Until recently, a Caltech postdoctoral astronomy researcher Ilaria Caiazzo it was just hunting the skies for the bodies of dead stars. She did not expect to discover an exotic white dwarf with two faces.
“It was an amazing discovery,” says Caiazzo Inverse. “We didn’t expect to find something like this. It was not predicted. It’s a new class [stellar] object.”
White dwarf stars are stellar remnants, the bright, hot remnants of a star that has exhausted its supply of thermonuclear fuel and blown most of its mass out into space, leaving the former dense core of the old star. Instead, they glow weakly with residual heat and cool slowly over time, usually exhibiting a composition of hydrogen or helium.
Apart from the white dwarf ZTF J203349.8+322901.1. While Caiazzo was searching for signs of white dwarfs that may have recently merged (dim stars that have rotated, magnetized and changed brightness), she found ZTF J203349.8+322901.1 – and gave noticed something strange. It appeared to have a surface composed of hydrogen on one side, and helium on the other.
“I found it because it fluctuates up and down in luminosity and because it looks like a white dwarf so I thought it was one of those things,” Caiazzo says. “Then I took a spectrum and analyzed it more and realized it’s a whole different class of object.”
I new paper published Wednesday in the journal nature, of which Caiazzo is the lead author, she and her colleague nicknamed ZTF J203349.8+322901.1 “Janus,” after the Roman god of doors and transitions; the two-faced god.
It is a result that could change the way we understand the afterlife of relatively large stars, including our Sun, which is destined one day to take its place among the odd white ghosts of stars across the sky.
Looking for the face of Janus the white dwarf
The star Janus first appeared on Caiazzo’s radar when he looked at data from the Zwicky Transient Facility (the ZTF in the official name of Janus), which scans the night sky from the Palomar Catletch Observatory in California.
Additional observations from Palomar showed that the star Janus rotates every fifteen minutes. Additional observations were made with the Keck Observatory in Hawaii using a spectrograph, an instrument that can identify chemical compounds in light based on the way they absorb different aspects of different wavelengths, which showed the unique two-faceted nature of the star.
“I could take one spectrum every minute as it rotates, and it completely changes from showing only hydrogen to helium online,” Caiazzo says. “When people look at these spectrums, they are completely different. I’ve been talking about this thing with a lot of people and showing the plots and it’s a little bonkers.”
According to Caiazzo white dwarfs have three important characteristics:
The first is that they are the dead cores of former stars, meaning that no nuclear fusion takes place within them – only the residual heat keeps them luminous. The second peculiarity is that they are strongly magnetized, with stable and structured magnetic fields.
And the third peculiarity is that they are very close.
“This one is a bit bigger than our Sun – but it’s incredibly compact. The radius, I think, is about 3000 kilometers,” says Caiazzo. “And so the center of gravity on the surface is very high.” (For reference, the radius of Mars is about 3389 kilometers.)
This strong gravity causes elements in a white dwarf to line up vertically, so that heavier elements sink, and lighter ones rise. So for many white dwarfs, spectrographic observations show hydrogen surfaces, but this is likely just a thin veneer floating on top, Caiazzo says, with most of the former star’s hydrogen being blown into the space when he died.
But other white dwarfs show only helium surfaces, and those white dwarfs are always 30,000 degrees kelvin (about 53,500 degrees Fahrenheit) or cooler in temperature.
“What happens at 30,000 degrees is that the helium layer becomes very convective near the surface,” Caiazzo says, and the turbulence breaks up the hydrogen layer and mixes it with the helium. “The hydrogen is completely diluted so you don’t see it in the spectrum anymore.”
It is possible that it captured the Janus white dwarf as it transitioned from a hotter hydrogen-dominated white dwarf, to a cooler helium-dominated white dwarf, since Janus is currently estimated to be 35,000 degrees kelvin. However, it’s unclear how long that transition should take – Caiazzo has been observing Janus for more than seven years now and it remains stable – and why it wouldn’t occur across the white dwarf’s entire surface at the same time.
Two theories for a double-faced star
One theory is that Janus’ magnetic field is aberrant or uneven in some way, so that it is blocking the convection currents on one side of the white dwarf. This would allow a thicker-than-usual layer of hydrogen to accumulate, a sort of “hydrogen ocean,” to form on one hemisphere of Janus, an idea put forward by one of Caiazzo’s co-authors on the new paper, Caltech theoretical physicist James Fuller.
But that hydrogen ocean probably wouldn’t survive the convection currents inside a white dwarf when it cooled below 30,000 degrees Kelvin. Finding another Janus-type white dwarf that is 30,000 degrees or colder would dispel the hydrogen ocean hypothesis.
The other theory detailed in the paper by Caiazzo and her colleagues also draws on magnetic fields, this time suggesting that a weak field could block convection on one hemisphere of a white dwarf while preserving the thin layer of hydrogen floating on top. But this theory would only hold if the Janus white dwarf is very close to the 30,000 degree kelvin mark. Too hot, and they remain under the control of hydrogen, too cold, and convection will become too powerful and will mix and hide any hydrogen that was on the surface.
Either way, “it’s this interaction between the very strong gravity on the surface of the white dwarf, magnetic fields and convection, that can create this type of object,” Caiazzo says.
The next step for Caiazzo and her colleagues is to continue looking for more of these unusual white dwarfs. One earlier observation found at least one white dwarf with a similar, though less extreme, composition, and now that she knows what to look for, Caiazzo is already finding new candidates.
“We know about 500,000 white dwarfs, so it’s a long search,” she says. But it’s a search that will inspire future spectrographic surveys, such as the still-under-construction Vera Rubin Observatory, scanning the spectra of millions of objects in the night sky.
“If I see that there is at least a little bit of helium and hydrogen in the spectrum, I can narrow the search, finding more of these objects,” Caiazzo says. “But I’ve already found enough.”