Astronomers blown away by strange two-sided star

Two-sided star

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 of hydrogen on one side and helium on the other.

In a first for white dwarfs, the burnt-out cores of dead stars, astronomers have found 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 white dwarf’s surface changes completely from one side to the other, says Ilaria Caiazzo, a postdoctoral scholar at Caltech leading a new study of the journal findings Nature. When I show observations to people, they are blown away.

White dwarfs are the scalding remains of stars that were once like our sun. As stars age, they turn into red giants; eventually, their outer soft material is blown away and their cores contract into dense, fiery white dwarfs. Our sun will evolve into a white dwarf in about 5 billion years.

Illaria Caiazzo, a postdoctoral research associate in astronomy, explains how her team used ZTF to discover a very unusual two-sided white dwarf, one side is all helium and the other hydrogen. Credit: Caltech

The new white dwarf, dubbed Janus after the two-faced Roman god of transition, was initially discovered by the Zwicky Transient Facility (ZTF), an instrument that scans the skies every night from Caltech’s Palomar Observatory near San Diego. Caiazzo was looking for highly magnetized white dwarf stars, 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 at Palomar, as well as HiPERCAM on the Gran Telescopio Canarias in Spain’s Canary Islands. These data confirmed that Janus rotates on its axis every 15 minutes.

Subsequent observations made with the WM Keck Observatory atop Maunakea in Hawaii revealed the white dwarf’s dramatic double-sided nature. The team used an instrument called a spectrometer to scatter the white dwarf’s light into a rainbow of wavelengths that contain chemical signatures. The data revealed the presence of hydrogen when one side of the object was in view (with no signs of helium) and only helium when the other side was visible.

Two-sided white dwarf star

This concept art shows the two-faced white dwarf nicknamed Janus. The blue-colored 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 to a mixture of materials. On the helium side, which appears bubbly, convection has destroyed the thin layer of hydrogen on the surface and raised the helium underneath. Credit: K. Miller, Caltech/IPAC

What would make a white dwarf floating alone in space have such drastically different faces? The team acknowledges they are baffled but have come up with a few possible theories. One idea is that we may be seeing Janus going through a rare phase of white dwarf evolution.

Not all, but some white dwarfs go from being dominated by hydrogen to helium on their surfaces, Caiazzo explains. We may have caught one of these white dwarfs in the act.

After white dwarfs form, their heaviest elements sink into their cores and their lightest elements, hydrogen, being the lightest of all, float to the top. But over time, as white dwarfs cool, materials are thought to mix. In some cases, hydrogen is mixed in and diluted such that the helium becomes predominant. Janus may embody this transition phase, but an urgent question is: why is the transition so disjointed, with one part evolving before the other?

The answer, according to the science team, may lie in magnetic fields.

Magnetic fields of double-sided white dwarf stars

Scientists think magnetic fields may explain the unusual two-sided appearance of the white dwarf nicknamed Janus. One side of the surface of dead stars is composed mostly of hydrogen, while the other side is helium, as seen in this artist’s animation. The asymmetrical magnetic fields (seen as ring lines) could have affected the mixing of materials in the white dwarf in such a way as to cause their distribution to be uneven. The rotation of the white dwarfs has been sped up in this animation; it normally rotates around its axis every 15 minutes. Janus is located 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, 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 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, says co-author James Fuller, a professor of theoretical astrophysics at Caltech. We don’t know which of these theories is correct, but we can’t think of any 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 ZTF’s sky survey. ZTF is very good at finding strange objects, says Caiazzo. Future surveys, such as those to be conducted by the Vera C. Rubin Observatory in Chile, he 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. Gnsicke, JJ Hermes, Jeremy Heyl, Adela Kawka, SR Kulkarni, Thomas R. Marsh, Przemek Mrz, Thomas A. Prince, Harvey B. Richer, Antonio C. Rodriguez, Jan van Roestel, Zachary P. Vanderbosch, St phane Vennes, Dayal Wickramasinghe, Vikram S. Dhillon, Stuart P. Littlefair, James Munday, Ingrid Pelisoli, Daniel Perley, Eric C. Bellm, Elm 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. Riddle, Ben Rusholme, and Dave I Sahman, July 19, 2023, Nature.
DOI: 10.1038/s41586-023-06171-9

The study was funded by Caltech’s Walter Burke Institute for Theoretical Physics, the European Research Council, the Leverhulme Trust and the UK’s Science and Technology Facilities Council.

Remarks from NASAs Neils Gehrels Swift Observatory, renamed in honor of Gehrels, a Caltech alumnus (PhD 82) who passed away in 2017, were also used in the study to help narrow down the object’s temperature to a searing 35,000 Kelvin (about 35,000 degrees Celsius).

Caltechs ZTF is funded by the National Science Foundation and an international collaboration of partners. Additional support comes from the HeisingSimons Foundation and Caltech. ZTF data is processed and stored 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.

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