An international team of astronomers has directly detected four nearby white dwarfs that had remained hidden in binary star systems, though their presence had been posited for decades. The study used observations from the Hubble Space Telescope (HST) and the Swift Observatory to confirm the presence of white dwarfs in four binary systems within 20 parsecs (about 65 light-years) of Earth. The research, published today in the Monthly Notices of the Royal Astronomical Society, provides an important new window into the complex interactions that shape binary stars over billions of years.
White dwarfs are the dense remnants left behind when stars similar to our Sun exhaust their nuclear fuel. The white dwarfs were each found in a type of binary system—called “post-common envelope”—in which each white dwarf is paired with a low-mass red dwarf star. The pairs are known as post-common envelope binaries because they have survived a period in which one star expanded and engulfed its companion in a shared envelope of gas, before becoming the white dwarf seen today. Astronomers believe this process plays a critical role in shaping the evolution of many binary star systems, but many details remain poorly understood.
The team, led by Mairi O’Brien, research fellow at the University of Warwick, UK, included second author David J. Wilson and J. Sebastian Pineda, research scientists at the University of Colorado Boulder’s Laboratory for Atmospheric and Space Physics. “Nearby isolated white dwarfs are usually easy to find, but we couldn’t see these four stars directly in visible wavelengths because their red dwarf companions were drowning out their light,” O’Brien said in a statement. “It’s a reminder that even in our own cosmic neighborhood, we can still find surprises if we look in the right way, at the right wavelengths.”
To study the systems, researchers used one of the ultraviolet spectrographs aboard Hubble to separate the faint light of the white dwarfs from their much brighter red dwarf companions. This allowed the team to directly detect and characterize the white dwarfs for the first time.
The observations showed that the white dwarfs have temperatures ranging from about 5,300 to 6,300 Kelvin (about 9,000 to 11,000 degrees Fahrenheit). At these relatively cool temperatures the white dwarf barely contributes to the combined spectrum of the two stars, meaning that Hubble spectroscopy is vital to disentangle the two signals. The results also revealed that estimates based only on photometric measurements differed by 5 to 8 percent, highlighting the value of direct spectroscopic observations.
Nearly 30 years ago, astronomers first proposed that one of the systems, named G 203-47, contained a white dwarf, which the new Hubble observations have now confirmed, making it the ninth-closest known white dwarf to the Sun.
Analysis led by Wilson using observations from the Swift X-ray telescope revealed that G 203-47 emits far less X-ray radiation than expected, which emerged as one of the study’s most surprising findings. In close binary systems, magnetic activity typically produces strong X-ray emissions, particularly when stars are rotating rapidly. “The unexpectedly weak X-ray signal suggests that the red dwarf companion is rotating much more slowly than the binary orbital period, unlike similar systems,” Wilson said.
The analysis indicated the star may take more than 100 days to complete a rotation even though it orbits the white dwarf every 14.9 days, making it a rare example of a system of this type that is not tidally locked. “What’s fascinating is that G 203-47 shouldn’t be rotating this slowly if it formed the same way as similar systems,” Wilson said. “This suggests that these binaries have had very different evolutionary histories. Some underwent violent, prolonged interactions early on that locked them tidally. Others, like G 203-47, experienced gentler, briefer encounters that left them in this unusual state.”
Beyond the discovery of the individual systems, the findings are also helping to complete the census of white dwarfs in the Sun’s local neighborhood. “If we want to understand how stars form and evolve,” Wilson said, “we have to know how many of each type there are.”
By Sara Pratt, LASP Sr. Communications Specialist
Founded a decade before NASA, the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder (LASP) is revolutionizing human understanding of the cosmos. LASP is deeply committed to inspiring and educating the next generation of space explorers. From the first exploratory rocket measurements of Earth’s upper atmosphere to trailblazing observations of every planet in the solar system, LASP continues to build on its remarkable history with a nearly $1 billion portfolio of new research and engineering programs.
