Astronomers have spent decades cataloguing the stars closest to the Sun, on the reasonable assumption that the nearby sky holds few surprises left. Four of them turned up anyway.
A team from the University of Warwick and the University of Colorado Boulder has confirmed white dwarfs — the cooling cores left behind when stars like the Sun run out of fuel and collapse — orbiting four red dwarf stars, all within 65 light-years of Earth. Because a red dwarf is larger and brighter, each pair had looked like a single star in visible light. Writing in Monthly Notices of the Royal Astronomical Society, the researchers pinned down the hidden companions in G 203-47, GJ 207.1, LHS 1817 and Wolf 1130.
"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," said Mairi O'Brien of Warwick, the study's first author.
What gave them away was motion. Each visible star showed a pronounced radial wobble — a slight rocking toward and away from Earth that betrays the gravitational tug of something unseen. The team then turned the Hubble Space Telescope's imaging spectrograph on the systems in ultraviolet, where a white dwarf ought to stand out. The complication is that red dwarfs flare violently, and a flare can counterfeit the ultraviolet signature of a white dwarf. Custom calibration separated the real signals from the noise.
The stubborn one
G 203-47 is the strangest of the four, and the nearest: 25 light-years away, and now ranked the ninth-closest white dwarf to the Sun. Twenty-seven years passed between the first sign of its wobble and the identification of what was causing it. Its red dwarf circles the white dwarf every 14.9 days but takes more than 100 days to turn once on its own axis — so the two are not tidally locked, as the Earth and Moon are, and as similar pairs generally become.
"G 203-47 shouldn't be rotating this slowly if it formed the same way as similar systems," said co-author David Wilson of Colorado Boulder. Some such binaries, he suggested, went through violent, drawn-out interactions that locked them together; others had gentler, briefer encounters.
The tally matters beyond the four stars themselves. Population models had predicted roughly four to five closely orbiting white dwarf and red dwarf pairs in this volume of space, and four is what emerged — an unusually clean match between theory and a headcount. The survey is also far from finished: only about 30 percent of red dwarfs within 20 parsecs have been systematically checked for hidden companions, which suggests the Sun's neighbourhood still has things left to give up.