Space within 65 light-years of Earth is considered well surveyed. Even so, researchers have now directly detected four stars there that had escaped observation for decades: white dwarfs lost in the glare of their companions.
White dwarfs are the dense, hot remnants of stars that have burned through their fuel and shed their outer layers. Standing alone, they are comparatively easy to find. But each of the four newly confirmed objects shares its system with a red dwarf — a smaller, cooler star that is nonetheless far brighter in visible light and simply outshines its dead partner. From outside, the systems looked like single stars.
The team around Mairi O'Brien of the University of Warwick, working with researchers at the University of Colorado Boulder, singled out the four systems because they showed a pronounced radial wobble: a slight rocking back and forth that points to a massive unseen companion. Using an ultraviolet spectrograph aboard the Hubble Space Telescope, the group separated the faint light of the white dwarfs from that of their companions. The task is complicated by the fierce flaring of red dwarfs, which can mimic a similar ultraviolet signal; a purpose-built calibration technique made the split possible. The study appeared in the Monthly Notices of the Royal Astronomical Society.
Nearby isolated white dwarfs are usually easy to find, O'Brien says — but these four could not be seen at visible wavelengths, because their red dwarf companions were drowning out their light. It is a reminder, she says, that even our own cosmic neighbourhood holds surprises if we look in the right way, at the right wavelengths.
A star rotating too slowly
The most puzzling is G 203-47, just 25 light-years away. Twenty-seven years passed between the first hint of a wobble and the detection; it is now the ninth-closest known white dwarf to the Sun. Its red dwarf takes more than 100 days to turn once on its axis, yet circles the white dwarf every 14.9 days. Gravity normally forces such pairs into locked rotation, as in the Earth-Moon system, where the Moon always presents the same face. Here it has not.
Analysis with the Swift X-ray telescope also showed that G 203-47 emits far less X-ray radiation than expected — one of the study's most surprising findings, and a clue to the unusually slow rotation. What is fascinating, says co-author David Wilson, is that G 203-47 should not be rotating this slowly if it had formed the same way as similar systems. Some of these binaries went through violent, prolonged interactions early on; others had gentler, briefer encounters.
The find closes a gap in the census of the Sun's surroundings: population models predicted four to five close white dwarf-red dwarf pairs within 20 parsecs, and exactly four were found. Room remains. Only about 30 per cent of the red dwarfs in that range have been checked systematically for hidden companions, says Pier-Emmanuel Tremblay of the University of Warwick; nine or ten further systems could still be out there.