of other inhomogeneous white dwarfs? Do any of the ~30 known white dwarfs with stratified atmospheres (but no detected inhomogeneity) also have non-uniform hydrogen layers? We’ll need more observations and models to find out!
Astrobite edited by Cole Meldorf and Magnus L’Argent.
Featured image credit: Background image, Paul Volkmer on Unsplash; white dwarf, NASA/CXC/M. Weiss; drawings added by Alexandra Masegian.
