For the first time, astronomers have discovered a white dwarf with two different faces, one of hydrogen and the other of helium. The discovery raises new theories about the evolution of white dwarfs and the role of magnetic fields in the formation of celestial bodies. Credit: K. Miller, Caltech/IPAC
Unusual white dwarf star is made up of hydrogen on one side and helium on the other.
In a first for white dwarfs, the scorched cores of dead stars, astronomers have discovered that at least one member of this cosmic family is two-faced. One side of the white dwarf is made up of hydrogen, while the other is made up of helium.
“The surface of the white dwarf changes completely from side to side,” says Ilaria Caiazzo, a postdoctoral researcher at Caltech who is leading a new study of the findings in the journal Nature. “When I show the sightings to people, they are blown away.”
White dwarfs are the burning remnants of stars that once looked like our sun. As stars age, they swell into red giants; eventually their outer fluffy matter is blown away and their cores contract into dense, burning white dwarfs. Our sun will evolve into a white dwarf in about 5 billion years.
Illaria Caiazzo, associate postdoctoral fellow in astronomy, explains how her team used the ZTF to discover a very unusual “two-sided” white dwarf – one side is all helium and the other is all hydrogen. 1 credit
The new white dwarf, dubbed Janus after the two-faced Roman god of transition, was originally discovered by the Zwicky Transient Facility (ZTF), an instrument that scans the sky each night from Caltech’s Palomar Observatory near San Diego. Caiazzo was looking for strongly magnetized white dwarfs, such as the object known as ZTF J1901+1458, which she and her team previously found using ZTF. One candidate object stood out for its rapid changes in brightness, so Caiazzo decided to investigate further with the CHIMERA instrument in Palomar, as well as HiPERCAM on the Gran Telescopio Canarias in the Spanish Canary Islands. These data confirmed that Janus rotates on its axis every 15 minutes.
Later observations made with the WM Keck Observatory atop Hawaii’s Maunakea revealed the white dwarf’s dramatic two-sided nature. The team used an instrument called a spectrometer to scatter light from the white dwarf into a rainbow of wavelengths containing chemical fingerprints. The data revealed the presence of hydrogen when one side of the object was in view (with no sign of helium), and only helium when the other side came into view.
This artist’s concept shows the two-faced white dwarf nicknamed Janus. The blue-tinted dead ash of a star, which was once a star like our sun, is composed mostly of hydrogen on one side and helium on the other (the hydrogen side appears brighter). The peculiar double-sided nature of this white dwarf could be due to the interaction of magnetic fields and convection, or a mixture of materials. On the helium side, which appears fizzy, convection has destroyed the thin layer of hydrogen on the surface and pushed the helium up below. Credit: K. Miller, Caltech/IPAC
What would make a white dwarf floating alone in space have such radically different faces? The team admits they are confused but have offered a few possible theories. One idea is that we may be witnessing Janus undergoing a rare phase of white dwarf evolution.
“Not all of them, but some white dwarfs change from hydrogen to helium on their surface,” says Caiazzo. “We might have caught one of those white dwarfs in the act.”
After white dwarfs form, their heavier elements sink to their core and their lighter elements – hydrogen being the lightest of all – float upwards. But over time, as the white dwarfs cool, the materials are thought to mix together. In some cases, hydrogen is mixed in and diluted so that helium becomes more prevalent. Janus may embody this phase of transition, but a pressing question is: why is the transition happening so disjointedly, with one side moving before the other?
The answer, according to the scientific team, may lie in magnetic fields.
Scientists believe 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 influenced the mixing of materials in the white dwarf in such a way as to have caused their uneven distribution. The rotation of the white dwarf has been sped up in this animation; normally it rotates around 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 tend to be asymmetrical, 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 would 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, the fields are thought to alter the pressure and density of atmospheric gases.
“Magnetic fields can cause gas pressures in the atmosphere to drop, which can allow an ‘ocean’ of hydrogen to form where the magnetic fields are strongest,” says co-author James Fuller, professor of theoretical astrophysics at Caltech. “We don’t know which of these theories are correct, but we can’t think of any other way to explain asymmetric sides without magnetic fields.”
To help solve the mystery, the team hopes to find more Janus-like white dwarfs with ZTF’s sky survey. “ZTF is very good at finding strange objects,” says Caiazzo. Future surveys, such as those to be carried out by the Vera C. Rubin Observatory in Chile, she says, should make the search for variable white dwarfs even easier.
Reference: “A rotating white dwarf shows different compositions on its opposite faces” by Ilaria Caiazzo, Kevin B. Burdge, Pier-Emmanuel Tremblay, James Fuller, Lilia Ferrario, Boris T. Gänsicke, JJ Hermes, Jeremy Heyl, Adela Kawka, SR Kulkarni, Thomas R. Marsh, Przemek Mróz, Thomas A. Prince, Harvey B. Richer, Antonio C. Rodriguez, Jan van Roestel, Zach ary P. Vanderbosch, Stéphane Vennes, Dayal Wickramasinghe, Vikram S. Dhillon, Stuart P. Littlefair, James Munday, Ingrid Pelisoli, Daniel Perley, Eric C. Bellm, Elmé Breedt, Alex J. Brown, Richard Dekany, Andrew Drake, Martin J. Dyer, Matthew J. Graham, Matthew J. Green, Russ R. Laher, Paul Kerry, Steven G. Parsons, Reed L. R iddle, Ben Rushol me and Dave I. Sahman, July 19, 2023, Nature.
DOI: 10.1038/s41586-023-06171-9
The study was funded by the Walter Burke Institute for Theoretical Physics at Caltech, the European Research Council, the Leverhulme Trust and the UK’s Science and Technology Facilities Council.
Comments from Nasa‘s Neils Gehrels Swift Observatory – renamed in honor of Gehrels, a Caltech alumnus (PhD ’82) who died in 2017 – was also used in the study to help reduce the object’s temperature to 35,000 Kelvin (about 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. ZTF data is processed and archived by IPAC, a science and data center for astronomy at Caltech. NASA supports ZTF’s search for near-Earth objects through the Near-Earth Object Observations program.
