For the first time, astronomers have discovered a white dwarf with two distinct faces, one of hydrogen and the other of helium. The discovery raises new theories about the evolution of the white dwarf and the role of magnetic fields in the formation of celestial bodies. Credit: K. Miller, Caltech/IPAC
unusual white dwarf the star is made of hydrogen on one side and helium on the other.
In a first for white dwarfs, the burned-out cores of dead stars, astronomers have discovered that at least one member of this cosmic family is two-faced. One side of a white dwarf is made up of hydrogen and the other is made up of helium.
“The surface of the white dwarf completely changes from one side to the other,” said Ilaria Caiazzo, a Caltech postdoctoral fellow who led a new study of the findings in the journal Nature. “When I show people the observations, they’re blown away.”
White dwarfs are the burning remains of stars that were once like our sun. As stars age, they swell into red giants; eventually, their fluffy outer material is blown away and their cores shrink into dense, fiery-hot white dwarfs. Our sun will become a white dwarf in about 5 billion years.
Ilaria Caiazzo, a postdoctoral research fellow in astronomy, explains how her team used the ZTF to discover a very unusual white dwarf with two faces – one side made entirely of helium and the other side made of hydrogen. Credit: Caltech
The newly discovered white dwarf, named Janus after the two-faced Roman god of transition, was originally discovered by the Zwicky Transient Facility (ZTF), an instrument that scans the sky nightly from Caltech’s Palomar Observatory near San Diego. Caiazzo has been looking for highly magnetized white dwarfs, such as the object known as ZTF J1901+1458, which she and her team previously discovered using ZTF. One candidate object stood out for its rapid changes in brightness, so Caiazzo decided to investigate further with CHIMERA tool in Palomar as well HiPERCAM of the Gran Telescopio Canarias in the Canary Islands of Spain. These data confirmed that Janus rotates on its axis every 15 minutes.
Follow-up observations made with the WM Keck Observatory atop Maunakea in Hawaii revealed the dramatic two-faced nature of the white dwarf. The team used an instrument called a spectrometer to spread the white dwarf’s light into a rainbow of wavelengths that contain chemical signatures. The data reveal the presence of hydrogen when one side of the object is visible (no sign of helium) and only helium when the other side rotates into view.
This artist’s concept shows the two-faced white dwarf nicknamed Janus. The blue-colored dead ash of a star that was once a star like our sun is composed primarily of hydrogen on one side and helium on the other (the hydrogen side appears brighter). The strange two-faced nature of this white dwarf could be due to the interaction of magnetic fields and convection, or to mixing of materials. On the helium side that appears bubbly, convection has destroyed the thin hydrogen layer at the surface and brought up helium from below. Credit: K. Miller, Caltech/IPAC
What would cause a white dwarf floating alone in space to have such drastically different faces? The team admits they are confused, but have come up with some possible theories. One idea is that we may be witnessing Janus going through a rare phase of white dwarf evolution.
“Not all, but some white dwarfs switch from hydrogen to helium dominance on their surface,” Caiazzo explains. “We may have caught one such white dwarf in its tracks.”
After white dwarfs form, their heavier elements sink into their cores and their lighter elements – hydrogen being the lightest of all – rise to the top. But over time, as white dwarfs cool, the materials are thought to mix. In some cases, the hydrogen is mixed in and diluted so that the helium becomes more abundant. Janus may embody this transitional phase, but a pressing question is: why does the transition occur in such a disjointed way, with one side developing before the other?
The answer, according to the research team, may lie in magnetic fields.
Scientists believe that magnetic fields may explain the unusual two-faced appearance of the white dwarf, nicknamed Janus. One side of the dead star’s surface is composed mostly of hydrogen, while the other side is helium, as seen in this artist’s animation. Asymmetric magnetic fields (seen as looping lines) may have affected the mixing of materials in the white dwarf in such a way as to cause them to be unevenly distributed. The rotation of the white dwarf is accelerated in this animation; it usually rotates on its axis every 15 minutes. Janus is about 1,300 light years away in the constellation Cygnus. Credit: K. Miller, Caltech/IPAC
“Magnetic fields around cosmic bodies are usually asymmetric or stronger on one side,” Caiazzo explains. “Magnetic fields can prevent materials from mixing. So if the magnetic field is stronger on one side, then that side will have less mixing and therefore more hydrogen.
Another theory proposed by the team to explain the two faces also depends on magnetic fields. But in this scenario, fields are believed to change the pressure and density of atmospheric gases.
“Magnetic fields can lead to lower gas pressures in the atmosphere, and this can allow a hydrogen ‘ocean’ to form where the magnetic fields are strongest,” said co-author James Fuller, professor of theoretical astrophysics at Caltech. “We don’t know which of these theories is correct, but we can think of no other way to explain the asymmetric sides without magnetic fields.”
To help solve the mystery, the team hopes to find more Janus-like white dwarfs with the ZTF sky survey. “ZTF is very good at finding strange objects,” Caiazzo says. Future surveys, such as those being conducted by the Vera C. Rubin Observatory in Chile, she says, should make finding variable white dwarfs even easier.
Reference: “A rotating white dwarf shows different compositions of its opposite faces” by Ilaria Caiazzo, Kevin B. Burge, Pierre-Emmanuel Tremblay, James Fuller, Lilia Ferrario, Boris T. Gensike, JJ Hermes, Jeremy Hale, Adela Kavka, SR Kulkarni, Thomas R. Marsh, Przemek Mroz, Thomas A. Prince, Harvey B. Richer, Antonio S. Rodriguez , Jan van Rostel, Zachary P. Vanderbosch, Stefan Venes, Dayal Wickramasinghe, Vikram S. Dillon, Stuart P. Littlefair, James Munday, Ingrid Pelizoli, Daniel Perley, Eric S. Bellm, Elme Breed, Alex J. Brown, Richard Decani, Andrew Drake, Martin J. Dyer, Matthew J. Graham, Matthew J. Greene, Russ R. Laher, Paul Kerry, Steven G. Parsons, Reed L. Riddle, Ben Rusholme, and Dave I. Sahman, 19 Jul 2023, Nature.
DOI: 10.1038/s41586-023-06171-9
The research was funded by Caltech’s Walter Burke Institute for Theoretical Physics, the European Research Council, The Leverhulme Trust and the UK Science and Technology Facilities Council.
Observations from NASAThe Neils Gehrels Swift Observatory — renamed for Gehrels, a Caltech alumnus (Ph.D. ’82) who died in 2017 — was also used in the study to help narrow down the object’s temperature to a scorching 35,000 Kelvin (roughly 35,000 degrees Celsius).
Caltech’s ZTF is funded by the National Science Foundation and an international collaboration of partners. Additional support comes from the Heising–Simons Foundation and Caltech. The ZTF data are processed and archived by IPAC, the Astronomy Science and Data Center at Caltech. NASA supports the ZTF search for near-Earth objects through the Near-Earth Object Observation Program.