Unprecedented Discovery of a “Two-Faced” White Dwarf Leaves Astronomers in Awe

For the inaugural time, astronomers have stumbled upon a peculiar white dwarf showcasing two distinct faces — one comprising of hydrogen and the other, helium. This find stimulates fresh hypotheses regarding white dwarf evolution and the role magnetic fields play in the shaping of heavenly bodies. Image Courtesy: K. Miller, Caltech/IPAC

An anomalous white dwarf has been found to be composed of hydrogen on one side and helium on the other.

This groundbreaking discovery introduces a new dimension to the understanding of white dwarfs — the remnants of extinguished stars. Astronomers have found that a unique member of this cosmic species bears two distinct faces. One facet of the white dwarf is hydrogen-based, while the other side consists of helium.

“The white dwarf’s surface experiences a dramatic transformation from one side to the other,” remarks Ilaria Caiazzo, a postdoctoral scholar at Caltech who leads a new study on this discovery, published in the journal Nature. “People are astonished when I present the observations.”

White dwarfs represent the burning residues of stars that once resembled our sun. As these stars age, they expand into red giants, eventually losing their outer soft material, and their cores compact into dense, blazing white dwarfs. Our sun is predicted to evolve into a white dwarf approximately 5 billion years from now.

Illaria Caiazzo, a postdoctoral scholar and research associate in Astronomy, reveals how her team used ZTF to uncover an extraordinarily atypical “two-faced” white dwarf — one side entirely composed of helium and the other of hydrogen. Image Courtesy: Caltech

The recently discovered white dwarf, informally termed Janus after the dual-faced Roman god of transition, was initially spotted by the Zwicky Transient Facility (ZTF), an instrument that daily scans the skies from Caltech’s Palomar Observatory near San Diego. Caiazzo was in pursuit of highly magnetized white dwarfs, like the object named ZTF J1901+1458, which her team had previously identified using ZTF. One potential object distinguished itself with its swift changes in brightness, prompting Caiazzo to conduct a more thorough investigation with the CHIMERA instrument at Palomar and HiPERCAM at the Gran Telescopio Canarias in Spain’s Canary Islands. These data verified that Janus rotates on its axis every 15 minutes.

Further observations at the W. M. Keck Observatory atop Maunakea in Hawaiʻi revealed the extraordinary double-faced nature of the white dwarf. The team deployed a spectrometer to split the light of the white dwarf into a spectrum of wavelengths that carry chemical fingerprints. The data unveiled the presence of hydrogen when one side of the object was visible (with no indications of helium), and purely helium when the other side came into view.

This artist’s impression displays the two-faced white dwarf informally named Janus. The blue-hued burnt-out star, which was once akin to our sun, is primarily composed of hydrogen on one side and helium on the other (the hydrogen side appears brighter). The bizarre double-faced characteristic of this white dwarf could be the result of the interaction between magnetic fields and convection, or a commingling of materials. On the helium side, which appears to be bubbly, convection has annihilated the thin hydrogen layer on the surface, bringing the underlying helium to the surface. Image Courtesy: K. Miller, Caltech/IPAC

What could lead to a white dwarf isolated in space showcasing such drastically different faces? The team admits they are perplexed but have proposed a few potential theories. One suggestion is that we might be witnessing Janus undergoing an uncommon phase of white dwarf evolution.

“Not all, but some white dwarfs transition from being hydrogen-dominated to helium-dominated on their surface,” Caiazzo explains. “We might have possibly caught one such white dwarf in the act.”

After the formation of white dwarfs, their heavier elements descend to their cores and the lighter elements — with hydrogen being the lightest — float to the top. However, over time, as the white dwarfs cool, these materials are believed to intermix. In certain instances, the hydrogen is blended into the interior, diluting it to the extent that helium becomes more dominant. Janus could be representing this transitional phase, but one burning question remains: why is this transition occurring in such an irregular way, with one side evolving prior to the other?

According to the scientific team, the answer might reside in magnetic fields.

“Magnetic fields around cosmic bodies are generally asymmetric, or stronger on one side,” explains Caiazzo. “Magnetic fields can obstruct the mixing of materials. Hence, if the magnetic field is stronger on one side, then that side would experience lesser mixing and thus contain more hydrogen.”

Another theory suggested by the team to explain the double faces also depends on magnetic fields. In this case, the fields are believed to alter the pressure and density of the atmospheric gases.

“The magnetic fields could potentially reduce gas pressures in the atmosphere, permitting a hydrogen ‘ocean’ to form where the magnetic fields are strongest,” states co-author James Fuller, professor of theoretical astrophysics at Caltech. “We are unsure which of these theories is accurate, but we cannot conceive any other way to explain the asymmetric sides without magnetic fields.”

To unravel this mystery, the team aims to discover more Janus-like white dwarfs with the help of ZTF’s sky survey. “ZTF excels at identifying peculiar objects,” says Caiazzo. Future surveys, such as those to be carried out by the Vera C. Rubin Observatory in Chile, she believes, should facilitate the finding of variable white dwarfs.

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, J. J. Hermes, Jeremy Heyl, Adela Kawka, S. R. Kulkarni, Thomas R. Marsh, Przemek Mróz, Thomas A. Prince, Harvey B. Richer, Antonio C. Rodriguez, Jan van Roestel, Zachary 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. Riddle, Ben Rusholme and Dave I. Sahman, 19 July 2023, Nature.
DOI: 10.1038/s41586-023-06171-9

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

Data from NASA’s Neils Gehrels Swift Observatory—renamed after Gehrels, a Caltech alumnus (PhD ’82) who passed away in 2017—were also incorporated in the study to help ascertain the object’s temperature, an infernal 35,000 Kelvin (equivalent to roughly 35,000 degrees Celsius).

Caltech’s ZTF is financially

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More about Two-Faced White Dwarf Discovery

• Caltech
• Zwicky Transient Facility (ZTF)
• W. M. Keck Observatory
• Vera C. Rubin Observatory
• Research paper in Nature journal