But even the nearby cosmos had its secrets. New research led by astronomers at the University of Warwick and the University of Colorado Boulder uncovered four nearby “hidden” neighboring white dwarf stars, galactically locked into orbit with their brighter red dwarf companions just 65 light-years from Earth. Among them is G 203-47, which is now confirmed to be the ninth-closest white dwarf star to the Sun.
White dwarfs are the dense remnants of stars like our Sun, usually easy to spot in isolation. But in these systems, their faint glow was drowned out by the brilliance of their red dwarf partners.
“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,” explained Dr Mairi O’Brien of Warwick. “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.”
The finding builds on decades of surveys that had detected only subtle radial wobbles, tiny motions back and forth in the red dwarfs, suggesting they harbor unseen companions. By carefully calibrating against the flaring activity of red dwarfs, which can mimic such signals, the team confirmed the presence of white dwarfs using the ultraviolet spectrograph of NASA’s Hubble Space Telescope.
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But G 203-47 was particularly puzzling. Though only 25 light-years away, its companion white dwarf remained elusive for 27 years after the wobble was first detected. Even more oddly, while it takes its red dwarf 100+ days to rotate once, the two stars were orbiting a white dwarf every 14.9 days. Normally, tidal forces would synchronize the two, like Earth’s Moon always showing the same face.
But here, the red dwarf spins far too slowly. “What’s fascinating is that G 203-47 shouldn’t be rotating this slowly if it formed the same way as similar systems,” said Dr David Wilson of the University of Colorado Boulder. “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.”
The discovery updates the local white dwarf census within 20 parsecs (65 light-years). Models had predicted four to five such close white dwarf–red dwarf pairs should exist, and the team found exactly four, matching theory. Yet the search is far from over.
“Only about 30 percent of red dwarfs within 20 parsecs have been systematically surveyed for hidden white dwarf companions,” noted Professor Pier-Emmanuel Tremblay of Warwick. “We think there could be as many as 9 or 10 additional binary systems in our local stellar environment that we haven’t found yet. If we put more targeted effort into observing red dwarfs, perhaps we will find more surprises like this.”
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This is a work that reminds us that even in the “quiet” corners of our galaxy, the stars still have secrets to share: companions hidden in plain sight; strange and rapid rotations; evolutionary genealogies etched or written across timescales far beyond most observers’ ability to see with the naked eye.
Journal Reference:
Mairi W O’ Brien, David J Wilson, Pier-Emmanuel Tremblay, et al. Direct detections of white dwarfs in four WD+dM post-common envelope binaries within 20 pc. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stag1195