The strange white dwarf star has two faces

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Astronomers have made a first-of-its-kind discovery – a white dwarf star with two completely different faces.

White dwarfs are the remnants of dead, burnt stars. Our sun will become a white dwarf around us 5 billion years After it swells into a red giant star, it blows out its outer matter and, with only the core remaining, shrinks back into a white-hot remnant.

The newly discovered white dwarf has two faces, one made of hydrogen and the other made of helium. Researchers named the star Janus after the two-faced Roman god of transition. A detailed study of the results is published July 19 in the journal Nature nature.

“The surface of the white dwarf shifts completely from one side to the other,” said lead study author Ilaria Caiazzo, a postdoctoral researcher in astronomy at Caltech, in a statement. “When I show people the notes, they’re dumbfounded.”

White dwarfs are incredibly dense, compressing a similar mass to our Sun into something equivalent to an Earth-sized planet.

The strong gravitational effect during a star’s death means that the remaining heavy elements move toward the center while lighter elements such as hydrogen or helium rise to the upper layer. Given the scorching temperatures of white dwarfs, the hottest ones have hydrogen atmospheres. As stars cool over time, they tend to have helium atmospheres.

But typical white dwarfs don’t have one side of the star dedicated to one element, and the other dominated by the other.

The unusual stellar remnant was first detected by the Zwicky Transit Facility, located at Caltech’s Palomar Observatory. Caiazzo used the instrument, which scans the sky every night, to make a recent survey of highly magnetized white dwarfs when an object rapidly changing in brightness appeared.

Follow-up observations were made by Caiazzo and her team using Palomar’s CHIMERA instrument, HiPERCAM located at Gran Telescopio Canarias in Spain’s Canary Islands and WM Keck Observatory in Maunakea, Hawaii.

All three observatories showed that Janus rotated on its axis every 15 minutes – and showed the nature and composition of the double star. Astronomers used a spectrometer to separate the white dwarf’s light into different wavelengths, which revealed the chemical signature of hydrogen on one side and helium on the other.

The star has a temperature of 62,540 degrees Fahrenheit (34,726 degrees Celsius), which the researchers determined with help from the Neil Gehrels Swift Observatory.

Researchers aren’t entirely sure why a star has two very different sides. It is possible that Janus is undergoing a rare form of evolution.

“Not all, but some white dwarfs transition from being hydrogen to helium dominating their surface,” Kiazo said. “We may have caught a white dwarf in the act.”

As a white dwarf cools over time, heavier and lighter materials may mix together. During this transition, it is possible for hydrogen to become diluted inside, allowing helium to become the dominant element.

If this is happening in Janus, then one side of the star evolves before the other side.

“The magnetic fields around cosmic bodies tend to be asymmetric, or stronger on one side,” Kaizu said. Magnetic fields can prevent mixing of materials. So, if the magnetic field is stronger on one side, that side will have less mixing and therefore more hydrogen. ”

Another possibility is that magnetic fields shift the pressure and density of these atmospheric gases on Janus.

“Magnetic fields may lead to lower gas pressures in the atmosphere, and this may allow hydrogen oceans to form where magnetic fields are strongest,” study co-author James Fuller, professor of theoretical astrophysics at Caltech, said in a statement. “We don’t know which of these theories is correct, but we can’t think of any other way to explain the asymmetric aspects without magnetic fields.”

The team will continue to search for more white dwarfs like Janus using the Zwicky Transient Facility because the instrument is “very good at finding strange objects,” Kayazu said.