These Double White Dwarf Binary Star Systems Should Not Exist – But Scientists Just Discovered 98 of Them
A team of researchers working at the Center for Astrophysics | Harvard & Smithsonian has discovered almost 100 double white dwarf binary star systems that technically shouldn't exist.
"The universe isn't old enough to make such low mass white dwarfs on their own, and yet, here they are," marveled Dr. Warren Brown, an astronomer at the Center for Astrophysics and the study's lead author, speaking in the official release.
After over a decade of painstaking research, the scientists finally found out why.
"We targeted candidate low mass white dwarf stars and found that they are all ultra-compact binaries," Brown explained. "They have companions in close orbits. The universe can't make a low mass white dwarf unless it's part of a compact binary."
White dwarf stars are what stars like our Sun will turn into once they have burned through all of their nuclear fuel. When that happens, the star puffs up into what's called a "red giant," then collapses under its own weight to form the tiny, dense star we call a white dwarf.
"It makes sense…The stars we studied lost so much of their mass during their evolution that they ended up as a low mass white dwarf," said Brown.
There are probably a lot more of them, too:
"The models estimate there's an order of a hundred million white dwarf binaries in our galaxy," added Dr. Mukremin Kilic, a study co-author from the University of Oklahoma. "We've found and confirmed 100 of them. Our observations can anchor the models for future surveys, and allow us to observe a specific subset of white dwarfs and cut through the population."
Next, researchers plan to use the Laser Interferometer Space Antenna (LISA) gravitational wave observatory (which is currently slated to launch in 2034) to search for more double white dwarf binary star systems.
"There are things you can do if you have sources with both light and gravity waves," Brown explained. "With light we can measure temperature, distance, velocity, but we don't measure mass directly; gravity-wave measurements measure mass."
As for these double white dwarf binaries, nobody much about them yet – except that they definitely shouldn't be possible based on the age of the universe, they definitely still exist anyway, and that there are potentially hundreds of millions more of them out there.
"The traditional response to these binaries was to call them supernova progenitors. Someday they will merge together and become something else, and it's unclear what," Brown considered.
"If there's one thing we know for certain, it is that the stars we've listed in the survey will be great sources for the LISA mission and for future white dwarf star and gravitational wave studies; they are gravity wave sources, they are the signature multi-messenger systems of the future," concluded Brown.
The team's research has been published in The Astrophysical Journal.
Cover Image: Artist's conception of extremely low mass detached double white dwarf binary. Credit: Melissa Weiss.