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Astronomers discover two-faced white dwarf

by News Staff

In a groundbreaking discovery, astronomers have encountered an extraordinary member of the white dwarf family, the burnt-out cores of deceased stars. This celestial object exhibits a fascinating peculiarity: it is two-faced. One side of the white dwarf’s surface is composed of hydrogen, while the other side consists of helium.

Ilaria Caiazzo, a leading postdoctoral scholar at Caltech, shared the astonishing findings of the study published in the journal Nature, stating, “The surface of the white dwarf completely changes from one side to the other, leaving people astounded when they see the observations.”

White dwarfs are the remnants of stars that once resembled our sun. As these stars age, they expand into red giants until their outer layers are shed, leaving behind dense and intensely hot white dwarfs. Even our sun is destined to follow this evolution, transforming into a white dwarf in approximately 5 billion years.

The recently discovered white dwarf earned the nickname “Janus,” inspired by the two-faced Roman god of transitions. The Zwicky Transient Facility (ZTF), an instrument located at Caltech’s Palomar Observatory near San Diego, initially detected this enigmatic object during its nightly scans of the skies.

Ilaria Caiazzo and her team had been on the lookout for highly magnetized white dwarfs like the previously identified ZTF J1901+1458, utilizing the ZTF. However, one candidate stood out due to its rapid changes in brightness. Intrigued by this anomaly, Caiazzo conducted further investigations using the CHIMERA instrument at Palomar, in addition to HiPERCAM on the Gran Telescopio Canarias located in Spain’s Canary Islands. These data confirmed that Janus rotates on its axis every 15 minutes.

The discovery of this unique and mesmerizing two-faced white dwarf opens up new avenues for understanding the complexities of these celestial remnants and provides an unprecedented glimpse into the cosmic wonders that surround us.

This artist’s animation shows the two-faced white dwarf nicknamed Janus rotating on its axis. The blue-tinted dead cinder of a star, which 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 peculiar double-faced nature of this white dwarf might be due to the interplay of magnetic fields and convection, or a mixing of materials. On the helium side, which appears bubbly, convection has destroyed the thin hydrogen layer on the surface and brought up the helium underneath. The white dwarf’s rotation has been sped up in this animation; normally, it rotates around its axis every 15 minutes. Credit: K. Miller, Caltech/IPAC

Atop Maunakea in Hawaiʻi, astronomers continued their investigation of the extraordinary two-faced white dwarf using the W. M. Keck Observatory. Employing a powerful spectrometer, they spread the white dwarf’s light into a mesmerizing rainbow of wavelengths that carry unique chemical signatures. The data they gathered provided a stunning revelation: when one side of the white dwarf was observable, it exhibited the presence of hydrogen but no signs of helium, and vice versa when the other side came into view.

This puzzling phenomenon left the team perplexed, prompting them to develop various theories to explain the drastically different faces of this lone white dwarf in space. While they remain uncertain, they have proposed some plausible explanations. One intriguing idea is that Janus might be undergoing a rare phase of white dwarf evolution.

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

The enigmatic nature of Janus continues to captivate astronomers, urging them to delve deeper into the mysteries of white dwarf evolution. The possibility of witnessing such a unique cosmic event adds a sense of wonder and excitement to the field of astronomy, as it unravels the complexities of these celestial objects and unveils the hidden secrets of the universe.

Scientists think that magnetic fields may explain the unusual two-face appearance of the white dwarf nicknamed Janus. One side of the dead star’s surface is composed primarily 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 to have caused their uneven distribution. The white dwarf’s rotation has been sped up in this animation; normally, it rotates around its axis every 15 minutes. Credit: K. Miller, Caltech/IPAC

Once white dwarfs form, a natural process causes their heavier elements to sink to the core, while the lighter elements, including hydrogen, float to the surface. As these white dwarfs cool down over time, it is expected that the materials will eventually mix together, leading to a more uniform composition. However, in some instances, the hydrogen may be pushed into the interior and become diluted, causing helium to become more dominant. Janus, the two-faced white dwarf, may exemplify this transitional phase, but what remains puzzling is why this transition occurs in a disjointed manner, with one side evolving differently from the other.

The science team speculates that magnetic fields could hold the key to unraveling this enigma. Magnetic fields surrounding cosmic bodies tend to be asymmetrical, with one side being stronger than the other. These magnetic fields can hinder the mixing of materials within the white dwarf. As a result, if one side of the white dwarf possesses a stronger magnetic field, it will experience less mixing, leading to a higher concentration of hydrogen on that side.

Alternatively, another theory proposed by the team suggests that magnetic fields influence the pressure and density of the atmospheric gases on the white dwarf. This, in turn, could affect how the materials mix and distribute across the surface, resulting in the distinct division of hydrogen and helium on the two faces of Janus.

While these theories offer plausible explanations, the intricacies of white dwarfs and the role of magnetic fields in their evolution remain subjects of ongoing research and fascination for astronomers. Janus’s peculiar characteristics continue to intrigue scientists and ignite curiosity about the diverse and captivating phenomena present in our vast universe.

Credit: California Institute of Technology

“The presence of magnetic fields could result in lower gas pressures in the atmosphere, leading to the formation of a hydrogen ‘ocean’ where the magnetic fields are strongest,” reveals co-author James Fuller, a professor of theoretical astrophysics at Caltech. While uncertainty remains regarding which theory is accurate, the team finds it difficult to explain the asymmetrical sides of Janus without considering the influence of magnetic fields.

To unravel this celestial mystery, the team aims to discover more white dwarfs similar to Janus using the sky survey capabilities of ZTF. As Caiazzo explains, “ZTF excels at detecting peculiar objects,” making it an invaluable tool in their quest. Additionally, future surveys, including those conducted by the Vera C. Rubin Observatory in Chile, are expected to enhance the detection of variable white dwarfs, further advancing their understanding of these enigmatic cosmic entities. The pursuit of Janus-like white dwarfs promises to provide invaluable insights into the mechanisms governing their evolution and the roles magnetic fields play in shaping their intriguing characteristics.

Source: California Institute of Technology

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