Unusual two-faced white dwarf star discovered with help from W. M. Keck Observatory on Big Island’s Maunakea

There’s a dead star out there in the universe that is double-dealing with its composition. Nothing nefarious, but it is a first for the cosmic family of stars to which it belongs.

Nicknamed Janus, after the the two-faced Roman god of transition, the newfound white dwarf has one side composed of hydrogen and the other made up of helium.

“The surface of the white dwarf completely changes from one side to the other,” said Ilaria Caiazzo, a postdoctoral scholar at Caltech and lead author of a new study detailing the discovery. “When I show the observations to people, they are blown away.”

The team of astronomers behind the study used data from Zwicky Transient Facility at Caltech’s Palomar Observatory in San Diego, Calif., and the W. M. Keck Observatory atop Maunakea on the Big Island. The study was published in today’s online edition of the journal Nature.

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

Janus was initially discovered by the Zwicky Transient Facility, which scans the skies every night. Caiazzo was searching for highly magnetized white dwarfs and came upon one candidate — Janus — that stood out for its rapid changes in brightness. So, Caiazzo decided to investigate further.

Using the Caltech HIgh-speed Multi-color camERA instrument at Palomar as well as the camera HiPERCAM on the Gran Telescopio Canarias in Spain’s Canary Islands, Caiazzo was able to confirm that Janus is rotating on its axis every 15 minutes.

Subsequent observations made with Keck Observatory revealed the dramatic double-faced nature of the white dwarf.

The team used the Low Resolution Imaging Spectrometer on the Keck I Telescope to view Janus in optical wavelengths, which is light our eyes can see, as well as the Near-Infrared Echellette Spectrograph on the Keck II Telescope to observe the white dwarf in infrared wavelengths. The data revealed the dead star’s chemical fingerprints, which showed 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 swung into view.

What would cause a white dwarf floating alone in space to have such drastically different faces? The team acknowledges they are baffled, but they have some possible theories.

One idea is that Janus might be undergoing a rare phase of white dwarf evolution.

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

After white dwarfs are formed, their heavier elements sink to their cores. Lighter elements — hydrogen being the lightest of all — float to the top. With time, as a white dwarf cools, the materials are thought to mix together. In some cases, the hydrogen is mixed into the interior and diluted such that helium becomes more prevalent.

Janus could embody this transition phase, but one pressing question is why is the transition happening in such a disjointed way, with one side evolving before the other?

The answer, according to the science team, might be in magnetic fields.

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

Another theory proposed by the team also depends on magnetic fields. In this scenario, however, the fields are thought to change the pressure and density of the white dwarf’s atmospheric gasses.

“The magnetic fields may lead to lower gas pressures in the atmosphere, and this may allow a hydrogen ‘ocean’ to form where the magnetic fields are strongest,” said study 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 the asymmetric sides without magnetic fields.”

To help solve the mystery, the team hopes to find more Janus-like white dwarfs with Zwicky Transient Facility’s sky survey.

“[Zwicky Transient Facility] is very good at finding strange objects,” Caiazzo said.

Future surveys, such as those to be performed by the Vera C. Rubin Observatory in Chile, she added, should make finding variable white dwarfs even easier.