White dwarf stars may collide to create a new type of amazing, “inverted” star, according to two new studies published this week.
A team of astronomers have identified two small bright stars called hot subdwarfs that had as yet invisible makeup. Another research team found a mechanism to explain how such stars could have formed.
The two strange stars were “first of their kind” that have been identified and must be extremely rare, he says. Marcela Miller Bertalami, astrophysicist from the Institute of Astrophysics in La Plata, Argentina, who headed theoretical research on how such stars can form, published in Monthly reports of the Royal Astronomical Society.
White dwarfs are slowly cooling cores of dead stars. Hot sub-dwarfs are a fairly rare phenomenon, old stars that burn four times warmer than the surface of the Sun, and, unlike our Sun, fuse helium in their nuclei instead of hydrogen, says Miller Bertalami.
Most three-quarters of the stars are hydrogen, one-quarter helium and a small fraction of other elements, says Miller Berthalam. A star first fills its lightest elements, so it fills helium only if it has already burned hydrogen, or if the force of attraction of another object has distracted hydrogen from the star.
But the two newly discovered stars did not resemble other subdwarfs that burn helium, as noted by a group of astronomers, according to a published report in the same magazine this week. The team, led by Klaus Werner, an astronomer at the Center for Astrophysics and Elementary Particles in Germany, realized it had seen not only a star with a helium-rich surface, but also a star rich in carbon and oxygen.
“It’s an extremely, extremely rare phenomenon,” says Miller Bertalami. Typically, a star creates carbon and oxygen by burning helium in its core, and these elements will not be visible on the surface. But the team found that about 40 percent of the star’s surface is made up of carbon and oxygen.
“You need to find a way to bring all that carbon and oxygen to the surface, and it’s not easy,” says Miller Bertalam.
Werner’s team, which already knew Miller Bertali, turned to him to try to figure out how these strange objects could have formed. Miller Bertali’s team was already working on a similar project and was able to show how such an amazing star could have formed.
Miller Bertalami and his colleagues found that a heavier white dwarf, rich in helium, under the right conditions can interact with a lighter white dwarf, rich in carbon and oxygen. Together this pair could create a hot dwarf that combines the materials of both.
These two stars would exist in binary – meaning that they revolve around each other. Over time, they emitted gravitational waves and spiraled toward each other until they met, says Miller Berthalam. In the process, they will come close enough that tidal forces break one or both of them.
Typically, a more massive star destroys a less massive one and cannibalizes the material of a lighter star. In Miller Bertali’s model, a helium white dwarf destroyed a lighter carbon-oxygen dwarf, so the surviving star found itself with carbon and oxygen sprayed all over the surface.
White dwarfs used to be dead because they no longer merged atoms into heavier elements for energy. But the merger between them has resumed the merger in a new, “revived” star – the dwarf, says Miller Bertalami.
The two studies form a “pleasant connection between theory and observation.” Warren Brown, Astrophysicist Center for Astrophysics | Harvard and the Smithsonian, who were not involved in any of the studies. “The measurements are pretty simple,” he says, and the theoretical explanation seems like a “great solution” to the puzzle with observations.
What’s surprising, though, is that these stars are “kind of inverted,” Brown says. The heavy elements that scientists expected to create in their nuclei – carbon and oxygen – appear on the surface, while the nuclei are full of light helium.
“It’s physically possible, so it has to happen,” Brown says. The question is how often? Once astronomers get a large sample, hundreds or at least dozens of stars, they can begin to find out how common these types of stars are in the galaxy.
Although this observation is unprecedented, astronomers may soon have new tools to detect such stellar events. The Laser Interferometer Space Antenna (LISA) and other next-generation gravitational wave detectors will be able to capture tens of thousands of stellar doubles in our galaxy, Brown says. The fusion of such white dwarfs will emit gravitational waves and offer a different lens through which to study them.